DEPENDENCE OF THE ABDOMINAL WALL-MESH INTERFACIAL STRENGTH

ON THE FIXATION METHOD FOR VENTRAL HERNIA REPAIR

by

Hummad Mohammad Tasneem

A Thesis

Submitted in Partial Fulfillment of the

Requirements for the Degree of

Master of Science

Major: Biomedical Engineering

The University of Memphis

May 2014

ii

Copyright © 2014 Hummad Tasneem

All rights reserved

iii

ACKNOWLEDGEMENTS

Without the support of a number of people, completion of this research project

would not have been possible. First and foremost, I would like to thank my friends and

family for being the best support group a person could ask for, cheering me on every step

of the way. I also want to thank Dr. Elaina Paulus, Dr. Nate Stoikes, Dr. John Sharpe,

Mr. Samir Rustom, and the rest of the UT team for leading the animal study and surgical

portion of the experiments. I want to extend gratitude to Robert Jordan and Rick Voyles

for providing maintenance and technical support with the Instron machine. In addition, I

would like to thank my fellow graduate students, in particular Thien-Chuong Phung,

Corey Holt, Adentoun Komolafe, Jie Gao, and Dema Assaf, all of whom advised and

assisted me through my thesis work. Similarly, I want to extend thanks to Jenina Madrid,

Phillip Nuvue, Hadiya Khan, Bilal Tasneem, and Alex Richardson for reviewing my

writing and helping me to develop my final report. Finally, I would like to thank my

thesis committee members: Drs. Eugene Eckstein, Nate Stoikes, and Esra Roan. Their

guidance and support during the course of this thesis study has been indispensable. A

special thanks goes out to my head advisor, Dr. Esra Roan, for keeping me motivated

during this process.

iv

ABSTRACT

Tasneem, Hummad. M.S. The University of Memphis. May 2014. Dependence of the

Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia

Repair. Major Professor: Esra Roan, Ph.D.

Hernia occurrence is on the rise. The most common approach to repair today is a

hernioplasty repair using a surgical mesh for permanent reinforcement after repairing the

hernia defect. Different fixation techniques using materials such as tacks, staples, sutures,

or adhesives are utilized to provide initial fixation until tissue ingrowth occurs. Currently,

regarding ventral hernia mesh repair operations there is inadequate amount of

information available regarding the efficiency of a mesh repair using adhesives.

Consequently, this study compares the interface strength between mesh and tissue when

mesh is fixed with either of the two following techniques: a) adhesives or b) sutures. Lap

shear test conducted on excised tissue specimens determined the fixation strength of the

interface between tissue and mesh at 24 hours, 1 week, and 2 weeks post recovery.

Uniaxial experiments were used to obtain nonlinear material properties of mesh and

tissue. The material properties were then utilized toward building a computational model

of the mechanical experiments.

v

Table of Contents

List of Tables ................................................................................................................................. vii

List of Figures ............................................................................................................................... viii

Chapter 1: Introduction .................................................................................................................... 1

Chapter 2: Background .................................................................................................................... 5

2.1 Surgical Repair....................................................................................................................... 5

2.1. A. Surgical Mesh ............................................................................................................ 6

2.1. B. Adhesive Fixation Technique .................................................................................... 8

2.2. Hernia Repair Complications ................................................................................................ 9

2.3. The Mesh-Tissue Interfacial Strength ................................................................................. 13

2.4 Interfacial Strength Measurements ...................................................................................... 14

2.4. A. Lap Shear Test ......................................................................................................... 16

2.5. Material Properties .............................................................................................................. 17

2.5. A. Linear Material Properties ....................................................................................... 18

2.6 Finite Element Method in Biomechanics ............................................................................. 20

2.6. A. Non-Linear Mechanical Properties .......................................................................... 21

Chapter 3: Methods and Materials ................................................................................................. 24

3.1 Mechanical Testing Instrument and Software ..................................................................... 28

3.2 Biomechanical Evaluation ................................................................................................... 29

3.2. A. Uniaxial Test Method and Procedure ...................................................................... 29

3.2. B. Nonlinear Material Properties .................................................................................. 29

3.2. C. Lap Shear Test ......................................................................................................... 31

3.2. D. Data Analysis ........................................................................................................... 31

3.3 Finite Element Analyses of the Uniaxial Extension and Lap Shear Experiment ................. 33

3.3. A. Simulation of Uniaxial Extension Using FEA ......................................................... 34

3.3. B. Lap Shear Simulation Using FEA............................................................................ 36

Chapter 4: Results and Discussion ................................................................................................. 39

4.1 Uniaxial Extension Experiments .......................................................................................... 39

4.1. A. Average Normalized Force ...................................................................................... 40

4.1. B. Nonlinear Mechanical Properties for Mesh and Tissue ........................................... 42

vi

4.2 Lap Shear Tests for Obtaining Interfacial Strength ............................................................. 45

4.3 Computational Study using FEA to Simulate Mechanical Experiments ............................. 53

4.3. A. Uniaxial Simulation with FEA ................................................................................ 53

4.3. B. Lap Shear Simulation with FEA .............................................................................. 57

Chapter 5: Conclusions .................................................................................................................. 61

5.1. Conclusions ......................................................................................................................... 61

5.2. Clinical Significance ........................................................................................................... 61

Chapter 6 Future Work .................................................................................................................. 63

6.1 Mechanical Experiments and Sample Preparation ............................................................... 63

6.1. A. Future Work ............................................................................................................. 63

6.1. B. Limitations that need to be addressed in Future Studies .......................................... 64

6.2 Finite Element Models for Computational Simulations....................................................... 65

6.2. A. Next Step toward Developing Full Robust Model of a VHMR .............................. 65

6.2. B. Limitations in Current Models that should be addressed in Future Work ............... 66

References ...................................................................................................................................... 67

Appendices ..................................................................................................................................... 71

A. Experiment Results ............................................................................................................... 71

B. Matlab Code: F-D Data Evaluation ....................................................................................... 75

C. Matlab Code: Box Plots ........................................................................................................ 77

D. ABAQUS INP File: Tissue Uniaxial Model ......................................................................... 78

E. Strain Energy Model vs. Uniaxial Tissue Experiments ......................................................... 80

F. FEA Uniaxial Simulation vs. Uniaxial Tissue Experiments .................................................. 83

G. ABAQUS INP File: Surgical Mesh Uniaxial Model ............................................................ 86

H. Strain Energy Model vs. Uniaxial Surgical Mesh Experiments ............................................ 88

I. FEA Uniaxial Simulation vs. Uniaxial Surgical Mesh Experiments ...................................... 90

J. ABAQUS INP File: Lap Shear Model ................................................................................... 92

K. FEA Lap Shear Simulation vs. Lap Shear Experiments ....................................................... 92

L. Artist Permission ................................................................................................................... 92

vii

List of Tables

Table Page

Table 1: Literature Review On Interfacial Strength ..................................................................... 12

Table 2: Mechanical Strength Of Mesh And Tissue ..................................................................... 20

Table 3: FEA Model Part Characteristics ..................................................................................... 35

Table 4: Uniaxial Results With Averaged Normalized Forces ..................................................... 41

Table 5: Strain Energy Function Coefficients For Abdominal Wall Tissue ................................. 43

Table 6: Strain Energy Function Coefficients For Mesh .............................................................. 44

Table 7: Averaged Normalized Forces For Glued Vs. Sutured Specimens .................................. 49

Table 8: Statistical Analysis Using Mann-Whitney-Wilcoxon U-Test ........................................ 49

Table 9: 2-Way Anova Test .......................................................................................................... 49

Table 10: Lap Shear Test Failure Mode Occurrences .................................................................. 53

Table 11: Mesh Convergence Study For Tissue ........................................................................... 55

Table 12: Mesh Convergence Study For Surgical Mesh .............................................................. 56

Table 13: Goodness Of Fit Of Experimental Data With Uniaxial Simulation ............................. 57

Table 14: Goodness Of Fit Of FEA Simulation To Experimental Data ....................................... 59

Table 15: Mesh Convergence Study For Lap Shear Models ........................................................ 60

viii

List of Figures

Figure Page

Figure 1: Schematic Of A Typical Ventral Hernia With Intestinal Protrusion ............................................. 1

Figure 2: Hernioplasty Repair For An Onlay Ventral Hernia Surgery ......................................................... 6

Figure 3: Bard Soft Knitted Polypropylene Surgical Mesh .......................................................................... 7

Figure 4: Mesh Placement. Top) Onlay, Middle) Inlay, Bottom) Sublay ..................................................... 8

Figure 5: A) Peel Test Schematic, B) Indention Test Schematic ................................................................ 15

Figure 6: Lap Shear Test Schematic ........................................................................................................... 16

Figure 7: Mesh To Tissue Ratio For Mechanical Testing. ......................................................................... 17

Figure 8: Uniaxial Tension Test Schematic ............................................................................................... 19

Figure 9: Description Of The Orientation ................................................................................................... 25

Figure 10: Implanted Surgical Mesh With Different Fixation Techniques ................................................ 26

Figure 11: Lap Shear Test Specimens. Left) Glued Fixation, Right) Sutured Fixation .............................. 27

Figure 12: Typical Sample Division ........................................................................................................... 27

Figure 13: A) Instron 3380 B) Bluehill Readings ...................................................................................... 28

Figure 14: Uniaxial Mesh Tests. Left) Direction 1, Right) Direction 2 ...................................................... 29

Figure 15: Lap Shear Test Sample .............................................................................................................. 31

Figure 16: Zoomed In Image Of Surgical Mesh. Left) Threads; Right) Pores ........................................... 35

Figure 17: Partitioned Part For Uniaxial Simulation .................................................................................. 36

Figure 18: FEA Model Of Lap Shear Experiments .................................................................................... 38

Figure 19: Representative Uniaxial Test Data ............................................................................................ 40

Figure 20: Representative Tissue Experimental Response And The Material Model ................................ 44

Figure 21: Representative Mesh Experimental Response And The Material Model .................................. 45

Figure 22: Representative Lap Shear .......................................................................................................... 46

Figure 23: Summary Of Results From Lap Shear Experiments ................................................................. 48

ix

Figure 24: Typical Mesh-Tissue Response At 2 Weeks Vs. Excised Tissue Response ............................. 50

Figure 25: Failure Modes ............................................................................................................................ 51

Figure 26: Stress Contour Plot A) Surgical Mesh B) Abdominal Tissue ................................................... 54

Figure 27: Mesh Convergence Study For Tissue Model ............................................................................ 55

Figure 28: Representative Tissue Experimental Response And The FEA Model ...................................... 55

Figure 29: Mesh Convergence Study For Surgical Mesh Models .............................................................. 56

Figure 30: Representative Mesh Experimental Response And The FEA Model Results. .......................... 56

Figure 31: FEA Model Reaction Force Contour Plot ................................................................................. 58

Figure 32: Comparison Of Experimental And FEA Results Of Lap Shear Experiments ........................... 59

Figure 33: Mesh Convergence Study For Lap Shear Models ..................................................................... 60

x

Table of Abbreviations

Abbreviation Unabridged

HMR Hernia Mesh Repair

VHMR Ventral Hernia Mesh Repair

SEF Strain Energy Function

FEA Finite Element Analysis

FE Finite Element

ν Poisson’s Ratio

RSS Sum of the Residuals

TSS Sum of the Total Squares

R2 Residual Squared

F-D Force vs. Displacement

BC Boundary conditions

3D Three- Dimensions

3D Two- Dimensions

SEF 2nd

Order R.P. Second Order Reduced Polynomial Strain Energy Function

SEF 1st Order Ogden First Order Ogden Strain Energy Function

MPa Mega-Pascal

cm centimeters

σ Stress

ε Strain

A Cross-sectional Area

PP Polypropylene

N/cm Peak Force Per Unit Width

σ-ɛ Stress vs. Strain

UTS Ultimate Tensile Strength

N/cm2 Peak Force Per Unit Surface Area

Rs Sum of the Ranks

N Sample Size

S.C. Specimen bound in Stationary Clamp

D.C. Specimen bound in Dynamic Clamp

C.S. Center of Specimen

Sp. Specimen

IACUC Institutional Animal Care Unit Committee

1

Chapter 1: Introduction

Hernia is a defect in the tissue wall that surrounds various organs of the body. It

eventually leads to protrusion of organs through the weak spot in the muscle, or

connective tissue, called fascia, as illustrated in Figure 1. The two important factors

involved in the formation of hernia are i) local weakness of tissue and ii) increased intra-

abdominal pressure. Some hernias are asymptomatic, whereas some can cause significant

symptoms such as pain, nausea vomiting and change in bowel habits in the affected

individual. These symptoms can progress to surgical emergencies in the event of

incarceration, obstruction, or strangulation. This can instigate further complications as the

affected tissue begins to break down (Canziani et al., 2009; Carriquiry, 1996; Shell IV, de

la Torre, Andrades, & Vasconez, 2008).

Hernias are usually repaired surgically and require that the protruding organ be

pushed back into the human body and the defect zone repaired. This is followed by

closing of the defect in the soft tissue via suturing. Last, if the surgeon chooses to do so,

it is possible to reinforce the sealed hole by fixating a piece of mesh over it.

Complications that can occur after surgical repair of the hernia include reoccurrence of

the hernia, post-operative pain, and post-operative infection (Shell Iv et al., 2008).

A B

Hernia

Figure 1. Schematic of a Typical Ventral Hernia with Intestinal Protrusion.

A) Cross-Sectional View, B) Gross Appearance.

2

While there are different types of hernias that occur in the body, this specific

study applies to ventral hernias, which appear in the abdominal wall at a location of a

previous incision. In 2003, it was estimated that nearly one million abdominal wall hernia

repair operations were performed in the United States, with a predicted 1% annual growth

rate (Rutkow, 2003). Rutkow’s work inspired a more recent study by Poulose et al. which

determined that in 2006 there were 365,400 ventral hernia operations in the United

States, with an annual increase of 3% (Poulose et al., 2012; Rutkow, 2003). This number

is thought to have grown with the increase of obesity in the population, a well-

acknowledged factor in the development and recurrence of ventral hernias. Together,

these factors elevate the necessity of developing better ventral hernia repair techniques.

Traditional ventral hernia mesh repair (VHMR) surgery, using sutures as a mean

of mesh fixation, has proven very effective in practice in reducing recurrence rates

compared to non-mesh repair operations. The recurrence rate of non-mesh repairs is 23%

whereas the recurrence rate of mesh-repairs is 46% (Luijendijk Rw Fau - Hop et al., 2000

). However, post-operative pain associated with sutures still remains a major concern,

which has been postulated to be due to stress associated with sutures and tissue

penetration (Stoikes et al., 2013).

Recently, modern adhesives, such as biological glue, have been utilized as an

alternative fixation technique that does not require suturing to hold the mesh in place

(Canonico et al., 2005; Kaul et al., 2012). In Canziani’s (2009) study, a sutureless

incisional hernia repair technique was examined and found to result in a low occurrence

of chronic post-operative pain. Only 1 out of 40 patients reported recurring post-operative

pain, indicating that the use of staples, or sutures, for securing the mesh, was in fact

3

invasive and unnecessary (Canziani et al., 2009). This was previously indicated in a 2006

study by Petter-Puchner et al., where perforation of the bowel was noted in techniques

involving staples to fixate hernia meshes in surgeries performed on rats. This indicated

that fixation techniques involving tissue penetration raised the risk of damage to other

organs and also increased the risk of possible infections, or further post-op injury ( Petter-

Puchner et al., 2006).

One of the major determining factors of the strength and durability of the overall

repair is the mesh-tissue interface, which must remain intact during recovery such that

adequate tissue growth occurs to seal the hernia defect and embed the implanted mesh.

Grevious et al. stated that on average a typical adult requires a tensile strength of 16

N/cm to prevent a sealed and repaired abdominal wall from reopening (Cobb, Kercher, &

Heniford, 2005; Grevious, Cohen, Shah, & Rodriguez, 2006). On the other hand, it was

reported by Cobbs and Kercher that on average polypropylene mesh could withstand a

force of 32 N/cm (W. S. Cobb et al., 2005; Grevious et al., 2006). Ultimately, these prior

studies suggest that the use of fibrin glue fixation is both viable and generally preferred.

However, lack of data on fixation response of the sutureless methods, especially in

ventral HMR surgeries, is preventing most surgeons from adopting this technique

(Deeken, Faucher, & Matthews, 2012; Katkhouda et al., 2001).

The main goal of this master’s thesis project is to measure and compare the

strength of the mesh-tissue interface a) with sutures and b) with adhesive in a porcine

model. This study looks at the effect of biological glue, specifically human fibrin glue,

and 2-0 polypropylene sutures on the interface strength between the mesh and abdominal

wall. The goal being that with such information available there will be less controversy in

4

accepting the biological glue as an acceptable replacement in practice compared to

current traditional methods. Moreover, this experimental study provided data to validate a

preliminary computational model of the mesh-tissue interface.

5

Chapter 2: Background

Ventral hernia is classified such that the herniated tissue region occurs anywhere

on the abdominal wall where a previous incision has created a weakness in the tissue

layers. Some factors that contribute and increase the risk of ventral hernia manifestation

include abnormal collagen formation, malnutrition, vitamin deficiency, ageing, obesity,

pregnancy, previous invasive surgery, mechanical strains (i.e., chronic coughing or

constipation), and physical trauma (Norton et al., 2008). While environmental conditions

can result in hernia, most reported cases are generally due to uncontrollable congenital

factors (Shell IV et al., 2008). In the case where a ventral hernia is left untreated, patients

risk the possibility of the protruding organs to be subjected to incarceration and

strangulation leading to bowel obstruction and possibly death (Norton et al., 2008).

2.1 Surgical Repair

Hernias are usually repaired surgically and require that the protruding organ be

reduced into the human body before the defect is repaired. There are a handful of surgical

techniques that can be used when treating a hernia on an abdominal wall. The more

traditional method of repair is the tension repair technique (also known as

herniorrhaphy), which involves suturing together the edges of the tissue defect zone

(Norton et al., 2008). Tension repair has mostly been replaced in modern practices with

the tension-free repair technique (also known as hernioplasty), which involves fixating a

piece of mesh on top of the hernia to cover up and seal the defect while allowing a brace

to exist, on top of which tissue integration will occur (Figure 2) (Lau, Patil, & Yuen,

2006). A general consensus of tension-free repair being superior to traditional tension

repair is due to the significant reduction in post-operative morbidity and complications

6

(Luijendijk Rw Fau - Hop et al., 2000; Prieto-Diaz-Chavez et al., 2005).

Figure 2. Hernioplasty Repair for an Onlay Ventral Hernia Surgery

2.1. A. Surgical Mesh

The success of a surgical mesh in terms of preventing hernia-related

complications depends on several factors which include, among others, biocompatibility,

pore size, weight, and mechanical strength (Norton et al., 2008; Patel et al., 2012). While

surgeons are opt to select their preferred mesh product, it has been noted in literature that

polypropylene (PP) mesh (Figure 3), characterized as macro-porous, light-weight (LW),

with monofilament threads, is more commonly used in hernia related surgeries then other

mesh types (Canonico et al., 2005). A tensile test study performed on eight different

types of surgical meshes showed that not all the meshes behaved as effectively in vivo

when compared to a PP mesh (Patel, Ostergard, & Sternschuss, 2012).

7

Figure 3. Bard Soft Knitted Polypropylene Surgical Mesh. Left) Scaled, Right) Close Up

Mesh fixation with the host tissue can occur using any combination of sutures,

staples, clips, tacks, and biological glues; while mesh placement can be done at either

ends, or within, the site of the herniated tissue (i.e. onlay, sublay, inlay mesh placement)

(Clarke et al., 2011) (see Figure 4). There is no specific surgical approach that has been

acknowledged as the gold standard for a hernia repair, and as such it is the operating

surgeon’s preference on mesh type, placement, and fixation technique used for repair

(Clarke et al., 2011; Israelsson, Smedberg, Montgomery, Nordin, & Spangen, 2006;

Kingsnorth, Shahid, Valliattu, Hadden, & Porter, 2008).

8

Figure 4. Mesh Placement. Top) Onlay, Middle) Inlay, Bottom) Sublay

2.1. B. Adhesive Fixation Technique

The use of biological glue for fixating mesh onto tissue is still relatively new to

VHMR operations in comparison to the standard suturing techniques. One common

product applied for adhesive fixation is the off label use of Tisseel (Baxter, Deerfield, IL,

USA), fibrin-based glue. This adhesive is biodegradable and formed by combining

human-derived fibrinogen, calcium chloride, and thrombin to create a matrix of

polymerized fibrin fibers. The biological properties of Tisseel allow homeostasis, wound

healing, and fibroblast proliferation to occur at the fixation site, all of which are added

benefits over the sutured or stapled mesh-fixation techniques (Campanelli et al., 2012). It

is important to note that there are currently no FDA approved adhesive products in the

Rectus Abdominis Peritoneum

Onlay Mesh Placement

Inlay Mesh Placement

Sublay Mesh Placement

Rectus Sheath

9

market for this surgical application.

Katkhouda et al. has demonstrated the comparable efficacy of fibrin glue to staple

fixation in a 2001 study using porcine tissue for inguinal hernia repair. Samples prepared

after a 12-day recovery showed the two methods to be generally comparable in tensile

strength when subjected to a make-shift pull-off test (Katkhouda et al., 2001). Regarding

data for ventral hernia repair, Chevrel and Rath reported in a trial including 389 patients

that there was a recurrence rate of 18.4% when no mesh was used to reinforce the repair,

5.5% with mesh onlay reinforcement, and 0.97% when fibrin glue was used to fixate the

mesh in place (Chevrel & Rath, 1997). The use of fibrin glue was further supported by

Canonico, (2005), who presented an argument for the sutureless repair technique (n =

80), showing drastic reduction in surgical time and greater surgeon satisfaction,

especially the perceived ease of the operation and reduced reports of post-operative pain

(Canonico et al., 2005).

2.2. Hernia Repair Complications

Some general complications associated with hernia repair operations include

recurrence of the hernia, nerve entrapment (chronic pain), bowel obstruction, seroma

build-up, fistula formation, post-operative pain, and wound infection (Norton et al.,

2008). Failure modes of the mesh upon being implanted into the host include mesh

mechanical failure, mesh contraction, mesh migration, mesh curling/buckling, mesh

infection, undesired adhesion of mesh with other organs or local tissue, fistula formation,

erosion of mesh, and seroma formation (Chevrel & Rath, 1997; Robinson, Clarke,

Schoen, & Walsh, 2005). Variations in fixation techniques for tension free repair will

additionally alter the likelihood of particular complications from occurring.

10

Traditionally, the tension-free technique was performed with fixating mesh onto

the abdomen using any combination of sutures, staples, clips, or tacks; the mesh suturing

fixation method being the most popular in traditional tension-free repair operations. The

one thing all of these fixation methods have in common is that they require tissue

penetration for mesh anchoring. Consequently, any location where such an incision is

made into the abdominal tissue wall will become susceptible to the reoccurrence of future

incisional ventral hernias. A brief summary of the fixation strength obtained from

different fixation techniques for VHMR is shown in Table 1. In comparison to the

tension repair procedure any combination of these fixation techniques are superior in

reducing complications.

As stated previously in this document, the main concerns associated with sutured

mesh-fixation is the post-operative, acute and chronic, pain speculated to be caused by

the excess tissue penetration necessary for mesh-anchoring (Canonico et al., 2013). A

detailed investigation comparing post-operative pain associated with fixation techniques

was done in a 2009 study by Canziani’s. In this study, a sutureless incisional hernia repair

technique was examined and found to result in an extremely low occurrence of chronic

post-operative pain (only one out of 40 reported recurring post-operative pain), indicating

that the use of staples or sutures for securing the mesh was in fact invasive and

unnecessary (Canziani et al., 2009). Further support toward suture alternatives was

provided by a 2012 meta-analysis of PubMed data for inguinal hernias which

demonstrated comparable recovery times and recurrence rates of sutureless mesh (e.g.

fibrin glue) fixation to sutured mesh fixation, as well as reduced occurrences of post-

operative pain in the sutureless reinforcement (4% sutureless compared to 12% sutured)

11

which is a clear advantage over the traditional sutured technique (Kaul et al., 2012). In

the end, these prior studies suggest that the use of fibrin glue fixation has an upper hand

in i) reducing acute and chronic post-operative pain ii) recurrence rate iii) surgical time

iv) seroma formation v) better patient acceptability, and is both viable and generally

preferred. Even so, the amount of data available in literature has proven to be insufficient

in convincing many surgeons from adopting this sutureless technique in VHMR

operations and therefore additional experiments, such as this study, are being performed

to provide greater support.

12

Table 1. Literature Review on Interfacial Strength

Literature Animal Model/

Mesh

Fixation

Technique

Healing

Time

Mechanical

Testing

Procedure

Interfacial

Strength

(as reported)

Translated values for

comparison

(N/cm)

(Katkhouda et al.,

2001) Pigs / PPm

Tiseel

Nothing

Staples

12 Days Pull-off

0.955 kg

1.03 kg

0.46 kg

1.87 N/cm

2.02 N/cm

0.90 N/cm

(Clarke et al., 2011) Pigs / PPm

Tiseel

Tiseel & Tacks

Tacks

Suture & Tack

4 Weeks Pull-off

5.2 x 104 N/m

2

5.0 x 104 N/m

2

6.4 x 104 N/m

2

6.8 x 104 N/m

2

31.20 N/cm

30.00 N/cm

38.40 N/cm

40.80 N/cm

(A. Petter-Puchner,

Fortelny, Mittermayr,

Öhlinger, & Redl,

2005)

Rats / Ti mesh

Rats / VYPROII

Tiseel

Staples 17 Days

Burst

(80 mmHg)

Pull-off

(300 g pull

force)

No failure in any

interface No failure in any interface

(Eriksen, Bech,

Linnemann, &

Rosenberg, 2008)

Pigs/ Motif

Pigs / Proceed

Tiseel

Ti Tacks

Tiseel

Ti Tacks

30 Days Peel Test

3.0 ± 1.5 N/cm

3.87 ± 1.2 N/cm

3.44 ± 0.7 N/cm

2.69 ± 1.3 N/cm

3.00 ± 1.50 N/cm

3.87 ± 1.20 N/cm

3.44 ± 0.70 N/cm

2.69 ± 1.30 N/cm

(McGinty, Hogle,

McCarthy, & Fowler,

2005)

Pigs/ PPm

Pigs / ePTFE

Pigs / PCO

Suture & Tack 28 Days Peel Test

2.1 N/cm

1.3 N/cm

2.8 N/cm

2.10 N/cm

1.30 N/cm

2.80 N/cm

(Majercik, Tsikitis,

& Iannitti, 2006) Pigs/ ePTFE Tacks

2 weeks

4 weeks 6

weeks 12

week

Lap Shear

0.83 ± 0.06 lbs

1.06 ± 0.07 lbs

0.88 ± 0.08 lbs

1.13 ± 0.07 lbs

1.85 ± 0.13 N/cm

2.36 ± 0.16 N/cm

1.96 ± 0.18 N/cm

2.51 ± 0.16 N/cm

(d'Acampora,

Kestering, Soldi, &

Rossi, 2007)

Rats/ PPm

Rats / Vypro Nothing 28 Days Lap Shear

48.05 ± 9.05 N

45.32 ± 16.8 N

24.03 ± 4.53 N/cm

22.66 ± 8.40 N/cm

(Schug-Pass, Lippert,

& Köckerling, 2010)

Pig/ PPm

(Bard soft)

Nothing

Tiseel 0 hour

Indention

3.17 ± 0.50 N

73.6 ± 13.4 N

0.32 ± 0.05 N/cm

7.36 ± 1.34 N/cm

(Gonzalez et al.,

2005)

Pig / PPm

Pig / PEm Sutures 3 Months Pull-off

159 N

194 N

23.73 N/cm

22.30 N/cm

Polyester mesh (PEm); Polypropylene mesh (PPm); Expanded polytetrafluoroethylene mesh (ePTFE) ; Polyester mesh with anti-adhesive collagen layer (PCO)

13

2.3. The Mesh-Tissue Interfacial Strength

Before the hernia is fully healed, stress due to intra-abdominal pressure will

concentrate and build up at the hernia repair site. Respectively, the mesh-tissue repair is

responsible in resisting the internal loads until a time arrives that the wound has fully

healed. In most cases it is not as much necessary for the repair site to be fully healed to

resist the internal loads as much as for complete tissue ingrowth to occur. While waiting

for adequate tissue in-growth, the mesh-tissue interface becomes an important

determining factor in preventing recurrence (Majercik, Tsikitis, & Iannitti, 2006). In

light of this, it was determined in a previous study by Majercik et al., that nearly 70% of

the full tissue ingrowth occurred within 2 weeks of wound healing (Majercik et al.,

2006). It was therefore suggested that prior to this time the mesh-tissue interface was a

significant contributor when predicting the possibility of particular failure modes

occurring (i.e., mesh migration, curling/buckling, and reoccurrence) (Eriksen, Bech,

Linnemann, & Rosenberg, 2008).

Since the fixation strength of the mesh-tissue interface is reliant on the fixation

technique used for binding mesh and tissue together. There becomes a relative need to

better understand the fixation techniques in order to understand and improve the mesh-

tissue interface and respectively the mesh-tissue repair. More specifically, in any system

subjected to loading there is a buildup of stresses. How the system distributed these

stresses provides insight on which part of the system is most responsible toward resisting

failure. In regards to mesh-tissue fixation there becomes a fundamental difference in

stress distribution, which is specific to the fixation technique.

14

In the case of sutured mesh fixation the stresses will concentrate at the site where

the sutures are embedded within the tissue. On the other hand, when adhesive fixation is

used, the stress will distribute uniformly over the entire mesh-adhesive surface area. As

such, suture fixation prior to adequate tissue ingrowth will be reliant on the tensile

strength of the sutured material and the surrounding fascia it is embedded within;

whereas glued fixation will be dependent on the adhesive strength of the product and the

surface area of the mesh-adhesive interface. Because of this, one of the added benefits of

using sutures over adhesives is that sutures have a relatively stronger raw tensile strength

than biological glue and will be able to withstand a greater load before failing (Klinge et

al., 1996; Klinge, Klosterhalfen, Muller, Ottinger, & Schumpelick, 1998).

2.4 Interfacial Strength Measurements

Due to the importance of the fixation strength of a mesh-tissue repair it is

necessary to perform mechanical tests, which can provide quantitative data of the

interfacial strength. This information would make it possible to compare different

fixation techniques and better evaluate the added benefits of fixation strength to other

related complications. One mechanical approach that is often utilized for determining

interfacial strength is called adhesion tests. A few of the most common types of adhesion

tests performed, among others, include indention, lap shear, peel-off, pull-off, and burst.

For meaningful results, the mechanical test chosen must be designed to subject the mesh-

tissue specimen to the stresses it would encounter in vivo within the abdominal wall.

Mechanical failure of the interface due to stress build-up initially may occur by

developing a defect within the interface at a reentrant corner of the bonded stiffener (i.e.

the mesh). A bonded stiffener is the substrate material in the system that increases the

15

global stiffness of the composite structure (Lacombe, 2005). Consequently, once this

initial weakness occurs within the interface the failure site will propagate and increase in

size over the distance of the implanted mesh piece. For mechanical tests where a tensile

load is applied laterally through the gauge length the failure will occur along the samples

width. Therefore, it is common practice within literature for report values, from such

mechanical tests, to be normalized in the form of peak force per unit width (N/cm).

In vivo, when internal pressure of the abdominal wall is applied to the mesh

interface, failure propagation can occur along any direction. For adhesive tests such as

peel and lap shear the load is applied on the specimen in one particular direction, which is

labeled as the length (Figure 5A). For mechanical tests such as indention or burst were

the load is applied over the entire surface area of the specimen the resulting max force

cannot be normalized using the width since failure could have occurred along any

direction (Figure 5B). For those mechanical tests the data is generally normalized using

the surface area of the mesh and reported as UTS (N/cm2).

Figure 5. A) Peel Test Schematic, B) Indention Test Schematic

A B

16

2.4. A. Lap Shear Test

For this study, the mesh-tissue interface was evaluated using a lap shear test

procedure. Similar to peel, the lap shear test is performed by having a tension load

applied on the specimen such that stress concentration occurs at the interfacial region. In

lap shear tests the loading occurs differently in both ends of the sample, such that a

different substrate is loaded on either end (see Figure 6). Specifically, one substrate is

displaced at a particular velocity while the second substrate is fixed in place, preventing

all movement and rotation. This will cause the stress propagation to occur within the

interface; in an ideal case, failure would occur due to shearing of the two substrates.

Figure 6. Lap Shear Test Schematic

In respect to the previous testing methods mentioned in this text, the main

advantage of using a lap shear test over an indention test is that it does not require a

17

larger tissue to mesh ratio of surface area for individual test specimens (Figure 7).

However, the loading can only be applied in one specific direction versus all directions as

would have occurred in an indention test. On the other hand, compared to the peel test the

advantage of the lap shear experiment is that the axial loading is applied in the same

direction as shear stresses that occur in the interfacial region in vivo; one failure mode

directly related to shearing is mesh migration.

Figure 7. Mesh to Tissue Ratio for Mechanical Testing.

Green) Surgical Mesh, Red) Tissue Specimen

2.5. Material Properties

Biocompatibility of an implant is not limited to the chemical reaction of the host

body but also its biomechanical compatibility to the unique environmental loads of the

biological system. The presence of embedded mesh within a tissue will ultimately make

the repaired tissue stiffer than its healthy counterpart. Consequently, a stiffer tissue

response in the abdominal wall is associated with a reduction in the physiological

compliance of the tissue (Hernández et al., 2011).

The compliance of the abdominal wall is very important as it enables the

18

abdominal wall to withstand and adjust to increased stresses and minor pressure changes

due to simple and/or strenuous activities. Since the abdominal wall is anisotropic, it has

become essential to consider the following two factors to achieve a superior repair: 1) the

finest material and 2) accurate mesh orientation for each hernia repair. This would ensure

that the mesh has characteristics that match closely to the natural characteristics of the

abdominal wall (Hernández-Gascón et al., 2011). The mechanical anisotropic behavior of

the surgical mesh is determined by the filament composition, mesh weave, and spatial

arrangement of the filaments, while in the abdominal wall, it is the collagen fibers rather

than muscle fibers responsible for determining passive tissue tensile strength (Hernández

et al., 2011). It is therefore important, to determine the material properties of the

implanted prosthetics and the associated tissues as to determine if the presence of a

particular prosthetic will cause any biomechanical change in the system. In this study,

material properties were obtained by performing uniaxial mechanical tests on samples of

excised abdominal tissue and surgical mesh.

2.5. A. Linear Material Properties

Uniaxial tension experiments are a type of tensile test used for obtaining

mechanical properties of materials (Lacombe, 2005). These tests cannot account for the

anisotropic nature of a material and instead obtain isotropic material properties, which are

characterized by a specific orientation. The test is done by loading the ends of a uniform

material specimen such that the sample is stretched until failure along a specific axial

direction. Figure 8 shows a schematic of a uniaxial tensile test, when done using a testing

apparatus raw data will be in the form of force vs. displacement (F-D).

19

Figure 8. Uniaxial Tension Test Schematic

Material properties cannot be directly derived from the F-D data and must first be

translated to stress vs. strain (σ-ɛ), which is done using Equation 1. In this study the stress

(σ) was translated from this data by dividing every force (F) data point by the specimen’s

initial cross-sectional area (A) (i.e. Sample width x thickness). Similarly the strain (ε)

was translated from the data by dividing every displacement (Δl) data point by the initial

gauge length (lo) (i.e. specimen height subtracted by specimen height bounded within

clamps). Comparably, other studies have also obtained material properties of abdominal

tissue, PP surgical mesh, and PP sutures. Table 2 provides a brief list of some of the

material properties reported in literature regarding similar studies to this thesis.

(1)

20

Table 2. Mechanical Strength of Mesh and Tissue

Literature Material Test Type Average Strength

± STD

(Klinge et al., 1996) Mesh Indention Test 40-100 N/cm

(W. S. Cobb et al., 2005) PP Mesh Indention Test 43.2 N/cm

(Schug-Pass et al., 2010) PP Mesh

(BARD soft) Indention Test 29 N/cm

Manufacturer’s Information PP Mesh

(BARD soft) Indention Test 36 N/cm

(Cobb, Harris, Lokey, McGill, & Klove,

2003) PP Mesh Indention Test 32 N/cm

(Hernández-Gascón et al., 2011) PP Mesh

(Optilene)

Uniaxial Tests:

Direction 1

Direction 2

7.57 ± 0.74 N/mm

10.79 ± 1.05 N/mm

(Klinge et al., 1996) PP sutures Uniaxial 30 N/cm

(Klinge et al., 1996) Human

Abdomen

Uniaxial Test:

Horizontal Direction

Vertical Direction

Theoretical Value:

(rupture strength)

60-80 N/cm

20-30 N/cm

4-16 N/cm

(William S. Cobb et al., 2005) Human

Abdomen

Theoretical strength:

(at max intra-

abdominal pressure)

11 – 27 N/cm

2.6 Finite Element Method in Biomechanics

Computational modeling refers to an engineering application at which material,

loading, and environmental conditions are simulated virtually giving a visual

representation of how mechanical loads would propagate within a composite structure

(Reddy, 2004). One common method for computational modeling is finite element

analysis (FEA), which is a numerical technique used to compute approximate solutions to

boundary-value problems (i.e., a differential equation with specific boundary conditions)

(Venkatesh, 2011).

In this approach for theoretical computations, the stress distribution is evaluated

by creating a virtual simulation of the system with particular characteristics, such as

loading, geometry, material properties, boundary constraints, and material interface

conditions, translated into the form of mathematical equations (Reddy, 2004). The

21

mathematical equations used are generally determined from known material values or

experimental data which is translated into an approximate numerical solution using

various principles of solid mechanics. The goal is to find values for unknown stresses at

points on or within the system (Holzapfel, 2000; Seshu, 2004). These stresses are known

as field variables which are infinite in number due to the continuous nature of the body

(Holzapfel, 2000).

2.6. A. Non-Linear Mechanical Properties

In FEA it is required to give material properties for each simulated part. In

regards to surgical mesh and abdominal wall tissue these properties cannot be described

as linearly elastic materials and require a relatively more complex application of

continuum mechanics. In brief, continuum mechanics is an area of mathematical physics

which describes the fundamental laws governing motion and deformation of a structure

as a continuum mass rather than as discrete particles (Reddy, 2004). Within the field of

continuum mechanics, nonlinear materials such as hyperelastic materials are often

described using the constitutive laws with corresponding constitutive equations

(Holzapfel, 2000). Respectively, the constitutive equations are further described in the

field of solid mechanics (Holzapfel, 2000).

For constitutive theories, it is important to note that the resulting mathematical

model created to represent a real-life material behavior is based off of actual

experimental data obtained from mechanical testing (e.g. excised tissue and surgical

mesh). Constitutive models or material models are very important and necessary when

creating a computational model. These models are generally quite simple for elastic

behavior but become relatively more complex and less likely to predict realistic behavior

22

when used toward describing nonlinear continuum mechanics (Holzapfel, 2000). One

approach to modeling the behavior of nonlinear hyperelastic material is using a strain

energy function (SEF) to describe a mechanical response. Some of the more well-known

strain energy functions that are readily available to be used in engineering software

packages include Hookean, Mooney-Rivlin, Polynomial, Reduced Polynomial, and

Ogden descriptions (Holzapfel, 2000).

Modern software’s used for computational studies such as finite element analysis

(FEA) have built in curve fitting generators that can take experimental stress-strain values

and relate potential fits with various strain energy functions (ABAQUS/CAE user's

manual : version 6.4, 2003). Two of the more relevant strain energy functions used in this

study were the 2nd

order reduced polynomial and the 1st order Ogden SEF’s. The

corresponding equations and characteristics of these functions are described in the

Equation 2 and Equation 3 (ABAQUS/CAE user's manual : version 6.4, 2003). In the

equations below “W” is work, “U” is energy, “J” is Jacobian and is used to describe the

total volume change at any given point, is stretch ratio, “μ” and “α”, and “N” are

material constants, “D” is the constant used to describe rate of deformation and is

dependent on the Poisson’s ratio, the different “I” values are strain invariants, and “Ci” is

a temperature-dependent material parameters.

23

∑

∑

∑

(First Order Ogden SEF)

∑

∑

∑

(Second Order Reduced Polynomial SEF)

(2)

(3)

24

Chapter 3: Methods and Materials

The aim of this thesis was to obtain a more in-depth understanding of the load

carrying capacity of the mesh-tissue interface resulting from a VHMR operation. The

major interest in this research is in whether there is a difference in the resulting interfacial

strength if fixation of the prosthesis is performed using fibrin glue instead of sutures. The

end goal is to shed light towards the application of adhesive fixation techniques in

VHMR surgeries so that surgeons can better weigh the benefits of either technique when

making an informed decision regarding their surgical approach.

Animal Model

Dr. Stoikes and his team at the University of Tennessee Health Science Center

(UTHSC) obtained porcine abdominal specimens through an animal study. All

procedures were approved by the Institutional Animal Care Unit Committee, IACUC ID

# 12-103.0-A. Female mongrel pigs that weighted 25-30 kg were used as the animal

models. Polypropylene soft knitted (BARDTM

) surgical mesh was implanted onto the

abdominal wall using the onlay mesh placement. Specific tissue layer onto which mesh

was fixated was generally the rectus sheet and rectus abdominis. Two prosthetic mesh

pieces with an area of 6 by 4 cm were implanted in each animal model with 2-3 cm of

space between them. Both soft tissue and surgical mesh are anisotropic materials (see

Figure 9). In this particular study the mesh was implanted by the surgeon without

consideration of mesh/tissue alignment.

25

Of the two implanted mesh pieces, one was fixated using 4 ml of the Tisseel fibrin

glue (Baxter Healthcare), spread uniformly on and throughout the mesh surface by the

surgeon’s finger (see Figure 10). The second mesh piece was fixated onto the abdominal

wall using 4, 2-0 Prolene PP sutures (Ethicon); which were sewn near the edges of the

mesh as shown in Figure 10. After the mesh was implanted the animal specimens were

allowed to recover for a given amount of time so that tissue ingrowth into the mesh could

occur. The amount of time allowed for recovery was regulated at 24 hours, 1 week, and 2

weeks for 8 animal models per group. For these experiments, animal care and operations

were carried out at the UTHSC and only excised samples provided by Dr. Stoikes and his

team was brought over to the University of Memphis for testing. Samples of mesh-tissue

specimens are shown in Figure 11.

Figure 9. Description of the Orientation

Direction 1

Directio

n 2

Direction 1

Directio

n 2

26

Figure 10. Implanted Surgical Mesh with Different Fixation Techniques

The excised abdominal wall for each animal specimen was used to create two

specimens for mechanical testing, as shown in Figure 11. The individual samples

included the 6 by 4 cm2 mesh piece fixated onto tissue that on average had a surface area

of 8 by 5 cm, the extra 2 cm of tissue height extending from one end of the sample;

thickness of samples were on average 0.75 cm. An area of approximately 1 by 4 cm of

mesh-tissue was cut away from the sample for a separate study regarding histological,

mesh contraction, and tissue ingrowth evaluation at the Department of Pathology of the

UTHSC (see Figure 12A) (Stoikes et al., 2013). The removal of part of the mesh-tissue

sample damaged the end of the mesh and tissue, which was cut away, creating defects

where stress propagation under loading could occur. This defect zone was accounted for

in the mechanical tests by binding and directly loading that specific region (see Figure

Sutured

Fixation

Mesh

Placement

Prior to

Fixation

Glued

Fixation

27

12B). Biomechanical testing of the interfacial strength was done using a lap shear testing

procedure and uniaxial testing procedure for determining material properties of tissue and

mesh.

Figure 11. Lap Shear Test Specimens. Left) Glued Fixation, Right) Sutured Fixation

Figure 12. Typical Sample Division

Zone A:

grips only

mesh

Zone B:

grips only

tissue

28

3.1 Mechanical Testing Instrument and Software

Mechanical testing was done using an Instron 3380 (Canton, MA) testing device

(see Figure 13A). The device functioned by displacing the position of a single clamp.

This moving clamp was capable of exerting either a tension or compressive load on the

sample, depending on the direction of the clamps movement. The load developed by

stretching the tissue is measured using a load cell and the displacement of the clamp

measured with a built in extensometer. The proposed test procedure used a 5 kN load cell

and a software protocol programmed to provide a displacement velocity of 0.42 mm/s.

The specific velocity was used to apply a relatively slow loading so that a near quasi-

static material response could be recorded. Raw force and displacement data was

recorded by a digital output reader running the Bluehill 2 software (see Figure 13B), and

extracted as an excel comma separated spread sheath, .csv file type. The multiple

peaks/spikes in the F-D data for sutured fixation, in Figure 13B, marks the point of the

lap shear test when a single suture pulls out of the mesh-tissue specimen.

Figure 13. A) Instron 3380 Mechanical Testing Apparatus with Pneumatic Clamps

B) Bluehill 2 Force vs. Extension Readings

A B

29

3.2 Biomechanical Evaluation

3.2. A. Uniaxial Test Method and Procedure

The mechanical response of the mesh and abdominal tissue under loading was

measured through uniaxial extension experiments. The tissue was cut so that each tested

sample had dimensions approximately 4 cm in width, 5 cm in height, and on average 0.75

cm in thickness. The thickness of individual samples varied between 0.5 and 1 cm

depending on the topological location of the specimen and was never uniform. The tested

mesh pieces were 3 cm in width, 6 cm in height, and 0.044 cm in thickness. Figure 14

shows a sample uniaxial test on surgical mesh in both directions; in these tests the fiber

orientation strongly influenced the load bearing capacity of the specimen.

Figure 14. Uniaxial Mesh Tests. Left) Direction 1, Right) Direction 2

3.2. B. Nonlinear Material Properties

The non-linear hyperelastic material model was determined in ABAQUS by curve

fitting various SEF’s with the experimental data to obtain a best fit. In this step, the curve

fitting was done to the experimental data up to a strain of 1, ϵ = 1, such that the

corresponding SEF modeled the initial mechanical response of the material. Additionally,

SEF’s in ABAQUS require the input of a Poisson’s ratio for the material. These values

30

were not determined in the scope of this study and were instead approximated from

known information. For surgical mesh a Poisson’s ratio of 0.45 was applied, ν = 0.45,

this value corresponds to the poisons ratio of a solid block of polypropylene. In regards to

soft tissue, previous literature has modeled this material with a nearly incompressible

Poisson’s ratio (0.4 < ν < 0.5). For the sake of this study, different Poisson’s ratios

between 0.46 and 0.498 were tested and a best fit was selected.

The best fit was decided by evaluating the goodness of fit as determined by the R2

value between the experimental and theoretical data sets. This was done by first plotting

the material response of the SEF in Excel (Microsoft, Redmond, WA) and curve fitting it

with a tread-line so that an equation which matched the data set could be obtained. This

equation was then used to determine the stress from the material model for all strains in

the uniaxial experimental data sets, such that the size of the array would be identical for

both data sets. The array of values for experimental stress and theoretical stress were

compared for a goodness of fit using a coefficient of determinates method, 0 < R2 ≤ 1

where a value of 1 portrays identical data sets. The outputted R2 value was determined by

first solving the sum of the squares (TSS) using Equation 4A, followed by the residual

sum of the squares (RSS) using Equation 4B, and finally using TSS and RSS values to

solve for R2 as denoted in Equation 4C.

∑ ∑

B C A (4)

31

3.2. C. Lap Shear Test

The interfacial strength between mesh and tissue were measured using a lap shear

test procedure. Individual specimens were prepared for testing by shaving off excess

tissue from the edges and the tissue thickness. The sample thicknesses were reduced at

times so that the specimen would be able to fit within the clamps. Approximately, 1x4

cm2 surface area of mesh was released from the tissue so that it could be held by the

upper clamp. The excess tissue, which remained untouched by the prosthetic mesh, was

held by the lower clamp. Eventually after enough imposed strain, the ultimate strength of

the mesh-tissue interface was observed. For each sample the initial gauge height between

the two clamps before testing was recorded as well as the initial width of the mesh, the

height of the overall sample, and the location at which failure occurred (i.e. interface,

mesh, and tissue). Figure 15 shows a sample of a mesh-tissue lap shear test.

Figure 15. Lap Shear Test Sample

3.2. D. Data Analysis

Interfacial strength and the relevant material strength were reported in the form of

peak force per unit width (N/cm). These values as well as other computations were done

32

in Matlab (MathWorks Inc., Natick, MA) and Microsoft Excel. In particular, a Matlab

code was written that graphed the force to displacement data and outputted all the

maximum forces for individual samples. These values were than stored and sorted in

Excel, where they were normalized with their respective initial mesh width. Statistical

analysis to determine significant difference between fixation techniques at evaluation

time was done through a 2 sample t-test for normally distributed variables and Mann-

Whitney-Wilcoxon statistical U-test, also known as a Wilcoxon’s Rank Sum test, for

ordinal variables.

Student t-tests were done within excel spreadsheet while Mann-U tests were done

using an online statistical package called VarrarStats (VarrarStats, New York). Samples

were evaluated at a confidence level of 95%; such that there was a significant difference

reported from the t-test if P < 0.05. On the other hand for the Mann-U test significant

difference was evaluated using the UA value as the determinate factor (Equation 5).

Which was done by comparing the UA value with the lower and upper limit of the U-test;

if the value falls outside the range provided, lower-upper limit, the two data sets are

significantly different. An additional statistical test was done using a 2-way analysis of

variance (Anova), performed in SigmaPlot (Systat Inc., Illinois), which was able to

compare the influence of 2 different independent variables on one dependent variable. In

this case the two independent variables were the evaluated time points (24 hours, 1 week,

and 2 weeks) and the fixation method (glued or sutured), where the dependent variable

was the fixation strength reported. Other analysis included identifying outliers in Matlab

and removing them from calculations regarding average normalized loads.

33

3.3 Finite Element Analyses of the Uniaxial Extension and Lap Shear Experiment

The long-term goal of the computational study is to generate a simplified, robust

model of hernias with mesh repair, which would help researchers better understand the

biomechanics of a repaired abdomen. Wound healing after mesh fixation includes a

biological response of hemostasis, inflammation, proliferation, granulation, remodeling,

and maturation. At two weeks of recovery it has been noted in literature that nearly 70%

of tissue ingrowth has already occurred (Majercik et al., 2006). The tissue ingrowth

corresponds to the proliferation step where collagen formation and new tissue fibers are

created. Therefore, a computational model mimicking interfacial properties at two weeks

would provide insight on a repaired abdominal wall where the mesh has been almost

completely embedded within the tissue. The first step in creating this model is attempting

to create a method to model the mechanical responses of both surgical mesh and the

excised abdominal tissue. After which a coupling method was sought for that could

accurately mimic the interfacial strength of a repaired abdomen at two- week recovery

post operation.

Individual tissue and mesh material models were made on the assumption of the

continuum theory of finite strain. It was also assumed that the materials were best

represented by nonlinear hyper-elastic strain energy functions. The anisotropic response

was defined by identifying a preferred material direction (same orientation as recorded

during experiments). Lastly, the tissue response was modeled to mimic a passive

abdominal wall.

(5)

34

3.3. A. Simulation of Uniaxial Extension Using FEA

A commercially available FEA software ABAQUS (Providence, Rhode Island)

was used for this work. Parts for the mesh and tissue specimens were drawn to the

corresponding dimensions recorded for each of the uniaxial experiments. The shapes for

both parts were simplified when modeling; for tissue it was assumed that the shape of the

specimens were uniform and the material homogenous such that the part could be drawn

simply as 3D solid rectangular pieces. In real life surgical mesh is formed of woven

threads and contains many pores, see Figure 16, such that the surface area is

exceptionally smaller than that of a uniform rectangular block with the same dimensions

of length, width, and thickness. Solid parts in FEA, such as those used to model tissue,

cannot be in contact with wired elements which would be required to model surgical

mesh formed of woven threads (ABAQUS/CAE user's manual : version 6.4, 2003).

Therefore, a compromise was made and the surgical mesh was simplified as a

homogenous 3D shell rectangular piece. Parts varied in dimension from sample to sample

but remained constant in element and design characteristics as listed in Table 3. A sample

of the ABAQUS standard input file for these simulations is available in Appendix D and

Appendix G.

35

Table 3. FEA Model Part Characteristics

Abdominal Tissue Bard Soft PP Mesh

Part Type 3D Solid Homogenous 3D Shell/Continuum Homogenous

Element Shape Hex Dominated Quad Dominated

Element Type

C3D8H Elements: quadratic

elements, 8 node linear

bricks, hybrid formulation,

with constant pressure

S4R Elements: Quadratic Elements, 4

node doubly curved think or thick shell

structure, reduced integration, hourglass

control, finite membrane strains

Material SEF Reduced Polynomial 2

nd

Order Ogden 1

st Order

Poisson’s Ratio Nearly Incompressible:

0.46 ≤ v ≤ 0.498

Polypropylene:

v = 0.45

Dimensions: 5 x 4 x 0.75 cm 6 x 3 x 0.044 cm

Figure 16. Zoomed In Image of Surgical Mesh. Left) Pores; Right) Threads

The modeled parts were partitioned into three regions; the first region

representing the area bound in the upper clamp, the second being the gauge area, and the

third being the area bound by the bottom clamp (see Figure 17). The first region was

given a displacement boundary condition. The magnitude of this displacement was

equivalent to the value obtained in the experimental data at which this specific simulation

supposedly failed. The third partitioned region was given a boundary condition to be

pinned in all directions such that there was no movement.

36

Figure 17. Partitioned Part for Uniaxial Simulation.

Arrows) Upward Displacement, X’s) Fixed Region

The computational test was run and the sum of the reaction forces in the y-

direction for each node within the fixed region was collected. The reaction forces were

divided by the initial cross-sectional area which allowed for the FEA results to output in

the form of stress and strain. To compare the computational predictions from FEA with

the experimental results, both data sets were graphed simultaneously and a goodness of

fit determined by calculating the R2 value. Mesh convergence studies were conducted to

determine the number of elements necessary for optimal results with respect to

computational time and cost.

3.3. B. Lap Shear Simulation Using FEA

This FEA simulation used customized dimensions for each specimen to match

recorded dimensions during testing. Each part remained consistent with the part and

element characteristics determined by the uniaxial FEA simulations (see Table 3). These

parts were then assigned the material SEF coefficients for the orientation where the

37

original test load had been applied. The mesh and tissue were adhered together using an

interaction constraint, surface-surface contact. This constraint was further customized to

adjust over closures and to tie adjusted surfaces, i.e., surface area of both mesh and tissue

part depicted by yellow squares in Figure 18. Contact properties comprised of “normal

behavior”, which included a pressure over closure, “Hard Contact”. The full FEA model

with associated constraints can be seen in Figure 18. The loading in ABAQUS was done

by applying a displacement boundary condition onto the mesh shell edge and having it

moved upward equivalent to the displacement of the first peak for that specimen from the

lap shear experiments. The ABAQUS standard input file for this simulation is available

in Appendix J.

One other boundary condition was applied; fixing in place the bottom partitioned

region of the tissue. The sum of the reaction force at the fixed tissue region was recorded

at various extensions and compared to the load vs. extension data of the lap shear

experiments. The two data sets were graphed simultaneously and compared to each other

by analyzing the goodness of fit. Last, a mesh convergence study was undertaken to

determine the number of elements necessary for optimal results with respect to

computational time and cost.

38

Figure 18. FEA Model of Lap Shear Experiments. Blue) Mesh, White) Tissue, Arrows)

Upward Displacement, X’s) Fixed Region, Squares) Tied Surfaces

39

Chapter 4: Results and Discussion

The aim of this thesis is to determine whether the use of fibrin glue as an

alternative fixation method for mesh placement on the abdominal wall results in adequate

interfacial strength between mesh and tissue. This was accomplished by measuring and

comparing the fixation strength of mesh-tissue interface with fibrin glue or classical

sutures. Three major activities were undertaken: (1) uniaxial extension experiments with

only the abdominal tissue and only mesh (BARD, New Jersey) to identify the material

properties, (2) lap shear test to determine interface strength between abdominal wall and

mesh without hernia defect, and (3) FE modeling of the lap shear experiment to initiate a

computational modeling effort to model hernia.

4.1 Uniaxial Extension Experiments

Uniaxial experiments were conducted to measure the mechanical response of the

mesh and tissue. Both Bard soft mesh and abdominal tissue are known to display

anisotropy. Due to this characteristic, two different orientations were tested and used as a

means of comparison for this study. Representative raw data from the uniaxial

experiments from tissue and mesh are shown in Figure 19.

40

Figure 19. Representative Uniaxial Test Data.

Tissue Specimen (Direction 1), Mesh Specimen (Direction 2)

4.1. A. Average Normalized Force

Uniaxial experiments were performed and F-D response of surgical mesh and

excised abdominal tissue collected. This data was used to obtain the peak force per unit

width used to describe the material strengths (Table 4). The average normalized force for

direction 2, the weaker of the two orientations for tissue, was 16 N/cm +/- 3.0 STD (Table

4). A similar value was reported by Grevious et al. as a minimum strength required in

mesh repair for a successful operation of an average adult (Grevious et al., 2006).

Direction 1 of tissue is characterized as following the fibers along the transverse

abdominis and transverse to the muscle fibers in the external oblique. This orientation

was previously reported to have a stiffer response than tissue tested in direction 2

(Hernández et al., 2011; Song, Alijani, Frank, Hanna, & Cuschieri, 2006). While the

uniaxial results for tissue indicate an average value relatively stronger in Direction 1 then

0

10

20

30

40

50

60

0 10 20 30 40 50

Load

(N

)

Extension (mm)

Uniaxial Test Samples

Tissue Uniaxial Test

Mesh Uniaxial Test

41

Direction 2, it was revealed in a 2 sample statistical T-test that there was actually no

significant difference between the two orientations. On the other hand, a similar statistical

test confirmed that there is a significant difference in the mechanical response of surgical

mesh with respect to the orientation tested.

As stated previously, a relatively stiffer F-D response of the prosthetic mesh in

comparison to the surrounding tissue will result in a stiffer regional response in the

abdomen. However, the fittingly stronger mechanical response is necessary in providing

reinforcement to resist internal loads at the hernia repair site. Specifically, the axial and

radial stresses relevant to a hernia will cause stretching perpendicular to the direction that

the hernia defect is propagating. Consequently, in a hernia repair operations, optimal

results will be best achieved if anisotropy of the prosthetic implant is orientated such that

the weaker direction is aligned to the direction the hernia is propagation. For this study

however, where there is no hernia defect, optimal results will be best achieved if

anisotropy of the prosthetic implant closely resembles the fixated tissue (i.e. direction 2

of mesh is aligned to direction 2 of tissue).

Table 4. Uniaxial Results with Averaged Normalized Forces

Material Average +/- STD

Direction 1

Average +/- STD

Direction 2 P-Value

Abdominal Wall 22.0 +/- 7.4

n = 6

16.0 +/- 3.0

n = 6

P = 0.21

No Significant

Difference

Bard Soft Mesh 48.0 +/- 1.3

n = 5

17.0 +/- 1.6

n = 5

P < 0.01

Significant

Difference

42

4.1. B. Nonlinear Mechanical Properties for Mesh and Tissue

When curve fitting a strain energy function with experimental data the ABAQUS

software will provide coefficient values for the specific SEF’s such that the equation will

be able to simulate the non-linear material response. Each strain energy functions used a

total of 3 coefficients. For the 1st order Ogden SEF the coefficients were µ, α, and D1 and

for 2nd

order reduced polynomial the coefficients were C10, C20, and D1. These

coefficients represented specific material properties. For example µ, C10, and C20 were

coefficient related to shear modulus and were expressed in the units of MPa. Alpha is a

dimensionless and unitless constant which plays a role in the theory of finite elasticity.

Last the D1 constant called “Dashpot” is the time derivative of strain, resisting changes in

length; the units are in MPa-1

. All coefficients for the material models have been

summarized in Table 5 and Table 6. The goodness of fit between the material model and

experimental data is shown in Figure 20 for tissue and in Figure 21 for surgical mesh. All

curve fitting results for both mesh and tissues are included in Appendix E and H.

The Poisson’s ratio of the abdominal wall soft tissue was not solved for in our

experimental tests. However, other studies indicate that soft tissue can be modeled using

a nearly incompressible Poisson’s ratio (0.45 < ν ≤ 0.499). Poisson’s ratios were tested

and a best fit was selected for each individual model, such that the uniaxial simulation in

ABAQUS gave similar results to the experimental data. Best fit at this stage was

determined through visual inspections, but quantitative values were determined for the

final fit using the coefficient of determinate method. It is worthwhile to note, that

Poisson’s ratio for best fit varied between 0.46 and 0.499 between models of individual

specimens. It is very unlikely that the compressibility of soft tissue is ever at a value of

43

0.499, which suggest that further investigation should be done on the significance of the

Poisson’s ratio for abdominal soft tissue with respect to our current experimental set-up.

In using this method a few assumptions were made to simplify the problem. In

reality the abdominal tissue samples included many different types of tissue including

superficial fascia, rectus sheath, and rectus abdominis muscle tissue as well as possibly

fascia from the linea alba and tendinous intersections. However, for the simplification of

determining a function, which could represent the material properties of the tissue sample

it, was assumed that the tissue was a homogenous structure. Similarly, surgical mesh

which is a woven structure with many pores was modeled in this study as a continuous

homogenous shell part. Additionally, the surgical mesh model used a 1st order Ogden

SEF, with a Poisson’s ratio of 0.45, to model mechanical response. This Poisson’s ratio is

the material property value for a block of polypropylene and would be significantly lower

for a woven material. Consequently, because of these simplifications there is a chance the

material model will be unable to fully account for the mechanical response of different

fiber orientations in the structure.

Table 5. Strain Energy Function Coefficients for Abdominal Wall Tissue

SEF: Reduced Polynomial n=2

Tissue Direction 1 Tissue Direction 2

Coefficients C10 C20 D1 R2 C10 C20 D1 R

2

Units MPa MPa Mpa-1

MPa MPa Mpa-1

Specimen 1 4.40E-03 2.62E-02 1.14 0.985 1.02E-02 1.28E-02 0.99 0.972

Specimen 2 2.00E-04 4.50E-03 3.34 0.993 1.00E-02 1.24E-02 1.57 0.989

Specimen 3 7.82E-03 1.06E-02 2.58 0.999 1.50E-03 3.90E-03 3.66 0.954

Specimen 4 8.60E-04 2.00E-02 2.33 0.994 3.04E-02 4.32E-02 1.60 0.998

Specimen 5 1.80E-02 1.10E-02 1.12 0.934 1.76E-03 3.24E-02 1.14 0.995

Specimen 6 1.0E-09* 4.25E-03 1.54 0.913 1.30E-02 1.50E-02 1.40 0.997

Average 5.21E-03 1.27E-02 1.15 1.12E-02 1.99E-02 1.34

STD 0.01 0.01 0.01 0.01

* Unusual output value from ABAQUS software

44

Figure 20. Representative Tissue Experimental Response and the Material Model Fit.

A) Direction 1 B) Direction 2

Table 6. Strain Energy Function Coefficients for Polypropylene Soft Knit Mesh

SEF: Ogden n=1

Soft Mesh Direction 1 Soft Mesh Direction 2

Coefficients μ α D1 R2

μ α D1 R2

Units MPa MPa-1

MPa MPa-1

Specimen 1 2.80 7.20 0.134 0.976 0.817 4.74 0.253 0.992

Specimen 2 7.00 6.70 0.031 0.972 0.780 5.20 0.339 0.986

Specimen 3 4.80 7.20 0.050 0.867 0.840 5.00 0.266 0.953

Specimen 4 5.20 7.00 0.043 0.891 0.730 4.40 0.325 0.984

Specimen 5 1.100 4.40 0.234 0.968

Average 4.95 7.03 0.065 0.792 4.835 0.296

STD 1.72 0.24 0.047 0.05 0.35 0.04

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1

Str

ess

(MP

a)

Strain (mm/mm)

Specimen 3

Tissue Direction 1

Uniaxial Data

SEF R.P. n=2

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 6

Tissue Direction 2

Uniaxial Data

SEF R.P. n=2

A B

45

Figure 21. Representative Mesh Experimental Response and the Material Model Fit.

A) Direction 1 B) Direction 2

4.2 Lap Shear Tests for Obtaining Interfacial Strength

Biomechanical analyses conducted by lap shear tests measured the mechanical

strength of the mesh-tissue interface of the specimens. Maximum force required to cause

failure in the specimens, Figure 22, was normalized by the width of the mesh-tissue

specimen. Results of these experiments are displayed as a boxplot in

Figure 23 numerically summarized in Table 7. Comparisons of normalized force of

fixation technique at each individual time point was evaluated using a statistical Mann-

Whitney-Wilcoxon test with 95%, Table 8. An additional statistical test was performed

evaluating all fixation techniques and evaluation time points against each other using a 2-

way Anova test with 95% confidence, Table 9.

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 2

Mesh Direction 1

Uniaxial Data

SEF Ogden n=1

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 2

Mesh Direction 2

Uniaxial Data

SEF Ogden n=1

A B

46

Figure 22. Representative Lap Shear

This box plot in Figure 23 shows the spread of fixation strength resulting from the

lap shear experiments. At each later time point there is a clear increase in the average

fixation strength corresponding to the amount of tissue ingrowth which has occurred.

However, the sutured boxplots are clearly stronger then the glued counterparts. The solid

orange line marks the 16 N/cm load suggested by Grevious et al., to be the minimal

strength required for a successful abdominal repair (Grevious et al., 2006). Both fixation

methods pass this benchmark at 2 weeks while at 1 week only sutured specimens do.

Supporting this benchmark value, our uniaxial results also indicated that on average the

mesh and tissue could withstand a minimum strength around 16 N/cm before failing.

Therefore, it can be predicted that once a specimens normalized forces exceed the

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80

Load

(N

)

Extension (mm)

Lap Shear Test

24 Hours

1 Week

2 Weeks

47

benchmark value there is a higher likelihood that failure in the mesh-tissue specimen will

occur in either the mesh or tissue rather than in the interface.

Only at 24 hours did tested specimens fail most frequently in the mesh-tissue

interface. The sutured interface had a mean strength relatively higher than the glued

interface [9.4 +/- 2.4

STD and 5.3 +/- 3.6

STD]. The mean of the normalized

strength remained stronger in the sutured interface for each higher time point measured.

More notably, the load vs. extension graphs revealed a stiffer response in the sutured

specimens than to the glued specimens. This outcome was most prominent in the two-

week specimens, where the mesh was fully incorporated into the abdominal wall. Table 7

shows the average normalized force values from the lap shear tests. A complete table of

results from all specimens can be found in Appendix A.

The lap shear experiments failed the test for normal distribution performed in

Sigmaplot. Therefor the Mann-U test which compares ordinal sample distributions of two

populations was utilized for evaluating these tests. The analysis revealed a significant

difference between glued and sutured fixation at all-time points were sutured specimens

were relatively stronger. This difference in average strength was between 44-47 % higher

during early recovery, t ≤ 1 week; suggesting, that the comparably weaker fixation

strength reported in literature for fibrin glue fixation is nearly half of that associated with

sutured fixation.

An additional statistical test was done using a 2-way analysis of variance

(Anova), with 95% confidence, which compared fixation strength with both evaluated

time points and fixation techniques. The summary of this statistical tests are as follows.

There was a statistical difference between both fixation techniques and all evaluation

48

times. In addition there was a statistical difference between glued and sutured fixation, at

both, 1 and 2 weeks but none at 24 hours. Next, there was a statistical difference between

glued fixations at all time points except for 1 week comparison to 24 hours, where no

difference was reported. Last, there was a statistical difference between sutured fixations

between all-time points. A complete tabulated result from this Anova test is shown in

Table 9. Matlab code for determining peak force is shown in Appendix B and Matlab

code for obtaining the boxplot in Figure 23 is shown in Appendix C.

Figure 23. Summary of Results from Lap Shear Experiments

49

Table 7. Averaged Normalized Forces for Glued vs. Sutured Specimens

Healing Time Glue

Average +/- STD

Suture

Average +/- STD

24 Hours 5.7 ± 3.6

; (n = 8) 9.4 ± 2.4

; (n = 8)

1 Week 12.2 ± 4.0

; (n = 7) 23.1 ± 12.2

; (n = 8)

2 Weeks 22 ± 7.4

; (n = 9) 30.9 ± 7.4

; (n = 8)

Table 8. Statistical Analysis using Mann-Whitney-Wilcoxon U-test

Glue Suture

Z-

value P-Value UA UB 95 % Confidence Level

24

Hours n = 8 n = 8 -2.73 0.003 < P < 0.006 109 19 36 < U < 92 S.D.

1

Week n = 7 n = 8 2.03 0.021 < P < 0.042 10 46 13 < U < 43 S.D.

2

Weeks n = 9 n = 8 -1.97 0.024 < P < 0.049 57 15 18 < U < 54 S.D.

S.D. = Significant Difference; N.S.D. = No Significant Difference

Table 9. 2-Way Anova Test

Two Way Analysis of Variance

All Pairwise Multiple Comparison Procedures (POC with Holm-Sidak method) Comparisons

for factor: Comparison

Diff of Means

t P Critical Level

Significant

Time points

2 week vs. 24 hours 18.965 7.847 <0.001 0.017 Yes

1 week vs. 24 hours 10.292 4.126 <0.001 0.025 Yes

2 week vs. 1 week 8.673 3.525 0.001 0.05 Yes

Fixation T. Suture vs. Glue 7.86 3.917 <0.001 0.05 Yes

24 hours Suture vs. Glue 3.578 1.032 0.308 0.05 No

1 week Suture vs. Glue 11.129 3.102 0.003 0.05 Yes

2 week Suture vs. Glue 8.875 2.635 0.012 0.05 Yes

Glued

Fixation

2 week vs. 24 hours 16.316 4.844 <0.001 0.017 Yes

2 week vs. 1 week 9.8 2.805 0.008 0.025 Yes

1 week vs. 24 hours 6.516 1.816 0.076 0.05 No

Suture

Fixation

2 week vs. 24 hours 21.614 6.235 <0.001 0.017 Yes

1 week vs. 24 hours 14.068 4.058 <0.001 0.025 Yes

2 week vs. 1 week 7.546 2.177 0.035 0.05 Yes

50

One additional evaluation was performed comparing the lap shear and uniaxial

results. The mean strength for failure in the sutured specimens was 31.0 +/- 7.4

STD,

while the glued specimens failed on average at 22.0 +/- 7.4

STD. These averages

indicate that the tissue with glued mesh had the same mechanical response as excised

abdominal tissue, whereas the sutured specimens had relatively stiffer F-D mechanical

response (See Figure 24). Such differences in relative material strength of mesh-tissue

specimens and abdominal wall tissue indicate a regionally stiffer tissue response when

using sutured fixation (with Prolene sutures).

Figure 24. Typical Mesh-Tissue Response at 2 weeks vs. Excised Tissue Response

In the lap shear experiments the reported normalized max force indicates the

sample has begun to fail. Failures however does not necessary have to occur in the

interface and can just as easily occur in the materials, i.e., the bound tissue or mesh, as

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1

Norm

ali

zed

Load

(N/c

m)

Extension (cm)

2 Week Sutured

2 Week Glued

Direction 1 Tissue Uniaxial

Direction 2 Tissue Uniaxial

51

seen in Figure 25. These locations vary during different stages of wound healing and play

a role in identifying how well the mesh was incorporated into the tissue structure. In the

case when a majority of failure is no longer occurring in the interface the normalized

loads can no longer be reported as the mesh-tissue interfacial strength. The locations at

which the tested specimens failed are summarized in Table 10.

.

Figure 25. Failure modes. Left: Mesh failure, Center: Interface failure, Right: Tissue

failure

At 24 hours, the interface failed for glued samples 87.5% of the time, as was

previously predicted for this level of tissue ingrowth. For the sutured samples the

interface failed 60% of the time, while the other 40% was failure in the tissue region.

Two reasons exist that could cause failure at the tissue region. The first reason for failure

would be that enough tissue ingrowth had occurred that the stress was no longer

concentrated in the interface, and the second reason would be that there was some level

52

of tissue damage caused by the suturing which resulted in stress build up at the damaged

tissue region and ultimately led to failure in the tissue. On the other hand, at 24 hours the

glued specimens still had an observable glossy layer of glue surrounding the mesh pieces

and had not fully dissolved, this protective layer would limit the amount of tissue

ingrowth possible between the mesh and tissue. The sutured specimens had no such layer

and tissue ingrowth with mesh was able to occur immediately.

At 1 week, both types of specimens had enough time to form an adequately strong

interface. However, the level of tissue ingrowth varied from specimen to specimen and

depended on the animal model and that individual animal’s rate of healing. It would be

predicted at this stage for results to be in a binomial distribution where the average

failures occurred within the tissue. This was indeed observed in the glued specimens;

nearly 55% of the time the tissue failed, 30% of these failures were at the interface and

15% of the time they were in the mesh. Results were far more variable in the sutured

samples; where 38% of the time the mesh failed, 24% the tissue did, and 38% the

interface failed. Standard deviation of the normalized force at failure was also high for

sutured specimens at 1 week, with the samples ranged from mesh almost fully embedded

to samples with barely any tissue ingrowth. One major concern of sutured fixation, which

could cause a delay in tissue ingrowth, was mesh curling; all samples with failed

interfaces at 1 week time points had nearly no tissue ingrowth and the sutured specimens

had observable mesh curling. Regardless, our failure mode analyses suggest that as early

as 1 week there was an adequate amount of tissue ingrowth such that the interfacial

strength was stronger than the bound material substrates.

53

At 2 weeks mesh pieces were well embedded into the tissue and presented great

difficulty in separating mesh from tissue when preparing samples for lap shear tests.

Nearly full tissue ingrowth would be characterized at this stage and stress propagation

would be expected to occur along the mesh in the lap shear test. In the glued specimens,

there was a 100% failure at the mesh. For the sutured samples there was a 75% failure in

the mesh and 25% failure in the tissue.

Table 10. Lap Shear Test Failure Mode Occurrences

Mesh Interface Tissue

# Failed Percentage # Failed Percentage # Failed Percentage

24 Hours

Glue 0 0% 7 87.5% 1 12.5%

Suture 0 0% 5 62.5% 3 37.5%

1 Week

Glue 1 14% 2 29% 4 57%

Suture 3 38% 2 24% 3 38%

2 Weeks

Glue 9 100% 0 0% 0 0%

Suture 6 75% 2 25% 0 0%

4.3 Computational Study using FEA to Simulate Mechanical Experiments

4.3. A. Uniaxial Simulation with FEA

Finite element analysis was used to create a uniaxial simulation of the mesh and

tissue specimens (Figure 26). The force-displacement data obtained from the uniaxial

tests were compared to the sum of the reaction forces at the pinned tissue region of an

FEA model created in ABAQUS (Figure 28). The uniaxial model showed a reasonable

fit to the experimental results, which validated the individual models for mesh and tissue.

The goodness of fit was determined using the coefficient of determinate value R2, Table

13. All results are included in Appendix F and Appendix I. FE mesh size was optimized

54

such that computational time was reduced without drastically influencing the output

results. This was done by performing a mesh convergence study on surgical mesh and

abdominal tissue uniaxial simulations, Figure 27/Table 11 and Figure 29/Table 12. It was

decided from these convergence studies that 1500 elements for tissue models and 160

elements for the surgical mesh was sufficient for the simulations.

Figure 26. Stress contour plot A) Surgical Mesh B) Abdominal Tissue

A B

55

Figure 28. Representative Tissue Experimental Response and the FEA Model Results.

A) Direction 1 B) Direction 2

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1

Str

ess

(MP

a)

Strain (mm/mm)

Specimen 3

Tissue Direction 1

Uniaxial Data

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 6

Tissue Direction 2

Uniaxial Data

FEA Simulation

Table 11. Mesh Convergence Study for Tissue

ELEMENTS Sum(σ) Run Time

24486 3.11E+00 < 10 min

10000 3.11E+00 < 5 min

1914 3.13E+00 < 30 sec

1500 3.14E+00 < 30 sec

864 3.15E+00 < 30 sec

640 3.16E+00 < 30 sec

540 3.17E+00 < 30 sec

416 3.18E+00 < 30 sec

165 3.19E+00 < 30 sec

140 3.21E+00 < 30 sec

80 3.25E+00 < 30 sec

A B

3.1

3.1

3.2

3.2

3.3

3.3

0 10000 20000

Su

m o

f N

om

inal

Str

ess

(N/c

m2)

# of Elements

Mesh Convergence for

Tissue Model

Figure 27. Mesh Convergence Study for Tissue

Model

56

Figure 29. Mesh Convergence Study for Surgical Mesh Models

Figure 30. Representative Mesh Experimental Response and the FEA Model Results.

13.0940

13.0945

13.0950

13.0955

13.0960

13.0965

13.0970

13.0975

13.0980

13.0985

0 1000 2000 3000Su

m o

f N

om

inal

Str

ess

(N/c

m2)

# of Elements

Mesh Convergence for Surgical

Mesh

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 2

Mesh Direction 1

Uniaxial Data

FEA Simulation

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 2

Mesh Direction 2

Uniaxial Data

FEA Simulation

Table 12. Mesh Convergence Study

for Surgical Mesh

ELEMENTS Sum(stress) Time

3240 1.31E+01 < 5 min

1800 1.31E+01 < 1 min

561 1.31E+01 < 30 sec

450 1.31E+01 < 30 sec

378 1.31E+01 < 30 sec

288 1.31E+01 < 30 sec

210 1.31E+01 < 30 sec

162 1.31E+01 < 30 sec

120 1.31E+01 < 30 sec

84 1.31E+01 < 30 sec

45 1.31E+01 < 30 sec

18 1.31E+01 < 30 sec

6 1.31E+01 < 30 sec

A B

57

4.3. B. Lap Shear Simulation with FEA

Finite element analysis (FEA) was used to create a lap shear simulation of the 2

week glued test specimens, Figure 31. The load to extension data obtained from the lap

shear tests were compared to the FEA simulation. In Figure 32A, there is a noticeable

offset between the simulation and the actual test data. The initial toe in region of the lap

shear results where rate of load to extension is exceptionally low can be associated with

both material (i.e. some uncontrolled property of healing tissue) and experimental

conditions (i.e. slack in the sample, fluid leakage from specimen, etc.). Whatever the

condition may be that causes this response, it did not occur in the uniaxial experiments

(mesh and tissue) and so was not captured in the SEF’s mechanical response.

For the most part, it was clear the FEA simulation had a stiffer response in

comparison to the actual response. Representative results are shown in Figure 32. All

results are included in Appendix K. Goodness of fit, R2 value, was determined using the

coefficient of determinate method as described in the Equation 4. There was a significant

variation between the R2 values reported for all models. Most frequently the simulation

Table 13. Goodness of Fit of Experimental Data with Uniaxial Simulation

Specimen

Tissue

Direction 1

R2

Tissue

Direction 2

R2

Mesh

Direction 1

R2

Mesh

Direction 2

R2

1 0.992 0.983 0.584 0.826

2 0.976 0.979 0.978 0.843

3 0.990 0.910 0.974 0.932

4 0.942 0.995 0.984 0.862

5 0.977 0.983 0.942

6 0.987 0.998

58

and experimental results were within 6% difference of one another; however R2 varied

significantly between models ranging from worst case of 64.3% difference to best case at

1.6% difference. The full list of R2 values for lap shear simulations are given in Table 14.

The number of elements used in these models was approximately 1500 for the

tissue part and 460 for the surgical mesh part; these values were determined by a mesh

convergence study performed for each lap shear models created. Since there was a

separate model with customized parts created for every single specimen all the individual

mesh convergence studies are not shown in this thesis. However, Figure 33 and Table 15

provide a sample of one of the mesh convergence studies, specimen 2_4.

Figure 31. FEA Model Reaction Force Contour Plot

59

Figure 32. Comparison of Experimental and FEA Results of Lap Shear Experiments.

Lap Shear Model A) Sub-Optimal Fit and B) A Good Fit by Visual Inspection.

Table 14. Goodness of Fit of FEA Simulation to Experimental Data

Specimen # R2 % Difference

Specimen 1_1 0.357 (64.3%)

Specimen 1_3 0.836 16.4%

Specimen 1_8 0.938 6.2%

Specimen 2_3 0.946 5.4%

Specimen 2_4 0.469 53.1%

Specimen 2_7 0.944 5.6%

Specimen 2_8 0.546 45.4%

Specimen 9_4 0.984 (1.6%)

Specimen 9_5 0.831 16.9%

0

5

10

15

20

25

30

35

40

0 10 20

Load

, N

Displacement, mm

Specimen 2_8

Lap Shear…FEA…

0

10

20

30

40

50

60

70

0 5 10

Load

, N

Displacement, mm

Specimen 9_4

Lap Shear Data

A B

60

Figure 33. Mesh Convergence Study for Lap Shear Models

112

113

114

115

116

117

118

0 2000 4000 6000Su

m o

f R

eact

ion

Forc

e (N

)

# of Elements

Mesh Convergence for Lap Shear

Model

Specimen 2_4

Table 15. Mesh Convergence Study for Lap Shear Models

Elements

in tissue

Elements

in mesh

Total # of

elements

Sum(RF) Run time

4070 700 4770 113.023 < 7 min 3400 460 3860 113.074 < 5 min 1500 460 1960 113.816 < 2 min 858 195 1053 114.896 < 1 min 627 154 781 115.033 < 30 sec 540 130 670 115.132 < 30 sec 288 108 396 117.29 < 30 sec 225 80 305 117.335 < 30 sec 168 63 231 117.707 < 30 sec

61

Chapter 5: Conclusions

5.1. Conclusions

The average normalized load with sutures is stronger than glued at all-time points.

At or greater than 1 week time point, the fixation strength became independent of the

technique and failure occurred in the materials rather than the interface.

After 2 weeks of recovery, the glued specimens show nearly identical load

measurement readings to the material strength of healthy tissue.

At 2 week recovery, the sutured samples exhibited interface strength globally stiffer

than what is characterized with healthy tissue while glued specimens showed a nearly

identical F-D response.

The strain energy model used to mimic the material response under loading was

comparably similar to experimental data such that FEA uniaxial simulations

frequently reported σ-ɛ values less than 9% difference of the experimental data.

Modeling approach used for mimicking lap shear experiments was not always

comparable to the experimental data and would require further investigation to

develop a consistent model.

5.2. Clinical Significance

Lower interfacial strength at earlier time points, 24 hours, has a significant

relevance for susceptibility of reoccurrence due to mesh migration. However both

techniques (sutures or adhesive) lead to a mesh-tissue interfacial strength lower than the

benchmark value of 16 N/cm, at this early stage. At two weeks, every sample tested had a

mesh-tissue interface exceedingly stronger than the mesh or tissue materials. Samples

fixated using sutures had a far stiffer global F-D response and higher strength value than

62

associated with healthy tissue, such that the strength values more closely resembled

relative material strength of polypropylene sutures.

63

Chapter 6 Future Work

6.1 Mechanical Experiments and Sample Preparation

6.1. A. Future Work

It would be clinically significant to repeat experiments using earlier time points

(t ≤ 1 week), where fixation technique is still relevant toward the interfacial

strength.

Additionally, it would be clinically significant to perform experiments where the

advantages of using adhesive fixation are compared to the disadvantages of the

relatively weaker fixation strength at these earlier time points.

For future studies, it may be beneficial to perform tests comparing the efficiency

of biodegradable suture fixation with fibrin glue fixation post 1 week recovery, in

order to determine if the higher strength values obtained with sutured specimens

was due to the use of non-degradable sutures.

Other potential studies that could be informative and helpful for medical

practitioners include: A) Testing different coating techniques for glue applications

in order to determine if there is any relation with coating technique and fixation

strength; B) Testing different biological glues other than Tisseel fibrin glue to see

if another product is more efficient for mesh repair operations; C) Testing

alternative surgical meshes besides BARD soft knit polypropylene mesh to see if

another product reacts better with adhesive glue for fixation on soft tissue.

Isotropic mesh should be utilized in any future studies where mesh is implanted

onto healthy tissue. The significance of anisotropic mesh is in resisting the higher

stress concentration along the tissue direction that is perpendicular to the direction

64

the hernia is propagating. In the case where a study is not done on a herniated

tissue region the benefit of having the anisotropic mesh becomes irrelevant.

6.1. B. Limitations that need to be addressed in Future Studies

Surgical procedures used for suture fixation have four evenly-spread surgical

knots tying the corners of the mesh piece to the abdominal tissue. In common

surgical practice for VHMR operations, sutures are generally knitted into the

mesh and abdominal wall such that the sutures follow through the entire perimeter

of the mesh. This technique would uniformly holds the mesh flat onto the

abdominal wall and could potentially influence better fixation.

Future work should have control samples, which evaluate mesh-tissue interface

with no fixation. This would determine the level of tissue ingrowth that would

have occurred naturally under the absence of any anchoring or adhesive materials

used for fixation.

Greater precautions can be taken in future studies to prevent infection, blood

clotting, or other such biological responses that artificially reduce the natural

strength of the implant.

For the lap shear clamping tests, a scalpel was used to free the mesh from tissue,

possibly creating many artificial microscopic tears and nicks in the tissue

interface before testing. Additionally, the level of tissue ingrowth after 1 week

made it difficult to clear all the tissue from the mesh before clamping. It is

unlikely that the mesh cleared for clamping was completely unbound from all

tissue fibers.

65

Inconsistency on amount of tissue shaved off samples to allow fitting in clamps

also added potential error to the tests. Too much tissue shaved off specimens

would create an area of weakness where failure could occur while too little tissue

would cause samples to slip out of the clamps during clamp tightening. Future

studies should consider a more consistent method of preparing specimens for

testing.

The most common source of loading on the mesh-tissue repair site of the

abdominal wall is due to intra-abdominal pressure. Therefore, future studies may

consider using burst tests for biomechanical evaluation to accompany shear

experiments.

Lastly, one limitation during sample preparation was the difficulty in visually

determining the mesh piece location of 2 week specimens. Use of color coated

mesh or other identification method would counter this limitation. Chances of

accidently cutting and damaging mesh as well as accidently clamping portions of

tissue or mesh into the wrong clamp is significantly higher in the 2 week samples.

6.2 Finite Element Models for Computational Simulations

6.2. A. Next Step toward Developing Full Robust Model of a VHMR

A future step is using the material models and interfacial fit to developing a more

robust model of a VHMR abdomen. A full model of an abdomen with a hernia

defect zone and mesh sealant would help provide a macro scale visualization of

how stress distribution or potential failure would occur. The clinical significance

of this will be that such a model would be able to provide a general idea on how

variation in internal pressure and other loading mechanisms due to trunk

66

movement will influence the stress concentration at the repair site. It can also

provide practitioners with a general idea of whether the patient is at high risk of

suffering from a hernia reoccurrence due to mesh failure.

6.2. B. Limitations in Current Models that should be addressed in Future Work

Further investigation should be done to determine the true Poisson’s ratio of the

surgical mesh and abdominal tissue.

Surgical mesh was modeled as a 3D continuous homogenous shell part instead of

as a knitted fiber with large pores. Similarly, abdominal tissue was modeled as an

isotropic homogenous material rather than a heterogeneous one. Future models

should better account for the heterogeneity of tissue and monofilament fiber

structure of mesh.

The interfacial fit was performed by tying the surface of the mesh and tissue

together instead of embedding the mesh in tissue as it occurs in vivo. Interfacial fit

will have a different mechanical response than that of an embedded implant.

Goodness of fit between FEA simulation and lap shear experiments were

inconsistent. FEA models should be further improved so that computational

simulations better resemble the experimental data.

Future models could also be improved by taking into account the material

properties associated with muscle-active tension

67

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71

Appendix A

Experiment Results

The following is tables include the results and specimen dimensions recorded for all the

mechanical tests performed.

2 Weeks Lap Shear Experiments

Date Sampl

e Fixation

Techniq

ue

Heigh

t Wid

th Gauge

Length Max

Force Normalized Max

Force Slop

e Failure

Mode

Remove

d

cm cm cm N

5/1/12 73-1 Glue 2.8 1.5 1.5 27.83 18.55 3.86 Mesh

5/1/12 74-1 Glue 3 3 2.5 50.31 16.77 1.87 Mesh

5/2/12 75-1a Glue 3.5 2 2.3 27.06 13.53 9.36 Mesh

5/2/12 75-1b Glue 3.5 2 2.4 43.27 21.63 4.35 Mesh

5/2/12 76-1a Glue

1.5 2.5 26.62 17.75 5.45 Mesh

5/2/12 76-1b Glue

1.5 2.3 39.41 26.27 5.29 Mesh

5/9/12 80-1 Glue 2.5 2.5 1.8 48.21 19.28 6.21 Mesh

5/9/12 79-1a Glue 3.3 2 3 50.88 25.44 8.86 Mesh

5/9/12 79-1b Glue 3.3 1.5 2.8 57.91 38.60 10.7

9 Mesh

5/1/12 73-2 Suture 3 1.5 2.5 55.56 37.04 3.00 Tissue

5/1/12 74-2 Suture

3 3.7 53.37 17.79 8.00 Mesh

5/2/12 75-2a Suture 2.5 1.5 1.5 41.30 27.53 4.64 Mesh

5/2/12 75-2b Suture 2.5 1.7 1.5 40.17 23.63 20.3 Mesh

5/2/12 76-2a Suture

1.5 3.6 57.66 38.44 0.99 Tissue

5/2/12 76-2b Suture

1.5 4.3 75.04 50.03 5.82 Tissue Outlier

5/8/12 78-2 Suture 5 3 3 102.0 34.01 7.61 Tissue

5/9/12 80-2 Suture 4 3 2.5 92.45 30.82 10.6 Mesh

5/9/12 79-2 Suture 3 2 2.6 75.17 37.58 9.16 Mesh

5/8/12 77-2 Suture 5 3.8 3 61.76 16.25 2.78 Mesh Damag

ed

72

1 Week Lap Shear Experiments

Date Sam

ple

Fixation

Techniq

ue

Hei

ght

Wi

dth

Gauge

Length

Max

Force

Normalized

Max Force Slope

Failure

Mode

Remo

ved

cm cm cm N

6/6/12 53-1 Glue 4 3 3.3 31.10 10.37 2.18 Tissue

6/12/12 55-1 Glue 5 4 4 50.82 12.70 4.49 Interface

6/12/12 56-1 Glue

3.9 5 51.05 13.09 3.03 Mesh

6/13/12 58-1 Glue 5 4 4.2 58.18 14.54 1.48 Tissue

6/27/12 59-1 Glue 5.5 4.5 4.5 35.86 7.97 3.01 Tissue

6/27/12 60-1 Glue 4.5 5 3.5 76.53 15.31 6.75 Interface

6/27/12 61-1 Glue 5 4 4.5 45.12 11.28 3.28 Tissue

6/13/12 57-1 Glue 4.5 4.2 4.1 90.10 21.45 8.32 Tissue Outlie

r

6/12/12 56-2 Suture

3 5 53.39 17.80 2.41 Interface

6/13/12 57-2 Suture

3 4.1 58.08 19.36 6.05 Mesh

6/27/12 59-2 Suture 5 4 4.5 47.39 11.85 4.35 Tissue

6/27/12 60-2 Suture 4.5 3.2 4 61.52 19.23 5.23 Interface

6/27/12 61-2 Suture

3.5 4 34.71 9.92 3.13 Tissue

6/6/12 53-2 Suture 3 2 2.2 78.18 39.09 4.91 Mesh

6/12/12 55-2 Suture 4.5 2.5 4.8 108.6 43.45 7.47 Interface

6/13/12 58-2 Suture 3.7 4 3.3 103.1 25.77 8.56 Mesh

6/5/12 51-2 Suture 4.5 2.6 2.5 55.24 21.25 2.76 Interface Infect

ed

73

24 Hours Lap Shear Experiments

Date Sampl

e

Fixatio

n

Techni

que

Hei

ght

Wi

dth

Gauge

Length

Max

Force

Normalized

Max Force

Slo

pe

Failure

Mode

Remo

ved

cm cm cm N

5/8/12 82-1 Glue 5.2 3.5 4.3 12.02 3.43 0.67 Interface

5/8/12 81-1 Glue 5.1 3.4 4 9.49 2.79 1.11 Interface

5/11/12 83-1 Glue 5.1 3.6 4.1 18.45 5.13 1.48 Interface

5/11/12 84-1 Glue 5.1 3.6 4 7.74 2.15 0.58 Interface

5/15/12 97-1 Glue 5 3.8 3.5 32.51 8.56 2.65 Interface

5/15/12 98-1 Glue 4.5 4 4 30.23 7.56 1.31 Interface

5/17/12 99-1 Glue

3.7 4.5 12.30 3.32 1.67 Tissue

5/17/12 100-1 Glue 5.2 3.8 4.2 47.01 12.37 3.49 Interface

5/8/12 81-2 Suture 5 3 3 29.20 9.73 2.00 Interface

5/8/12 82-2 Suture 5 3.4 3 18.97 5.58 1.21 Interface

5/11/12 83-2 Suture 5.2 3.4 4 39.87 11.73 1.69 Interface

5/11/12 84-2 Suture 5.1 3.6 3.6 27.92 7.75 1.21 Tissue

5/15/12 98-2 Suture 4.5 3.8 4 37.80 9.95 1.21 Tissue

5/17/12 100-2 Suture 5 3.7 3.2 43.72 11.82 1.60 Interface

5/17/12 99-2 Suture 4.8 3.6 3.5 33.86 9.41 1.32 Tissue

5/17/12 97-2 Suture 5 3.5 4.5 81.64 23.32 3.28 Interface Outlier

Uniaxial Experiments for Abdominal Tissue Material Strength Sample

Type

Orie

nt.

S

p.

Heig

ht

Wid

th

Thickn

ess

Gauge

Length

Max

Force

Normalized Max

Force

Slo

pe Failu

re

Site # (cm) (cm) (cm) (cm) (N)

Tissue D1 1 5 4 0.5 < t

< 1 3.5 122.2 30.55

5.2

0 S.C.

Tissue D1 2 5 4 0.5 < t

< 1 2.5 48.2 12.04

3.8

1 S.C.

Tissue D1 3 5 4 0.5 < t

< 1 2.5 110.3 27.57

3.3

6 S.C.

Tissue D1 4 5 4 0.5 < t

< 1 3 100.2 25.04

4.7

7 C.S.

Tissue D1 5 5 4 0.5 < t

< 1 2.5 89.7 22.43

3.1

2 D.C.

Tissue D1 6 5 4 0.5 < t

< 1 2.5 56.0 14.00

4.0

5 D.C.

Average Normalized Force +/- Standard Deviation

21.94 +/- 7.44 (N/cm)

74

Uniaxial Experiments for Abdominal Tissue Material Strength Sample

Type

Orie

nt.

S

p.

Heig

ht

Wid

th

Thickn

ess

Gauge

Length

Max

Force

Normalized Max

Force

Slo

pe

Failu

re

# (cm) (cm) (cm) (cm) (N)

Site

Tissue D2 1 5 4 0.5 < t

< 1 3 55.09 13.77

2.41

S.C.

Tissue D2 2 5 4 0.5 < t

< 1 3 72.33 18.08

5.27

C.S.

Tissue D2 3 5 4 0.5 < t

< 1 3.5 57.48 14.37

3.02

D.C.

Tissue D2 4 5 4 0.5 < t

< 1 2 78.28 19.57

3.40

S.C.

Tissue D2 5 5 4 0.5 < t

< 1 3.5 55.62 13.90

4.34

S.C.

Tissue D2 6 5 4 0.5 < t

< 1 2.5 75.41 18.85

4.15

S.C.

Average Normalized Force +/- Standard Deviation

16.43 +/- 2.69 (N/cm)

Orient. = Orientation; SP. = specimen; S.C. = Specimen region at Stationary Clamp; D.C. = Specimen Region at Dynamic Clamp; C.S. = Center of specimen

Uniaxial Experiments for BARD Soft Knitted Polypropylene Surgical

Mesh Material Strength Sample

Type

Ori

ent.

Sp

. Height Width

Thick

ness

Gauge

Length

Max

Force

Normalized Max

Force Slope

Failu

re

# (cm) (cm) (cm) (cm) (N)

Site

PP Mesh D1 1 6 3 0.044 2 142.9 47.6 16.4 S.C.

PP Mesh D1 2 6 3 0.044 1.9 145.7 48.6 21.7 S.C.

PP Mesh D1 3 5.9 3 0.044 1.8 138.6 46.2 19.9 S.C.

PP Mesh D1 4 6 3 0.044 2 136.3 45.4 14.6 D.C.

PP Mesh D1 5 6 3 0.044 2 153.9 51.3 16.8 C.S.

Average Normalized Force +/- STDev 47.83 +/- 2. 29 (N/cm)

PP Mesh D

2 1 6 3 0.044 2 53.0 17.7 3.8 S.C.

PP Mesh D

2 2 6 3 0.044 2 57.0 19.0 2.8 S.C.

PP Mesh D

2 3 6 3 0.044 2 45.7 15.2 2.8 C.S.

PP Mesh D

2 4 6 3 0.044 2 46.4 15.5 2.5 S.C.

PP Mesh D

2 5 6 3 0.044 2 53.4 17.8 3.1 D.C.

Average Normalized Force +/- STDev 17.03 +/- 1.62 (N/cm)

Orient. = Orientation; SP. = specimen; S.C. = Specimen region at Stationary Clamp; D.C. = Specimen Region at Dynamic Clamp; C.S. = Center of specimen

75

Appendix B

Matlab Code: F-D Data Evaluation

The following is the Matlab code used for evaluating the raw data files retrieved

from the Bluhill2 software for the mechanical tests. This graph evaluated the force and

displacement data points to determine the maximum force at failure, the slope of the F-D

graph to the first peak, and the area under the graph up to the first peak.

% This file opens the .csv files (Raw data files from Bluehill2 software) and obtains F-D results

clear all

clc

close all

dirName = ('03082013');

filesInDir = dir(dirName);

numFiles = length(filesInDir);

%Bard mesh max load:

maxLoad = 25*9.81 / 2.2;

cc=0;

for ii=1:14;

if length(filesInDir(ii).name) > 2 & filesInDir(ii).name(1:2)=='Sp'

cc=cc+1;

fileName = [dirName '/' filesInDir(ii).name];

[num txt raw] = xlsread(fileName);

data = num(:,:);

data(:,1) = num(:,2);

data(:,2) = num(:,3);

sizeN(cc,:) = size(data);

figure(1)

plot(data(:,1),data(:,2),'b')

[maxMag(cc) maxInd(cc)] = max(data(:,2));

[maxX(cc) maxY(cc)] = ginput(1);

[minX(cc) minY(cc)] = ginput(1);

indMax(cc) = min(find(((maxX(cc)-0.01) < data(:,1)) & (data(:,1) < (maxX(cc)+0.01 ))));

indMin(cc) = min(find( minX(cc)-0.01 < data(:,1) & data(:,1) < minX(cc)+0.01 ));

dataNew = data(indMin(cc):indMax(cc),:);

76

% The slope:

P(cc,:) = polyfit(dataNew(:,1),dataNew(:,2),1);

Y = polyval(P(cc,:),dataNew(:,1));

figure(3)

plot(dataNew(:,1),dataNew(:,2),'r')

hold on

title(num2str(P(cc,1)))

plot(dataNew(:,1),Y,'k')

hold off

figure(10)

plot(data(:,1),data(:,2),'b')

axis([ 0 100 0 60])

grid on

figure(2)

subplot(7,2,cc)

plot(data(:,1),data(:,2),'b')

axis([ 0 100 0 60])

grid on

% Finding the first maxima that is within 40mm.

[yMax(cc) yInd] = max(data(:,2) .* (data(:,1) < 40))

% Area under the curve:

z(cc) = trapz(data(1:yInd,1),data(1:yInd,2))

hold on

plot(dataNew(:,1),Y,'r')

title([fileName(10:17) ' Slope =' num2str(P(cc,1)) ' N/mm'])

hold off

figure(5)

plot(data(:,1),data(:,2))

hold on

plot(data(yInd,1),data(yInd,2),'ro')

pause(1)

else

continue

end

end

77

Appendix C

Matlab Code: Box Plots

The following is the Matlab code used for evaluating the normalized mas force

results from the lap shear experiments. This code performed an anova test which was

used as an initial statistical approach to determine difference between data sets. This test

was also used to identify and remove outliers from the data set before final averages were

determined.

close all

%

24 Hour Glue Vs Suture

glue24 = ...

[ 3.43 2.79 5.13 2.15 8.56 7.56 3.32 12.37 4.16 9.79 1.33 6.02 6.89 1.77 4.72 4.55]; %Normalized Max

Force 24 hours Glue

suture24 = ...

[ 9.73 5.58 11.73 7.75 9.95 11.82 9.41 7.96 NaN NaN NaN NaN NaN NaN NaN NaN];%Normalized Max

Force 24 hours Suture

%%

close all

glue1w = ...

[ 10.37 12.70 13.09 14.54 7.97 15.31 11.28 NaN NaN NaN NaN NaN NaN NaN NaN NaN];%Normalized

Max Force 1 weeks Glue

suture1w = ...

[ 17.80 19.36 11.85 19.23 9.92 39.09 43.45 25.77 NaN NaN NaN NaN NaN NaN NaN

NaN];%Normalized Max Force 1 weeks Suture

%%

close all

glue2w = ...

[ 18.55 16.77 13.53 21.63 17.75 26.27 19.28 25.44 28.60 38.60 NaN NaN NaN NaN NaN

NaN];%Normalized Max Force 2 weeks Glue

suture2w = ...

[ 37.04 17.79 27.53 23.63 38.44 50.03 34.01 30.82 37.58 NaN NaN NaN NaN NaN NaN

NaN];%Normalized Max Force 2 weeks Suture

%%

figure

[h pi ci] = anova1([glue24(1,:); suture24(1,:); glue1w(1,:); suture1w(1,:); glue2w(1,:); suture2w(1,:)]'); %

Anova, T-test

title('Normalized Force')

axis([0 7 -5 65])% 0-7 x-axis (catagories), -5-65 Y-axis (normalized force magnitude)

78

Appendix D

ABAQUS INP File: Tissue Uniaxial Model

The following is an ABAQUS Standard input file for the computational uniaxial

simulation of the excised abdominal tissue. The complete list of nodes and elements has

been removed in order to shorten the document.

*End Part

**

**

** ASSEMBLY

**

*Assembly, name=Assembly

**

*Instance, name=TissueUniaxial-1, part=TissueUniaxial

*Node

[REMOVED]

*Element, type=C3D8H

[REMOVED]

*Nset, nset=Set-1, generate

1, 2184, 1

*Elset, elset=Set-1, generate

1, 1500, 1

** Section: Section-1

*Solid Section, elset=Set-1, material=Material-1

,

*End Instance

**

*Nset, nset=Set-1, instance=TissueUniaxial-1

[REMOVED]

*Elset, elset=Set-1, instance=TissueUniaxial-1, generate

901, 1200, 1

*Nset, nset=Set-2, instance=TissueUniaxial-1

[REMOVED]

*Elset, elset=Set-2, instance=TissueUniaxial-1, generate

1201, 1500, 1

79

*End Assembly

**

** MATERIALS

**

*Material, name=Material-1

*Hyperelastic, n=2, reduced polynomial

0.0102, 0.0128, 0.99, 0.

** ----------------------------------------------------------------

**

** STEP: Step-1

**

*Step, name=Step-1, nlgeom=YES

*Static, direct

0.1, 1.,

**

** BOUNDARY CONDITIONS

**

** Name: BC-1 Type: Displacement/Rotation

*Boundary

Set-1, 2, 2, 3.

** Name: BC-2 Type: Symmetry/Antisymmetry/Encastre

*Boundary

Set-2, ENCASTRE

**

** OUTPUT REQUESTS

**

*Restart, write, frequency=0

**

** FIELD OUTPUT: F-Output-1

**

*Output, field, variable=PRESELECT

**

** HISTORY OUTPUT: H-Output-1

**

*Output, history, variable=PRESELECT

*End Step

80

Appendix E

Strain Energy Model vs. Uniaxial Tissue Experiments

The following content include all the graphs for each individual strain energy

material model created for each individual abdominal tissue specimen.

81

82

83

Appendix F

FEA Uniaxial Simulation vs. Uniaxial Tissue Experiments

The following content include all the graphs for each individual FEA abdominal

tissue uniaxial simulation and relates it to the experimental data.

84

85

86

Appendix G

ABAQUS INP File: Surgical Mesh Uniaxial Model

The following is an ABAQUS Standard input file for the computational uniaxial

simulation of the surgical mesh. The complete list of nodes and elements has been

removed in order to shorten the document.

*Heading

** Job name: Surgical Mesh Model name: Model-1

** Generated by: Abaqus/CAE 6.12-1

*Preprint, echo=NO, model=NO, history=NO, contact=NO

**

** PARTS

**

*Part, name=Mesh

*End Part

**

**

** ASSEMBLY

**

*Assembly, name=Assembly

**

*Instance, name=Mesh-1, part=Mesh

*Node

[REMOVED]

*Element, type=S4R

[REMOVED]

*Nset, nset=Set-2, generate

1, 496, 1

*Elset, elset=Set-2, generate

1, 450, 1

** Section: Mesh

*Shell Section, elset=Set-2, material=Mesh

0.044, 5

*End Instance

**

*Nset, nset=Set-1, instance=Mesh-1

[REMOVED]

*Elset, elset=Set-1, instance=Mesh-1, generate

87

151, 300, 1

*Nset, nset=Set-2, instance=Mesh-1

[REMOVED]

*Elset, elset=Set-2, instance=Mesh-1, generate

1, 150, 1

*End Assembly

**

** MATERIALS

**

*Material, name=Mesh

*Hyperelastic, ogden

2.7, 7.4, 0.19

** ----------------------------------------------------------------

**

** STEP: Step-1

**

*Step, name=Step-1, nlgeom=YES

*Static, direct

0.1, 1.,

**

** BOUNDARY CONDITIONS

**

** Name: BC-1 Type: Displacement/Rotation

*Boundary

Set-1, 2, 2, 2.

** Name: BC-2 Type: Symmetry/Antisymmetry/Encastre

*Boundary

Set-2, ENCASTRE

**

** OUTPUT REQUESTS

**

*Restart, write, frequency=0

**

** FIELD OUTPUT: F-Output-1

**

*Output, field, variable=PRESELECT

**

** HISTORY OUTPUT: H-Output-1

**

*Output, history, variable=PRESELECT

*End Step

88

Appendix H

Strain Energy Model vs. Uniaxial Surgical Mesh Experiments

The following content include all the graphs for each individual strain energy

material model created for each individual surgical mesh specimen.

89

)

90

Appendix I

FEA Uniaxial Simulation vs. Uniaxial Surgical Mesh Experiments

The following content include all the graphs for each individual FEA surgical mesh

uniaxial simulation and relates it to the experimental data.

91

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1

Str

ess,

(M

Pa)

Strain, (mm/mm)

Specimen 5

Mesh Direction 2

(R2 = 0.942)

Uniaxial Data

92

Appendix J

ABAQUS INP File: Lap Shear Model

The following is a sample ABAQUS Standard input file for the computational lap

shear simulation (specimen 2_7). The complete list of nodes and elements has been

removed in order to shorten the document.

*Heading

** Job name: sp27 Model name: Model-1

** Generated by: Abaqus/CAE 6.12-1

*Preprint, echo=NO, model=NO, history=NO, contact=NO

** PARTS

*Part, name=Mesh

*End Part

*Part, name=Tissue

*End Part

** ASSEMBLY

*Assembly, name=Assembly

*Instance, name=Mesh-1, part=Mesh

*Node

[REMOVED]

*Element, type=S4R

[REMOVED]

*Nset, nset=Mesh, generate

1, 468, 1

*Elset, elset=Mesh, generate

1, 425, 1

** Section: Mesh

*Shell Section, elset=Mesh, material=Mesh

0.044, 5

*End Instance

*Instance, name=Tissue-1, part=Tissue

*Node

[REMOVED]

*Element, type=C3D8H

93

[REMOVED]

*Nset, nset=Tissue, generate

1, 1890, 1

*Elset, elset=Tissue, generate

1, 1352, 1

** Section: Tissue

*Solid Section, elset=Tissue, material=Tissue,

*End Instance

*Nset, nset="Tissue Tied Region", instance=Tissue-1

[REMOVED]

*Elset, elset="Tissue Tied Region", instance=Tissue-1, generate

1093, 1352, 1

*Nset, nset="Mesh Edge", instance=Mesh-1

5, 6, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98

99, 100

*Elset, elset="Mesh Edge", instance=Mesh-1, generate

409, 425, 1

*Nset, nset=Set-7, instance=Mesh-1

1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18

19, 20, 21, 22, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96

97, 98, 99, 100

*Elset, elset=Set-7, instance=Mesh-1, generate

409, 425, 1

*Nset, nset=Set-9, instance=Tissue-1

9, 11, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182

*Elset, elset=Set-9, instance=Tissue-1, generate

132, 1092, 80

*Elset, elset="_MeshTied Face_SPOS", internal, instance=Mesh-1, generate

1, 408, 1

*Surface, type=ELEMENT, name="MeshTied Face"

"_MeshTied Face_SPOS", SPOS

*Elset, elset="_TissueTied Face_S4", internal, instance=Tissue-1, generate

56, 1092, 4

*Surface, type=ELEMENT, name="TissueTied Face"

"_TissueTied Face_S4", S4

*End Assembly

** MATERIALS

*Material, name=Mesh

*Hyperelastic, ogden, test data input, poisson=0.45

*Uniaxial Test Data

[REMOVED]

*Material, name=Tissue

*Hyperelastic, n=2, reduced polynomial

0.521, 1.275, 0.15, 0.

94

** INTERACTION PROPERTIES

*Surface Interaction, name=IntProp-1

1.,

*Surface Behavior, pressure-overclosure=HARD

** BOUNDARY CONDITIONS

** Name: Tied Region Type: Symmetry/Antisymmetry/Encastre

*Boundary

"Tissue Tied Region", ENCASTRE

**

** INTERACTIONS

** Interaction: Int-2

*Contact Pair, interaction=IntProp-1, small sliding, type=SURFACE TO SURFACE, adjust=0.0,

tied

"MeshTied Face", "TissueTied Face"

** ----------------------------------------------------------------

** STEP: Step-1

*Step, name=Step-1, nlgeom=YES

*Static

0.05, 1., 1e-05, 0.1

** BOUNDARY CONDITIONS

** Name: Displacement Type: Displacement/Rotation

*Boundary

"Mesh Edge", 1, 1

"Mesh Edge", 2, 2, 2.3

"Mesh Edge", 3, 3

** OUTPUT REQUESTS

*Restart, write, frequency=0

** FIELD OUTPUT: F-Output-1

*Output, field, variable=PRESELECT

** HISTORY OUTPUT: H-Output-1

*Output, history, variable=PRESELECT

*End Step

95

Appendix K

FEA Lap Shear Simulation vs. Lap Shear Experiments

The following content include all the graphs for each individual FEA lap shear

simulation and relates it to the experimental data.

96

0

10

20

30

40

50

0 5 10

Load

, N

Displacement, mm

Specimen 9_4

(R2 = 0.984)

Lap Shear Data

FEA Simulation

97

Appendix L

Artist Permission

Artist permission to use specific figures in this thesis

12/20/2013

Dear Ms. Coalinn Golden

I am writing to request some images from your collection entitled:

Schematic, cross-sectional view, of a typical ventral hernia with intestinal protrusion.

Schematic, cosmetic defect, of a typical ventral hernia with intestinal protrusion.

Hernioplasty repair for an onlay ventral hernia surgery

Mesh placement (onlay, inlay, sublay)

Abdominal wall description of orientation This image will appear in a book by Hummad Tasneem currently entitled “Dependence of the

Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia Repair” to

be published by the University of Memphis Press in the Spring of 2014. This is a scholarly

undertaking that will reach a limited and specialized academic audience.

I am requesting permission to use the image as both an interior illustration and other forms of

illustration connected with this volume, including but not limited to advertising, publicity, and

direct mail, or other similar uses, but excluding use as a cover illustration. I ask that you grant

nonexclusive world rights for the reproduction, as part of this thesis only, in all languages and

for all editions (including ebook).

Please sign and return this letter to me along with the image in question. Please contact me if

you have any questions regarding this request.

Sincerely yours,

Hummad Tasneem

Approved: _________________________________ Date: ____________

(signature)

98

12/20/2013

Dear Ms. Kathryn Hicks

I am writing to request some images from your collection entitled:

Surgical mesh close up view of mesh pores

This image will appear in a book by Hummad Tasneem currently entitled “Dependence of the

Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia Repair” to

be published by the University of Memphis Press in the Spring of 2014. This is a scholarly

undertaking that will reach a limited and specialized academic audience.

I am requesting permission to use the image as both an interior illustration and other forms of

illustration connected with this volume, including but not limited to advertising, publicity, and

direct mail, or other similar uses, but excluding use as a cover illustration. I ask that you grant

nonexclusive world rights for the reproduction, as part of this thesis only, in all languages and

for all editions (including ebook).

Please sign and return this letter to me along with the image in question. Please contact me if

you have any questions regarding this request.

Sincerely yours,

Hummad Tasneem

Approved: _________________________________ Date: ____________

(signature)