Bioactive Interference Screws for ACL Reconstruction

Dana Nadler – Communicator

Cole Kreofsky – BSAC

Katherine Davis – BSAC

Aaron Huser – BWIG

Joe Poblocki – Team Leader

Client – Professor William Murphy

Advisor – Professor Kristyn Masters

Problem Statement The primary objective of this project is to

design an interference screw for ACL reconstruction that will simultaneously promote the growth of tissue while the screw degrades.

Background Information - ACL 90,000 annual ACL reconstruction surgeries worldwide Reconstructions use patellar or hamstring tendon grafts Grafts are implanted in the femur and tibia Grafts secured with interference screws

http://miranda.ingentaselect.com/vl=2967323/cl=12/nw=1/fm=docpdf/rpsv/cw/pep/09544119/v217n1/s9/p59

Background Information - Screws Titanium:

Extremely strong Biocompatible Reliable Second surgery may be

necessary Degradable Plastics:

Strength and shape Degradable Multiple polymers used

Poly(L-Lactic) Acid (PLLA)

Poly(Lactic-co-Glycolic) Acid (PLGA)

http://www.arthrotek.com/products/aclpcl_interference.cfm

http://www.jnjgateway.com/home.jhtml?loc=USENG&page=viewContent&contentId=09008b9880a6f43e&parentId=09008b9880a6f43e

Problem Motivation Problems:

Current screw may cause unwanted debris in the knee

Current material does not promote tissue growth

A new interference screw is needed that: Mimics bone scaffold structure Promotes tissue growth

Design ConstraintsThe screw must be: Bioactive Biocompatible Biphasic Easily sterilized or autoclaved Structurally sound

Design Materials Alginate Hydrogel:

Can be doped with growth factors, nutrients, metabolites, etc.

Provides three dimensional scaffolding Promotes cell proliferation and tissue growth

Mold InvestigationsPlastic

Transition temperature must be larger then melting point of thermoplastic

Data:Plastic Tm(oC) Tg(oC)PLGA 210 37PLA 185 57PGA 225 40

PaperWill burn when thermoplastic is added

MetalMay be only alternative that can handle the temperature of thermoplastic

Design Evolution Limitations with

mechanical properties of mineralized hydrogel

Change geometry of thermoplastic to increase structural strength

Minimize stress on mineralized hydrogel portions of screw

PLGA

Alginate

Hexagon Core Design

Triangle Core Design

Shear stress at point M:

Where B varies with a/r over the range

Alginate Calculations3r

TB

432

49.130506.99713.339697.22135.1

r

a

r

a

r

a

r

aB

ra 5.01.0

r

a

M

Maximum Shear Stress of PLGA: ~14 MPa

Insertion Torque: 1.5 N*m

Maximum Radius of Screw: 5mm

32

0534.79501.25427.10055.1

r

a

r

a

r

aB

For two grooves:

For four grooves:

Alginate Radius = 0.5mm 2% of total area can be alginate

For 4 Grooves: r=5mm

a/ r BRadius of

Alginate (mm)Shear Stress (MPa)

0.5 3.874275 2.5 46.50.3 1.72 1.5 20.640.1 1.167 0.5 14.01

For 2 Grooves: r=5mm

a/ r BRadius of

Alginate (mm)Shear Stress (MPa)

0.5 1.921 2.5 23.010.3 1.393 1.5 16.70.1 1.137 0.5 13.64

Torque Analysis

BL

BLL

AA

BA

LLA

HT

H

T

45.22

33)4/3)((2/1

2

33

)4/3)((2/1

2

2

Torque AnalysisT

rr

F

F

F

F

F

F

F

F

F

T

L

BFF

BFLF

BFTM

LFTM

HT

HT

HH

TT

2

)6)(2/()3)(2/(

)6)(2/(0

)3)(2/(0

HT

HT

FF

BLL

BFF

)225.1(

45.2;2

Combining the two equations gives:

Future Work Find thermoplastic with:

Adequate melting point Structural integrity FDA approval

Investigate further alginate addition Quantitatively analyze designs Mold Development Prototype Development Documentation