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What happens when you sit? Explaining seated buttocks deformation.

Learning Objectives

• Describe 2 possible ways that tissue deformation leads to tissue breakdown.

• Identify the loaded anatomy of the buttocks.

• Compare and contrast tissue deformation across cushions and postures.

WHAT DO WE KNOW?

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Magnitude and Duration of Pressure

Reswick, J.B. and J. Rogers, in Bedsore Biomechanics, R.M. Kenedi, J.M. Cowden, and J.T. Scales, Editors. 1976, University Park Press: Baltimore. p. 301-310.

From the outside in…

skinSub-q adipose

muscle

bone

Pressure Injury Aetiology

1. Cell damage from prolonged deformation (strain)

2. (Deformation-induced) ischaemia of soft tissues

3. A disruption in the equilibrium of the lymphatic system

Figure 9. Schematic of DTI AeotiologyOomens, C.W., et al., Pressure induced deep tissue injury explained.

Ann Biomed Eng, 2015. 43(2): p. 297-305.

Bouten, C.V., et al., The etiology of pressure ulcers: skin deep or

muscle bound? Arch Phys Med Rehabil, 2003. 84(4): p. 616-9.

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WHAT DO PEOPLE THINK THEY KNOW?And why some believe sitting through this 1 hour webinar will kill you…

#1 Pressure Myth: 32mmHg

• Myth: any load exceeding 32 mmHg is harmful.

• Study which measured the pressure within the capillary loop of a fingernail bed (Landis , 1930).

• Landis’ protocol did not include inducing occlusion.

8

Landis, 1930- Heart

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How much normal loading causes ischemia?(remember ischemic damage has magnitude & duration factors)

• Studies report widely varying results– Lassen and Holstein, 1974

• occlusion pressure approximated diastolic pressures – Holloway, Daly, Kennedy, & Chimoskey, 1976)

• blood flow decreased at mean arterial pressure • occlusion reached around 120 mmHg.

– Ek, Gustavsson, & Lewis, 1987• ‘weak positive correlation’ between blood flow during loading and systolic blood pressure.

– Sangeorzan, Harrington, Wyss, Czerniecki, & Matsen, 1989) • 71 mmHg was needed to occlude flow over 'soft' sites • 42 mmHg occluded flow over 'hard' sites.

– Bennett, Kavner, Lee, & Trainor, 1979) • 100-120 mmHg necessary to occlude vessels in ‘low shear’ conditions • 60-80 mmHg needed in the presence of ‘high shear’

– Goosens, Zegers, van Dijke & Snijders, 1994• 85.5 mmHg necessary to induce ischemia in ‘no shear’ condition• 64 mmHg needed the presence of ‘high shear’

“50% Strain damages tissue”

• In bio-artificial muscle (petri-dish) loaded directly

• Strain of an individual cell (not bulk tissue strain)

• Instantaneous cell death at ~57% compressive strain

• Cells survive 50% strain for 1 hour

• Cells die after 4.75 hours at 35% strain

Gefen, A., B. van Nierop, D.L. Bader, and C.W. Oomens, Strain-time cell-death threshold for skeletal muscle in a tissue-engineered model system for deep tissue injury. J Biomech, 2008. 41(9): p. 2003-12.

What loads cause damage?

• Dinsdale 1974– normal pressure and normal pressure + friction to swine– 3 hours with normal pressures below 150 mmHg → no damage– 3 hours with Normal pressures at 45 mmHg + friction → tissue changes

• Daniel, Wheatley, and Priest 1985– 200 mmHg to troch of swine for 15 hours → no PrU– 500 mmHg for 4 hours → PrU

• Linder-Ganz and Gefen 2004– Rat hind limbs loaded up to 6 hours– 6 hours at 100 mmHg → damage, 2 hours at 300 mmHg → damage

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Gefen, A., Reswick and Rogers pressure-time curve for pressure ulcer risk. Part 2. Nurs Stand, 2009. 23(46): p. 40-4.

So, if someone tries to tell you that 32mmHg or 50% strain will cause damage…

RUNor

Poke their arm for an hour and wait for the cells to die.

BACK TO THE SEATED BUTTOCKS…

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MRI Studies of the Buttocks: FONAR “Stand-up” MRI

Poll: What are we sitting on?

A. Muscle

B. Fat

C. Connective Tissue and Skin

D. An uncomfortable office chair

Typical Assumption: People are Sitting on Muscle

Sopher, R., et al. J Biomech, 2010. 43(2): p. 280-6.

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Gluteus Maximus Characteristics

Wu & Bogie, 2013. J Tissue Viability. 2013 Aug;22(3):74-82.

Muscle coverage of ischial tuberosities using 3D MRI measurements

Wheelchair Users

Non-disabled cohort

Total

0 % Glut Coverage 13 3 16

<25% Glut Coverage 2 3 5

25-50% Glut Coverage 0 1 1

>50% Glut Coverage 1 2 3

Totals 16 9 25

Example of the Gluteus Maximus While Seated: T12 SCI Male

Inferior View Medial View

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Able-Bodied Buttocks

SCI (Atrophied) Buttocks

Natural anatomical variation during sitting

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Soft tissues at the Support Surfaces

Shear

Strain

Undeformed

Normal forces can

deform tissue

Shear forces can

deform tissue

Image courtesy of Dr. Nicola Petrone

Tissue Deformation Under Load

Displacement + Distortion (strain)

Interface Pressure Case Study

Matrx Vi Roho HP

44 yo male, T5-6 SCI, 6’1”, 163 lbs, PrU Hx at Right IT

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Case Study Continued: What’s Inside?

Matrx Vi Roho HP

Case Study

• What story did the IPM tell?– Normal or perpendicular force over area (pressure)

• What story did the MRI tell?– Strain, displacement

• Different IPM, similar shape and strains• He’s not a great candidate for either cushion, but it took skin

inspection to figure that out.

Does interface pressure relate to what’s going on inside at all?

Case Study 2

J2 Deep Contour Matrx Vi

47 yo male, 6’1”, 220 lb, C5-6 SCI, no Hx PrU

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Case Study 2

J2 Deep Contour Matrx Vi

Case Study 2

J2 Deep Contour Matrx Vi

Case Study 2

J2 Deep Contour Matrx ViUnloaded

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Case Study 2

• Very similar pressure maps, but very different buttocks shapes

• Very anterior presentation of the ischial tuberosity – all the tissue mass seems to be behind him

• Evidence of significant internal shear strains in the adipose

Does interface pressure relate to what’s going on inside at all?

Interface Pressure vs. Buttocks Shape (lots of people on similar cushions)

300250200150100

0.030

0.025

0.020

0.015

0.010

0.005

Peak Pressure Index (mmHg)

Sa

git

tal

Cu

rva

ture

(1

/m

m)

Results exclude rows where isFoam=0.

Sagittal Curvature

3002502001 501 00

0.030

0.025

0.020

0.01 5

0.01 0

0.005

Peak Pressure Index (mmHg)

Sa

git

tal

Cu

rva

ture

(1

/m

m) AB ContourFoam

AB HR45

WC ContourFoam

WC HR45

Status CushionGeneral

Results exclude rows where isFoam=0.

Sagittal Curvature

Interface Pressure vs. Buttocks Shape Why is the relationship so complicated?

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Interface Pressure vs. Buttocks Shape Why is the relationship so complicated?

• Amount and distribution of tissue

• Type of tissue

• Quality / mechanics of tissue

• Shape and presentation of bone

Biomechanical Risk!

• Differences in cushion construction: materials and design

Low Biomechanical

Risk

High Biomechanical

Risk

Does this mean I shouldn’t use IPM?(hint… no)

• Contact area• Peak pressure index• Dispersion index• Symmetry• Answering the question: “is this cushion achieving the goal related to

pressure management?”• Ideal for feedback for subject posture, weight shifts• Pressure measurements alone are not

sufficient to alert the clinician to potential areas of tissue breakdown

Back to the confusing part… Can we measure what is going on inside?

• Internal stress and strain in vivo

– Strain measurement techniques in the works

– Stress measurements would be invasive

• Computational models of stress and strain

– Require validation

– Sensitive to assumptions

• Buttocks models and phantoms

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DIFFERENCES IN TISSUE DEFORMATION

ACCORDING TO BODY TYPE & BIOMECHANICAL

RISK

Risk Spectrum

Pro

bab

ly:

Som

e In

com

ple

te S

CI,

oth

er d

iagn

ose

s

Risk Spectrum

Pro

bab

ly:

Som

e In

com

ple

te S

CI,

oth

er d

iagn

ose

s

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Risk Spectrum

Pro

bab

ly:

Som

e In

com

ple

te S

CI,

oth

er d

iagn

ose

s

Visualizing Biomechanical Risk

Can Sit on Anything!

Needs to be attentive to what he sits on!

Strain: The Inner Adipose Surface

Lowest

Biomechanical Risk

Highest

Biomechanical Risk

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Strain: The Inner Adipose Surface

Lowest

Biomechanical Risk

Highest

Biomechanical Risk

95908580757065

15.0

12.5

10.0

7.5

5.0

% Maximum Displacement

Bu

tto

cks D

isp

lace

me

nt

(mm

)

AB

PUS

SCI

Study

Tissue Compliance

HOW DIFFERENT SURFACES SUPPORT

THE BUTTOCKS

Choosing a cushion

You are trying to juggle many things:

• Comfort

• Stability

• Function

• Pressure Injury prevention

Cushion performance for pressure injury prevention:

• Microclimate management

• Durability

• Ability to correct or accommodate obliquity

• Shape Compliance

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Shape Compliance

• The ability of a cushion to support the buttocks with minimal buttocks deformation

• Can be considered a metric of cushion performance

• Currently no method available to evaluate shape compliance

Shape Compliance Pilot Study

• Goals

– To describe the deformation of highly atrophied seated buttocks tissue under load on different types of cushions in an effort to explain how different types of wheelchair cushions support the buttocks.

– To provide supporting data for an eventual shape compliance test

Loading Conditions Tested

Java (Ride Designs): Offloading

Orthotic approach

Matrx Vi (Invacare): Immersion

Compression of Foam

Roho HP (Permobil):

Immersion

Compression of Bladders

Aims to behave like a fluid

Pelvis Unloaded:

Tissue under the

pelvis sags due

to gravity

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Subjects Studied

• N=4 Complete SCI– 1 M, 3 F– T2-T11– 32-46 yo– 5-24 years post injury– 2 no history of PrI, 1 recurrent PrI, 1 PrI immediately after injury only– No fixed deformities, able to be seated safely on a variety of skin protection cushions,

can sit with no obliquity from the frontal plane

• N=3 a/b– 21-52 yo– 2 M, 1 F

Distance from the Seat Base (meat to metal?)

RohoMatrxJava

SCIa/bSCIa/bSCIa/b

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Dis

tan

ce (

cm

)

Distance to Seat Base

Posterior Coronal

View of Skin

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Posterior Coronal

View of Skin

Posterior Coronal View of Gluteus Maximus and Pelvis

Lateral Sagittal

View of Skin

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Bulk Tissue Thickness Under the Ischium

0

5

10

15

20

25

30

35

40

45

Subject 3 Subject 4 Subject 5 Subject 6

Thic

knes

s (m

m)

Bulk Tissue Thickness Under the Ischium (Average in 50mm Region)

Java Matrx Roho Unloaded

Sacrum and Coccyx

Sacrum and Coccyx

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Sacrum and Coccyx

Sacrum and Coccyx

Tissue Under the Greater Trochanter

0

10

20

30

40

50

60

Subject 3 Subject 4 Subject 5 Subject 6

Thic

knes

s (m

m)

Bulk Tissue Thickness Under the Trochanter (Average in 50mm Region)

Java Matrx Roho Unloaded

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What’s it all mean?

• On all surfaces, load is supported somewhere, and tissue deforms (no cushion had perfect shape compliance everywhere)– Keep an eye on those regions with most deformation.– Remember the coccyx!

• Even with expert skills, it’s tough to seat people perfectly.– How do they seat themselves?– How much variation can they get away with?

• These are *highly* atrophied butts. Other butts are going to look different on these surfaces. Remember Biomechanical Risk.

• Posture! These were best efforts at neutral posture. But people slouch. And lean to the side. And cross their legs. etc

DIFFERENCES IN TISSUE DEFORMATION

WITH CHANGES IN POSTURE

© 2017, Permobil

Sliding Tendency and Slouching

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© 2017, Permobil

Internal Motion During Posterior Pelvic Rotation

Image courtesy of Permobil/Sunrise

SubjectID

Posture

13121098

Slouch

Upright

Slouch

Upright

Slouch

Upright

Slouch

Upright

Slouch

Upright

70

60

50

40

30

20

10

0

13121098

Slouch

Upright

Slouch

Upright

Slouch

Upright

Slouch

Upright

Slouch

Upright

Embrace

Dis

tan

ce

(m

m)

Vicair

Distance to Seat Base Under IT

Distance to Seat Base with Slouching?

What happens with slouching?

66 yo M, T12 SCI

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© 2017, Permobil

• No change in thickness

• Only small changes to buttocks curvature

• Changes to immersion depended on cushion and participant

Internal Responses to Slouching

Sacrum and Coccyx

Pelvic Tilt on the Java

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Summary

• Relationship between external load and internal response is complicated– Tissue amount, distribution, shape, quality

– Biomechanical risk

• Biomechanical risk – Exposure to loading

– Subject and injury specific factors that influence tissue deformation

• age, gender, smoking, BMI, etc.

• Cushions– Shape compliance

• Posture– Impacts tissue deformation

– Is not constant