Laboratory for cell and tissue engineering

Institute for Complex Molecular Systems (ICMS)

Department of Biomedical Engineering

The role of shear forces and shear strains in the development of pressure ulcers.

Cees Oomens, Dan Bader

“Shear and tissue integrity – the state of the science” London, October 2014

definition of a pressure ulcer

A pressure ulcer is a localized injury to the skin and/or underlying tissue, usually over a bony prominence, resulting from sustained pressure (including pressure associated with shear). A number of contributing or confounding factors are also associated with pressure ulcers; the primary of which are impaired mobility and impaired sensory perception.

EPUAP/NPUAP/PPPIA, 2014

definition of a pressure ulcer

A pressure ulcer is a localized injury to the skin and/or underlying tissue, usually over a bony prominence, resulting from sustained pressure (including pressure associated with shear).

EPUAP/NPUAP/PPPIA, 2014

Problem with definition

• The definition suggests that we have to focus on pressures and shear forces when we study pressure ulcers

• It even suggests that it is primarily the interface pressure and shear force that are relevant

Normal force (pressure)

Shear force

Danger of shear forces – parallel to the skin

Reichel , 1985 Dinsdale, 1974

Why are shear forces more dangerous than pressure alone ? Because the mechanical load is higher?

Probably NOT

Hypothesis of damage mechanisms due to shear forces

Skin wrinkles: This leads to very high deformations in the skin

Hypothesis of damage mechanisms due to shear forces

Delamination of skin layers

Hagisawa, 2005

shear modulus [kPa]

Indentation modulus [kPa]

Young’smodulus [kPa]

humidity

Stratum corneum 25% 30 600 0.04-10∙106

98%

10 ? 6-10∙104

Viable epidermis 25% 30 600 ?

98%

10 ? ?

Dermis 8 1-10 1-20∙103

Hypodermis 24 20-30 ?

Red: Geerligs et al. 2011,2012, Black: literature values

Hypothesis of damage mechanisms due to shear forces

Shear modulus much lower than Young’s modulus – Anisotropic Behaviour

Hypothesis of damage mechanisms due to shear forces

Easier to block blood vessel by shearing

Hypothesis of damage mechanisms due to shear forces

Effect of frictional sliding on skin

Heat generation Abrasion of tiny particles from Stratum Corneum at each sliding Up-regulation of cytokines in skin

Effect of shear forces seems important near surface

• All mechanisms described so far are important near the surface layers of the skin

• But what is the effect of pressure and shear in deeper layers?

How about strain/deformation?

How about strain/deformation?

Shear deformation is everywhere

Inhomogeneous shear strains – highest near bones

Chow,Odell, 1978

Todd, Thacker, 1994

Dabnichki et al., 1994

Oomens et al., 2003

Inhomogeneous shear strains – highest near bones

Chow,Odell, 1978

Todd, Thacker, 1994

Dabnichki et al., 1994

Oomens et al., 2003

muscle

skin

fat

Internal strains in a sitting person

Linder-Ganz et al.. J Biomech, 2007.

18

High strain often found near bony prominence

MR image during sitting

Linder-Ganz et al.. J Biomech, 2007.

Strain distribution

Internal strain and stress are very nonuniform

The initiation of damage is a very local phenomenon – at cellular level – and we need to know the effects of mechanical loading at a very local level.

Model systems to study etiology of pressure ulcers

Model system

single cell studies

Tissue engineered muscle

Rat model human volunteers

Cells and tissues

PAGE 21

Bouten et al. Ann. Biomed. Eng. (2001)

Breuls et al. Ann. Biomed. Eng. (2003)

Peeters et al. Med. Biol. Engng. Comp. (2003)

10 µm

Cells die as a result of moderate, but sustained

deformation even if oxygen and nutrient

supply are normal

Tissue engineered muscle

Damagemarkers

fluorescent markers

Gawlitta et al. J. Appl. Physiol. (2007) Gawlitta et al. Ann. Biomed. Eng (2007)

Deformation damage starts

fast

Hypoxia leads to change in metabolism

Low pH and lack of nutrients cause cell death at a longer time scale

Gefen et al. J.Biomech. 2008

23

The concept of animal studies

A. Stekelenburg et al. Med. Eng. Phys. (2006)

T2 – weighted MRI Perfusion measurements Animal specific theoretical models

In vivo studies confirm in-vitro studies

deformation damage fast and strong

correlation with level of deformation

Ischaemic damage slower process

Time scale is different from in-vitro scale

Two mechanisms for damage development

PAGE 24 17-11-2014

strain

Two mechanisms for damage development

PAGE 25 17-11-2014

strain

Two mechanisms for damage development

PAGE 26 17-11-2014

strain

Occlusion of blood vessel

Change in metabolism

Accumulation of waste products

pH decrease

CELL DEATH

Two mechanisms for damage development

PAGE 27 17-11-2014

strain

Occlusion of blood vessel

Change in metabolism

Accumulation of waste products

pH decrease

CELL DEATH

Direct deformation related damage

Membrane failure

Disruption of cytoskeleton

?

time

Exte

rnal

Pre

ssu

re

Reswick and Rogers, 1976

Recent research suggests a new risk curve

High Risk

Low Risk

INTE

RN

AL

LOC

AL

STR

AIN

/DEF

OR

MAT

ION

Clinical relevance

If mechanical loads causing ischaemia (threshold 1) cannot be avoided relieve the load at regular times (within 4 hours) to reset metabolic equilibrium.

Indeed try to avoid shear forces as much as possible

Avoid high loads above threshold 2.

17-11-2014

Clinical relevance

PAGE 30 17-11-2014

Do not pull or push the soft tissue around the wound at the time of body transfer, repositioning, diaper changing and bed operation.

(T.Ohura, 2012)

Acknowledgments

Dan Bader

Frank Baaijens

Klaas Nicolay

Gustav Strijkers

Jo Habets

Carlijn Bouten

Amit Gefen

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Marielle Bosboom Roel Breuls Emiel Peeters Debby Gawlitta Debbie Bronneberg Lisette Cornelissen Karlien Ceelen Anke Stekelenburg Sandra Loerakker Willeke Traa Jules Nelissen Kevin Moerman Jibbe Soetens