A presentation aimed at cliniciansand associated professionals

ShearA contributory factor inpressure ulceration

Introduction

Although shear is understood to be a major contributorto the formation of pressure ulcers, the measurement ofshear and how shear contributes to the formation ofpressure ulcers have not yet been fully established.

In 2005, an international consensus group, the ShearForce Initiative (SFI) was created in order to explore thecurrent level of knowledge and to shape future research.

Mission Statement

To define, study and measure shear and its effect

on the various physiological, metabolic and physical

characteristics that influence the assessment,

prevention, treatment and development of pressure

ulcers in order to improve patient management.

This section represents the evidence for the role of shearin pressure ulceration based upon an extensive literaturereview and the agreement of the expert panel

Shear:What DO we know?

We know thatShear: What DO we know?

shear is a normal mechanical forcewith physiological effects

An action or stress resulting from applied forces which causes ortends to cause two contiguous internal parts of the body to deformin the transverse plane (i.e. shear strain)

Ohura T (Sapporo, Japan) provided in 2007

Defining ShearShear: What DO we know?

Ohura T (Sapporo, Japan) provided in 2007

Visualizing ShearShear: What DO we know?

We know thatShear: What DO we know?

load has an internal effect orinfluence on the tissue

Distribution of stresses inside tissuefrom the view of biomechanics

Shear: What DO we know?

Surface Pressure

InsidetissueTensile

stress

Compressionstress

Shearstress

Bone

We know thatShear: What DO we know?

shear occurseven when lying flat

Cross section through the bodyat the level of the trochanter Gluteal

muscle

Bone

MagneticResonanceImaging (MRI)

A. The inset slideshows the tissueover the buttockswhen completelyoff-loaded.

B. When the bodyis resting on a solidsurface, the extentof distortion(spread) in themuscle can bevisualised.

B.

Shear occurs even when laying flatShear: What DO we know?

A.

We know thatShear: What DO we know?

if pressure remains constant andwe increase shear, the deformation

of soft tissue may be greater.

Movement influences shear forcesShear: What DO we know?

Makoto Takahashi,Yuta Tamura(Sapporo, Japan) provided in 2007

Movement increasesstrain in deep tissues

bone

Pressure: 8mmHgLateral movement: None

bone

Direction ofmovement

Pressure: 8mmHgLateral movement: Only 5mm!

We know thatShear: What DO we know?

changes in position or postureare likely to change shearexternally and internally

Shear force occurring when the headof the bed is raised and lowered

Shear: What DO we know?

Ohura T, Takahashi M, et alSapporo, Japan. J Pressure Ulcers. 9(1): 21-27, 2007.

Click image toplay animation

An example of residual shearShear: What DO we know?

Deformation and ischemia can occur whilethe elbow is bending and stretching.

Shear: What DO we know?

Before movement After movement

Deformation Blanching

Ohura T, Takahashi M, et alJpn J Pressure Ulcers. 9121-27, 2007.

Measurement of shear is not easyShear: What DO we know?

The complexity of shear measurements requires closeattention to both patient safety and potentiallyconfounding influences.

A guidance document has been produced to aid in: Calibration and validation of shear measurement devices Laboratory / bench measurement of shear Clinical measurement of shear with respect to:

Patient safety Data validity

See the document Suggested Shear Measurement Guidanceavailable at the SFI (http://shearforceinitiative.com/) andNPUAP (www.npuap.org) websites

This section illustrates the areas where evidence islacking and highlights the areas where furtherresearch is needed

Shear:What we DO NOT know

We dont knowShear:

how shear actually causestissue damage.

We dont knowShear:

what is the relationship betweenshear force and time?

Although pressure ulceration is associated with shear,it is as yet unclear as to the exact relationship betweenshear and its duration that causes damage

We dont knowShear:

whether shear damages musclemore than skin or fat.

It is unclear whether certain tissues are moresusceptible to shear forces

We dont knowShear:

which patients are at greatest riskof shear injury.

There is insufficient knowledge about the characteristics ofshear, and the influence it has on tissue, to reliably predictthose at greatest risk.

We dont knowShear:

the relationship between externaland internal shear.

While tools have been developed to try to measure shear at theskin surface there is insufficient knowledge about its effect indeeper tissue.

We dont knowShear:

what effect postural changes(frequency and/or speed)

have on shear force.

Shear models show that shear changes with movement butwhether these effects influence tissue damage is unknown.

Shear:Practical Suggestions

Practical suggestions to reducethe amount of shear forceexerted on the tissue: Assist the patient to adopt a stable

posture using aids when necessary Reduce the chance of sliding down in

the bed Reduce the chance of sliding in a chair When repositioning patients LIFT

dont DRAG

Summary

Shear is a normal mechanical force withphysiological effects

Shear is difficult to measure It is unclear how shear actually causes

tissue damage Changes in position or posture are likely

to change shear externally (friction) andinternally (shear strain)

Further ReadingGuttmann L.: Spinal cord injuries: comprehensive management and research. Oxford, Blackwell Scientific, 1973

Hobson D.A.: Comparative effects of posture on pressure and shear at the body-seat interface. Journal of RehabilitationResearch & Development. Fall 92, 29(4):21-, 2003

Bennett L., Kavner D., Lee B., Trainor F.A.: Shear vs pressure as causative factors in skin blood flow occlusion. Arch PhysMed Rehabil. 60: 309-314, 1979

Le KM, Madsen BL, Barth PW, et al. : An in-depth look at pressure sores using monolithic silicon pressure sensors. PlastReconstr Surg. 74:745-756, 1984

Ohura T, Ohura N. Jr.: Pathogenic Mechanisms and Classification of Undermining in Pressure Ulcers-Elucidation ofRelationship with Deep Tissue Injuries. WOUNDS. 18(12):329-339, 2006

Ohura T, Takahashi M, Ohura N. Jr. : Influence of External Force (Pressure and Shear Force) on Superficial Layer andSubcutis of Porcine Skin and Effects of Dressing Materials. Wound Repair and Regeneration, 2007; in press.

Ohura N, Ichioka S, Nakatsuka T, Shibata M.: Evaluating dressing materials for the prevention of shear force in thetreatment of pressure ulcers. J Wound Care. 14(9):401-4, 2005

FOR A COMPREHENSIVE LIST OF SHEAR-RELATED REFERENCES GO TO www.shearforceinitiative.com

COEFFICIENT OF FRICTION :A measurement of the amount of friction existing between twosurfaces for a given force tending to hold the surfaces together.

FRICTION :The resistance to motion of the external tissue sliding in a parallel directionrelative to the support surface resulting in external tissue damage.

PRESSURE :The force per unit area exerted perpendicular to a surface.

SHEAR :An action or stress resulting from applied forces which causes or tends tocause two contiguous internal parts of the body to deform in thetransverse plane (i.e., shear strain).

Glossary