What is shear?

12/08/2015 11:41:22

The final post in our 'Pressure, Friction, Shear' series deals with shear, which has become a buzzword in pressure ulcer prevention in recent times.

So what is shear?
A shear force or shear stress can be defined as, ‘Unaligned forces pushing one part of a body in one direction, and another part of the body in the opposite direction’(1)
Shear forces can lead to shear strain, which is the deformation of a cell. 
Whereas pressure acts at right angles with the skin’s surface, shear forces act in a direction parallel to the skin’s surface. Skin is much more easily sheared than compressed.
Shear is a key force in the development of pressure ulcers. The 2014 International NPUAP-EPUAP Pressure Ulcer Prevention and Treatment Clinical Practice Guideline defines a pressure ulcer as “… a localized injury to the skin and/or underlying tissue usually over a bony prominence, as a result of pressure, or pressure in combination with shear(2).”  
How can shear be prevented?
As mentioned in the first post in this series, where friction is present, shear forces will be present too.  
Shear forces work in conjunction with pressure and friction, therefore the three can easily be confused. See the previous blog post on pressure to recep.
So the answer should be simple – lie very still and don’t move?
Actually – no. 
A common misconception is that you need movement for shear to be present: NOT TRUE! 
A patient lying on a support surface will still experience shear forces that act across the skin’s surface due to the pressure of lying down, even when no movement occurs.
As we have already learnt, for every action there is an equal and opposite reaction – in the case of the diagram below, where pressure is applied above an irregularly shaped bony prominence, the reaction forces (red arrows) will have components of both shear and pressure.
This is compounded by friction when the patient does move, as explained in our ‘What is Friction’ blog.
It’s a complicated process, and means that shear is a very difficult force to measure.
Moreover, while we do know that shear is associated with pressure ulcer development, it is still unclear as to the relative damage caused by shear over time and whether it cause more damage to skin cells or muscle cells, and its impact on the skin’s surface compared to friction or pressure.
Ultimately, all three forces – friction, pressure, shear - act on skin cells, causing distortion and damage. 
Cells will die as a result of moderate but sustained deformation even if oxygen and food supplies are maintained. 
Choosing a medical support surface such as Dartex® that has been developed specifically to work in conjunction with your mattress core, will significantly reduce the effect of these forces and ultimately lower the risk of pressure ulcers on vulnerable areas of skin.
Want to know more? 
Speak with one of our technical experts for more information on the impacts of pressure, shear and friction enquiries@dartexcoatings.com 
(1)    https://en.wikipedia.org/wiki/Shear_force 
(2) National Pressure Ulcer Advisory Panel, European Pressure Ulcer Advisory Panel and Pan Pacific Pressure Injury Alliance. Prevention and Treatment of Pressure Ulcers: Quick Reference Guide. Emily Haesler (Ed.). Cambridge Media: Perth, Australia; 2014 

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