While the major factor in the development of pressure injury is pressure itself shear and friction forces also play a part.
What are shear and friction?
When you move over a surface and remain in contact with it, the skin gets pulled along that surface. You can feel this ‘drag’ effect if you run your hand over something.
This ‘drag’ is caused by friction and leads to a difference in the movement of the skin and the deeper tissues.
This difference is called shear and is an important factor in injury development.
How can shear and friction lead to a pressure ulcer?
Shear causes stretching and tearing of blood vessels which reduces blood flow, increases blood pooling and can lead to cell damage.
Shearing stops or reduces blood flow more easily than pressure alone (for example, it is easier to cut off flow in a water hose by bending than by pinching it).
Although shear and friction causes tissue damage slightly differently to pressure, the damage produced is the same as with pressure.
Importantly, it is difficult to create pressure without shear, and shear without pressure, so the two usually go hand-in-hand.
For information on how pressure leads to pressure injury, see our article How Pressure Causes Pressure Ulcers.
How can repeated friction cause a pressure ulcer?
As well as being a key element of making shear force, friction between the skin and the stationary surfaces such as bed clothes can result in the loss of the top layer of the skin (the stratum corneum).
Think about repeatedly rubbing your skin against a surface – after a time, the skin would start to become red and sore. This is due to friction.
This inflammation can lead to deeper injury or could be the initial damage needed to start pressure ulcer formation.
When there is loss of the top skin layer, there is also a breach in the barrier against infection.
How can friction and shear be reduced?
There are several strategies to reduce shear and friction forces:
- Use low-friction clothing such as Parafricta undergarments and socks to reduce friction on the vulnerable heel and pelvic areas. These can be especially useful for people who push themselves back up the bed using their heels, or who have involuntary leg movement.
- Use positioning aids to maintain comfortable positioning, without using friction to maintain body position.
- Use a profiling bed to maintain position with a knee break – this stops people sliding down the bed and needing to move themselves across the bed surface.
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