What is the difference between pressure, shearing and friction?




is where there is direct pressure over a bony prominence that is sustained over a long period without moving or slipping down. 

Shear pressure injury is related to pressure but is due to gravity pushing the skeleton down inside the skin or due to movement without releasing the pressure.


is better understood if you take a knuckle of one hand and place it in the palm of the other hand. Move the knuckle backward and forward over the hand and you will see that the skin stays in one place while the bone inside moves within the skin. 

Relate that to a sacrum or heel. The skin is held in place by the surface that it is on (sheet or cushion etc.). The skeleton obeys gravity and, being heavy, will gradually slip inside the skin (as you would have found with the knuckle bone). This causes the skin to stretch in front of the bone and pinches up skin behind the bone. This means the tiny blood vessels will be pinched, preventing the blood reaching areas away from the bony prominence. The pinching of the vessels will cause micro thrombi to occur if the pressure is not relieved every 2 to 3 hours. Once this happens, the blood can no longer reach the areas that have been ischaemic and the tissue will die (necrosis). 

At the same time, the pressure from the bone over the internal tissues will also be excluding blood and will be wearing away tissue inside the skin as it rubs internally. Shearing will only occur if there is pressure and is actually more dangerous than pressure alone.

Shear injury will not be seen at the skin level because it happens beneath the skin.


will be seen as it is the result of dragging the tissues over a surface, pulling off the cells from the epidermis and exposing the dermis. Often referred to as ‘carpet burns’.

Pressure is where the skeleton bony prominences press down on the skin internally and the injury will be the shape of the bone that caused the pressure when pressed against a firmer surface.



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