Here are the key forces that cause pressure ulcers

Four physical forces are culprits in the formation of pressure ulcers:direct pressure, shearing, friction, and maceration. Here is a summary of how these forces act on tissue: 

Direct pressure. 

This is the compression or squeezing together of soft tissues, especially against bony prominences such as the sacrum, scapula, elbows, head, heels, and pelvis. It begins with the vertical force of a person pulled by gravity onto a supporting surface. Tissue compression of this nature over prolonged periods shuts down capillary blood flow and leads to tissue damage from oxygen deprivation. Tissue necrosis results. Ulcers caused by direct pressure often present as a small surface wound, but the sore often develops in a cone shape, with a wide base of unseen deep tissue damage and a far smaller presentation at the skin. 

Shear. 

This is a horizontal force perpendicular to pressure and refers to the internal, opposing motions of tissue layers and bones. It is caused by the combination of friction and movement. For instance, shear is at work when the patient’s skin sticks to a mattress surface and the patient or surface moves. Gravity pulls down on the deep tissue while friction against the mattress holds the skin in place. This often occurs with patients who elevate the head of the bed more than 30 degrees — a situation in which the spine may slide down while the skin sticks to the sheets. Shearing forces stretch and tear blood vessels and are the primary reason why heels often break down on support surfaces that deliver interface pressures below capillary closure. One researcher uses a garden hose analogy to explain shear: "Assume pressure is like stepping on the hose, then shear is like bending it. It is easier to shut off blood by bending capillaries [shear], just as it is easier to stop water flow through a hose by bending it.1 Patient movement, second-party movement of the patient and bed movement are all potential [causes] of shear." 

Friction. 

This is also caused by opposing motions, but by the movement of one surface across another. This occurs, for instance, when a patient is pulled across a bed linen surface. Whereas shear forces are exerted internally, friction works on the surface of the skin, resulting in dermal wounds. 

Maceration. 

This condition results from prolonged exposure to moisture, which causes tissue softening. As the skin and underlying tissues soften, skin integrity decreases, and the skin becomes susceptible to the various mechanical forces described above, especially friction. The most common source of moisture for wound patients is urinary and fecal incontinence.

Reference
1. Jay R. Pressure and shear: Their effects on support surface choice. Ostomy Wound Mgmt 1995; 41:36-45. n