McClemont Cone Effect

When a person is lying or sitting, pressure is transferred from the external surface, through the layers of the skin, toward the underlying bone. Skin, blood vessels, subcutaneous fat and muscle are compressed between the bone (which acts as a counter pressure) and the external surface. 

This results in a cone, or pyramid shaped, pressure gradient. The apex of the cone equates to the bony surface where tissue interface pressures are highest. This leads to the intensity of pressure being up to five times greater on deep tissues (muscles/bony surfaces) than that on the epidermis. 

Deep tissue necrosis often occurs first at the bony interface as a result of this pressure, and the fact that muscle tissue is more sensitive and less resistant to pressure than the skin. Pressure exerted at the bony interface then emerges at a point in the surface of the skin. A small, inflamed area, over a bony prominence, may indicate tissue breakdown that is much deeper and wider than indicated at the surface of the skin. How these effects can be assessed will be discussed in Module 2.