 |
The term shear
is commonly used to refer to the effect of a loading condition in which the skin surface
remains stuck to a support surface while the underlying bony structure moves in a
direction tangential to the surface. For example, when the head of a bed is raised or
lowered, if the skin over the sacrum does not slide along the surface of the bed or the
bed does not absorb the resulting shear force by deforming in the horizontal direction,
the effect will be a shearing of the soft tissue between the sacrum and the support
surface (Reichel, 1958). In engineering terms, the resulting shearing or deformation of
the soft tissue would be referred to as shear strain. The characteristics of the support
surface affecting this potentially harmful situation are the coefficient of friction of
the surface and its ability to deform horizontally. Certain support surface technologies
protect the skin from shear better than others do. Friction is a tangential force acting
at the interface that opposes shear force. In a static condition where the skin is not
sliding along the support surface, the friction is equivalent to the shear force. The
maximum friction is determined by the coefficient of friction of the support surface and
the pressure. This is why surfaces with high coefficients of friction have the potential
for high shear. |
