shear stress on the skin is relieved at cavity edges in a resilient foam pad by cavity walls shaped to define a region of diminished support under the cavity edge . the edge material yields more easily under loading and may roll into the cavity to avoid pressing into body tissues . the interior wall or walls of the cavity are slanted outwardly to undercut all or part of the cavity edge to reduce the thickness of material under the edge . the shape and curvature of the cavity and cavity walls can be adapted and tailored to achieve particular pressure gradients and patterns .