A major difficulty in developing pressure relief surfaces is that the pressure at which injury occurs is poorly understood. Despite attention to reduction, interface pressure cannot be accurately measured. There are three key questions for investigation: what fraction of the capillaries must remain occluded to induce irreversible tissue injury; what duration of occlusion results in tissue injury; and, what frequency of capillary occlusion for a given duration and fraction of capillaries can induce irreversible tissue injury?