Our analysis has several limitations. All the trials evaluating pressure-redistribution overlays for operating tables included in Table I report incidence of pressure ulcers. Because of the low validity of tissue interface pressure as a surrogate outcome for pressure ulcer,60 we excluded studies using tissue interface pressure as the primary outcome measure.17, 61 and 62 Our natural history model of pressure ulcers does not take anatomic site into account. In the case of multiple ulcers, the model represents only the greatest stage pressure ulcer. Pressure ulcers were staged according to a simplified version of the National Pressure Ulcer Advisory Panel classification that did not include deep tissue injury or unstageable pressure ulcers.31