Pressure ulcers are soft-tissue damage associated with tissue exposure to sustained deformations and stress concentrations. In patients who are proned for ventilation or surgery, such damage may occur in the superficial chest tissues that are compressed between the rib cage and the support surface. Prophylactic dressings have been previously proven as generally effective for pressure ulcer prevention. In this study, our goal was to develop a novel computational modelling framework to investigate the biomechanical efficacy of a dressing with a soft cellulose fluff core in protecting proned surgical patients from chest pressure ulcers occurring on the operating table, due to body fixation by the Relton-Hall frame. We compared the levels of mechanical compressive stresses developing in the soft chest tissues, above the sternum and ribs, due to the trunk weight, whilst the body is supported by the Relton-Hall frame pads, with versus without the prophylactically applied bilateral dressings. The protective efficacy index for the extremely high stresses, above the 95th-percentile, were 40.5%, 25.6% and 24.2% for skin, adipose and muscle, respectively, indicating that the dressings dispersed elevated soft-tissue stresses. The current results provide additional support for using soft cellulose fluff core dressings for pressure ulcer prophylaxis, including during surgery.