Decubitus ulcers (pressure sores) on the buttocks from sitting in a wheelchair for extended periods is a form of the condition called "Ischial bursitis"
This form of condition arises as a result of injury or overuse of ISCHIAL TUBEROSITY a form of rounded bone that extends from the ischium.
Hence, considering the nature of the sores described above in the question, the bony protuberance responsible is ISCHIAL TUBEROSITY
