Upper sacrum from an old man who had been bed–ridden for several years after a stroke (note the advanced hypotrophy of the musculature). The sacrum is cut parallel to the L5–S1 joint space, exposing the dorsal St sacral foramina. Much of the bone marrow in this specimen has been replaced by fat (fatty degeneration), and while the SI "body" and lamina still are composed of sparse cancellous trabeculae, the alar bone is replaced by a body of fat and the cortical bone is severely attenuated. The posterior wing of the ilium swings in more medial than usual, this obviously would make it impossible to insert sacrum screws converging toward the promontorium. Penetration of the anterior cortex of the sacrum would in this case carry considerable risk of violating nerves and blood vessels.