The sacrum is a large, triangular bone located between the lumbar and coccygeal portions of the caudal spinal axis.
A healthy sacral region is rarely fractured except in instances of serious injury, such as a fall or trauma to the area. However, patients with osteoporosis are inclined to develop stress fractures and fatigue fractures in the sacrum.
Sacroiliac Joint Pain
Inflammation of the SI joint (sacroiliitis) can be the source of buttock and upper thigh pain. This is seen classically as ankylosing spondylitis and the other seronegative spondyloarthropathies. The combination of maximal pain below L5 plus pain in the region of the posterior superior iliac spine (PSIS) and tenderness in the sacral sulcus region has a positive predictive value of 60%. The classic feature of sacroiliitis on history is that the back pain is improved with activity and exacerbated by rest. This pain will usually waken a patient from sleep at night. SI imaging, plain films, or MRI can be used for definitive evaluation of the SI joint. On physical examination it will be important to stress the SI joint with a flexion abduction external rotation (FABER) test.