ABSTRACT
The pelvic girdle is formed by the hip bones (1) articulating with each other and with the sacrum (2).
The sacro-iliac joints (3) are synovial with a fibrous capsule supported by strong anterior, posterior and intra-articular ligaments. Movement is limited. The ligaments relax a little during pregnancy, allowing a wider pelvis for delivery, but possibly causing back pain. (also caused by arthritis of the joints). The body weight tends to tilt the upper sacrum down and forward, but the lower sacrum is prevented from consequently swivelling up and backward by the sacrotuberous (4) and sacrospinous (5) ligaments. The former passes to the ischial tuberosity from the posterior aspects of the ilium, sacrum and coccyx, lying external to the sacrospinous ligament that passes to the ischial spine from a smaller, sacral origin. The greater sciatic foramen (6) transmits nerves and vessels from the pelvis to the buttock. The lesser sciatic foramen (7) is inferior to the sacrospinous ligament, therefore inferior to the pelvic floor. Nerves and vessels passing through it enter the perineum.
