The sacrotuberous ligament is a ligament of the posterior pelvis that is situated alongside and that runs roughly parallel to the sacrum at the base of the spine. Connecting the sacrum to the ischium bone of the pelvis, it helps to stabilize these bones. Also known as the posterior sacrosciatic ligament, its fibers blend with those of several other tissues, most notably the tendon of the biceps femoris muscle, a major muscle of the hamstrings on the back of the thigh, as well as the fibers of the posterior sacroiliac ligament. The sacrotuberous ligament also is a ligament of the sacroiliac joint, a synovial or movable joint linking the sacrum to either iliac bone of the pelvis.
This ligament, a dense bundle of connective tissue consisting largely of collagen fibers, originates about midway down the back side of the sacrum, a stacked bone at the base of the spine consisting of five fused vertebrae. Contained entirely within the pelvis, the sacrum supports the weight of the spine above and transfers it to the pelvic bones: the ilium to either side of the sacrum, and below the ilium, the ischium, and pubis. The sacrotuberous ligament begins on the sacrum, crosses the medial or inner border of the iliac bone — thus crossing the sacroiliac joint — and attaches to the tuberosity of the ischium bone.
At the lowest point on the pelvis, the tuberosity of the ischium is a thickened portion of this boomerang-shaped bone. Found where the bone curves in the middle, the tuberosity, one on either side of the pelvis, is often referred to as the body’s “sit bones,” as this is the structure upon which one’s weight is placed when sitting. The sacrotuberous ligament connects to the tuberosity, thus helping to link the sacrum to the pelvis, even though the sacrum does not actually come into contact with any pelvic bone but the ilium.
One of five ligaments holding the sacroiliac (SI) joint together, the sacrotuberous ligament is considered one of two extrinsic ligaments of the joint. This means that it does not cross the joint directly but only indirectly maintains joint integrity. It does have a role to play, however. Along with the other extrinsic ligament of the joint, the sacrospinous ligament, it is responsible for controlling the motion of nutation, or sacral flexion. In other words, it limits how far forward the sacrum, in conjunction with a similar motion of the iliac bones, can flex relative to the ischia below.