The sacrospinous ligament is a ligament of the human pelvis. It forms a triangle, with fibers attaching on one end to the sacrum and coccyx bones of the lower spine, then tapering to attach more narrowly at the other end to the ischium bone of the lower posterior pelvis. The sacrospinous ligament is responsible for stabilizing the ischium relative to the sacrum alongside it, so that it does not tilt too far posteriorly when leaning the torso forward upon the hips. It also pulls the sacrum back to a neutral position within the pelvis during each step of a walking gait, thus helping to prevent the pelvis from rotating too much when walking.
Consisting of three bones — the ilium, ischium, and pubis — the pelvis supports a large number of movements, allowing humans to sit upright, transferring forces between the torso and legs when walking and running, and far more. Joined to both the spine and the legs, it is linked to these bones by multiple ligaments that not only hold the bones together at their various joints but also stabilize them relative to one another. In doing so, these fibrous bands of connective tissue allow the body to withstand all kinds of forces acting on the human skeleton.
One such ligament is the sacrospinous ligament, which along with the sacrotuberous ligament joins the ischium to the sacrum. The sacrum, essentially, is five stacked vertebral bones that have become fused and that narrow in width from top to bottom, achieving a wedge shape. Beneath the sacrum is a smaller and similarly tapered fused bone, the coccyx. It is often referred to as the tailbone. While the cervical, thoracic, and lumbar sections of the spine are found above the pelvis, the sacrum and coccyx are contained by the posterior pelvis, situated between the paired ilium bones above and the paired ischium bones below. In other words, the sacrum and coccyx form the vertical midline of the rear aspect of the pelvis and are found deep to the buttocks.
Forming a nearly horizontal triangular bridge between the sides of the sacrum and coccyx and a border on the inside of the posterior ischium called the ischial spine is the sacrospinous ligament. It arises from either margin of the sacrum along most of its length as well as from the sides of the coccyx below. The fibers of this ligament then cross a space in the pelvis to create two openings: the greater sciatic notch above and the lesser sciatic notch below. Converging as they run laterally, or toward either side of the pelvis, the fibers attach as a narrow band to the ischial spine, a sharp projection angling backward and slightly inward from the posterior ischium.
The sacrospinous ligament also conducts several muscles and vessels through the sciatic notches, which further classifies these openings as the greater and lesser sciatic foramens, or spaces through which bodily structures pass. Included among these structures are the piriformis muscle, a deep muscle of the posterior hip that works in conjunction with the ligament to resist rotation of the sacrum relative to the ischium when walking and running, as well as blood vessels and nerves serving that and other muscles of the deep posterior hip.