The sacrospinous ligament (small or anterior sacrosciatic ligament) is a thin, triangular ligament attached by its apex to the ischial spine, and medially, by its broad base, to the lateral margins of the sacrum and coccyx, in front of the sacrotuberous ligament with which its fibers are intermingled.
Course
Together with the sacrotuberous ligament, the sacrospinous ligament converts the greater sciatic notch into the greater sciatic foramen, and the lesser sciatic notch into the lesser sciatic foramen. The lateral part of its upper border forms the lower boundary of the greater foramen, while the lateral part of its lower border forms the upper part of the lesser foramen. Its posterior half is covered by the sacrotuberous ligament, and in its entire length it covers the equally triangular coccygeus muscle, to which its closely connected.
The pudendal vessels and nerve pass behind the sacrospinous ligament directly medially and inferiorly to the ischial spine. The inferior gluteal artery, from a branch of the internal iliac artery, passes behind the sciatic nerve and the sacrospinous ligament and is left uncovered in a small opening above the top of the sacrospinous ligament. The coccygeal branch of the inferior gluteal artery passes behind the mid-portion of the sacrospinous ligament and pierces the sacrotuberous ligament at multiple locations. The main body of the inferior gluteal artery leaves the pelvis posteriorly to the upper border of the sacrospinous ligament, to follow the inferior portion of the sciatic nerve out of the greater sciatic foramen.
Function
The main function of the sacrospinous ligament is to prevent posterior rotation of the ilium with respect to the sacrum. Laxity of this ligament along with the sacrotuberous ligament allows for posterior rotation to occur. Stresses to these ligaments occur most often when leaning forward or getting out of a chair.
When pelvic ligaments and connective tissue supports are severally attenuated or absent in women, the sacrospinous ligament provides a consistently strong site for fixation of the vaginal apex, so-called sacrospinous ligament suspension, in which the vaginal apex is suspended posteriorly and laterally to the ligament on either side or both sides.
