Sacrospinous ligament

The sacrospinous ligament (small or anterior sacrosciatic ligament) is thin, and triangular in form; it is 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.

Relations
It is in relation, anteriorly, with the Coccygeus muscle, to which it is closely connected; posteriorly, it is covered by the sacrotuberous ligament, and crossed by the internal pudendal vessels and nerve.

Its upper border forms the lower boundary of the greater sciatic foramen; its lower border, part of the margin of the lesser sciatic foramen.

Its main function is to prevent posterior rotation of the ilia with respect to the sacrum. Laxity of this ligament alomg with the sacrotuberous ligament allows for this posterior rotation to occur. Stresses to these ligaments occur most often when leaning forward or getting out of a chair.