This capsule is strengthened by the ventral, interosseous, and dorsal sacroiliac ligaments.

http://en.wikipedia.org/wiki/Pelvis

This capsule is strengthened by the ventral, interosseous, and dorsal sacroiliac ligaments. The most important accessory ligaments of the sacroiliac joint are the sacrospinous and sacrotuberous ligaments which stabilize the hip bone on the sacrum and prevent the promonotory from tilting forward.