Sacro-Iliac Ligaments
Stability of the sacrum in the pelvic ring is dependent on the function of the sacro-iliac ligaments.  The posterior ligaments act as a ‘Suspension Bridge’, and are some of the strongest ligaments in the body.  There are three sets of ligaments.  The most superficial are the dorsal (posterior) sacro-iliac ligaments of which there are short and long forms.  The strongest are the interosseous ligaments.  The ventral (anterior) sacro-iliac ligaments join the bones above and below the pelvic brim.  Dissociation of the sacro-iliac joint by trauma implies a large degree of violence.  Injuries such as these are often accompanied by avulsion of the transverse process of L5 (the iliolumbar ligament insertion point).