Posterior Sacro-Ilac Ligament

The sacro-iliac joints are located just medial (and anterior) to the osseous landmarks of the posterior superior spines of the ilia.  The ligament bridges the short distance between the posterior superior spines and the lateral aspect of the posterior surface of the sacrum, immediately posterior to the joint.  It is this portion of the ligament that is palapable and can be used for analysis.  Roll the thumbs over both posterior superior spines, moving medial toward the sacrum.  As the thumbs (kept in contact with the posterior superior pines) approach the sacrum, run the thumbs up and then down this small region, this is the location of the sacro-iliac ligaments.

As the base of the sacrum wedges posterior on either side (or both), the sacro-iliac ligament is stretched over the sacro-iliac joint on the posterior side (or sides).

When stretched over the joint due to a misalignment of the sacrum, the sacro-iliac ligament is tight, and flattened. 

Spano 2015