The interosseous sacro-iliac ligament is a strong, thick structure; it fills the narrow irregular space between the roughened areas of bone above and behind the auricular surfaces of the sacro-iliac joint.  It is the major bond between the two bones.  The posterior sacro-iliac ligament covers the ligament superficially.  The deeper part has superior and inferior bands, which pass from depressions behind the sacral articular surface to those on the iliac tuberosity.  One or two small accessory joint cavities may sometimes be found within the ligament near the posterior inferior iliac spine and transverse tubercles of the sacrum.