Type: Synovial PLANE joint (almost no movement).

Articulation: Auricular (ear-shaped) surface of ILIUM with the wing of SACRUM.

Function: Transmits weight from spine to pelvis to hindlimbs.

Ligaments:
- Dorsal Sacroiliac ligament (short) — between tuber sacrale and sacral spines.
- Ventral Sacroiliac ligament (capsule).
- Sacrosciatic ligament (broad sheet) — between sacrum + first coccygeals and ischiatic spine. Forms the lateral wall of the pelvic cavity.

Greater & Lesser Sciatic Foramina are openings in this ligament — sciatic nerve & gluteal vessels pass through.

SACROILIAC JOINT — link between spine & pelvis

This joint is the only bony connection between the spine and the hindlimb. It joins the ear-shaped (auricular) surface on the wing of the ilium with the matching surface on the sacrum.

TYPE: Synovial plane joint, but movement is almost zero — the joint is locked by very tight ligaments because it must rigidly transmit body weight.

LIGAMENTS:
• Dorsal sacroiliac ligament: Between tuber sacrale and the median sacral crest.
• Ventral sacroiliac ligament: Reinforces joint capsule below.
• Sacrosciatic ligament (broad pelvic ligament): A large sheet stretching from the sides of the sacrum and first 2-3 coccygeal vertebrae down to the ischiatic spine and tuber ischii. It forms the LATERAL WALL of the bony pelvic cavity (since the bones don't reach there). The ligament is pierced by:
– Greater Sciatic Foramen (cranial) — sciatic nerve & cranial gluteal vessels pass through.
– Lesser Sciatic Foramen (caudal) — internal obturator tendon & pudendal vessels pass through.

CLINICAL — Sacroiliac Subluxation: Common in horses ('hunter's bump', 'jumper's bump') after a fall or jumping injury — visible asymmetry of the rump, the tuber sacrale on one side becomes higher than the other.