Overview
Each hip bone (os coxae) results from the fusion of three bones, the ilium, the ischium and the pubis, which meet at the acetabulum. The two hip bones join ventrally at the pelvic symphysis and dorsally with the sacrum at the sacroiliac joints, forming the pelvis. In the horse the ilium is broad and slopes craniolaterally, and its landmarks shape the outline of the croup.
Ilium
- Wing of the ilium (ala ossis ilii): broad and laterally concave; the gluteal muscles, very large in the horse, arise from its gluteal surface.
- Tuber coxae (tuber coxae): the lateral angle of the wing; it forms the visible, palpable point of the hip.
- Tuber sacrale (tuber sacrale): the medial angle of the wing; it rises beside the sacral spinous processes and forms the highest point of the croup.
- Body: narrow, joining the wing to the acetabulum.
Ischium and pubis
- Ischial tuberosity (tuber ischiadicum): forms the point of the buttock; origin of the hamstring muscles (biceps femoris, semitendinosus and semimembranosus).
- Ischial arch: the caudal border of the two ischia, between the ischial tuberosities.
- Pubis: forms the cranial part of the pelvic floor; the prepubic tendon attaches to its cranial border.
- Obturator foramen (foramen obturatum): large, between pubis and ischium, closed by muscles and membrane.
Acetabulum
A deep cavity that holds the head of the femur. Through its acetabular notch pass the ligament of the head of the femur and the accessory ligament of the femur, unique to the horse, which arises from the prepubic tendon and limits abduction of the limb.
Clinical relevance
- Fracture of the tuber coxae ("knocked-down hip"): from hitting door frames or falling sideways. The point of the hip is flattened and asymmetric; most heal with rest, although the asymmetry remains.
- Stress fractures of the ilium: in racehorses, in the wing or body of the ilium; they cause pelvic limb lameness and can become complete and injure the iliac artery.
- Pelvic fractures: in falls; they are examined by rectal palpation and ultrasound, looking for crepitus, asymmetry or haematomas.