Overview
The hip bone (os coxae) results from fusion of the ilium, ischium and pubis, which meet at the acetabulum, the socket that receives the head of the femur. The two hip bones join ventrally at the pelvic symphysis and articulate dorsally with the sacrum; with it and the first caudal vertebrae they form the pelvis. In cattle the pelvis is large and angular, and its bony prominences are visible and palpable under the skin.
Ilium and ischium
- Tuber coxae: the lateral angle of the iliac wing, large and prominent; it is the "hook bone" seen on each side of the loin.
- Tuber sacrale: the medial angle of the iliac wing, next to the midline, close to its opposite partner.
- Ischial tuber (tuber ischiadicum): the caudal end of the ischium, on either side of the tail head; in cattle it is triangular, with three prominences, and forms the "pin bone".
- Ischial arch (arcus ischiadicus): the caudal border between the two ischia.
- Obturator foramen (foramen obturatum): large, between the pubis and the ischium.
Pelvic cavity
- Pelvic inlet: oval and oblique.
- Pelvic floor: formed by the pubis and ischium; in the cow it is concave and rises caudally towards the ischial arch, so the calf must travel upwards during expulsion, unlike in the mare, whose pelvic floor is flatter.
- Lateral wall: closed by the broad sacrotuberous ligament, which relaxes in the days before calving.
Clinical relevance
- Body condition: the tuber coxae and the ischial tuber, and the line between them, are basic landmarks for body condition scoring in the cow.
- Dystocia: disproportion between the size of the calf and the pelvic canal is a common cause of difficult calving, especially in first-calf heifers. Pelvimetry, rectal measurement of the height and width of the pelvic inlet, is used as an aid to select heifers for breeding.
- Fracture of the tuber coxae: from knocks against doorways or narrow passages; it leaves the rump asymmetric, with the point on the affected side flattened.
- Calving paralysis: compression of the obturator and sciatic nerves inside the pelvis during difficult calvings can stop the cow from rising or from adducting the limbs.