Overview
The sacrum (os sacrum) results from the fusion of the five sacral vertebrae of cattle. It is triangular: the cranial base articulates with the last lumbar vertebra, the caudal apex with the first caudal vertebra, and the wings articulate with the ilia at the sacroiliac joint. It forms the roof of the pelvic cavity.
Key features
- Promontory (promontorium): cranioventral edge of the body of the first sacral vertebra; it marks the dorsal limit of the pelvic inlet.
- Wings of the sacrum (alae sacri): their auricular surface articulates with the ilium. The sacroiliac joint is almost immobile and is reinforced by strong ligaments.
- Median sacral crest (crista sacralis mediana): in cattle the spinous processes fuse into a continuous crest, unlike the horse, in which they remain separate.
- Lateral sacral crests: formed by the fused transverse processes.
- Dorsal and ventral sacral foramina: four pairs of each, between the fused vertebrae; they transmit the branches of the sacral nerves.
- Pelvic surface: concave; the rectum lies on it.
- Broad sacrotuberous ligament (ligamentum sacrotuberale latum): wide sheet running from the lateral border of the sacrum and the first caudal vertebrae to the ischial spine and the ischial tuber, forming the lateral wall of the pelvis.
Clinical relevance
- Signs of impending calving: in the days before parturition the pelvic ligaments relax and the area on each side of the tail head sinks.
- Epidural anaesthesia: caudal epidural is given at the sacrococcygeal space or, more often, at the first intercoccygeal space, and anaesthetises the tail, perineum, vagina and rectum without affecting gait; lumbosacral epidural, between L6 and the sacrum, reaches more cranial regions.
- Sacrocaudal injuries: fractures or luxations of the sacrum and the sacrocaudal junction, typical of cows mounted by others in heat; they cause tail paralysis and sometimes disturbances of urination and defecation.