Overview
The sacrum (os sacrum) results from the fusion of five sacral vertebrae and forms the roof of the pelvic cavity. Its base articulates cranially with the last lumbar vertebra, and its wings with the ilium at the sacroiliac joints, through which the thrust of the pelvic limbs is transmitted to the spine. In the horse the wings also articulate with the transverse processes of L6 (intertransverse joint).
Key features
- Base and promontory: the cranial surface of the body of S1; the promontory projects ventrally at the pelvic inlet.
- Wings (alae ossis sacri): broad and triangular, with the auricular surface for the ilium on their dorsolateral side.
- Spinous processes: joined at their base but free at the tips, unlike the continuous sacral crest of cattle. Beside them rise the tubera sacralia of the ilium, which in the horse lie level with the tips of the spinous processes or slightly above them.
- Dorsal and ventral sacral foramina: transmit the branches of the sacral nerves.
- Ligaments: the dorsal and ventral sacroiliac ligaments join the sacrum to the ilium, and the broad sacrotuberous ligament closes the pelvic cavity laterally.
Clinical relevance
- Sacroiliac joint disease: a recognised cause of croup pain and poor performance in sport horses; diagnosis is difficult because the joint is deeply covered by muscle.
- Asymmetry of the tubera sacralia ("hunter's bump"): prominence of one or both tubera sacralia, which can point to an old injury of the sacroiliac ligaments, although it is also seen in horses without signs.
- Sacral fractures: in horses that fall over backwards or onto the croup; they can injure the sacral and caudal nerves, causing tail paralysis, urinary and faecal incontinence and loss of perineal sensation.