Overview
The sacrum is the segment of the vertebral column that articulates with the hip bone. In the dog it is formed by three sacral vertebrae that fuse during the first year and a half of life into a single short, wedge-shaped bone, wider cranially than caudally. It forms the roof of the pelvic cavity and transmits the propulsive force of the pelvic limb to the trunk.
Base and wings
- Base (basis ossis sacri): cranial surface that articulates with the body of the seventh lumbar vertebra. Its slightly prominent ventral edge is the promontory (promontorium), a landmark for measuring the diameter of the pelvic canal.
- Wings (alae sacrales): lateral expansions of the first sacral vertebra. Each wing carries the auricular surface (facies auricularis), a rough face that articulates with the matching surface of the ilium to form the sacroiliac joint, which allows very little movement and is reinforced by strong ligaments.
- Apex (apex ossis sacri): caudal end, which articulates with the first caudal vertebra.
Dorsal surface
The fused spinous processes form the median sacral crest (crista sacralis mediana); in the dog the processes usually remain partly separate. On each side lie the intermediate sacral crest (articular processes) and the lateral sacral crest (transverse processes). Two pairs of dorsal sacral foramina (foramina sacralia dorsalia) transmit the dorsal branches of the sacral nerves.
Pelvic surface
Smooth and concave. It shows the transverse lines, remnants of the fusion of the vertebral bodies, and the pelvic sacral foramina (foramina sacralia pelvina), through which the ventral branches of the sacral nerves leave to join the lumbosacral plexus (sciatic, pudendal and pelvic nerves).
Sacral canal
The sacral canal (canalis sacralis) continues the vertebral canal. In the dog the spinal cord ends at the level of the last lumbar vertebrae, so the sacral canal contains only the nerve roots of the cauda equina.
Clinical relevance
- Lumbosacral space: epidural anaesthesia is performed between L7 and the sacrum; the promontory and the wings help locate it by palpation.
- Sacroiliac luxation: common in pelvic trauma (road traffic accidents); it usually accompanies fractures of the hip bone and is fixed with a screw driven through the wing of the ilium into the body of the first sacral vertebra.
- Degenerative lumbosacral stenosis (cauda equina syndrome): compression of the nerve roots at the L7–S1 junction causes lumbar pain, lameness and deficits of the tail, bladder and anus, especially in the German shepherd.
- Sacral fractures: may injure the sacral nerves and cause urinary or faecal incontinence.