Overview
The cat sacrum (os sacrum) is formed by the fusion of three sacral vertebrae. It lies between the wings of the ilia, with which it forms the sacroiliac joints, and transmits the thrust of the pelvic limbs to the spine. Cranially it articulates with L7 and caudally with the first caudal vertebra.
Key features
- Base and promontory: the cranial end of the body of S1; its ventral edge, the promontory, marks the dorsal boundary of the pelvic inlet.
- Wings of the sacrum: lateral expansions of S1 carrying the auricular surface, which articulates with the ilium.
- Median sacral crest: formed by the fused spinous processes.
- Dorsal and ventral sacral foramina: two pairs of each, through which the dorsal and ventral branches of the sacral nerves leave.
- Sacral canal: continuation of the vertebral canal; it houses the nerves of the cauda equina and, in the cat, may contain the end of the dural sac.
- Apex: the caudal end, which articulates with the first caudal vertebra.
Clinical relevance
- Sacroiliac luxation: very common in feline pelvic trauma (road accidents, falls), often together with fractures of the hip bone. It is stabilised with screws through the ilium into the body of the sacrum.
- Sacrocaudal luxation ("tail pull" injury): stretching of the sacral and caudal nerves causes a flaccid tail and, in severe cases, urinary retention and incontinence. The prognosis depends on perineal sensation and anal tone.
- Megacolon: malunion of sacral and pelvic fractures can narrow the pelvic canal and lead to chronic constipation.
- Manx cats: the mutation that causes taillessness can come with malformations of the sacrum and sacral spinal cord, leading to incontinence.