SN7 SHORT NOTE 5M

Describe the sacrum with its features. Add a note on caudal anaesthesia and the sacro-iliac joint.

Anatomy · 1st Professional — Pre-Clinical

The sacrum is a large triangular bone formed by the fusion of five sacral vertebrae, wedged between the two hip bones.

Feature	Detail
Base	Above; its anterior projecting margin is the sacral promontory, an important obstetric landmark
Apex	Below, articulating with the coccyx
Pelvic (anterior) surface	Concave; four pairs of anterior sacral foramina transmitting the ventral rami
Dorsal surface	Convex; median, intermediate and lateral sacral crests; four pairs of posterior sacral foramina
Sacral hiatus	The lower opening of the sacral canal, from failure of fusion of the laminae of S5 (and often S4), bounded by the sacral cornua
Auricular surface	Ear-shaped, for the sacro-iliac joint

The sacral canal contains the cauda equina, the filum terminale and the meninges
The dural sac ends at S2, so the canal below that level contains only the sacral and coccygeal nerve roots and epidural fat

The needle must not be advanced above S2, or the dural sac is punctured and a total spinal block may result
The hiatus is absent or abnormal in about 5% of people

A synovial (plane) joint between the auricular surfaces of the sacrum and ilium, becoming largely fibrous and often ankylosed with age
Very strong ligaments — anterior, posterior and interosseous sacro-iliac, with the sacrotuberous and sacrospinous ligaments preventing rotation of the sacrum
Weight transmission tends to push the upper sacrum downward and forward; this is resisted by the interosseous ligaments and the accessory ligaments
Movements are minimal — slight nodding (nutation), increased in pregnancy under relaxin

Formed by four fused rudimentary vertebrae
Gives attachment to the anococcygeal ligament, coccygeus, levator ani, gluteus maximus and the sacrotuberous ligament
It moves backward during defaecation and childbirth, increasing the anteroposterior diameter of the outlet

Sacro-iliac joint disease is an early feature of ankylosing spondylitis, and sacroiliitis on radiography is a diagnostic criterion
Coccydynia — pain from injury to the coccyx, often after a fall onto the buttocks or after childbirth; notoriously persistent
Spina bifida occulta is commonest in the lumbosacral region, from failure of fusion of the laminae; often marked by a tuft of hair or a dimple
Sacral fractures may injure the sacral nerve roots and cause bladder, bowel and sexual dysfunction; the sacral foramina must be examined on imaging
Bone marrow aspiration from the posterior iliac crest uses the landmarks of this region, and the posterior superior iliac spine is easily palpable at the dimple of Venus

All questions in Pelvis & Perineum