ANATOMY

© 2004 The Medicine Publishing Company Ltd1ANAESTHESIA AND INTENSIVE CARE MEDICINE

The sacrum is a large triangular bone formed from the five fused sacral vertebrae (Figure 1). Two rows of foramina on its dorsal and pelvic surfaces divide it into a median portion, the body and two lateral masses. The paired pelvic and dorsal sacral foramina communicate by intervertebral foramina with the central sacral canal and convey the ventral and dorsal rami of the sacral nerves. The base, directed upwards and forwards is formed from the upper surface of the first sacral vertebra. Lateral to this, the lateral masses of the sacrum are at their widest and form the prominent ala. The protruding central part of the superior surface is known as the promontory. The pelvic surface is concave and the smooth lateral masses give attachment to the piriformis muscle. The posterior surface is convex and rough, and in the midline where it roofs the sacral canal it bears a median sacral crest, which is subcutaneous and palpable, and two less prominent intermediate sacral crests more laterally. Each row of dorsal sacral foramina lies lateral to the intermediate sacral crests. The lateral surface is the roughened articular surface forming part of the sacroiliac joint. Strong interosseous ligaments connect the sacrum and ilium.

Sacrum and sacral hiatusJohn Craven

John Craven was formerly Consultant Surgeon at York District Hospital,

York, UK. He trained in Manchester, Uganda and Cardiff. He is past

chairman of the primary examiners of the Royal College of Surgeons of

England.

The sacral canal is triangular in transverse section and contains the cauda equina and CSF with the meninges. It runs the length of the sacrum, following its curve, from the lumbar canal to the sacral hiatus. The sacral hiatus is closed by the posterior sacrococcygeal membrane. Fibrous strands often divide the extradural space of the canal into compartments (which accounts for some failures to produce uniform analgesia after caudal block). The anterior wall of the canal is formed of sacral vertebrae, the posterior wall by the sacral laminae. Laterally, four pairs of foraminae are present. The contents of the sacral canal are: • the dural sac, which ends by the 2nd sacral vertebra (marked

by a line joining the posterior superior iliac spines); the pia mater continues as the filum terminale

• the sacral nerves, the coccygeal nerve and their dorsal root ganglia

• the internal vertebral venous plexus, which is more marked anteriorly than posteriorly; the epidural needle should therefore be kept as posterior as possible to avoid inadvertent puncture

• areolar tissue. The sacral hiatus is triangular, the result of a failed fusion of the fifth, and occasionally the fourth, laminar arch to fuse. The fourth sacral spine forms its rounded apex and, on each side is a sacral cornu (Figure 1). It is covered by the sacrococcygeal membrane, which is pierced by the fifth sacral and coccygeal nerves. It lies superior to the sacrococcygeal junction, about 4 cm proximal to the tip of the coccyx, beneath the upper limit of the intergluteal cleft. The technique of caudal anaesthesia is described in Anaesthesia and Intensive Care Medicine 4:12: 413. It is so commonly used that the possible anatomical abnormalities should be known:• the sacral hiatus may be displaced upwards or downwards • the sacral canal may be narrowed or obliterated• the fibrous sheet covering the hiatus may be ossified• the dural sac extends distally as far as S3 or, occasionally, S4• the hiatus may be long and narrow or broad and deep and the

epidural space beneath it may be very shallow or very deep.The average capacity of the canal is about 30 ml and its length ranges from 10 to 15 cm.

Posterior and anterior views of sacrumPosterior Anterior

Median crest

Posterior sacral foramen

Superior articular processes

Tubercles

Intermediate crest

Cornu

Sacral hiatus

Lateral massPromontory

Ala

Anterior sacral foramen

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