The sacrum is an irregular, triangular shaped piece of bone made up of the five fused sacral vertebrae. It is concave anteriorly, convex posteriorly, and is wedged and stabilized between the iliac bones by the sacroiliac joints.
The superior part of the sacrum articulates with the fifth lumbar vertebra via an intervertebral disc and articular (facet) joints, just as the other vertebrae do.
Some of the component parts of the original vertebrae can still be recognized within the sacrum.
The sacrum has five surfaces, anterior, posterior, two lateral surfaces and a base.
The pedicles, laminae, transverse and spinous processes fuse to form an irregular surfaced plate. From medial to lateral you will find:
- Median sacral crest
- This is formed by the fused spinous processes
- Intermediate or articular crests
- Each crest is a series of tubercles formed by the fused articular processes . The superior articular processes of S1 articulate with the inferior articular processes of L5. The inferior articular processes of the fifth sacral vertebra appear as elongated, rounded tubercles, the sacral cornua
- Dorsal sacral foramina
The dorsal and ventral rami of the sacral nerves emerge from the sacral canal (the sacral portion of the vertebral canal) through four pairs of foramina, the dorsal sacral foramina on the posterior surface of the sacrum and the ventral sacral foramina on the anterior surface. The dorsal and ventral foramina are also known as the posterior and anterior sacral foramina respectively.
- Lateral sacral crests
The lateral sacral crests lie lateral to the dorsal sacral foramina and are formed by the fused transverse processes. The tips form a crest of transverse tubercles
- Lateral mass
The mass of bone lateral to the sacral foramina forms a lateral mass, one either side of the midline
Anterior and Lateral Surface
From medial to lateral the anterior (pelvic/ventral) surface of the sacrum has the following features:
- Transverse ridges
Four transverse ridges demarcate the points at which the bodies of the five sacral vertebrae are fused
- Ventral sacral foramina
Four pairs of ventral (or anterior) sacral foramina lie at the ends of the transverse ridges and immediately anterior to their corresponding posterior pairs They open into the pelvis and transmit the anterior rami of the sacral spinal nerves from the sacral canal.
- Lateral mass
The mass of bone lateral to the sacral foramina forms a lateral mass, one either side of the midline. The lateral surface of each lateral mass forms the lateral surface of the sacrum. The upper part of the lateral surface is covered with cartilage forming the auricular surface, which articulates with the ilium
The base of the sacrum is actually its upper surface and comprises:
- The upper surface of the body of the first sacral vertebra
This articulates with the lower surface of the body of the fifth lumbar vertebra by an intervertebral disc. The anterior edge projects forwards to form the sacral promontory
- The triangular upper opening of the sacral canal
The sacral canal is the sacral portion of the vertebral canal and runs the length of the sacrum. The upper opening lies behind the body of S1.
- The ala
The ala forms a large triangular surface on either side of the body of S1. It is grooved by the passage of the lumbosacral trunk of the sacral plexus.
The sacral canal is the sacral portion of the vertebral canal and runs the length of the sacrum
It is bounded anteriorly by the fused S1-S4 vertebral bodies, and posteriorly by the fused laminae and spinous processes.
The upper opening forms part of the base of the sacrum and it ends inferiorly at the sacral hiatus.
It opens anteriorly into the pelvis via the four pairs of ventral sacral foramina and posteriorly via the four pairs of dorsal sacral foramina.
The laminae of the fifth sacral vertebra fail to fuse in the midline in over 90% of people leaving a triangular gap at the lower end of the median crest. This triangular gap is called the sacral hiatus
Borders of the sacral hiatus:
In most people it is the laminae of the fifth sacral vertebra which fail to fuse so the superior border of the sacral hiatus is formed by the lower borders of the fused laminae of S4.More rarely, the apex of the sacral hiatus lies more superiorly, in which case the superior border is formed by the laminae of either S3, S2 or S1.
The medial edges of the laminae of the fifth sacral vertebra, with the sacral cornua. The sacral cornua are rudimentary articular processes of the S5 vertebra.
The hiatus is covered superficially by skin, subcutaneous fat and the fibrous posterior sacrococcygeal ligament, which extends from the sacral cornua to the coccyx
The coccyx and the posterior surface of the body of S5
Caudal epidural injection
When performing a caudal epidural injection, a needle is passed through the sacral hiatus. The dural sac usually terminates at the level of S2 although this can vary. The epidural space extends down to the sacral hiatus, so a caudal epidural injection can be performed by passing a needle through the sacrococcygeal ligament and sacral hiatus. The distance from the tip of the dura to the sacral hiatus ranges from 1.5-7.5 cm.
Locating the sacral hiatus
The position of the sacral hiatus can be identified from surface markings. A horizontal line drawn between the posterior superior iliac spines forms the base of an equilateral triangle, with the hiatus lying at the point of the triangle inferiorly. The posterior superior iliac spines can be identified by two skin dimples which overlie them. The sacral hiatus can be palpated as a depression. To identify the position of the hiatus more accurately, the inferior end of the median sacral crest, the two cornua of the sacrum and the tip of the coccyx can also be palpated through the skin at the top of the natal cleft. In an adult the distance from the sacral hiatus to the tip of the coccyx is approximately the same as the distance from the tip of their index finger to their proximal interphalangeal joint.
Once the sacral hiatus is identified the overlying skin is carefully cleaned with antiseptic solution. A 22 gauge short bevelled cannula or needle is directed at about 45o to the skin and inserted till a ‘click’ is felt as the sacrococcygeal ligament is pierced. The needle is then carefully directed in a cephalad direction at an angle approaching the long axis of the spinal canal.