The sacrum is an irregularly shaped bone, made up of a group of five fused vertebrae in the area of what is commonly known as the base of the spine. Regarded as the keystone of the human body, the sacrum is important because it forms a link between the spine and the iliac bones, and also has an important part to play in hip stability.
The lumbosacral (L5/S1) joint comprises the superior articular process of the sacrum which articulates with the inferior articular facets of L5. There is a great deal of pressure occurring at this joint, as the weight of all of the vertebrae above bears down on it, relying on it for their stability.
To bear this weight and to achieve this stability, the orientation of the L5/S1 facet joint is mostly in the coronal plane, providing more support to the joint. This articulation is united by an intervertebral disc, which is deeper anteriorly along with the vertebral body of L5. This contributes to the lumbosacral angle.
The sacrococcygeal joint (S5/C1) is an amphiarthrodial joint that lies between the sacral apex and the base of the coccyx. The disc in this joint is much thinner owing to the reduction in pressure as the majority of the pressure at this point has been distributed superior to the S5/C1 joint, across the appendages.
The sacroiliac joint (SIJ) is a synovial joint occurring between the sacral and iliac articular surfaces. There is a lot of debate about the range of movement that occurs at this joint, with research suggesting it could be between 2 and 18 degrees. One firm belief is that the movement that does occur is a secondary movement, important for the mechanical movements of the hip.
The SIJ allows for simultaneous rotational and translational movement associated with walking gait. The slight movements of the SIJ relieve pressure on the hips, as the natural swinging of the hips that occurs during walking would increase the risk of pelvic fracture.
There is a delicate balance between the right and left SIJs, because the origin point of the axis of motion is precisely in the middle of the posterior iliac spines. A dysfunctional posture can cause the axis of motion to become displaced, which could compromise the soft tissue holding the two articular surfaces in place. As a result, SIJ pain is relatively common.
At the base of the sacrum is located the first of the five fused sacral vertebrae. The S1 vertebra is the biggest in the sacrum, whose concave superior articular facets project posteromedially to communicate with L5. Although it is fused it still possesses transverse processes and pedicles.
The pelvic surface, located inside the curve of the sacrum contains four pairs of foramina. These foramina allow the ventral rami of the first four sacral spinal nerves to pass through.
The midline of the dorsal surface of the sacrum bears a projection of bone known as the median sacral crest. This crest is palpable and is formed by the fusion of sacral spines of S1 to S4. The landmark of the sacral hiatus, which is caused by the lamina of S5 failing to meet in the median plane resulting in an exposed dorsal surface, is located just below the S4 tubercle. Just lateral to this are the four pairs of sacral foramina.
The sacral lateral surface consists of the fusion of the transverse processes which narrows as it travels inferiorly. There is an auricular surface on the lateral part of the sacrum that articulates with the ilium. Just posterior to the auricular surface is a rough prominence that contains three depressions intended for the attachment of strong ligaments.
The triangular sacral canal is formed by the sacral vertebral foramina, and descends from the opening on the basal surface to the sacral hiatus. Since the spinal cord finishes at around L2 the canal does not carry the spinal cord. It instead carries the filum terminale, long threadlike bands of connective tissue and the cauda equina, long sacral roots of spinal nerves.
Finally, the apex of the sacrum is the inferior segment. It is the fifth sacral vertebral body and has an oval shaped facet for articulation with the coccyx. The apex projects posteriorly to increase the size of the pelvic cavity.
The piriformis muscle, attaching onto the pelvic surface (S2-S4), is one of the major muscles on the sacrum. The iliacus and coccygeus also attach to the pelvic surface superolaterally and inferolaterally respectively. The erector spinae aponeurosis and multifidus cover the dorsal surface while the gluteus maximus attaches to the lateral borders of the sacrum.
Spondylolisthesis is a slippage of one vertebra on another, usually L5 on S1 as a result of the extreme pressure upon that joint. There are five different causes of the slippage, these being:
- Dysplastic spondylolisthesis: congenital defect.
- Isthmic spondylolisthesis: a fracture of the isthmus (or pars interarticularis), reducing the strength of the facet joint in the sagittal plane causing the anteriorization of the vertebrae.
- Degenerative spondylolisthesis: due to degeneration of the discs and the facet joints.
- Traumatic spondylolisthesis: caused by acute fractures in the neural arch. Separate to fractures in the isthmus.
- Pathologic spondylolisthesis: bone disease weakening the bone. Possibly due to metastatic disease, osteoporosis or tumours.
Along with slippage, there are other more typical dysfunctions that occur around the sacrum due to the load placed on it, such as disc herniation and degeneration, facet joint pain, and sciatica due to L5 nerve root (part of long sciatic nerve) impingement.