It is a strong, weight-bearing joint, which acts to transfer forces from the lower limb to the vertebral column.
In this article, we shall look at the anatomy of the sacroiliac joint – its structure, neurovascular supply, and clinical correlations.
The sacroiliac joint is an articulation between the ilium of the pelvis and the sacrum of the vertebral column. The articular surfaces are irregular in shape and interlock to produce a stable joint. They are lined with hyaline cartilage.
In later life, the sacroiliac joints may become completely fused, resulting in a loss of the joint cavity.
The sacroiliac joint is a synovial joint. It is encompassed by a fibrous joint capsule, which is lined by a synovial membrane.
The ligaments of the sacroiliac joint reinforce the synovial capsule. There are three main ligaments:
- Interosseous sacroiliac ligament – located posteriorly and superiorly to the joint, spanning between the ilium and sacrum. It is the strongest of the ligaments.
- Posterior sacroiliac ligament – also located posteriorly to the joint, covering the interosseous ligament.
- Anterior sacroiliac ligament – thickening of the anterior component of the joint capsule. It is relatively thin and weak.
The primary function of the sacroiliac joint is to transmit forces from the lower limb to the vertebral column. Therefore, the joint is extremely strong with limited movement possible.
There is a small degree of gliding and rotational movement that can occur between the interlocking articular surfaces.
During pregnancy, the ligaments of the sacroiliac joint become slightly lax – this encourages movement within the joint in childbirth.
Arterial supply to the sacroiliac joint is via the iliolumbar artery and the medial and lateral sacral arteries. Venous drainage is by accompanying veins into the internal iliac vein.
The sacroiliac joint is innervated by branches of the sacral spinal nerves.
Clinical Relevance – Sacroiliitis
Sacroiliitis refers to inflammation of the sacroiliac joint. Ankylosing spondylitis (AS) is the most common type of inflammatory arthropathy which affects the joint.
It causes both spondylitis (inflammation of the vertebral joint) and sacroiliitis. Classically, patients complain of early morning lower back pain and stiffness which improves with exercise and movement.
A plain X-ray typically demonstrates sacroiliitis (blurring of the joint margins, subchondral erosions, sclerosis, and changes in joint space). Furthermore, the vertebrae can demonstrate squaring of lumbar vertebrae, bamboo spine (late and uncommon) and syndesmophytes.