The sacroiliac joint is a synovial joint that forms between the lateral articulating surfaces of the sacrum, and the articulating surfaces of the ilium. The joint is supported by a number of strong ligaments. The joint allows for some mobility during walking and other movements, and gives the pelvis some flexibility, but maintains its strength.
In this article we will discuss the gross and functional anatomy of the sacroiliac joint. We will also discuss the clinical relevance of this structure at the end.
This is a synovial joint formed between the lateral articulating surfaces of the sacrum and the articulating surfaces of the ilium. The bone is supported by a number of strong ligaments.
The sacrum forms the posterior portion of the bony pelvis, as well as the supporting structure of the vertebral column. The articulating surfaces of the sacrum are L-shaped while those of the ilium are C-shaped. They allow for a small amount of movement.
The sacral surface of the joint is covered by hyaline cartilage, and the ilium is covered by fibrocartilage.
As the person ages, the sacroiliac ligaments and sacroiliac bony structure degrades. The sacroiliac joint structure changes shape over time. When we are born the joint is relatively planar. As the walking process begins, the joint develops angular orientations and a more congruent joint structure.
The human body develops an elevated ridge over the posterior surface of the ilium. A depression also develops along the sacral surface of the joint. The ridge and corresponding depression allow for the attachment and insertion of the strong sacroiliac ligaments, which increase joint stability and reduce the incidence of dislocations.
- Anterior sacroiliac ligament - This ligament is better considered as a thickening of the anterior aspect of the sacroiliac joint capsule. It connects the anterior surface of the lateral sacrum to the auricular margin of the ilium. It is also considerably thinner than the posterior sacroiliac ligament.
- Posterior sacroiliac ligament - This can be further divided into the short (deep ) and long (superficial) components. The deep/short fibres run almost horizontally. They connect the first and second transverse tubercles, that arise from the posterior surface of the sacrum, to the iliac tubercle. The superficial/long fibres of the posterior sacroiliac ligament run in an oblique vertical direction. They attach the third and fourth transverse tubercles, that arise from the posterior surface of the sacrum, to the posterior superior iliac spine.
- Interosseus sacroiliac ligament - The dorsal interosseus ligaments are a series of short and strong ligaments that run in a nearly horizontal direction to prevent subluxation or dislocation of the sacroiliac joint.
Other ligaments of the sacroiliac joint
- Iliolumbar ligament- This ligament extends from the lateral surface of the transverse process of the fifth lumbar vertebrae, and inserts onto the inner lip of the iliac crest. It stabilizes the joint superiorly and strengthens the joint overall.
- Sacrotuberous ligament- This is a triangular-shaped flat ligament that runs from the lower transverse tubercles of the sacrum and the superior part of the coccyx, to the ischial tuberosity. The broad base of the ligament arises from the posterior superior iliac spine, as well as the posterior sacroiliac ligaments. The fibres run in an oblique direction, and converge to a narrow point of fibres that insert onto the medial surface of the ischial tuberosity. The ligament is pierced by the coccygeal branches of the inferior gluteal nerve, as well as cutaneous branches of the coccygeal plexus. The most inferior part of the gluteus maximus muscle also attaches to the sacrotuberous ligament, as do some of the fibres of biceps femoris. The sacrotuberous ligament runs behind the sacrospinous ligament along its entire course.
- Sacrospinous ligament- This is another thin triangular-shaped ligament of the pelvis. It arises from the lateral margin of the sacrum and the coccyx, and inserts onto the ischial spine. The fibres do mingle and merge with the sacrotuberous ligament. The sacrospinous ligament covers the coccygeus muscle along its entire length. The greater sciatic notch forms above the ligament, and the lesser sciatic notch forms below the ligament. The presence of the ligament divides the notches into greater and lesser sciatic foramina.
Greater sciatic foramen and blood supply
The greater sciatic foramen is how the majority of structures leave the pelvis (sciatic nerve, superior and inferior gluteal arteries, veins and nerves). The greater sciatic foramen also allows the pudendal vessels and nerve to leave the pelvis, and they wrap around the sacrospinous ligament, and then re-enter the pelvis via the lesser sciatic foramen. The primary purpose of the sacrospinous ligament is to limit the extent of rotation of the ilium around the sacrum.
The blood supply to the sacroiliac joint comes from the superior gluteal arteries, lateral sacral arteries, and the iliolumbar arteries (all three are branches of the internal iliac artery). The nerve supply likely comes from the superior gluteal nerve and the ventral and dorsal rami of the first two sacral nerves. The lymphatic drainage of the joint follows the course of the arteries to the lumbar and iliac lymph nodes.
Ankylosing spondylitis - This is a chronic inflammatory autoimmune condition that affects the axial skeleton and can affect some joints, with the sacroiliac joint being primarily affected. The patient will present with chronic dull backache in the lumbar or gluteal regions. Stiffness in the morning is also commonly found. Gradually, the patient will lose mobility, as their joints stiffen. It is classified as one of the seronegative spondyloarthropathies, as it does not raise levels of autoantibodies as can be found in some inflammatory rheumatological conditions, e.g. rheumatoid arthritis. Bamboo spine is a condition that can develop as the intervertebral joints fuse, resulting in spinal rigidity. Treatment includes non-steroidal anti-inflammatory drugs (e.g. ibuprofen) and disease modifying anti-rheumatic drugs (e.g. sulfasalazine). Physiotherapy can help, and surgery can be used to replace affected joints. The condition is associated with inflammation of the anterior chamber of the eye (anterior uveitis).
Sacroiliitis - The inflammation of the sacroiliac joint is referred to as sacroiliitis. It occurs in inflammatory conditions (bowel diseases i.e. crohn’s and Ulcerative colitis, ankylosing spondylitis, psoriatic arthritis etc).
Role of relaxin in the loosening of the joint - The relaxin hormone is released in pregnancy, and relaxes the ligaments of the body, and most importantly the pelvis. This means that during the second stage of labour (when the fetus is being passed through the pelvis), the pelvis is able to mould (the baby’s head also moulds with the skull bones passing over each other, as they are not yet fused).
Fusion of the SI joint in the management of low back pain - Ankylosing spondylitis causes bamboo spine, a condition where the vertebrae and discs fuse into one continues not flexible structure. The spine also fuses with the sacrum, and severely limits spinal mobility.