Located between the pelvis and base of the spine, the sacroiliac joints, or SI joints, are strong, stable joints that allow for little movement. While SI joints do not bend like a knuckle or knee, they are susceptible to degenerative arthritis.
Sacroiliac Point Function and Dysfunction
The sacroiliac joints are C- or L-shaped joints that connect the sacrum at the base of the spine to the iliac bones in the pelvis. The sacrum is shaped like an upside down triangle. It is a strong bone that supports the spine.
See Hip Anatomy
SI joints act as shock absorbers, transmitting and absorbing the forces of the upper body to the pelvis and legs. A dense network of muscles and ligaments support these joints and allow very little movement. These SI joint ligaments are the strongest in the body:1
- Anterior sacroiliac ligament
- Interosseous sacroiliac ligament
- Posterior (or dorsal) sacroiliac ligament
- Sacrotuberous ligament
- Sacrospinous ligament
Injury to these ligaments can cause SI joint pain. For example, small tears through years of overuse or strain can eventually allow for too much movement in the joint, resulting in pain.
The SI joints are synovial joints, meaning each joint is lubricated with a viscous fluid, called synovial fluid, and encapsulated by a thin membrane, called a synovium. In addition, the back and lower portions of the SI joints’ surfaces are covered in articular cartilage. Degeneration of this cartilage is called osteoarthritis and can also cause SI joint pain.
Sacroiliac Joint Dysfunction and Sacroiliitis
Pain in one or both of the SI joints is often called sacroiliac joint dysfunction, but is also known as sacroiliac joint syndrome, disease, or disorder. This pain is thought to be caused by abnormal motion of the SI joints:
- Too much movement in the joint. If the SI joint has too much flexibility, the resulting instability can be a lower back pain and/or hip or groin pain.
- Too little movement in the joint. Pain can also be caused by inflexibility; an overly stiff joint can lead can lead to lower back pain and/or pain that radiates down the leg and mimics sciatica pain.
Sacroiliac joint dysfunction often leads to inflammation of one or both of the SI joints. Any type of SI joint inflammation is called sacroiliitis. The inflammation may be caused by wear-and-tear on the joints (degenerative arthritis), or be a symptom of a larger inflammatory condition, such as ankylosing spondylitis.