The sacroiliac joint (SI joint) is a pair of joints located in the lower back, connecting the spine to the pelvis. The SI joint links the sacrum, the triangular bone at the base of the spine, with the ilium, the large, flared bone of the pelvis. Despite its relatively small size, the SI joint acts as a bridge between the upper body and the lower limbs. Its movement is far more restricted than most other joints in the body.
The Sacroiliac Joint’s Dual Classification
The sacroiliac joint is difficult to classify because it exhibits characteristics of two different joint types. Anatomically, it is classified as a synovial joint, surrounded by a fibrous capsule and containing lubricating synovial fluid, placing it in the diarthrosis category (freely movable joints). Functionally, however, it is categorized as an amphiarthrosis (slightly movable joint). This dual nature reflects the SI joint’s limited range of motion. The SI joint is best described as a modified synovial joint, designed for strength and stability rather than mobility.
The articulation surfaces display a unique combination of cartilage types. The sacral side is covered in hyaline cartilage, while the iliac side is lined with fibrocartilage, a tougher tissue that contributes significantly to stability. This unusual pairing underscores the SI joint’s dual function of allowing minimal movement while maintaining strength.
Key Structural Components Limiting Movement
The limited movement of the sacroiliac joint is due to the irregular shape of the articulating surfaces and a dense network of surrounding ligaments. The auricular surfaces of the sacrum and ilium interlock tightly, often described as C-shaped or L-shaped, providing stability. Unlike typical synovial joints, these surfaces feature elevations and depressions that become more pronounced with age.
This rough, interlocking nature physically resists sliding or shearing forces, creating a form-fitting mechanism that restricts rotation and translation. The narrow joint space, typically 0.5 to 4 millimeters, further contributes to the joint’s stiffness.
The ligamentous architecture provides the primary extrinsic stability, preventing excessive motion. The interosseous sacroiliac ligament is one of the strongest ligaments in the human body. Located posteriorly and superiorly, it spans the gap between the sacrum and the ilium, preventing anterior and inferior movement. The posterior sacroiliac ligaments are also thicker and stronger than the anterior sacroiliac ligament. These powerful complexes work with the bony contours to ensure the joint remains stable.
Primary Role in Stability and Load Transfer
The main function of the sacroiliac joint is to transfer mechanical load between the spine and the lower extremities. It manages substantial forces generated by the upper body’s weight, distributing them downward into the legs, and transmits ground reaction forces upward into the vertebral column. The minimal motion permitted allows the joint to function effectively as a shock absorber, shielding the spine from impacts.
The body relies on a self-bracing effect, called the “form and force closure” mechanism, to maintain stability under weight. Form closure is provided by the interlocking bony structure, while force closure is supplied by the tension of the ligaments and surrounding musculature. This combined stability allows the SI joint to manage high vertical shear loads, locking the sacrum between the two halves of the pelvis.
Understanding Sacroiliac Joint Pain
As a major load-bearing structure, the sacroiliac joint is a common source of lower back pain. Pain originating here is often called sacroiliac joint dysfunction (SIJD) or sacroiliitis (inflammation of the joint). Dysfunction can arise from hypermobility (too much movement and instability) or hypomobility (too little movement, causing fixation and stiffness).
The pain is typically localized to the lower back and buttock region, but frequently radiates. Patients often report discomfort extending down the back of the thigh, groin, or foot, sometimes mimicking sciatica. This referred pain is due to the SI joint’s proximity to and shared innervation with surrounding nerves.
Symptoms are often aggravated by activities that stress the joint, such as prolonged sitting, stair climbing, or standing on one leg. Up to 25% of all low back pain cases may originate from the SI joint, making diagnosis challenging due to symptom overlap with hip and lumbar spine issues. Conditions like pregnancy, trauma, or inflammatory arthritis can contribute to SI joint pain by affecting joint stability.