The Science of Sacroiliac (SI) Joint Dysfunction
Sacroiliac (SI) joint dysfunction involves abnormal movement or positioning of the joint between your sacrum and ilium bones. This joint normally allows only small amounts of movement (2-4mm translation and 2-4 degrees rotation) but plays a crucial role in transferring forces between your spine and lower extremities. The SI joint is surrounded by some of the strongest ligaments in the body, which can become either too loose (hypermobile) or too tight (hypomobile), both of which can cause pain and dysfunction. The joint surfaces are irregular and interlocking, designed more for stability than mobility, which makes them vulnerable to dysfunction when normal mechanics are disturbed. Muscle imbalances around the pelvis significantly contribute to SI joint problems. When muscles like the deep abdominals, multifidus, gluteus maximus, or latissimus dorsi don't function properly, it alters the force distribution across the joint and can lead to compensatory stress patterns. The joint is richly innervated with pain receptors, which explains why SI dysfunction can be extremely painful. The pain pattern often involves the posterior pelvis but can refer to the groin, hip, thigh, and even down to the foot, making diagnosis challenging.
Contributing Factors
Your SI joint functions as part of the closed kinetic chain that includes your lumbar spine, pelvis, and hip joints. During walking, the SI joint must allow small rotational movements while maintaining stability to transfer forces effectively from your legs to your spine.
The joint's movement pattern is complex and involves nutation (sacrum tilting forward) and counter-nutation (sacrum tilting backward) that must be coordinated with hip and spine movement. When this coordination is disrupted, abnormal stresses develop that can lead to joint irritation and surrounding muscle guarding.
Form closure refers to the passive stability provided by the joint's bony architecture and ligaments, while force closure describes the active stability created by muscle activation. Problems can develop in either system, but force closure dysfunction is more common and typically responds well to physiotherapy intervention.
The posterior oblique sling (latissimus dorsi and opposite gluteus maximus connected through the thoracolumbar fascia) and anterior oblique sling (internal oblique and opposite adductor muscles) are crucial muscle chains that provide dynamic stability to the SI joint during functional activities.