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.