Sacroiliac Joint Instability: Causes, Diagnosis, and Rehabilitation

What Is the Sacroiliac Joint?

The sacroiliac joint (SIJ) is the articulation between the sacrum — the wedge-shaped bone at the base of the spine — and the ilium of the pelvis. It is a synovial joint anteriorly and a syndesmosis posteriorly, supported by some of the strongest ligaments in the human body: the posterior sacroiliac, interosseous sacroiliac, sacrotuberous, and sacrospinous ligaments. The SIJ transmits the entire load of the axial skeleton to the lower limbs and absorbs ground reaction forces travelling upward through the body. Under normal conditions it moves minimally — approximately two to four degrees of rotation and one to two millimetres of translation — a movement called nutation and counternutation. When this movement becomes excessive, poorly controlled, or asymmetrical, the joint becomes a source of pain and functional impairment.

How Does SIJ Instability Develop?

SIJ instability arises when the passive (ligamentous), active (muscular), and neural control systems that govern joint load transfer are insufficient. Pregnancy and the postpartum period is the most common precipitating context: relaxin-mediated ligamentous laxity increases SIJ mobility, while the altered centre of mass and reduced intra-abdominal pressure capacity challenge active stability. Traumatic loading — falls onto one leg, MVA impacts, or direct pelvic trauma — can strain the posterior ligament complex acutely. Repetitive asymmetrical loading from unilateral sport, sustained single-leg postures, or repetitive rotation gradually compromises the force-closure mechanism. Hypermobility disorders (e.g. Ehlers-Danlos syndrome), inflammatory arthropathies, and prior lumbar fusion surgery also predispose to SIJ dysfunction.

Recognising SIJ Instability

The hallmark symptom is unilateral posterior pelvic pain located just medial and inferior to the posterior superior iliac spine (PSIS), often referring into the buttock, groin, or posterior thigh. Pain is consistently aggravated by single-leg loading: rolling over in bed, climbing stairs, stepping off a kerb, and prolonged standing on one leg. The active straight leg raise (ASLR) test is the most sensitive clinical indicator of impaired SIJ load transfer: difficulty raising one leg from supine, which is immediately reduced when the clinician manually compresses the ilia, indicates force-closure insufficiency. The cluster of SIJ provocation tests — distraction, compression, thigh thrust, FABER, and Gaenslen's — when three or more are positive, provides high diagnostic confidence.

Form Closure and Force Closure

SIJ stability depends on two complementary mechanisms. Form closure refers to inherent joint stability from the interlocking geometry of the irregular, ridged sacral and iliac surfaces. Force closure refers to the compressive force across the joint produced by coordinated muscular contraction. The muscles most critical to force closure include the gluteus maximus (acting via the thoracolumbar fascia and sacrotuberous ligament), biceps femoris, deep multifidus, transversus abdominis, and pelvic floor musculature. These structures act as a unit — anticipatory co-contraction that pre-stiffens the SIJ before load is applied. Pain-related inhibition, deconditioning, or postpartum changes disrupt this coordination, leaving the joint vulnerable.

Rehabilitation Principles

Rehabilitation must restore neuromuscular coordination before progressing to load tolerance. Phase one addresses pain education, activity modification to reduce provocative loading, and isolated activation of deep stabilisers — transversus abdominis, pelvic floor, deep multifidus — in comfortable positions. A pelvic compression belt worn at the level of the greater trochanters provides immediate symptomatic relief by augmenting passive force closure in the early stages. Phase two integrates stabiliser activation into single-leg loading: step-ups, controlled single-leg stance, modified deadlift patterns. Phase three progressively loads the restored system through functional movement patterns relevant to the individual. Manual therapy — SIJ manipulation, muscle energy technique, or soft tissue work to the posterior ligament complex — reduces pain and restores normal joint kinematics, improving exercise quality.

Prognosis and Outlook

With appropriate identification and progressive rehabilitation, SIJ instability responds well to conservative management. Postpartum SIJ pain typically resolves within three to six months. Chronic instability from trauma or hypermobility requires longer-term commitment to neuromuscular training but achieves excellent functional outcomes. A small percentage of patients with refractory instability unresponsive to extended conservative management may be considered for SIJ fusion — but this represents a minority, and thorough rehabilitation should always precede surgical consideration.