Classification
- Type: Compound — synovial anteriorly (lower two-thirds), syndesmosis (fibrous) posteriorly (upper one-third)
- Degrees of freedom: Minimal — 1–4° of rotation (nutation/counternutation) and 1–2 mm of translation
- Region: Pelvis (along with the anatomy/joints/pubic-symphysis and anatomy/joints/lumbosacral articulations)
Articular Surfaces
- Sacral articular surface (concave in the sagittal plane): The auricular (ear-shaped) surface on the lateral sacrum. Covered with hyaline cartilage (thicker than the iliac cartilage). The surface is irregular — with ridges and depressions that interlock with the ilium, increasing stability.
- Iliac articular surface (convex): The reciprocal auricular surface on the medial ilium. Covered with fibrocartilage (thinner than the sacral surface). The surface ridges interlock with the sacral depressions — this interlocking increases with age as the joint surfaces become more irregular.
- Age-related changes: In young adults, the surfaces are relatively smooth. By the fourth decade, surface irregularities, fibrous bridging, and osteophyte formation progressively limit motion. By the seventh decade, partial or complete ankylosis is common (more in males than females).
Movements and ROM
| Movement |
Normal ROM |
Description |
| Nutation |
1–4° |
The sacral base tips anteriorly (sacral promontory moves inferiorly and anteriorly) while the coccyx moves posteriorly. This is the "close-packed" movement that tightens the SI ligaments and increases stability. Occurs during: standing, trunk extension, single-leg stance. |
| Counternutation |
1–4° |
The sacral base tips posteriorly while the coccyx moves anteriorly. This loosens the SI ligaments and opens the pelvic inlet. Occurs during: sitting, trunk flexion, end-range hip flexion. |
| Translation |
1–2 mm |
Minimal translatory glide, primarily inferior-superior |
Form closure vs. force closure. SI joint stability comes from two mechanisms: form closure (the bony interlocking of the irregular articular surfaces and the wedge shape of the sacrum between the ilia) and force closure (the dynamic compression produced by muscles, fascia, and ligaments — particularly the thoracolumbar fascia, multifidus, transversus abdominis, gluteus maximus, and pelvic floor). Rehabilitation of SI dysfunction must address force closure — muscles that compress the SI joint.
Capsular Pattern
Pain when stress is applied to the joint (provocation tests); typically pain with unilateral movements
The SI joint does not have a traditional capsular pattern in the peripheral joint sense. SI dysfunction is identified primarily through provocation testing (cluster of tests) rather than through range limitation proportions.
Resting Position
- Not clearly defined — the SI joint has so little mobility that a distinct resting position is not clinically relevant
Close-Packed Position
- Nutation (sacral base anterior)
- Maximum surface interlocking, ligaments taut
- The SI joint is in close-packed position during standing and single-leg stance
End-Feels
The SI joint has so little motion that end-feel assessment is not performed in the traditional sense. Mobility is assessed through provocation tests and spring testing (PA pressure on the sacrum).
Ligaments
Anterior Sacroiliac Ligament
- Attachments: Anterior and inferior sacrum → anterior ilium
- Function: Thin and relatively weak. Resists counternutation (posterior sacral base tipping) and some lateral sacral separation. The weakest SI ligament.
Posterior Sacroiliac Ligament (Short and Long)
- Attachments: Short posterior: sacral tubercles → PSIS and iliac tuberosity. Long posterior: S3-S4 → PSIS (blends with the sacrotuberous ligament).
- Function: The strongest SI ligaments. Resist counternutation and nutation (depending on the fiber direction). The long posterior SI ligament is the most commonly tender ligament in SI joint dysfunction — palpation tenderness just medial and inferior to the PSIS is a key clinical finding.
Interosseous Sacroiliac Ligament
- Attachments: Between the sacral and iliac tuberosities (deep to the posterior SI ligament)
- Function: The strongest ligament in the body. Fills the space between the sacrum and ilium posteriorly. Resists all SI joint motion. It is so strong that the sacrum will fracture before this ligament ruptures.
Sacrotuberous Ligament
- Attachments: Lateral sacrum and coccyx → ischial tuberosity
- Function: Resists nutation (prevents the sacral base from tipping too far anteriorly). Taut during nutation, slack during counternutation. The gluteus maximus and biceps femoris (long head) have direct fascial connections to this ligament — their contraction increases SI stability.
Sacrospinous Ligament
- Attachments: Lateral sacrum and coccyx → ischial spine
- Function: Resists nutation. Shorter and deeper than the sacrotuberous ligament. Together with the sacrotuberous ligament, it converts the greater and lesser sciatic notches into foramina.
Mobilization Techniques
Hands-on instruction is required. The descriptions below provide clinical reference detail for understanding and supervised practice. They are not a substitute for instructor-led technique training. Correct hand placement, force dosage, and tissue response interpretation require hands-on coaching and feedback.
General Contraindications
- Absolute: Acute sacral fracture, sacral malignancy, active infection (septic sacroiliitis — can occur in IV drug users and immunocompromised patients), ankylosing spondylitis with fused SI joints, acute inflammatory sacroiliitis (RA, psoriatic arthritis, reactive arthritis)
- Relative: Pregnancy (hormonal ligament laxity — the SI joint is already more mobile; avoid aggressive techniques), severe osteoporosis, pelvic ring instability (pubic symphysis disruption)
Assessment Before Treatment
Provocation test cluster (at least 3 of 5 positive = SI joint involvement):
- Distraction test (posterior gapping)
- Compression test (lateral compression in side-lying)
- Thigh thrust (posterior shear — sacral base stressed)
- Sacral thrust (PA force on sacrum in prone)
- Gaenslen's test (hip hyperextension off table edge)
A cluster approach is necessary because individual SI joint tests have poor reliability. Three or more positive tests significantly increases diagnostic confidence.
PA Sacral Glide (Nutation Mobilization)
Purpose: Produces nutation (anterior sacral base tipping). Useful when the sacrum is held in relative counternutation and nutation mobility is restricted.
Patient position:
- Prone on the treatment table
Hand placement:
- Mobilizing hand: Heel of the hand or crossed pisiform contacts placed on the sacral base (upper sacrum, just below L5). Force directed anteriorly and slightly inferiorly.
Technique execution:
- Apply an oscillatory force directed anteriorly on the sacral base, producing nutation
- Grade I–II: Gentle oscillations for pain modulation and assessment of sacral spring
- Grade III: Oscillations into the nutation resistance — the posterior SI ligaments and sacrotuberous/sacrospinous ligaments provide the barrier
- Duration: 30–60 seconds, 2–3 sets
Indications:
- SI joint pain confirmed by provocation test cluster
- Restricted sacral spring on PA testing
- Post-partum SI dysfunction (after hormonal ligament laxity has resolved — typically >3 months postpartum)
Technique notes:
- Common error: Pressing on L5 instead of the sacral base — produces lumbar extension, not sacral nutation. The sacral base is broad and flat, easily distinguished from the L5 spinous process.
- Reassessment: Repeat the most positive provocation test. Reduced pain confirms SI involvement.
Iliac Posterior Rotation (Gapping)
Purpose: Produces a posterior rotation of the ilium on the sacrum, which gaps the anterior SI joint and stretches the anterior SI ligament. Useful when the ilium is held in anterior rotation (common with psoas/hip flexor tightness).
Patient position:
Hand placement:
- Stabilizing hand: On the opposite ASIS, stabilizing the pelvis
- Mobilizing hand: On the ASIS of the affected side. Force directed posteriorly and inferiorly (pushing the ASIS toward the table and toward the feet).
Technique execution:
- Apply an oscillatory force directing the ASIS posteriorly — producing posterior iliac rotation
- Grade I–II: Pain modulation
- Grade III: End-range oscillations
- Duration: 30–60 seconds, 2–3 sets
Indications:
- Clinical finding of anterior iliac rotation (ASIS lower on the affected side, PSIS higher)
- SI joint pain with positive provocation tests
- Hip flexor tightness producing anterior pelvic tilt
Technique notes:
- Integration: Release the psoas and rectus femoris first — hip flexor tightness holds the ilium in anterior rotation and resists posterior rotation mobilization.
- Reassessment: Re-assess ASIS/PSIS symmetry and provocation test pain.
Muscles Crossing This Joint
Primary Force Closure Muscles
- anatomy/muscles/gluteus-maximus — attaches to the sacrotuberous ligament and thoracolumbar fascia; its contraction compresses the SI joint via the fascial sling
- anatomy/muscles/multifidus — crosses the SI joint posteriorly; co-contracts with transversus abdominis to compress the SI joint
- anatomy/muscles/transversus-abdominis — wraps around the pelvis via the thoracolumbar fascia; its contraction directly compresses the SI joints bilaterally
- anatomy/muscles/biceps-femoris (long head) — attaches to the sacrotuberous ligament; its tension increases SI stability during gait
- anatomy/muscles/pelvic-floor-muscles — attach to the sacrum and coccyx; contribute to force closure from below
Muscles That Affect Pelvic Position
- anatomy/muscles/psoas-major — anterior tilt → anterior iliac rotation → nutation bias
- anatomy/muscles/rectus-femoris — anterior tilt via its AIIS attachment
- anatomy/muscles/hamstrings — posterior tilt → posterior iliac rotation → counternutation bias
- anatomy/muscles/piriformis — originates from the anterior sacrum; its tension directly affects sacral position
Conditions Affecting This Joint
- SI joint dysfunction — the most common SI condition in manual therapy practice; pain typically over the PSIS region, may refer to the buttock and posterior thigh (does not radiate below the knee)
- Ankylosing spondylitis — inflammatory spondyloarthropathy that begins at the SI joints; bilateral sacroiliitis is the hallmark radiographic finding; morning stiffness >30 minutes improves with exercise; HLA-B27 positive
- Sacroiliitis — inflammation of the SI joint from any cause (RA, psoriatic arthritis, reactive arthritis, infection); bilateral inflammatory sacroiliitis = ankylosing spondylitis; unilateral = consider infection or other spondyloarthropathy
- Pelvic girdle pain (pregnancy-related) — hormonal ligament laxity (relaxin) increases SI mobility; pain over the SI joint and pubic symphysis; most common in the third trimester and postpartum
- Pelvic ring fracture — high-energy trauma (motor vehicle accident, fall from height); SI joint disruption as part of pelvic ring instability; absolute contraindication to manual treatment
Clinical Notes
- Assessment reliability is the central challenge. Individual SI joint tests (ASIS/PSIS palpation, seated flexion test, standing flexion test, leg length comparison) have poor inter-rater reliability. The provocation test cluster (3 of 5 tests positive) is the most reliable clinical approach and should be the basis for SI diagnosis.
- Force closure rehabilitation is the treatment foundation. Even when mobilization reduces pain, lasting improvement requires restoring the force closure mechanism. Key exercises: gluteus maximus activation (hip extension, bridging), transversus abdominis activation (drawing-in maneuver), multifidus retraining. Pelvic belts can provide temporary external force closure while muscles are rehabilitated.
- SI dysfunction vs. lumbar radiculopathy. SI joint pain refers to the buttock and posterior thigh but typically does not radiate below the knee. Lumbar radiculopathy follows a dermatomal distribution below the knee. This is the primary differentiating feature. If the patient has calf or foot symptoms, investigate the lumbar spine, not the SI joint.
- Red flag: bilateral morning stiffness. If a young adult (typically male, age 15–35) presents with bilateral SI pain, morning stiffness >30 minutes that improves with activity, and reduced lumbar flexion — suspect ankylosing spondylitis. Refer for HLA-B27 testing and SI joint imaging.
Key Takeaways
- SI joint stability depends on form closure (bony interlocking) and force closure (muscular compression via gluteus maximus, transversus abdominis, multifidus, and the thoracolumbar fascia) — rehabilitation must address force closure.
- Use the provocation test cluster (3 of 5 positive) for diagnosis — individual SI tests have poor reliability.
- SI joint pain refers to the buttock and posterior thigh but does not radiate below the knee — below-knee symptoms suggest lumbar radiculopathy.
- Bilateral sacroiliitis in a young adult with morning stiffness is ankylosing spondylitis until proven otherwise — refer for testing.
