What is the diagnosis and treatment for sacroiliitis of the sacral iliac joint?

Diagnosis and Treatment of Sacroiliitis

Diagnostic Imaging Algorithm

Begin with conventional radiography of the pelvis to evaluate the sacroiliac joints, followed by MRI if radiographs are negative or equivocal but clinical suspicion remains high. 1, 2

Initial Imaging: Radiography

• Obtain anteroposterior pelvis radiographs as the first-line imaging modality to assess for chronic structural changes including erosions, sclerosis, joint space narrowing, and ankylosis 1, 2
• Oblique views provide no additional benefit over standard anteroposterior views 1
• Add cervical and lumbar spine radiographs if symptoms are referable to the spine to assess for syndesmophytes, erosions, shiny corners, vertebral body squaring, and ankylosis 1
• Thoracic spine radiographs are not useful for diagnosis due to overlying structures, unless specific symptoms warrant evaluation 1

Critical limitations of radiography:

• Sensitivity ranges only 19-72% with specificity 47-84.5% 1
• Misses more than half of patients with structural changes when compared to CT 1
• Interobserver agreement is only fair to moderate 1
• Early inflammatory changes remain radiographically occult for 3-7 years after symptom onset 1, 2
• One study showed 41.3% of radiography reports gave incorrect diagnoses using CT as reference 1

Second-Line Imaging: MRI

MRI of the sacroiliac joints is the next imaging technique of choice after radiography, with sensitivity of 79% and specificity of 89% for axial spondyloarthritis. 1, 2

MRI Protocol Requirements:

• STIR (short tau inversion recovery) sequences to detect bone marrow edema and active inflammation 1
• T1-weighted sequences to identify chronic structural lesions including sclerosis, erosions, fat deposition, and ankylosis 1, 2
• Contrast is generally not necessary as noncontrast and contrast-enhanced MRI have similar diagnostic utility 1, 2
• Contrast may increase diagnostic confidence in select cases but does not significantly improve accuracy 1, 2

Key MRI Findings:

Active inflammatory lesions:

• Bone marrow edema (primary finding for ASAS criteria) 1
• Capsulitis, synovitis, and enthesitis 1

Chronic structural lesions:

• Erosions, sclerosis, fat deposition (backfill), and ankylosis 1
• Fat metaplasia is highly specific (95-98% specificity) for axial spondyloarthritis 1
• Intra-articular signal changes including increased T1 signal and ankylosis are highly specific 1

Important caveat: Bone marrow edema alone can occur in up to 30% of healthy controls, postpartum patients, athletes, and degenerative conditions 1, 2. However, deep bone marrow edema extending at least 1 cm deep to the articular surface is more specific for axial spondyloarthritis 1, 2.

Spine MRI Considerations

• MRI of the spine is not routinely recommended for initial diagnosis 2
• Consider adding spine MRI when symptoms are referable to the spine, as 5% of patients have inflammatory changes isolated to the spine and 41% have involvement of both sacroiliac joints and spine 1
• If obtained, image at minimum the cervical and lumbar spine 1

Imaging Modalities NOT Recommended

Do not routinely use:

• Bone scintigraphy/SPECT: Low to moderate sensitivity with variable specificity 1, 2
• Ultrasound: No relevant literature supporting its use 1, 2
• F-18-fluoride PET/CT: Insufficient evidence with poor interreader reliability 1, 2

CT has limited role:

• CT demonstrates structural changes with higher sensitivity than radiography and excellent interreader reliability 1
• However, CT cannot demonstrate active inflammation, making MRI superior for diagnosis 1
• CT may be useful when MRI is contraindicated, particularly low-dose CT protocols 1

Treatment Approach

Conservative Management (First-Line)

• Physical therapy targeting sacroiliac joint stabilization and mobility 3
• NSAIDs and analgesics for symptom relief 3
• This conservative approach should be attempted before escalating to interventional strategies 3

Interventional Management (Refractory Cases)

When conservative treatment fails:

• Fluoroscopy-guided diagnostic joint block is the gold standard for confirming SI joint as pain source; reduction in pain following anesthetic confirms diagnosis 3
• Corticosteroid injections into the SI joint 3
• Prolotherapy for ligamentous stabilization 3
• Radiofrequency ablation of sensory nerves 3

Surgical Management

• SI joint fusion surgery reserved for cases where previous conservative and interventional methods provide no significant relief 3

Disease-Specific Treatment

For inflammatory sacroiliitis associated with axial spondyloarthritis:

• TNF-inhibitors have been shown to reduce inflammation on MRI in multiple studies 1
• Biologic therapies show promise, particularly for ankylosing spondylitis 4
• MRI may be repeated to assess treatment response when disease activity is unclear, clinical/laboratory data are conflicting, or findings would alter treatment decisions 1

Follow-Up Imaging

• No standard method exists for routine follow-up imaging 1
• Repeat MRI only when clinically indicated: unclear disease activity, conflicting data, or when results would change management 1
• If assessing structural progression with radiographs, use intervals of no less than 2 years as structural changes develop slowly 1