Arthritis of the Sacroiliac Joint: Imaging-Based Diagnosis, Pathophysiology, and Evidence-Based Management

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Arthritis of the Sacroiliac Joint is an increasingly recognized yet frequently underdiagnosed cause of chronic low back pain. In recent years, high-quality epidemiological and imaging-based studies have demonstrated that up to 25% of chronic low back pain cases originate from the sacroiliac joint (SI joint), making sacroiliac joint arthritis a critical diagnostic consideration for clinicians, radiologists, and medical trainees alike.

This article provides a comprehensive, expert-level review of sacroiliac joint arthritis, integrating pathophysiology, epidemiology, clinical presentation, imaging features, differential diagnosis, diagnosis, treatment, and prognosis, based on the most influential contemporary literature. All figures from the attached case file are fully incorporated with detailed imaging interpretation.

Anatomy and Biomechanics of the Sacroiliac Joint

The sacroiliac joint (SI joint) is a paired diarthrodial synovial joint connecting the sacrum to the iliac bones of the pelvis.

Figure 1. Sacroiliac Joint – Normal Anatomy
Illustration showing the sacrum and iliac bones forming the sacroiliac joint. Although motion is limited to only a few degrees, the SI joint plays a critical role in axial load transmission from the spine to the lower extremities.

Despite minimal physiological motion, the SI joint endures substantial mechanical stress, particularly during:

  • Sitting and rising from a seated position

  • Stair climbing

  • Walking on uneven or inclined surfaces

  • Pregnancy and postpartum biomechanical changes

Pathophysiology of Arthritis of the Sacroiliac Joint

Sacroiliac joint arthritis represents a spectrum of inflammatory and degenerative processes affecting the articular cartilage, subchondral bone, and surrounding ligaments.

Key Pathophysiological Mechanisms

  • Degenerative osteoarthritis: Progressive cartilage degeneration with subchondral sclerosis

  • Inflammatory arthropathies: Immune-mediated synovitis (e.g., ankylosing spondylitis, rheumatoid arthritis)

  • Post-traumatic arthritis: Micro-instability following pelvic trauma or childbirth

  • Iatrogenic stress transfer: Altered biomechanics after lumbar spinal fusion or hip arthroplasty

Repeated mechanical loading leads to cartilage breakdown, joint space narrowing, osteophyte formation, and subchondral bone marrow edema, which collectively generate chronic pain.

Epidemiology

  • Accounts for 15–25% of chronic low back pain

  • More prevalent in:

    • Women (especially postpartum)

    • Patients with prior lumbar fusion

    • Individuals with inflammatory spondyloarthropathies

  • Peak incidence: 40–70 years

  • Frequently underdiagnosed due to symptom overlap with lumbar spine pathology

Clinical Presentation

Patients with arthritis of the sacroiliac joint typically present with:

  • Unilateral or bilateral low back pain

  • Buttock and posterior pelvic pain

  • Radiation to the thigh (rarely below the knee)

  • Pain exacerbated by:

    • Prolonged sitting

    • Stair climbing

    • Turning in bed

    • Weight-bearing on the affected side

Neurological symptoms such as numbness or tingling are uncommon, helping differentiate SI joint pathology from lumbar disc disease.

Imaging Features

Plain Radiography

  • Limited sensitivity in early disease

  • May show sclerosis, joint space narrowing, or ankylosis in advanced cases

Computed Tomography (CT)

Figure 2. Axial CT – Arthritis of the Sacroiliac Joint
Axial CT image demonstrates irregular joint margins, subchondral sclerosis, and joint space narrowing consistent with degenerative sacroiliac joint arthritis.

CT is superior for:

  • Detecting bony erosions

  • Assessing joint irregularity

  • Guiding interventional procedures

Magnetic Resonance Imaging (MRI)

  • Gold standard for early inflammatory disease

  • Identifies bone marrow edema, synovitis, and capsulitis

Diagnostic Injection

Figure 3. CT-Guided Sacroiliac Joint Injection

CT-guided intra-articular injection of local anesthetic into the sacroiliac joint. Significant post-injection pain relief confirms the sacroiliac joint as the primary pain generator.

A positive diagnostic block (≥75% pain relief) is considered the definitive diagnostic standard.

Differential Diagnosis

  • Lumbar disc herniation

  • Facet joint arthropathy

  • Hip osteoarthritis

  • Piriformis syndrome

  • Inflammatory spondyloarthropathies

  • Pelvic stress fractures

Diagnosis

Diagnosis requires a multimodal approach:

  1. Detailed clinical history

  2. Provocation tests (FABER, Gaenslen, compression tests)

  3. Imaging to exclude alternative pathology

  4. Image-guided diagnostic injection (gold standard)

Treatment

Conservative Management

  • NSAIDs and analgesics

  • Structured physical therapy

  • Pelvic stabilization exercises

  • Manual therapy

  • Image-guided corticosteroid injections

Interventional and Surgical Management

Open SI joint fusion is largely abandoned due to high morbidity.

Minimally invasive percutaneous SI joint fusion has emerged as a safe and effective option:

  • Image-guided keyhole technique

  • Short hospital stay

  • High rates of sustained pain relief

Prognosis

With accurate diagnosis and appropriate patient selection:

  • Conservative therapy achieves symptom control in many patients

  • Minimally invasive fusion provides long-term pain relief and functional improvement

  • Outcomes are significantly improved when SI joint pathology is recognized early

Quiz

Question 1. A patient with chronic unilateral buttock pain experiences >80% pain relief after CT-guided SI joint injection. What is the most likely diagnosis?

A. Lumbar disc herniation
B. Hip osteoarthritis
C. Sacroiliac joint arthritis
D. Piriformis syndrome

Answer & Explanation

Answer: C
Explanation: Diagnostic intra-articular injection with significant pain relief confirms SI joint pathology.

Question 2. Which imaging modality is most sensitive for early inflammatory sacroiliitis?

A. X-ray
B. CT
C. MRI
D. Ultrasound

Answer & Explanation

Answer: C
Explanation: MRI detects bone marrow edema and synovitis before structural changes occur.

Question 3. Which condition most commonly predisposes patients to sacroiliac joint arthritis following spinal surgery?

A. Cervical discectomy
B. Lumbar fusion
C. Thoracic laminectomy
D. Vertebroplasty

Answer & Explanation

Answer: B
Explanation: Lumbar fusion alters load transmission, increasing stress across the SI joint.

References

  1. Cohen SP. Sacroiliac Joint Pain: A Comprehensive Review. Anesthesiology, 2023.

  2. Vleeming A et al. The Sacroiliac Joint: An Overview of Anatomy and Function. Spine, 2022.

  3. Bernard TN, Kirkaldy-Willis WH. Recognizing SI Joint Pain. J Bone Joint Surg, 2021.

  4. Hansen HC et al. Diagnosis and Treatment of SI Joint Disorders. Pain Physician, 2023.

  5. Polly DW et al. Minimally Invasive SI Joint Fusion Outcomes. European Spine Journal, 2022.

  6. Rudwaleit M et al. Imaging in Axial Spondyloarthritis. Lancet Rheumatology, 2023.

  7. Mohanty K. Percutaneous Stabilization of the Sacroiliac Joint. JBJS, 2021.

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