Sacroiliitis is inflammation of the sacroiliac (SI) joint and is the imaging hallmark of axial spondyloarthritis (axSpA). On MRI, subchondral bone marrow oedema (BME)/osteitis is the single defining feature of active sacroiliitis, while erosion, fat metaplasia, backfill and ankylosis represent structural damage. This article summarises the current MRI and radiographic findings, the ASAS definition of a positive MRI (including the 2016 update and the 2021 data-driven lesion cut-offs), the modified New York radiographic grading, and the key differential diagnoses.
Quiz
Sacroiliitis is associated with all the following diseases EXCEPT?
- Ankylosing spondylitis
- Diffuse idiopathic skeletal hyperostosis
- Crohn’s disease
- Reactive arthritis (Reiter’s syndrome)
Click here for the answer
Answer: B. Diffuse idiopathic skeletal hyperostosis (DISH) involves the sacroiliac ligaments and can mimic sacroiliitis, but there are no subchondral erosions or true synovial-joint sclerosis, and the synovial portion of the joint is spared.
Pathophysiology and clinical context
Sacroiliitis is inflammation of the sacroiliac joint associated with a range of spondyloarthropathies. Inflammation begins in the synovial (lower and middle) third of the joint, and the thinner iliac cartilage is affected earlier and more severely than the sacral side.
The Assessment of SpondyloArthritis international Society (ASAS) criteria for inflammatory back pain (in patients with back pain for >3 months) require 4 of the following 5 features:
- Age at onset <40 years
- Insidious onset
- Improvement with exercise
- No improvement with rest
- Pain at night (with improvement on getting up)
Distribution as a clue to the cause
The pattern of joint involvement narrows the differential diagnosis:
| Distribution | Typical causes |
|---|---|
| Bilateral, symmetric | Ankylosing spondylitis, inflammatory bowel disease–associated, advanced psoriatic arthritis |
| Bilateral, asymmetric | Reactive arthritis, psoriatic arthritis, gout, osteoarthritis |
| Unilateral | Infection (pyogenic or tuberculous), early reactive arthritis, osteoarthritis, neoplasm |
Imaging modalities
| Modality | Role | Strengths | Limitations |
|---|---|---|---|
| Radiograph (AP pelvis / Ferguson view) | First-line assessment of structural change; modified New York grading | Cheap, widely available | Insensitive to early and active disease; oblique joint and overlap limit reproducibility |
| CT | Reference standard for erosion, sclerosis and ankylosis | Excellent cortical bone detail; low-dose and dual-energy protocols now available | Ionising radiation; cannot depict active bone-marrow inflammation |
| MRI | Modality of choice; detects both active inflammation and structural lesions | Shows subchondral BME/osteitis; no radiation; enables early diagnosis | Cost/availability; false-positive BME in athletes, postpartum women and mechanical stress |
| Bone scintigraphy | Historical localisation of disease to the SI joint | — | Non-specific; largely replaced by MRI |
Key MRI imaging features
Slide to see the annotated image.
The ASAS MRI working group divides findings into active (inflammatory) lesions and structural (damage) lesions. The 2019 update revised the definitions of erosion, fat lesion, capsulitis and enthesitis and added new definitions for joint-space enhancement, joint-space fluid, fat metaplasia in an erosion cavity (“backfill”), ankylosis and bone bud.
| Active (inflammatory) lesions | Structural (damage) lesions |
|---|---|
| Subchondral BME/osteitis – high signal on STIR/T2 fat-sat with enhancement on post-contrast T1; the defining lesion of active sacroiliitis | Erosion – subchondral bone loss, best on T1; usually iliac and anteroinferior, causing pseudo-widening |
| Capsulitis – enhancement/oedema of the anterior or posterior joint capsule | Sclerosis – low signal on all sequences in the subchondral bone |
| Enthesitis – oedema/enhancement at ligamentous attachments (sacrospinous, sacrotuberous, interosseous) | Fat metaplasia (fat lesion) – bright, sharply marginated homogeneous T1 signal; a “deep” (>1 cm) fat lesion is highly specific |
| Joint-space enhancement / fluid – abnormal enhancement or fluid within the synovial cleft (2019 definition) | Backfill (fat metaplasia in an erosion cavity) – bright T1 signal filling an erosion, bounded by sclerosis; specific for spondyloarthritis |
| Ankylosis / bone bud – bony bridging across the joint; the end stage of the disease |
Radiograph and CT depict chronic bony change – joint-space pseudo-widening, erosion, sclerosis and ankylosis. CT is the reference standard for structural lesions but cannot show active inflammation.
The ASAS definition of a “positive MRI”
ASAS first defined active sacroiliitis on MRI in 2009: clear BME/osteitis in subchondral bone that is highly suggestive of axSpA is required; other active lesions (synovitis, enthesitis, capsulitis) support but do not replace it. The original quantitative rule (≥2 BME lesions on a single semicoronal slice, or one lesion on ≥2 consecutive slices) was retained.
The 2016 ASAS MRI working-group update confirmed that subchondral BME remains the defining observation and that structural lesions may increase the reader’s confidence but are not required. It de-emphasised the fixed quantitative cut-off in favour of BME being “highly suggestive of axSpA.”
The 2021 data-driven ASAS analysis proposed reproducible lesion cut-offs (each with ≥95% specificity for axSpA):
| Lesion | Cut-off highly suggestive of axSpA |
|---|---|
| BME (active) | ≥4 SI-joint quadrants at any location, OR the same location on ≥3 consecutive slices |
| Erosion (structural) | ≥3 SI-joint quadrants, OR the same location on ≥2 consecutive slices |
| Fat lesion (structural) | ≥5 SI-joint quadrants, OR ≥3 consecutive slices, OR a single deep (>1 cm) fat lesion |
Important pitfall: the ASAS definition is a classification tool for research, not a clinical diagnostic criterion. Isolated low-grade BME occurs in 20–40% of healthy people, athletes and postpartum women, so over-reliance on BME alone leads to over-diagnosis. Correlate with structural lesions, distribution and the clinical picture.
Radiographic grading: modified New York criteria
| Grade | Radiographic finding |
|---|---|
| 0 | Normal SI joints with well-defined margins |
| 1 | Suspicious changes with incipient sclerosis |
| 2 | Loss of definition of articular margins, subchondral osteoporosis and areas of reactive sclerosis |
| 3 | Subchondral sclerosis of both sacral and iliac margins, erosions, reduced/widened joint space, partial ankylosis |
| 4 | Complete ankylosis with residual sclerosis |
Imaging recommendation and protocol
MRI is the most sensitive modality for early marrow oedema and for characterising and staging SI-joint disease. A dedicated SI-joint protocol uses semicoronal oblique planes aligned to the long axis of the sacrum with:
- STIR or T2 fat-saturated sequence for active inflammation (BME)
- T1-weighted sequence for structural lesions (erosion, fat metaplasia, backfill, ankylosis)
- Contrast is usually not required; STIR is sufficient for BME
Top differential diagnoses and mimics
- Osteitis condensans ilii – typically in women of child-bearing age, related to abnormal mechanical stress. Bilateral, symmetric, sharply defined triangular subchondral sclerosis on the iliac side, without erosions or joint-space change.
- Degenerative osteoarthritis – joint-space narrowing, osteophytes and vacuum phenomenon, without true erosive/inflammatory change.
- Infective (septic) sacroiliitis – usually unilateral with intense BME, intra-articular fluid, periarticular soft-tissue inflammation and abscess. Fat metaplasia and backfill are absent.
- Hyperparathyroidism – subchondral bone resorption and joint-space widening without active inflammatory lesions.
- Stress/mechanical and postpartum change – a frequent source of false-positive BME; look for the absence of erosion, backfill and typical distribution.
Clinical features
- Symptoms – low back pain radiating to the buttocks and hips, with morning stiffness and inflammatory features
- Age/sex – axial spondyloarthritis presents predominantly in young adults; radiographic ankylosing spondylitis is more common in men
- Risk factor – HLA-B27 positivity
Treatment
- Physiotherapy and structured exercise (first-line, evidence-based)
- NSAIDs as first-line pharmacotherapy
- Biologic and targeted therapy for active disease refractory to NSAIDs – TNF inhibitors, IL-17 inhibitors and JAK inhibitors (managed by rheumatology)
- Image-guided intra-articular steroid injection for focal symptomatic disease
- SI-joint fusion reserved for refractory chronic pain
Frequently asked questions
What is the most reliable MRI sign of active sacroiliitis?
Subchondral bone marrow oedema (osteitis) on STIR or T2 fat-saturated sequences is the single defining feature. Under the ASAS definition it must be clearly present in subchondral bone and highly suggestive of axial spondyloarthritis; structural lesions add confidence but are not required.
What is an “ASAS-positive MRI”?
It is a classification concept: subchondral BME highly suggestive of axSpA. The 2021 data-driven analysis suggests BME in ≥4 SI-joint quadrants, or the same location on ≥3 consecutive slices, as a reproducible cut-off. It is designed for research classification, not clinical diagnosis.
Is MRI or CT better for sacroiliitis?
MRI is the modality of choice because it shows active inflammation (BME) that radiography and CT cannot. CT remains the reference standard for structural lesions such as erosion, sclerosis and ankylosis.
What is backfill on MRI?
Backfill is fat metaplasia within an erosion cavity – bright T1 signal filling an erosion and bounded by a rim of sclerosis. It is a relatively specific sign of spondyloarthritis and represents an intermediate step between erosion and ankylosis.
Can bone marrow oedema be normal?
Yes. Low-grade subchondral BME is seen in 20–40% of healthy adults, athletes and postpartum women. Diagnosing sacroiliitis on BME alone over-calls disease; correlate with structural lesions, distribution and clinical findings.
References
Best single review:
Co-Authors: Dr. Bhargavi Sovani. Illustration by Dr. Disha Lokhandwala.
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