Sacroiliitis is inflammation of one or both sacroiliac (SI) joints — the paired joints that connect the base of the spine (sacrum) to the pelvis. It is a common and frequently underdiagnosed cause of low back, buttock, and posterior hip pain, and may occur in isolation or as the defining early manifestation of an underlying inflammatory arthritis such as ankylosing spondylitis.
Sacroiliitis is an important diagnosis to distinguish from other causes of posterior hip and buttock pain. The deep buttock pain it produces is frequently confused with lumbar disc disease, piriformis syndrome, or trochanteric bursitis — all of which can refer pain to overlapping areas. Inflammatory sacroiliitis is the hallmark of ankylosing spondylitis and related spondyloarthropathies, which are also associated with rheumatoid-spectrum inflammatory arthritis of peripheral joints. In athletes with groin and pelvic pain, sacroiliitis overlaps clinically with osteitis pubis and sports hernia — and a dedicated pelvis MRI is the only study that can evaluate all three simultaneously.
Causes
Inflammatory arthritis (spondyloarthropathy).
Sacroiliitis is the cardinal and often earliest manifestation of the spondyloarthropathy family — a group of interrelated autoimmune conditions that includes ankylosing spondylitis (AS), psoriatic arthritis, reactive arthritis (formerly Reiter's syndrome), and arthritis associated with inflammatory bowel disease (Crohn's disease and ulcerative colitis). In ankylosing spondylitis — the prototype — the HLA-B27 gene is present in over 90% of patients and is thought to contribute to immune-mediated targeting of entheses (ligament and tendon insertion sites) throughout the axial skeleton, beginning at the SI joints. Inflammatory sacroiliitis is typically bilateral and symmetric, begins in young adulthood, and — if untreated — can progress through a characteristic sequence of bone marrow edema, erosion, sclerosis, and ultimately bony fusion (ankylosis) of the SI joints and spine. The window for effective treatment to prevent structural damage is widest in the early inflammatory stage, making early MRI diagnosis critical.
Other causes.
Mechanical and degenerative sacroiliitis results from excessive or asymmetric loading of the SI joint — due to leg length discrepancy, abnormal gait mechanics, prior lumbar fusion surgery that transfers increased motion and stress to the adjacent SI joint, or simply age-related degeneration of the fibrocartilaginous joint surface. Pregnancy and early postpartum are common triggers, as relaxin-mediated ligamentous laxity dramatically increases SI joint mobility and the associated biomechanical stress. Direct pelvic trauma can cause acute SI joint injury and subsequent post-traumatic sacroiliitis. Septic (infectious) sacroiliitis — most commonly caused by Staphylococcus aureus — is uncommon but serious, producing unilateral, rapidly progressive symptoms with systemic signs of infection and requiring urgent aspiration, culture, and intravenous antibiotic treatment.
Symptoms
The hallmark symptom is deep, aching pain in the low back, buttock, or posterior hip — often localized to the area just medial to and above the posterior superior iliac spine (the "SI joint region"). Pain may radiate into the upper thigh or groin but typically does not extend below the knee, which helps distinguish SI joint pain from true lumbar nerve root compression. The inflammatory vs. mechanical distinction is critically important for treatment: inflammatory sacroiliitis characteristically produces pain that is worse with rest and prolonged inactivity, worse in the morning with stiffness lasting more than 30–60 minutes, and paradoxically improves with movement and activity — a pattern known as inflammatory back pain that is the defining clinical feature of axial spondyloarthritis. Mechanical sacroiliitis produces the opposite pattern — pain that worsens with activity, prolonged sitting on hard surfaces, stair climbing, and positional transitions. Constitutional symptoms including fatigue, weight loss, and extra-articular features such as anterior uveitis (eye inflammation), psoriatic skin plaques, or gastrointestinal symptoms may be present in inflammatory disease and should be specifically asked about.
Diagnosis
Physical exam includes a battery of specific SI joint provocation tests — the FABER test (Flexion, ABduction, External Rotation with the ankle resting on the opposite knee), the thigh thrust test (posterior shear force through the SI joint), the Gaenslen test, and compression and distraction tests — with pain reproduction on at least three of these tests increasing diagnostic confidence significantly. Morning stiffness duration and the inflammatory vs. mechanical pain pattern are important historical features. Laboratory testing for HLA-B27, CRP, ESR, and ANA/RF helps characterize the underlying systemic diagnosis. X-rays of the pelvis can show the characteristic changes of established sacroiliitis — subchondral sclerosis, erosions, joint space irregularity, and ultimately fusion — but are frequently normal in the first 7–10 years of inflammatory disease, delaying diagnosis by an average of nearly a decade in some series.
An MRI of the pelvis is the gold standard for early diagnosis of sacroiliitis and is the only imaging modality capable of detecting the earliest pathological change — subchondral bone marrow edema on fluid-sensitive sequences — long before any structural changes appear on X-ray. This is the key contribution MRI makes: identifying active inflammation at a stage when disease-modifying treatment can prevent irreversible structural damage, fusion, and spinal deformity. MRI also characterizes structural lesions (erosions, sclerosis, fat metaplasia, ankylosis) and allows grading of disease severity. For mechanical and post-surgical sacroiliitis, MRI identifies the pattern and severity of joint degeneration and guides injection planning. Septic sacroiliitis on MRI demonstrates aggressive bone marrow edema, periarticular soft tissue edema, and joint fluid — prompting urgent aspiration for culture.
Classification
Sacroiliitis is classified by underlying cause, which determines the treatment approach entirely.
- Inflammatory (axial spondyloarthritis): Chronic autoimmune sacroiliitis, typically bilateral and symmetric. Often HLA-B27 positive. The cardinal feature of ankylosing spondylitis — requires systemic disease-modifying treatment with NSAIDs and biologics to prevent progressive fusion.
- Mechanical/degenerative: Typically unilateral or asymmetric. Caused by biomechanical overload, leg length discrepancy, adjacent-segment disease after lumbar fusion, or age-related degeneration. Responds to physical therapy, activity modification, and targeted SI joint injection.
- Pregnancy-related: Bilateral laxity-driven inflammation during late pregnancy and the postpartum period. Managed conservatively with pelvic support belts, physical therapy, and anti-inflammatory medications safe in the clinical context.
- Infectious (septic): Almost always unilateral. Bacterial infection requiring urgent aspiration, blood cultures, and intravenous antibiotics. Surgical drainage may be necessary for abscess formation.
- Post-traumatic or post-surgical: Follows direct pelvic injury or is an adjacent-segment consequence of lumbar spinal fusion. Managed with injections, physical therapy, and in refractory cases, SI joint fusion.
Treatments
Treatment is entirely tailored to the underlying cause — the treatment of inflammatory sacroiliitis and mechanical sacroiliitis are fundamentally different and must not be conflated.
Conservative care: Anti-inflammatory medications (NSAIDs) are first-line for both inflammatory and mechanical sacroiliitis, though they are more consistently effective in the inflammatory subtype. Physical therapy targeting pelvic and core stabilization, hip rotator and abductor strengthening, and stretching of the hip flexors and piriformis reduces biomechanical stress on the SI joint. SI joint belts can provide proprioceptive support and reduce excessive mobility during the acute phase, particularly in pregnancy-related cases. Heat before activity and ice after help manage symptoms.
Medical management of inflammatory disease.
For sacroiliitis due to ankylosing spondylitis or other spondyloarthropathies, sustained suppression of inflammation with daily NSAIDs is the foundation of treatment — continuous NSAID use (rather than as-needed dosing) has been shown to slow radiographic progression. When NSAIDs provide inadequate control, biologic agents targeting TNF-? (adalimumab, etanercept, certolizumab) or IL-17A (secukinumab, ixekizumab) are highly effective at reducing active inflammation, improving function, and preventing structural damage — including the bony fusion that ultimately disables untreated patients. Early initiation of biologics in patients with active MRI inflammation but no radiographic damage (non-radiographic axial spondyloarthritis) is increasingly supported by evidence as the optimal window for disease modification.
Injections.
Image-guided corticosteroid injection directly into the SI joint — performed under fluoroscopic or CT guidance to precisely access the irregular joint space — provides both diagnostic confirmation (pain relief confirms the SI joint as the source) and meaningful therapeutic relief lasting weeks to months. In inflammatory sacroiliitis, injections complement systemic therapy during flares. Radiofrequency ablation (RFA) of the lateral branch nerves supplying the SI joint is an effective option for chronic mechanical sacroiliac pain in patients who respond to diagnostic nerve blocks but have not benefited from conservative measures — providing longer-lasting pain relief without the need for surgery.
Surgery.
Minimally invasive SI joint fusion — using titanium implants placed percutaneously across the joint under fluoroscopic guidance — is an established and increasingly performed procedure for severe, refractory mechanical sacroiliac pain in patients who have failed comprehensive conservative care and demonstrated clear SI joint pain on diagnostic injection. It is not indicated for inflammatory sacroiliitis, where systemic biologic treatment is the appropriate intervention. Fusion reliably reduces pain and improves function in well-selected patients, with high satisfaction rates in contemporary series.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Pelvis MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.