The sacroiliac (SI) joint is the most under-diagnosed cause of low back, buttock, and one-sided pelvic pain in adults — and one of the most rewarding to treat correctly. Modal Pain Management, located at 369 Lexington Avenue in Midtown Manhattan and led by Dr. Alex Movshis, specializes in image-guided diagnosis and treatment of SI joint dysfunction and sacroiliitis. We use the same examination framework, ultrasound and fluoroscopic guidance, and four-step treatment ladder used at academic spine centers, and we deliver them in a single-physician practice where the doctor who diagnoses you is the doctor who performs your procedure. If your pain has been called “low back pain” for months or years and conventional treatment has not worked, the SI joint is one of the first things we will systematically rule in or rule out at your consultation.
SI Joint Pain Is Not Lumbar Pain: The Modern Paradigm
For most of the twentieth century, pain in the lower back and buttock was assumed to come from the lumbar spine — discs, nerve roots, and facet joints. The sacroiliac joints were dismissed as essentially immobile and incapable of generating significant pain. That paradigm is wrong, and the change in understanding has been one of the most important shifts in modern interventional pain medicine.
Cadaver and live-imaging studies have demonstrated that the SI joint moves a small but biomechanically critical amount under load — typically 1–4 mm of translation and 1–4 degrees of rotation — and that this motion is accompanied by significant shear stress on the joint surfaces and the posterior interosseous and dorsal sacroiliac ligaments. The joint is densely innervated by the L5 dorsal ramus and the lateral branches of S1, S2, and S3, and it can be a powerful pain generator when those nerve afferents are activated by inflammation, capsular distension, or ligamentous strain.
The most important consequence of this updated anatomy is clinical: in patients presenting with chronic low back pain who have not responded to standard lumbar care, the SI joint is the actual pain generator in roughly 15–30% of cases — and the prevalence climbs to 40–43% in patients who have had a previous lumbar fusion (because the rigid lumbar segment transfers more load and shear to the adjacent SI joint over time, a well-described phenomenon called adjacent-segment SI joint dysfunction). Despite this, the SI joint is rarely examined in primary care and is frequently missed even on advanced lumbar imaging because MRI of the lumbar spine often does not include adequate coronal-oblique views of the joint. The result is that patients spend months or years receiving treatment for the wrong structure.
The Modal Pain Management approach inverts the default. When a new patient arrives with low back, buttock, or one-sided pelvic pain, we systematically test for SI joint dysfunction at the consultation visit using the validated battery of provocative maneuvers (FABER, Gaenslen, distraction, compression, thigh thrust), the Fortin finger sign, and a focused neurologic exam to separate SI from lumbar disc, facet, hip, and piriformis pain. When the picture is convincing, image-guided diagnostic injection is performed at the same visit or within the following week, and the result of that diagnostic block determines the next step on the ladder.
Why It Hurts There: The Anatomy
The sacroiliac joints are paired, irregularly shaped synovial-amphiarthrodial joints between the auricular surface of the sacrum and the iliac portion of the innominate bone. Each joint is stabilized by a network of ligaments — the anterior sacroiliac ligament, the posterior interosseous ligaments (which are some of the strongest in the human body), the long posterior sacroiliac ligament, and the sacrotuberous and sacrospinous ligaments — and is acted upon by the lumbar multifidus, the thoracolumbar fascia, the gluteus maximus and medius, the biceps femoris, the latissimus dorsi (via the posterior oblique sling), and the abdominal obliques (via the anterior oblique sling).
Pain arises through several distinct mechanisms. Mechanical capsular distension occurs when the joint capsule is stretched by abnormal motion — most often after pregnancy (relaxin-mediated ligament laxity that can persist for months postpartum), a fall onto the buttock, a misstep that produces an unexpected single-leg load, or repetitive twisting in golf, tennis, hockey, and rotational sports. Ligamentous strain and instability develop when the dorsal sacroiliac and interosseous ligaments are chronically overloaded by leg-length discrepancy, pelvic obliquity, prior lumbar fusion, or post-traumatic asymmetry. Inflammatory sacroiliitis is an immune-mediated process driven by ankylosing spondylitis, psoriatic arthritis, IBD-associated arthritis, or reactive arthritis, and involves bilateral joint inflammation, bone marrow edema on MRI, and over time, ankylosis. Septic sacroiliitis is rare but is a true emergency — it presents with fever, severe one-sided buttock pain, and elevated inflammatory markers, and is most often seen in IV drug users, the immunosuppressed, and the postpartum period. Post-fusion adjacent-segment dysfunction is a slow-developing mechanical pain that emerges months to years after a multilevel lumbar fusion as the SI joint absorbs the additional load.
The pain referral pattern from the SI joint is broad and is one of the reasons it is frequently misdiagnosed. The Slipman-Fortin map, derived from confirmed-source patients, demonstrates referral into the buttock (94%), the lower lumbar region (72%), the posterior thigh (50%), the lateral thigh (28%), the groin (14%), the foot (12%), and the abdomen (2%). Pain that radiates below the knee is uncommon from the SI joint alone and should prompt a careful neurologic exam to exclude a lumbar nerve root cause.
SI Joint Pain vs. Other Causes of Low Back, Buttock, and Groin Pain
The most clinically valuable thing a pain specialist can do at the first visit is correctly identify which structure is producing the pain. The differential diagnosis for one-sided lower-back-and-buttock pain is broad, and the wrong-structure mistake is common.
Lumbar disc herniation and radiculopathy produce pain that follows a dermatomal pattern, often radiates below the knee into the calf or foot, and is accompanied by numbness, tingling, weakness, or reflex changes. Pain typically worsens with sitting and improves with walking. The straight-leg raise test is positive. SI joint pain rarely produces dermatomal symptoms or true neurologic deficits.
Lumbar facet joint pain localizes paraspinally to one or both sides of the midline at the L4–L5 and L5–S1 levels and is reproduced by extension and rotation. It does not typically refer below the gluteal fold. A diagnostic medial branch block confirms the diagnosis when at least 80% pain relief is achieved within the anesthetic window. If both lumbar facet and SI joint pain are present, sequential diagnostic blocks are used to determine the relative contribution of each.
Hip osteoarthritis and femoroacetabular impingement (FAI) present with anterior groin pain, an antalgic gait with shortened stance phase on the painful side, restricted internal rotation, and a positive FADIR (flexion, adduction, internal rotation) test. SI joint pain may also refer into the groin, but hip pathology is reproduced by hip range-of-motion testing and confirmed on AP and lateral hip radiographs. When both are present, an intra-articular hip injection clarifies the relative contribution.
Piriformis syndrome (learn more) presents with deep buttock pain that worsens with sitting and reproduces a sciatic-pattern leg pain, with a positive FAIR test (flexion, adduction, internal rotation of the hip while seated). The pain is typically deeper in the buttock than SI joint pain and is reproduced by direct palpation midway between the sacrum and the greater trochanter rather than over the joint itself.
Inflammatory sacroiliitis (ankylosing spondylitis, psoriatic arthritis, IBD-associated arthritis, reactive arthritis) presents with morning stiffness lasting more than 30–60 minutes that improves with movement, alternating buttock pain, onset before age 45, family history of spondyloarthropathy, and elevated CRP and ESR. The HLA-B27 antigen is positive in roughly 90% of ankylosing spondylitis patients. MRI demonstrates bone marrow edema in the subchondral bone of one or both joints. Rheumatology referral and disease-modifying biologic therapy is the foundation of care.
Hip labral tear produces deep groin pain, mechanical clicking or catching, and is reproduced by the FADIR test. MR arthrography is the diagnostic study of choice. Many patients have both labral and SI joint pathology.
Greater trochanteric pain syndrome / hip bursitis (learn more) presents with lateral hip pain over the greater trochanter, pain when lying on the affected side, and tenderness with single-leg stance — a different anatomic location than SI joint pain, but the two coexist commonly because both share the same underlying gluteal weakness and pelvic biomechanics.
Pelvic floor dysfunction can produce posterior pelvic and SI-region pain, especially postpartum, and is reproduced by direct internal pelvic floor examination — usually performed by a pelvic floor physical therapist.
How SI Joint Pain Is Diagnosed
There is no single test that confirms SI joint pain in isolation, and imaging alone is not diagnostic — the SI joint can be the pain generator in patients with completely normal MRI scans, and degenerative changes on imaging are extremely common in asymptomatic adults. Diagnosis is made by combining a careful history, a focused physical examination, and (when indicated) a confirmatory image-guided diagnostic block.
The Fortin finger sign is the single most useful bedside test: the patient is asked to point with one finger to the area of maximum pain. When the patient consistently points to within 1 cm of the PSIS — one finger-breadth medial and inferior — the test is highly suggestive of an SI joint origin.
The provocative test battery — distraction, compression, thigh thrust, FABER (Patrick’s), and Gaenslen — should be performed in sequence. When three or more of these five tests are positive, the post-test probability of SI joint pain is high (positive likelihood ratio approximately 4–6) and the joint should be the target of subsequent treatment. When fewer than three are positive, the SI joint is unlikely to be the dominant pain source and other structures should be explored.
A focused neurologic exam is performed to exclude lumbar radiculopathy: straight-leg raise, dermatomal sensation, motor strength of L4 (tibialis anterior), L5 (extensor hallucis longus), and S1 (gastrocnemius and FHB), and reflexes (patellar at L4, Achilles at S1).
Imaging is used to exclude other diagnoses and to screen for inflammatory sacroiliitis. AP pelvic radiograph and lumbar MRI are typically the initial studies. Dedicated SI joint MRI with coronal-oblique STIR sequences is the imaging study of choice when inflammatory sacroiliitis is suspected — it can identify bone marrow edema before structural changes appear. CT is reserved for evaluating fusion, erosion, or post-traumatic anatomy.
The diagnostic image-guided SI joint injection is the gold standard when the clinical picture is uncertain or when interventional treatment is being considered. A small volume of local anesthetic is injected into the joint under fluoroscopic or ultrasound guidance. At least 75% pain relief within the anesthetic window (1–4 hours after injection) confirms the SI joint as the dominant pain generator and predicts a meaningful response to therapeutic injection or radiofrequency ablation.
The Image-Guided Treatment Ladder
SI joint pain responds best to a stepwise, evidence-based approach. The right step depends on chronicity, on whether the pain is mechanical or inflammatory, and on the response to prior treatment. At Modal Pain Management, we walk through this ladder with each patient at the consultation visit and customize the order based on the diagnostic findings.
Step 1 — Structured physical therapy and load management. Most acute mechanical SI joint pain — pregnancy-related, post-traumatic, post-overuse — resolves with a 6–12 week course of targeted physical therapy. The evidence-based components are: pelvic alignment correction (Mulligan SNAGs and muscle energy techniques for innominate rotation), gluteus medius and maximus strengthening (single-leg bridges, side planks, monster walks), lumbar multifidus activation, deep hip external-rotator strengthening, anterior and posterior oblique sling activation, and postural and ergonomic correction. Sleep position modification (a pillow between the knees in side-lying), avoidance of prolonged single-leg standing and asymmetric loading, and a graded return-to-activity progression are essential. Pelvic support belts (sacroiliac belts) provide useful symptom relief in pregnancy-associated and acute post-traumatic SI pain. We coordinate care with experienced pelvic and orthopedic physical therapists in the Midtown area.
Step 2 — Image-guided intra-articular SI joint corticosteroid injection. When pain has been present longer than 6–8 weeks, when it interferes with sleep or work, or when physical therapy has not produced adequate improvement, image-guided intra-articular corticosteroid injection is the next step. The procedure is performed under either fluoroscopic or ultrasound guidance — image guidance is non-negotiable because the SI joint is deep, the joint cleft is narrow and obliquely oriented, and blind landmark injections enter the joint correctly only 22–55% of the time, while image-guided injections succeed >90% of the time. A long-acting corticosteroid (typically methylprednisolone or triamcinolone) is combined with a small volume of local anesthetic. The procedure takes 5–10 minutes, the patient walks in and out, and most patients return to normal activity the next day. Approximately 70–90% of correctly diagnosed patients experience clinically meaningful pain relief, with median relief lasting 3–9 months and many patients reporting durable relief beyond one year when paired with structured rehabilitation. The injection is also diagnostic — at least 75% pain relief within the anesthetic window confirms the SI joint as the dominant pain generator and predicts a strong response to subsequent radiofrequency ablation, if needed.
Step 3 — Cooled or conventional radiofrequency ablation (RFA) of the lateral branches. For patients whose pain returns within months of injection, or who have recurring episodes that always respond to repeat injection but never stay quiet, radiofrequency ablation of the L5 dorsal ramus and the lateral branches of S1, S2, and S3 dorsal rami is the next step. The procedure denervates the posterior aspect of the joint and the dorsal sacroiliac ligaments while preserving motor function and sensation in the lower limbs. Cooled RFA uses a water-cooled electrode that produces a larger spherical lesion and has the strongest randomized evidence base for SI joint pain (the SInergy trial, Cohen and colleagues, the 2012 and 2018 systematic reviews); it typically provides 9–18 months of relief in well-selected patients. Conventional thermal RFA with multi-site lesioning of the lateral branches is a reasonable alternative when cooled RFA is not available. Both procedures are performed under fluoroscopic guidance with local anesthetic and (optional) light sedation; patients walk in and out and return to normal activity within 24–72 hours. RFA can be repeated as the nerves regenerate, which typically takes 12–18 months.
Step 4 — Minimally invasive SI joint fusion referral. For patients with severe, refractory SI joint pain who have failed structured physical therapy, who have had at least temporary relief from image-guided injection (confirming the diagnosis), and who have failed lateral-branch RFA — and particularly for patients with imaging-confirmed instability or post-fusion adjacent-segment dysfunction — minimally invasive SI joint fusion is a durable option. Modern triangular titanium implant systems (iFuse, SImpact, SIBone, and similar) are placed through a small lateral incision under fluoroscopic guidance and produce immediate joint stabilization with bony fusion over 6–12 months. Multiple randomized trials (INSITE, iMIA, SIFI) demonstrate sustained pain relief in 80%+ of well-selected patients out to 5 years. We coordinate this referral with experienced minimally invasive spine surgeons in the New York City area when the indication is clear.
A parallel track for inflammatory sacroiliitis. When the clinical picture, lab work (HLA-B27, CRP, ESR), and MRI suggest inflammatory sacroiliitis (ankylosing spondylitis, psoriatic arthritis, IBD-associated arthritis), the foundation of care is rheumatology co-management with disease-modifying anti-rheumatic drugs (DMARDs), TNF-alpha inhibitors, or IL-17 inhibitors. Image-guided corticosteroid injection is still useful for symptom flares, but the underlying immune disease must be treated systemically. We refer to and co-manage with rheumatologists in the Midtown and Upper East Side area.
Physical Therapy and Self-Care
The single most important thing a patient with SI joint pain can do is build and maintain pelvic stability through targeted exercise. The exercises that matter most are the ones that activate the gluteus medius and maximus (which control single-leg stance and pelvic drop), the lumbar multifidus (which provides segmental spinal stability), and the deep hip external rotators (which control the position of the femur in the acetabulum). A typical evidence-based program builds from isometric activation to isotonic strengthening to functional, sport-specific loading over 8–12 weeks.
Specific home strategies include: sleeping side-lying with a pillow between the knees to maintain neutral pelvic alignment, avoiding prolonged single-leg standing (no “hip-hanging” while waiting in line), avoiding sitting cross-legged for long periods, sitting on a wedge cushion or sit-stand desk to break up prolonged sitting, addressing leg-length discrepancy with a heel lift if confirmed by exam, and using a sacroiliac support belt during pregnancy or for the first weeks of an acute flare.
Heat is generally more useful than ice for chronic SI pain because it relaxes the surrounding paraspinal and gluteal musculature; ice is more useful in the first 48 hours after a clear acute injury. NSAIDs can be useful for short courses but should not become a long-term strategy because of well-documented gastric, renal, and cardiovascular risks with chronic use.
When SI Joint Pain Is an Emergency
While the vast majority of SI joint pain is not dangerous, several presentations require same-day or emergency-department evaluation:
- Fever with severe one-sided sacroiliac pain — concerning for septic sacroiliitis, particularly in IV drug users, the immunosuppressed, the postpartum period, or after a recent invasive procedure. Inflammatory markers (CRP, ESR), blood cultures, and dedicated SI joint MRI are urgent.
- New bowel or bladder dysfunction or saddle (perianal) numbness — concerning for cauda equina syndrome. Emergency department immediately.
- Unexplained weight loss with night pain that is unrelieved by position changes — concerning for malignancy. Urgent imaging and oncologic workup.
- Pain accompanied by inflammatory eye symptoms (uveitis), psoriasis, IBD, or a family history of ankylosing spondylitis — strongly suggestive of inflammatory spondyloarthropathy. Same-week rheumatology evaluation.
- Severe, sudden, post-traumatic pelvic pain — exclude pelvic ring fracture, particularly in osteoporotic patients. Emergency pelvic imaging.
Why Modal Pain Management for SI Joint Pain in NYC
We do three things differently. First, we examine for the SI joint on every patient who presents with low back, buttock, or one-sided pelvic pain — most practices do not. The five-test provocative battery, the Fortin finger sign, and the focused neurologic exam are completed at the consultation visit so that the right pain generator is identified before any procedure is offered. Second, we use image guidance for every SI joint injection — fluoroscopic or ultrasound — because the published accuracy of blind landmark injections is poor and image guidance is what separates a diagnostic and therapeutic procedure from a coin flip. Third, the doctor who diagnoses you is the doctor who performs your procedure. In a single-physician practice, there is no handoff to a fellow or a rotating proceduralist; Dr. Movshis personally performs every consultation, every injection, and every radiofrequency ablation.
Same-week consultation appointments are typically available at 369 Lexington Avenue in Midtown Manhattan. Image-guided injection, when indicated, is usually scheduled within 1–2 weeks at the same office. Most major insurance is accepted; benefits verification is free and is performed before your visit.
