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SI Joint Pain

A Patient Guide to Anatomy, Causes, Diagnosis, and Treatment Options

In This Guide

  • What SI joint pain is and how common it is
  • The anatomy and nerve supply of the SI joint
  • Causes and risk factors
  • Why SI joint pain is hard to diagnose
  • SI joint pain versus lumbar facet pain
  • Natural course and non-surgical treatment
  • When further procedures may help

What Is Sacroiliac (SI) Joint Pain?

The sacroiliac (SI) joints connect your sacrum, the triangular bone at the base of your spine, to the ilium, the large wing-shaped bone of your pelvis. You have two SI joints, one on each side of your lower back, just below your beltline. These joints carry the weight of your upper body down into your legs every time you stand, walk, or shift your weight. When an SI joint becomes irritated, unstable, or inflamed, it can cause deep, aching pain in the low back, buttock, and sometimes the back of the thigh.

SI joint pain is more common than many patients realize. Estimates vary by study, but sacroiliac joint dysfunction is thought to be the primary source of pain in roughly 15 percent to 30 percent of people with chronic low back pain, with several large reviews converging on a figure near 25 percent in adults with ongoing low back pain (NCBI StatPearls). Among patients who have already had lumbar spine surgery and still have pain, the SI joint is an even more common culprit, found in up to 40 percent of these cases in some series (PMC review, A Closer Look into the Association between the Sacroiliac Joint and Low Back Pain).

The Anatomy: Sacrum, Ilium, and the SI Joint's Unusual Design

The SI joint is unlike most joints in your body. It is not a single simple hinge. Instead, it has two connected parts working together. The lower, front portion of the joint is a true synovial joint, meaning the sacrum and ilium bones are covered with cartilage and separated by a thin layer of lubricating fluid, allowing a small amount of gliding motion. The upper, back portion of the joint is mostly ligamentous, meaning the bones are held together directly by strong bands of connective tissue rather than gliding cartilage surfaces (PMC, The sacroiliac joint: an overview of its anatomy, function and potential clinical implications).

The Posterior Ligamentous Complex

Behind the joint sits a dense network of connective tissue called the posterior ligamentous complex, anchored by the interosseous sacroiliac ligament, one of the strongest ligaments in the body. This complex is what gives the SI joint most of its stability. It resists the shearing and twisting forces that occur every time you take a step, and it is richly supplied with nerve endings, which is why injury or strain here can be a significant source of pain (PMC, sacroiliac joint anatomy review).

How the SI Joint Is Wired for Pain

The back of the SI joint and its ligaments receive their nerve supply mainly from the lateral branches of the S1 through S3 dorsal rami, small nerve branches that come off the sacral spinal nerves, with an additional contribution from the L5 dorsal ramus. This innervation pattern is the anatomic basis for both diagnostic nerve blocks and radiofrequency treatments aimed at this joint, and it is also why SI joint irritation can feel similar to a lumbar spine problem to the nervous system (PMC, Diagnosis and interventional pain management options for sacroiliac joint pain).

  • Sacrum: the triangular bone formed by fused vertebrae at the base of the spine; it forms the inner, sacral half of the SI joint.
  • Ilium: the large wing-shaped pelvic bone; it forms the outer, iliac half of the SI joint.
  • Synovial portion: the lower, front section of the joint, lined with cartilage and capable of small gliding movements.
  • Ligamentous (fibrous) portion: the upper, back section, held together by dense connective tissue rather than a gliding joint surface.
  • Nerve supply: lateral branches of the S1 to S3 dorsal rami, plus a contribution from the L5 dorsal ramus.

What Causes SI Joint Pain?

SI joint pain generally falls into two broad categories: mechanical dysfunction, where the joint's normal motion and load transfer are disrupted, and inflammatory sacroiliitis, where the joint itself is inflamed as part of a systemic disease. Distinguishing between these two is important because the treatment approach is different. Mechanical SI joint dysfunction is treated with the therapies described in this guide, while inflammatory sacroiliitis, often linked to conditions such as ankylosing spondylitis or other spondyloarthropathies, is primarily managed by a rheumatologist with disease-modifying medications, sometimes alongside orthopedic care (NCBI StatPearls).

Common Causes and Risk Factors

  • Trauma: a fall onto the buttocks, a motor vehicle collision, or a heavy, awkward lift can injure the SI joint ligaments directly (AAFP, Sacroiliac Joint Dysfunction: Diagnosis and Treatment).
  • Pregnancy and the postpartum period: hormonal changes loosen the SI joint ligaments to allow the pelvis to widen for childbirth. This normal laxity can lead to instability and pain, and pelvic girdle pain affects roughly 1 in 5 pregnant women; those with uneven (asymmetric) SI joint laxity during pregnancy have about three times the risk of pain that persists after delivery (PMC, sacroiliac joint anatomy and clinical implications review).
  • Prior lumbar spine fusion: fusing lumbar segments changes how load and motion are transferred down the spine, placing extra stress on the SI joints below the fusion. Reported rates of new or worsening SI joint pain after lumbar fusion vary widely across studies, roughly 6 percent to 75 percent, and risk tends to rise with the number of levels fused and when the fusion extends down to the sacrum (Springer, Incidence, diagnosis and management of sacroiliitis after spinal surgery).
  • Leg length discrepancy: an uneven leg length or gait abnormality causes the pelvis to bear weight asymmetrically with every step, which over time can overload one SI joint (PMC, Minimally Invasive and Conservative Interventions for the Treatment of Sacroiliac Joint Pain).
  • Female sex and joint mobility: women tend to have more SI joint mobility than men, an adaptation that helps with childbirth but can also make the joint more prone to instability and pain (AAFP; NCBI StatPearls).
  • Degenerative and inflammatory arthritis: osteoarthritis and inflammatory spondyloarthropathies, including ankylosing spondylitis, can damage the joint surfaces over time.
  • Repetitive loading activities: sports and activities with repeated twisting or shear forces on the pelvis, such as football, gymnastics, and golf, can gradually strain the joint (AAFP).

Inflammatory Sacroiliitis vs. Mechanical SI Joint Dysfunction

It is worth pausing on this distinction because the two conditions are treated very differently. Inflammatory sacroiliitis is a true inflammation of the joint, often part of a systemic autoimmune or rheumatologic disease, and it typically shows up on imaging as erosion or fusion of the joint over time. Mechanical SI joint dysfunction, sometimes called SI joint syndrome, refers to pain from abnormal motion, instability, or strain of an otherwise structurally normal joint, and it often does not show dramatic changes on imaging at all. Most patients seen in a spine surgery practice for SI joint pain have the mechanical type, but your surgeon will screen for signs pointing toward an inflammatory cause, since that changes the treatment plan (NCBI StatPearls).

Why SI Joint Pain Is Difficult to Diagnose

SI joint pain has a reputation among clinicians as one of the trickier diagnoses in spine care, for several honest reasons.

  • The referral pattern overlaps with other conditions: SI joint pain classically causes a deep ache in the buttock that can radiate into the back of the thigh, and sometimes into the groin or lower leg. This pattern looks a lot like lumbar facet joint pain or even a pinched nerve (lumbar radiculopathy), so history and exam alone often cannot tell them apart (Pain Medicine Review, Axial Low Back Pain: A Review of Facet and Sacroiliac Joint Syndromes; PMC, ASPN Best Practice Guideline).
  • No single exam maneuver is reliable on its own: Individual physical exam tests for the SI joint, such as the thigh thrust, compression, distraction, Gaenslen's, and Patrick's tests, each have only modest accuracy when used alone. A systematic review found that combining three or more positive provocative tests raised sensitivity to about 81 percent and specificity to about 68 percent, but no single isolated test performed reliably by itself (PMC, Accuracy of the Diagnostic Tests of Sacroiliac Joint Dysfunction).
  • Imaging is often unhelpful: X-rays, CT, and MRI frequently look normal in mechanical SI joint dysfunction, or show incidental degenerative changes that do not correlate with pain. One CT review found signs of SI joint degeneration in about 65 percent of people who had no low back pain at all, meaning imaging findings alone cannot confirm the joint as the pain source (PMC, ASPN Best Practice Guideline for the Treatment of Sacroiliac Disorders).
  • A controlled diagnostic injection is often needed to confirm the diagnosis: Because history, exam, and imaging each fall short individually, the accepted way to confirm the SI joint as the true pain generator is a fluoroscopically or CT-guided diagnostic injection of numbing medicine directly into the joint. A response of 75 percent or greater pain relief is generally used as a positive result, though some protocols use a lower threshold; using two blocks on separate days with different anesthetics increases confidence and reduces false positives (PMC, Diagnosis and interventional pain management options for sacroiliac joint pain).

SI Joint Pain vs. Lumbar Facet Joint Pain

Because both conditions can cause low back pain that spreads into the buttock and thigh, distinguishing SI joint pain from lumbar facet joint pain is one of the most common diagnostic challenges in spine care. The table below summarizes the key differences clinicians look for.

FeatureSI Joint PainLumbar Facet Joint Pain
Typical pain locationUsually one-sided, centered below the L5 level, often felt as a deep ache over the back of the pelvis near the posterior superior iliac spineUsually centered in the low back itself, at or above the L5 level, and can be on one or both sides
Referral patternButtock and posterior thigh most common; sometimes groin or lower leg, rarely below the kneeMay refer to the buttock, hip, or posterior thigh, but rarely extends below the knee
Aggravating activitiesTransitional movements: standing up from sitting, climbing stairs, rolling over in bed, prolonged sitting on the affected sideExtension and twisting of the low back, prolonged standing; often eased by sitting or bending forward
Notable exam findingFortin finger test (patient points with one finger directly over the joint); positive cluster of 3 or more provocative maneuversNo single reliable exam finding; pain reproduced with lumbar extension and rotation (extension-loading, such as Kemp's sign)
Imaging findingsOften normal; degenerative changes are common even in people without pain and do not reliably confirm the diagnosisMay show facet joint arthritis or hypertrophy on CT or MRI, but findings correlate poorly with pain and are common in pain-free adults
Confirmatory diagnostic blockIntra-articular SI joint injection with local anesthetic; positive result generally defined as 75 percent or greater pain reliefMedial branch nerve block; positive result generally defined as 80 percent or greater pain relief, ideally confirmed with two blocks on separate days

In practice, these two conditions can overlap or coexist in the same patient, which is exactly why a careful history, targeted exam, and, when needed, a controlled diagnostic block are so important before deciding on treatment (Pain Medicine Review; PMC, ASPN Best Practice Guideline).

When to Seek Care Right Away

  • New numbness, weakness, or loss of feeling in one or both legs
  • Loss of control of your bladder or bowels, or numbness in the saddle area (between your legs), which can signal a surgical emergency
  • Fever, chills, or unexplained weight loss along with your back or pelvic pain, which can point to an infection or other serious illness
  • Severe pain that began after a significant fall, motor vehicle collision, or other major trauma, especially if you cannot bear weight
  • Pain that steadily worsens at night or does not improve at all with rest or position changes

Natural History: What Happens Without Treatment

The good news is that, like most causes of acute back pain, acute SI joint pain resolves in the majority of patients with simple conservative care and time (PMC, ASPN Best Practice Guideline for the Treatment of Sacroiliac Disorders). Pain that has been present for a shorter time, and pain following a clear inciting event such as a fall or childbirth, tends to respond best to conservative measures. Pain that becomes chronic, however, is harder to resolve on its own and is associated with meaningfully reduced quality of life; some studies have found that patients with chronic SI joint pain report quality-of-life scores among the lowest of any chronic disease category studied (PMC, Minimally Invasive and Conservative Interventions for the Treatment of Sacroiliac Joint Pain).

Evidence-Based Treatment Options

Treatment for SI joint pain follows a stepwise approach, starting with the least invasive options and moving toward more targeted interventions only if needed.

Step 1: Conservative (Non-Surgical) Care

  • Activity modification: avoiding movements and positions that clearly provoke pain, especially in the first several days after a flare, while staying as active as tolerated overall.
  • Physical therapy: a program focused on pelvic and core stabilization, correcting muscle imbalances, and restoring normal movement patterns; in one randomized trial cited in current guidelines, structured manual therapy produced a favorable response in roughly 72 percent of patients.
  • NSAIDs and other medications: anti-inflammatory medications are commonly used to reduce pain and inflammation; muscle relaxants and topical agents may play a supporting role for some patients.
  • SI belt (pelvic support belt): a snug belt worn around the pelvis that limits excessive SI joint motion and can reduce pain during daily activity, particularly during pregnancy and the postpartum period.

Current clinical guidelines recommend optimizing these conservative measures, typically over a period of weeks, before moving on to interventional treatment (PMC, ASPN Best Practice Guideline for the Treatment of Sacroiliac Disorders).

Step 2: Interventional / Procedural Treatment

When conservative care is not enough, image-guided SI joint injections and, for select patients, radiofrequency ablation of the nerves supplying the joint (targeting the same S1 to S3 lateral branches and L5 dorsal ramus described earlier) are the next step. These procedures serve both a diagnostic and a therapeutic purpose. NextEra Spine Care covers these interventional treatments in full detail, including candidacy, what to expect, and recovery, in a separate patient guide dedicated to SI joint injections and radiofrequency ablation.

A Further Option for Select Patients

For patients with confirmed SI joint pain who do not get lasting relief from conservative care and interventional treatments, minimally invasive SI joint fusion surgery is an additional option that stabilizes the joint directly. This is a more involved decision with its own set of candidacy criteria, and it is covered in a separate NextEra Spine Care guide dedicated to SI joint fusion.

Bottom Line

SI joint pain is a common, often underdiagnosed cause of low back and buttock pain that can mimic other spine conditions, especially lumbar facet joint pain and nerve-related pain. Getting the diagnosis right, often with the help of a controlled diagnostic injection, matters because it points toward the treatment most likely to help you. Most patients improve substantially with conservative care alone, and effective interventional and surgical options exist for those who need more. If you have persistent low back or buttock pain that has not responded to initial treatment, Dr. Waldrop and the NextEra Spine Care team can help determine whether your SI joint is the source and build the right treatment plan for you.

Glossary of Medical Terms

TermWhat It Means
Sacroiliac (SI) jointThe joint connecting the sacrum (base of the spine) to the ilium (pelvic bone), one on each side of the lower back.
SacrumThe triangular bone formed by fused vertebrae at the very bottom of the spine, above the tailbone.
IliumThe large, wing-shaped bone that forms the upper part of the pelvis.
Synovial jointA joint with a fluid-filled space and cartilage-covered surfaces that allow gliding motion.
Posterior ligamentous complexThe dense network of ligaments behind the SI joint, including the interosseous sacroiliac ligament, that provides most of the joint's stability.
Dorsal ramus (plural: rami)A branch of a spinal nerve that supplies the muscles, joints, and skin of the back.
Lateral branchA smaller nerve branch off the dorsal ramus that specifically supplies sensation to the SI joint and posterior ligaments.
SacroiliitisInflammation of the SI joint, often related to an autoimmune or rheumatologic disease such as ankylosing spondylitis.
Mechanical SI joint dysfunctionPain from abnormal motion, instability, or strain in an SI joint without primary inflammatory disease.
SpondyloarthropathyA family of inflammatory diseases, including ankylosing spondylitis, that can affect the spine and SI joints.
Pelvic girdle painPain around the pelvis, including the SI joints and pubic symphysis, common during and after pregnancy.
Leg length discrepancyA difference in length between the two legs that can cause uneven loading of the pelvis and SI joints.
Lumbar facet jointOne of the small paired joints at the back of each spinal vertebra that helps guide and limit spine motion.
Provocative maneuverA physical exam test designed to reproduce a patient's pain by stressing a specific joint or structure.
Fortin finger testA physical exam test in which the patient points with one finger directly over the painful SI joint area.
Diagnostic blockAn injection of numbing medicine into or near a specific joint or nerve, used to confirm it as the source of pain.
Medial branch blockA diagnostic injection near the small nerve branches that supply a lumbar facet joint.
Radiofrequency ablationA procedure that uses heat generated by radio waves to disable specific pain-carrying nerves.
Lumbar fusionA surgery that permanently joins two or more vertebrae in the lower back to eliminate motion between them.
SI beltA supportive belt worn around the pelvis to reduce excess motion at the SI joints.

Sources

This handout is based on the following evidence-based, peer-reviewed, and professional-society sources:

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