Doctors diagnose sacroiliac joint pain from nine primary causes: osteoarthritis and degenerative changes, ankylosing spondylitis, trauma and injury, pregnancy and childbirth, muscle imbalances, leg length discrepancy, scoliosis, gait abnormalities, and rare infections. For example, a person might develop sacroiliac joint pain after a motor vehicle collision or a heavy lifting injury at work, but another person might experience the same pain from simple muscle imbalances that worsen their gait over years of sitting at a desk. Understanding which cause is driving your pain matters significantly because it determines treatment approach—a muscle imbalance needs different management than an autoimmune condition like ankylosing spondylitis.
The sacroiliac joint connects your spine to your pelvis and bears much of your body’s weight during movement. When something goes wrong in this joint, pain often radiates to the lower back, buttocks, or thighs. This article explains all nine causes doctors diagnose most frequently, how they identify which cause is behind your pain, and what this means for your next steps toward relief.
Table of Contents
- How Osteoarthritis and Degenerative Changes Damage the Sacroiliac Joint
- Ankylosing Spondylitis and Inflammatory Causes of Sacroiliac Joint Pain
- Structural and Anatomical Causes: Leg Length Discrepancy and Scoliosis
- Trauma and Injury as Direct Causes of Sacroiliac Joint Pain
- Muscle Imbalances and Movement Dysfunction
- Childbirth and Pregnancy-Related Sacroiliac Joint Pain
- Rare Infections and Advanced Causes
- Conclusion
How Osteoarthritis and Degenerative Changes Damage the Sacroiliac Joint
The most common cause of sacroiliac joint pain is osteoarthritis and degenerative changes in the joint itself. This happens when the cartilage that cushions the joint wears away over time, causing bone-on-bone friction and inflammation. The process is similar to what happens in other joints throughout the body, but the sacroiliac joint experiences particular stress because of the heavy loads it carries during walking, standing, and bending. Osteoarthritis develops gradually, often without a clear trigger moment. However, one notable pathway to sacroiliac joint osteoarthritis is post-surgical degeneration following lumbar fusion surgery on the lower spine.
When the spine is fused in one location, the sacroiliac joint below must absorb additional movement and stress, accelerating wear-and-tear changes. This means someone who underwent spine fusion years earlier might develop sacroiliac pain as a delayed consequence of their surgery. The challenge with degenerative changes is that they’re visible on imaging but don’t always correlate with pain levels. Some people with severe degenerative changes on X-ray or MRI experience minimal pain, while others with mild degeneration suffer significantly. This mismatch means imaging findings alone cannot confirm sacroiliac joint pain as the source of your symptoms.
Ankylosing Spondylitis and Inflammatory Causes of Sacroiliac Joint Pain
Ankylosing spondylitis (AS) is an inflammatory autoimmune disease that specifically targets the sacroiliac joint, causing a condition called sacroiliitis. Unlike osteoarthritis, which develops from wear and tear, AS is caused by the immune system mistakenly attacking the joint tissues. The inflammation spreads through the spine and other joints over time if left untreated, and it typically strikes people in their 20s, 30s, and 40s—much younger than typical osteoarthritis. Ankylosing spondylitis is more common in people with a genetic marker called HLA-B27, but the presence of this gene doesn’t guarantee development of the disease.
People with AS often report morning stiffness that improves with movement and exercise, which distinguishes it from mechanical sacroiliac pain that typically worsens with activity. However, if you have AS, your sacroiliac joint pain won’t improve simply with rest or physical therapy because the underlying cause is immune-mediated inflammation rather than a structural problem that rehab can fix. Treatment for AS-related sacroiliac pain typically involves disease-modifying antirheumatic drugs (DMARDs) or biologic medications that suppress the immune response. Diagnosis requires blood tests for inflammatory markers and HLA-B27, not just imaging, which is why rheumatologists rather than orthopedic surgeons often manage this condition.
Structural and Anatomical Causes: Leg Length Discrepancy and Scoliosis
Some people develop sacroiliac joint pain due to structural differences in their body that create uneven weight distribution. Leg length discrepancy—where one leg is functionally or structurally shorter than the other—forces the pelvis to tilt unevenly during walking and standing. Over months or years, this asymmetrical loading strains the sacroiliac joint on the longer-leg side, triggering pain and dysfunction. Leg length discrepancy can be measured clinically through physical examination or confirmed with imaging. The important distinction is that discrepancies smaller than half an inch often don’t cause problems, whereas larger differences frequently lead to sacroiliac pain.
Some people correct this with a shoe lift inserted in the shoe of the shorter leg to rebalance pelvic tilt. However, shoe lifts don’t work for everyone—if muscle imbalances or poor movement patterns are also contributing to your pain, the lift alone won’t resolve the problem. Scoliosis, an abnormal curvature of the spine, can also affect sacroiliac joint alignment and stress distribution. The spine curves sideways, which changes how weight flows through the pelvis to the sacroiliac joints. People with scoliosis-related sacroiliac pain often benefit from targeted exercises that strengthen stabilizer muscles around the joint, though the underlying curve cannot be reversed in adults.
Trauma and Injury as Direct Causes of Sacroiliac Joint Pain
Traumatic injuries are a straightforward but significant cause of sacroiliac joint pain. Heavy lifting—especially lifting awkwardly or with poor form—can strain the joint ligaments and surrounding muscles. Falls directly onto the buttocks, motor vehicle collisions, and other high-impact injuries can fracture the joint, dislocate it, or damage the ligaments that stabilize it. For example, someone might feel sharp pain immediately after a fall while hiking, or pain might develop gradually over days following a car accident as inflammation develops.
The distinction between traumatic sacroiliac pain and degenerative pain is that traumatic pain typically has a clear onset moment and often improves with conservative treatment like rest, ice, compression, and physical therapy. However, if an injury damages multiple ligaments or causes subtle fractures, healing takes months and rehab is crucial to prevent chronic instability. One important limitation: not all sacroiliac pain following trauma is purely mechanical. The injury itself triggers inflammation, and this inflammatory response can persist even after the structural damage heals. This explains why some people with sacroiliac injuries don’t fully recover with physical therapy alone and may need additional interventions like SI joint belts, injections, or other treatments.
Muscle Imbalances and Movement Dysfunction
Among all causes of sacroiliac joint pain, muscle imbalances and movement dysfunction are incredibly common yet often overlooked during initial evaluation. The muscles that stabilize the sacroiliac joint include the gluteus maximus, deep core muscles, and muscles in the hip and lower back. When these muscles are weak, overactive on one side, or unconditioned, they fail to stabilize the joint properly. The joint then moves excessively during normal activities, triggering pain and inflammation. Poor posture is a major contributor to muscle imbalances. Sitting for eight hours daily at a desk weakens glute muscles while tightening hip flexors—this exact pattern destabilizes the sacroiliac joint.
Over weeks and months, the repeated micro-movements in the unstable joint cause pain. Physical therapy targeting specific muscle groups—particularly the gluteus medius, gluteus maximus, and deep core stabilizers—can often resolve this type of pain completely without any imaging or injections. Gait abnormalities, meaning altered walking patterns, also fall under movement dysfunction. An abnormal gait can result from an Achilles tendon rupture, an anterior cruciate ligament (ACL) injury in the knee, ankle problems, or even old habits developed to avoid pain elsewhere. Your body compensates by walking differently, which changes how forces flow through the sacroiliac joint. However, fixing the original gait problem—through physical therapy, orthotic devices, or appropriate footwear—often resolves the secondary sacroiliac pain without directly treating the joint itself.
Childbirth and Pregnancy-Related Sacroiliac Joint Pain
Pregnancy is a unique cause of sacroiliac joint pain that affects a significant percentage of pregnant and postpartum women. During pregnancy, hormonal changes (particularly relaxin) loosen ligaments throughout the body to allow the pelvis to expand for childbirth. These same hormone-induced changes affect the sacroiliac joint ligaments, reducing their stiffness and stability. Additionally, the increasing weight gain and changed posture shift the center of gravity forward, increasing stress on the sacroiliac joint.
Natural childbirth itself can injure or strain the sacroiliac joint, and some women report the onset of pain immediately after delivery. The good news is that pregnancy-related sacroiliac pain often resolves within weeks to months postpartum as hormones normalize and ligaments tighten again. However, if it doesn’t resolve, the pain can persist for years, particularly if the woman engages in heavy lifting or activities that stress the joint during the postpartum period. For pregnant or postpartum women with sacroiliac pain, management typically focuses on pelvic support belts, physical therapy to strengthen stabilizer muscles, and modifications to daily activities. Medications are limited during pregnancy, so non-pharmacological approaches are essential.
Rare Infections and Advanced Causes
Infection of the sacroiliac joint, though rare, is a serious cause of sacroiliac pain that doctors must not miss. Bacterial infections, typically caused by Staphylococcus aureus, can reach the sacroiliac joint through the bloodstream or direct inoculation. Sacroiliac joint infections cause severe pain, fever, and systemic illness, and they progress rapidly if untreated. Diagnosis requires blood cultures, imaging showing joint destruction, and sometimes joint aspiration (withdrawing fluid from the joint for analysis).
The critical distinction is that infectious sacroiliitis requires antibiotics and sometimes surgical drainage, whereas the other causes typically don’t. Missing an infection while treating pain with physical therapy or injections can allow it to worsen. Red flags for infection include severe, unrelenting pain, fever, chills, night sweats, or rapid onset of symptoms in a person with a recent infection elsewhere in their body. Advanced causes also include rare conditions such as sacroiliac joint tuberculosis in endemic areas and inflammatory bowel disease-associated sacroiliitis. These require specialized diagnosis and management, but they account for only a tiny fraction of sacroiliac pain cases.
Conclusion
Sacroiliac joint pain has nine primary causes that doctors diagnose through physical examination, imaging, and sometimes specialized tests like joint injections. The cause matters greatly because it determines whether your treatment should focus on strengthening muscles, taking medications to suppress inflammation, correcting structural issues, recovering from trauma, or identifying and treating a rare infection.
Many cases of sacroiliac pain improve substantially with targeted physical therapy and lifestyle modifications, while others require medical or surgical intervention. If you’re experiencing sacroiliac joint pain, the first step is evaluation by a healthcare provider who can identify which cause is driving your symptoms. Once the underlying cause is clear, a treatment plan tailored to that specific problem offers the best chance of recovery and long-term relief.
