What Causes Sacroiliac Joint Pain? A Closer Look

The sacroiliac joint connects your sacrum — the triangular bone at the base of your spine — to the ilium, the large bone on each side of your pelvis. When this joint becomes irritated or moves too much or too little, it produces pain that many people describe as a deep ache on one side of the lower back or buttock. The causes range from a sudden injury to years of uneven wear and tear, and for many people, the exact trigger is never fully identified.

What Is the Sacroiliac Joint and What Does It Do?

The sacroiliac joint is one of the largest joints in the body, but it does not move much. It is designed for stability, not mobility. Its job is to transfer the weight of your upper body to your legs and to absorb shock when you walk, run, or jump.

Strong ligaments hold the joint together. Muscles in the buttock, hip, and lower back support it from the outside. The joint has a small amount of cartilage and a thin layer of fluid, but it is far less flexible than a knee or shoulder.

Because this joint sits deep inside the pelvis, pain from it is often mistaken for pain from the lumbar spine or the hip. This misdiagnosis is common and can delay proper treatment.

What Causes Sacroiliac Joint Pain?

Sacroiliac joint pain usually starts with one of three patterns: a single traumatic event, a repetitive strain over time, or a change in how the joint loads weight. In some cases, no clear cause can be found.

Trauma is a leading cause. A fall onto the buttock, a car accident, or a sudden heavy lift can stretch or tear the ligaments around the joint. The joint may then become unstable, and the pain can persist for months or years.

Repetitive strain is another common cause. Activities that involve uneven leg motion — such as running on a slanted road, stepping off a curb repeatedly, or doing lunges with poor form — can irritate the joint over time. Prolonged sitting with poor posture can also contribute.

Changes in gait are a major and often overlooked cause. When you limp because of a knee, foot, or hip problem, your sacroiliac joint takes on uneven stress with every step. This can inflame the joint even though the original problem was elsewhere.

How Pregnancy Affects the Sacroiliac Joint

Pregnancy is one of the most common times for sacroiliac joint pain to develop. The hormone relaxin softens ligaments throughout the pelvis in preparation for childbirth. This natural process also makes the sacroiliac joint less stable.

As the baby grows, the center of gravity shifts forward. The lower back and pelvis must work harder to maintain balance, and the sacroiliac joint absorbs much of that added load. Many women notice pain in the back of the pelvis during the second half of pregnancy.

For most women, the pain resolves within a few months after delivery. Research suggests that some women continue to have pelvic girdle pain for longer, and a small number develop chronic sacroiliac joint dysfunction. No clinical guidelines currently guarantee a specific timeline for recovery because individual outcomes vary widely.

Arthritis and Inflammatory Conditions

Arthritis can affect the sacroiliac joint just as it affects other joints in the body. Osteoarthritis from age-related wear and tear is one possibility, though it is less common in the sacroiliac joint than in the knees or hands.

Ankylosing spondylitis is a type of inflammatory arthritis that frequently involves the sacroiliac joints. In fact, inflammation of the sacroiliac joints is often one of the earliest signs of this condition. The pain tends to be worse in the morning, improves with movement, and may wake a person from sleep in the second half of the night.

Other inflammatory conditions, including psoriatic arthritis and reactive arthritis, can also cause sacroiliac joint inflammation. If your pain is accompanied by morning stiffness lasting more than 30 minutes, or if you have a family history of inflammatory arthritis, mention this to your doctor.

Leg Length Difference and Gait Problems

A difference in leg length — even a small one — can change how your sacroiliac joints load weight. When one leg is shorter, the pelvis tilts on that side. The sacroiliac joint on the longer side may become compressed, while the one on the shorter side may become stretched.

This does not mean everyone with a leg length difference develops sacroiliac pain. Many people have a small difference and never notice it. But when the difference is more than about half an inch, the risk of pelvic and lower back problems increases.

Gait problems from foot conditions can have a similar effect. Flat feet, overpronation, or a poorly healed ankle fracture can change how force travels up the leg and into the pelvis. The sacroiliac joint then works harder than it was designed to.

Sacroiliac Joint Dysfunction vs. SI Joint Inflammation

These two terms are often used interchangeably, but they describe different problems. Sacroiliac joint dysfunction means the joint moves abnormally — either too much or too little. Inflammation means the joint is swollen and irritated, regardless of how it moves.

In practice, the two often occur together. An unstable joint can become inflamed. An inflamed joint can alter movement patterns. Treatment usually addresses both issues at the same time.

Physical examination findings can help tell them apart. Pain that worsens when you stand on one leg or climb stairs suggests a mechanical problem. Pain that is constant, worse at night, or accompanied by stiffness suggests inflammation. Imaging such as MRI can show active inflammation in the joint, but it cannot directly measure joint motion.

How Is Sacroiliac Joint Pain Diagnosed?

Diagnosis of sacroiliac joint pain is challenging because the symptoms overlap heavily with other conditions. A herniated lumbar disc, sciatica, and hip bursitis can all produce pain in the same general area.

Doctors typically start with a physical exam. Several specific maneuvers — such as pressing on the joint, spreading the legs against resistance, or pulling the knee toward the opposite shoulder — can reproduce the pain. If three or more of these tests are positive, the sacroiliac joint is a likely source.

A diagnostic injection is often the most reliable way to confirm the diagnosis. A doctor injects a small amount of numbing medication into the joint under X-ray guidance. If your pain drops by 75 percent or more for a few hours, the sacroiliac joint is almost certainly the source. Some clinicians use this test routinely; others reserve it for unclear cases.

Treatment Options and What the Evidence Shows

Treatment for sacroiliac joint pain usually starts with physical therapy. The goal is to strengthen the muscles that support the joint, particularly the gluteal muscles and the deep core muscles. Manual therapy from a physical therapist or chiropractor may help restore normal joint motion.

Some research suggests that physical therapy improves symptoms for many people, but the evidence is not uniform. Results vary widely from person to person. What works for one individual may not work for another.

A sacroiliac belt — a firm strap worn around the pelvis — can provide temporary relief by compressing the joint and reducing motion. It is a supportive tool, not a cure. Most people use it for short periods during activities that aggravate the pain.

Nonsteroidal anti-inflammatory drugs such as ibuprofen or naproxen can reduce pain and swelling for short-term use. They do not fix the underlying problem, and long-term use carries risks to the stomach and kidneys.

For persistent pain, some doctors offer corticosteroid injections into the joint. These can provide relief for weeks to months, but the effect is temporary. No large human trials have confirmed that injections change the long-term course of the condition.

Radiofrequency ablation is a procedure that uses heat to disable the nerves that carry pain signals from the joint. Some studies show benefit, but the evidence is mixed and the effect may wear off after several months. Surgery to fuse the joint is reserved for severe cases that have not responded to any other treatment.

When to See a Doctor

See a doctor if your pain is severe, lasts longer than a few weeks, or interferes with daily activities. Seek urgent care if the pain follows a fall or accident, if you lose control of your bladder or bowels, or if you have numbness in the groin area. These symptoms can indicate a more serious problem.

Fevers, unexplained weight loss, or pain that is worse at night and not relieved by changing position should also be evaluated promptly. These can be signs of infection or inflammatory disease that requires specific treatment.