Where is Si Joint Pain Felt? Everything You Need to Know

The sacroiliac joint, often called the SI joint, connects your spine to your pelvis. When this joint becomes irritated, the pain is most often felt in the lower back and buttock on one side. Many people describe it as a deep, dull ache that can also shoot down the back of the thigh, sometimes into the groin or hip. Unlike typical low back pain, SI joint pain rarely travels below the knee.

What Exactly Is the SI Joint and Where Is It Located?

The sacroiliac joint sits between the sacrum and the ilium bones. The sacrum is the triangular bone at the base of your spine. The ilium is the large, wing-shaped bone on each side of your pelvis. You have two SI joints — one on the left and one on the right.

These joints are small but built for strength. Their main job is to transfer weight and forces between your upper body and your legs. They move very little — only a few millimeters. But when that small movement becomes too much or too little, pain can start.

The SI joint is located right above the tailbone and below the waist. If you place your hands on your hips and slide them toward your spine, you are close to the area. The dimples you can see just above your buttocks, known as the “dimples of Venus,” sit directly over the SI joints.

Where Is SI Joint Pain Felt Most Often?

SI joint pain is almost always one-sided. It is very rare for both sides to hurt at the same time. The most common location is the low back and buttock on the same side as the irritated joint.

People often describe the pain as deep and hard to pinpoint. It is not a sharp surface pain. It feels like it is coming from inside the body. The pain can spread to the groin area, the hip, or down the back of the thigh. Some people feel it in the lower abdomen or the foot on the same side.

Current research suggests that true SI joint pain rarely goes below the knee. If your pain travels past the knee into the calf or foot, it is more likely coming from a disc or nerve issue in the spine. This is one of the most useful clues for telling SI joint pain apart from other types of low back pain.

How to Tell SI Joint Pain From Sciatica or Hip Pain

This is where many people get confused. Sciatica, hip arthritis, and SI joint dysfunction all cause pain in similar areas. But they feel different and have different triggers.

One simple test doctors use is the FABER test. You lie on your back and place the ankle of your painful leg on the opposite knee. If pressing down on the bent knee reproduces your pain, the SI joint is a likely source. This is not a perfect test, but it is a good starting point.

Another clue is how the pain behaves with movement. SI joint pain often gets worse when you stand on one leg. Putting all your weight on the painful side while standing or walking up stairs can trigger sharp pain. Rolling over in bed at night is another common complaint.

What Causes SI Joint Pain in the First Place?

The causes fall into two groups: too much motion or too little motion. Hypermobility means the joint moves too much. Hypomobility means it is stiff and stuck. Both can cause pain, but they need different treatments.

Pregnancy is one of the most common causes. The hormone relaxin loosens ligaments throughout the body, including the SI joints. This allows the pelvis to expand for childbirth. But it can also make the joint unstable and painful. Many women first notice SI joint pain during or after pregnancy.

Injury is another common cause. A fall onto the buttocks, a car accident, or a sudden twist can strain the ligaments around the joint. Over time, repetitive activities like running on uneven surfaces can also irritate the joint.

Arthritis can affect the SI joint too. Ankylosing spondylitis, a type of inflammatory arthritis, often starts in the SI joints. This condition causes stiffness and pain that is worse in the morning and improves with movement. If you wake up stiff and feel better after moving around, inflammatory arthritis might be the cause.

Previous back surgery is a less known cause. After a spinal fusion, the segments below the fusion take on more stress. The SI joint is one of those segments. Studies have found that up to 40 percent of people who have had a lumbar fusion develop SI joint pain later.

What the Research Actually Says About Diagnosis and Treatment

Diagnosing SI joint pain is harder than it sounds. There is no single test that confirms it with certainty. Imaging like X-rays or MRI can rule out other problems, but they rarely show the SI joint dysfunction itself.

The gold standard for diagnosis is a diagnostic injection. A doctor injects a numbing medication into the SI joint under X-ray guidance. If your pain goes down by at least 75 percent for several hours, the SI joint is likely the source. This is the most reliable method as of 2026, though it is not perfect.

Physical therapy is the first line of treatment. The goal is to stabilize the joint by strengthening the muscles around it. The glutes, the core, and the hip rotators all play a role. A good therapist will also check for muscle imbalances — tight hip flexors or weak glutes are common contributors.

Manual therapy can help. Some people benefit from gentle manipulation of the joint by a chiropractor or physical therapist. But aggressive adjustments can make hypermobile joints worse. If you have too much motion, manipulation is usually not the right choice.

Bracing is another option. A sacroiliac belt worn around the hips can compress the joint and reduce painful motion. These belts are most helpful for people with hypermobility, such as pregnant women. They are not a long-term solution but can reduce symptoms during daily activities.

For severe cases that do not respond to therapy, injections of steroids or platelet-rich plasma are options. Steroid injections reduce inflammation but do not fix the underlying instability. PRP injections aim to strengthen the ligaments, but the evidence is still mixed. Some studies show benefit, others do not. As of 2026, PRP for SI joint pain is not a standard treatment.

Surgery is a last resort. SI joint fusion involves placing implants across the joint to stop it from moving. The procedure has become less invasive in recent years, but it is still major surgery. Success rates vary widely depending on the study. Some report 80 percent satisfaction, others much lower. The decision to fuse should not be taken lightly.

Common Misconceptions About SI Joint Pain

Many people believe SI joint pain is the same as sciatica. They are not the same. Sciatica is caused by irritation of the sciatic nerve, usually from a herniated disc. SI joint pain comes from the joint itself. The two can feel similar, but they require different treatments. Treating SI joint pain like sciatica often leads to months of wasted effort.

Another misconception is that stretching always helps. For hypermobile SI joints, stretching can actually make things worse. If the joint already moves too much, stretching the surrounding muscles removes the stability they provide. Strengthening, not stretching, is usually the answer for hypermobility.

Some people think that if an MRI is normal, the pain must be in their head. That is not true. SI joint dysfunction rarely shows up on MRI. A normal scan does not mean normal function. The joint is designed for stability, not large movements, so standard imaging often misses the problem.

There is also a belief that surgery is the only fix. Most people with SI joint pain improve with conservative care. Physical therapy, activity modification, and bracing work for the majority of patients. Surgery is reserved for the small group who fail all other treatments.

What to Avoid If You Have SI Joint Pain

Avoid sitting on your wallet. It tilts your pelvis and puts uneven pressure on the SI joint. Men often report significant relief just by removing their wallet from their back pocket.

Avoid sleeping on your stomach. This position arches the lower back and stresses the SI joints. Side sleeping with a pillow between the knees is better. The pillow keeps the top leg from pulling on the pelvis and irritating the joint.

Avoid high-impact activities until the pain settles. Running, jumping, and heavy squats can aggravate the joint. Swimming and walking on flat surfaces are safer options. If you must run, stick to soft surfaces like grass or a track.

Avoid sitting for long periods. The SI joint stiffens up when you stay still. Get up and move every 30 minutes. Even a short walk around the room can help keep the joint from locking up.

Avoid relying on pain relievers alone. NSAIDs like ibuprofen can reduce inflammation, but they do not fix the underlying problem. Using them long-term can mask symptoms while the joint continues to get worse. Use medication for short-term relief while you address the root cause.