What Is Sacroiliitis, and How Is It Different From SI Joint Dysfunction?

The sacroiliac joints sit at the back of your pelvis, connecting your spine to your hip bones. They carry a huge amount of load every time you stand, walk, or climb stairs. When something goes wrong here, the pain can be confusing. It often feels like lower back pain, deep buttock pain, or even pain down the leg.

The tricky part is working out whether that pain comes from inflammation or from a mechanical problem. The distinction matters because the right answer changes everything about how the problem is managed.

What are the symptoms of sacroiliitis?

Sacroiliitis typically causes a deep, aching pain in the lower back or buttocks that is worse after rest and improves with movement. The hallmark pattern is morning stiffness lasting more than 30 minutes, sometimes waking you at night.

Other symptoms can include fatigue, a general feeling of unwellness, and occasionally pain referring down the back of the thigh. The pain is sometimes mistaken for ischial bursitis or sciatica because the location overlaps.

What causes sacroiliitis?

Sacroiliitis is caused by inflammation within the SI joint itself. The most common driver is an autoimmune or inflammatory condition, though infection and mechanical overload can also trigger it.

A blood marker called HLA-B27 is found in many people with axial spondyloarthritis, though not everyone who carries it develops the condition. Your GP or rheumatologist can arrange this blood test alongside inflammatory markers like CRP and ESR.

How is sacroiliitis different from SI joint dysfunction?

The core difference is the cause of the pain. Sacroiliitis is driven by inflammation inside the joint. SI joint dysfunction is a mechanical problem, where the joint is stiff, hypermobile, or sitting out of its normal position because of a pelvic tilt, leg-length difference, or muscle imbalance.

In practice, the two can overlap. A pelvis that sits unevenly puts extra stress on one SI joint, which may then become inflamed. That is why understanding both the alignment picture and the inflammatory picture gives a clearer answer than either one alone.

How is sacroiliitis diagnosed with imaging?

MRI is the gold-standard imaging tool for detecting active sacroiliitis. It can show bone marrow oedema, the telltale sign of active inflammation inside the SI joint, long before structural damage appears on a standard X-ray.

Where standing imaging fits in

If your pain pattern sounds more mechanical, or if MRI has already ruled out active inflammation, the next question is whether your pelvis and spine are aligned under load. A standard X-ray or MRI is taken lying down. Lying flat relaxes your muscles and removes the effect of gravity, so it can hide alignment problems that only show up when you are standing and weight-bearing.

ScanAlign standing EOS imaging captures your full skeleton from head to toe while you stand naturally. It uses about 90% less radiation than a standard X-ray, around ten times less. This kind of image can reveal whether a pelvic tilt, leg-length difference, or spinal curve is overloading one SI joint and contributing to your pain.

What helps manage sacroiliitis?

Management depends entirely on the cause. ScanAlign is a diagnostic imaging clinic and does not provide treatment, but understanding the broad approaches can help you have a more informed conversation with your doctor or specialist.

The right starting point is the right diagnosis. If your symptoms sound inflammatory, ask your GP about HLA-B27 testing, inflammatory blood markers, and a referral to rheumatology. If they sound mechanical, standing alignment imaging can show what lying-down scans miss. You can book a standing pelvis scan to look at your alignment under load.

Frequently asked questions about sacroiliitis