Sacroiliac joint

What it means

At the base of the spine, the triangular sacrum slots between the two iliac bones of the pelvis. The places where they meet on each side are the sacroiliac joints, often abbreviated SI joints. Unlike the hip or knee, they are not designed for movement — their job is stability, transferring load between the spine, pelvis, and legs. They are wrapped in strong ligaments and have only a small amount of give. There is one on the left and one on the right; both are usually examined together.

Why it appears on a CT or MRI report

Reports comment on alignment, joint space, the bone on either side of the joint, the surrounding ligaments, and any signs of inflammation. You may see terms such as sclerosis (extra dense bone), erosions, bone marrow oedema, sacroiliitis (inflammation), ankylosis (fusion), or simply normal alignment with no inflammatory change. MRI is the most sensitive way to look for active inflammation, while CT shows the bony detail and any erosions, fusion, or fractures clearly.

What it usually means

Most reports mention the SI joints in passing as part of describing the pelvis, and most are normal or show mild age-related changes. Mild wear-and-tear changes are very common from middle age onwards and often have no symptoms. Active inflammation of the joint (sacroiliitis) can be a sign of an inflammatory spinal condition such as ankylosing spondylitis or another spondyloarthropathy, particularly in younger adults with longstanding lower back pain that improves with movement and is worse after rest. SI joint pain can also follow injury, pregnancy and delivery, repetitive load, or a leg-length difference. Less commonly, infections or fractures of the joint show up on imaging, especially in people with risk factors. The report alone rarely sorts these out; clinical history, blood tests for inflammation, and sometimes injection tests help confirm the source of pain.

When to follow up

Bring the report to your doctor along with the pattern of any symptoms — where the pain sits, when it is worst, whether stiffness eases with movement, and any history of psoriasis, inflammatory bowel disease, or eye inflammation. Talk to your doctor sooner if the report mentions active sacroiliitis, erosions, or bone marrow oedema, especially with persistent back or buttock pain. Seek urgent care for fever with severe lower back pain, loss of bowel or bladder control, or new severe pain after an injury.

A plain-language way to picture it

Imagine the spine as a stack of blocks resting on a triangular wedge. The wedge sits inside an open hinge formed by the two halves of the pelvis. The contact points on either side of the wedge are the SI joints — built for steadiness rather than swing, like the brackets bolting a heavy beam to a wall. When the brackets are calm, no one notices them. When they are irritated, every step across the floor reminds you they are there.