SI Joint Dysfunction

The pain that gets treated as something else

A recognisable version of this patient exists. They have had buttock and low back pain for a year or more. They have had an MRI showing degenerative change, physical therapy aimed at the lumbar spine, possibly an epidural injection, and none of it has made much difference.

The reason is that the problem was never in the lumbar spine. It was in the joint just below it.

Where the sacroiliac joint is and what it does

The sacrum, the triangular bone at the base of the spine, meets the pelvis on each side at the sacroiliac joint. These joints move very little. Their job is transferring load between the spine and the legs, and they are held together by some of the strongest ligaments in the body.

When one becomes painful, the pain is typically deep, on one side, and low: below the belt line rather than across the back. Patients often point to it with a single finger just inside the back of the pelvis, which is a more specific gesture than the open hand people use for lumbar pain.

How it differs from sciatica

Both can produce buttock pain and both can run into the thigh. The differences are worth knowing because they are what the examination looks for.

SI joint pain is usually provoked by transitions and by asymmetric loading: getting in or out of a car, rolling over in bed, standing from sitting, climbing stairs, standing on one leg. It tends to stay above the knee.

Sciatica follows a nerve. It more often travels below the knee, comes with numbness or tingling in a specific territory, and is frequently worse with sitting and with coughing or straining. Weakness in a specific muscle group points firmly away from the SI joint.

Hip arthritis is the third member of this group and produces groin pain more than buttock pain, with restricted and painful hip rotation on examination.

Confirming it

No single test settles this, and that is the honest position rather than a hedge.

The examination uses several provocative manoeuvres in combination, because individually they are not specific enough to rely on. Several positive tests in a patient whose history fits is meaningful; one positive test is not.

Imaging mainly serves to exclude other causes. Mechanical SI joint pain frequently looks unremarkable on an MRI, so a normal scan does not rule it out.

The most useful piece of evidence is a diagnostic injection: local anaesthetic placed into the joint under imaging guidance. Meaningful temporary relief tests the joint directly, which nothing else on the list does.

After a lumbar fusion

This deserves its own mention because the pattern is common and often unrecognised.

Fusing lumbar segments transfers load to the levels above and below, and the sacroiliac joint sits directly below. A patient whose original leg pain resolved after a fusion, and who later developed new one-sided buttock pain, may well be describing an SI joint that is now carrying more than it used to.

The treatment ladder

Treatment works through a sequence, and the order matters.

Physical therapy targeting the pelvis, hip and gluteal musculature, rather than generic lumbar work, comes first. Activity modification around the specific provocative movements helps. Some patients benefit from a pelvic belt.

Injections come next, and do two jobs at once: they can provide relief, and they confirm the diagnosis.

SI joint fusion sits at the top of the ladder. It is reserved for patients whose diagnosis has been confirmed, whose lumbar spine and hip have been properly excluded, and whose symptoms have persisted despite working through the steps below it. In a well-selected patient it can be a very satisfying operation. In a poorly selected one it treats a joint that was never the problem, which is why the confirmation matters more here than almost anywhere else on this site.