Sacroiliac (SI) Joint Dysfunction

The sacroiliac (SI) joints are two paired joints at the back of your pelvis where the bottom of the spine (the sacrum) meets the wing-like hip bones (the ilium) on each side. They're built to move barely at all — held together by thick ligaments — but they do transfer load from your spine into your legs every step you take. SI joint dysfunction is pain coming from one of those joints. It accounts for roughly 15-25% of chronic low back pain. It can flare up from a sudden twisting injury, pregnancy (the hormones that loosen the pelvis for childbirth), a difference in leg length, extra strain after a previous lumbar spinal fusion, or an inflammatory arthritis condition like ankylosing spondylitis.

The classic pattern is pain in the lower back, buttock, and sometimes the back of the thigh — almost always on just one side. Pressing on the dimple at the back of the pelvis usually reproduces it.

• Pain in the lower back and buttock — usually on one side, centered right over the dimple at the back of the pelvis
• Pain that may travel down the back of the thigh (but rarely below the knee, which helps distinguish it from sciatica)
• Aching that worsens with prolonged standing, stair climbing, or getting in and out of a car
• Pain that flares when you shift weight onto one leg or roll over in bed
• Stiffness in the lower back and hips first thing in the morning
• A catching or clicking sensation at the back of the pelvis with certain movements

No single exam test nails the diagnosis. Your provider runs through a cluster of maneuvers — moving your hip into specific positions, applying pressure to the joint — and counts how many reproduce your pain. The most reliable confirmation is an image-guided injection of numbing medicine directly into the SI joint: if your pain disappears for the few hours the medicine works, that's strong proof the SI joint is the source. An MRI is added when an inflammatory arthritis is suspected.

Surgery helps only a small minority of spine patients — usually those with a specific structural problem plus a nerve issue that isn’t getting better with a structured non-operative trial. When that step is genuinely warranted, OSI coordinates it the same way we coordinate every other part of your care: imaging, records, and the handoff are handled for you, so no part of the process falls on your shoulders.

External patient-education references and related OSI pages for additional background:

• Britannica — Vertebral column
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language