Sacroiliac pain — and how to treat it

Most people are completely unaware of their sacroiliac joint until it starts to cause them pain. Then it becomes hard to ignore. It often shows up as a deep ache on one side of the low back, just beside the tailbone, or a sharp catch when rolling over in bed or getting up from a chair. The pain can radiate into the buttock or the back of the thigh, and it's often mistaken for sciatica or generic low-back pain. But what leads to pain in the SI joint, and what can actually be done about it?

What the SI joint does

The sacroiliac joints are the two places where the sacrum — the triangular bone at the base of the spine — meets the ilia, the broad blades of the pelvis. They sit a few inches either side of the tailbone, roughly where the dimples appear on the lower back. These joints barely move. A few millimetres, and only in certain directions. But the job they do is significant: they're the bridge through which every step, every load, every force from the legs travels up into the spine, and everything from the spine travels down into the legs. When they stop moving the way they should, or move unevenly, the body notices.

1. Hip & pelvic imbalance

This is probably the most common driver I see, and the one that the traditional five-reason list often misses. The pelvis is a ring — two ilia joined to the sacrum at the back and to the pubic bone at the front. When that ring twists, the SI joints are where the twist shows up.

A few patterns I look for:

• Innominate rotation. One side of the pelvis rotates forward, the other back. Extremely common after a fall onto one hip, a pregnancy, a long phase of sitting cross-legged on the same side, or sleeping curled the same way night after night. It creates what feels like a leg-length difference — because functionally, it is one.
• Functional leg-length discrepancy. Most leg-length differences aren't structural. The bones are the same length; the pelvis is simply sitting crooked, making one leg appear longer. This matters, because no amount of stretching or strengthening will fix a bone-length issue, but a pelvis can be realigned.
• Hip joint restriction. If the hip joint itself doesn't rotate well, the SI joint ends up doing the hip's job — absorbing rotation that should have happened lower down. The SI wasn't designed for that, and it complains.

These three often travel together: a stiff hip leads to a rotated innominate, which produces a functional leg-length difference, which loads the SI unevenly. Untangling which came first is part of the evaluation.

2. Pelvic floor imbalance

Often there is an imbalance in the muscles of the pelvic floor that affects the sacrum directly. The pelvic floor attaches to the tailbone and the sit-bones, and when one side pulls harder than the other, it tilts the sacrum. The correction has two parts: releasing the side that's gripping too hard, and building tone — through targeted exercises — on the side that's gone quiet. Which is which can be figured out through osteopathic evaluation. This includes the gluteus maximus, which can be thought of as the backside of the pelvic floor — and is often where the asymmetry is most obvious.

3. Tension in the uterus

The uterus connects to the sacrum via the uterosacral ligament. Uterine tension can therefore affect the sacrum quite directly — and this is a common finding in patients whose SI pain tracks with their cycle, or who developed it during or after pregnancy. Often when a traditional sacral adjustment will not hold, releasing the tension in the uterus abdominally, just above the pubic bone, gives more lasting results.

4. Tailbone misalignment

A fall from as far back as childhood can tweak the tailbone into a position it never came back from. Most people don't even remember the fall. But as long as the tailbone is off, the ligaments running from it to the sacrum will keep pulling the sacrum out of position — and no amount of work on the SI itself will hold until the tailbone is addressed.

5. Foot & ankle issues

Because of the way the body stacks, what happens below affects everything above. A collapsed arch, a stiff ankle, an old sprain that never fully rehabilitated — any of these can change how force travels up the leg into the pelvis with every step. Over thousands of steps a day, small asymmetries at the foot become significant loads at the SI. If you're having trouble with the SI joint, it's worth making sure everything in the foot and ankle is articulating correctly.

6. Muscular imbalances

It's important that the body be balanced — left to right, top to bottom, front to back. Getting evaluated and given a program to balance your body's tone is key to supporting the position of the sacrum. One of the most common patterns I see is the glutes not doing enough of the work, with the low-back muscles taking over the job of hip extension. The SI sits right in the middle of that imbalance and absorbs the mismatch.

Why it's rarely one thing

SI pain is almost always a convergence. An old tailbone fall from childhood, plus years of desk-sitting that rotated one innominate forward, plus glutes that went quiet, plus a collapsed arch on the right — and then one day bending to pick up a bag of groceries becomes the thing that flares it. The grocery bag isn't the cause. It's just the moment the accumulated pattern ran out of room.

That's why a durable resolution usually means addressing more than one of the above at the same time. Adjusting the sacrum without releasing the uterosacral ligament, or mobilizing the SI without addressing the stiff hip feeding into it, tends to produce a good short-term result that doesn't hold. Find all the contributors, address them together, and the SI gets to stop compensating.

In health, Eli Mead, D.O.M.P.