Sacroiliac joint pain is felt low down, just below the belt line and off to one side. Many people can cover the sore spot with a single finger. It usually spreads into the buttock and down the back of the thigh, and it rarely travels past the knee.

That is why it gets overlooked. The pain sits lower than people expect a joint of the spine to be, so it is blamed on a disc, a muscle or the hip for years.

What and where is the sacroiliac joint?

You have two, one each side, where the sacrum, the triangular bone at the base of your spine, meets the ilium, the broad wing of your pelvis. The dimples at the back of your pelvis sit over them.

The joint barely moves, a few millimetres at most. What it does instead is transfer your upper body weight into your legs with every step. That is why a joint that hardly moves can hurt so much: little else is loaded this hard, this often.

Where is sacroiliac joint pain felt?

It is nearly always one sided, below the belt line, over or beside the dimple on that side:

• A deep ache in one buttock
• Pain low across the back of the pelvis
• Groin pain on the same side, in some people
• Pain down the back of the thigh, stopping above the knee

SI joint pain gets pointed at with one finger, while a broader back problem gets a hand swept across the low back. Pain running past the knee into the calf or foot points more often to a lumbar nerve root, covered on our back pain and lumbar radiculopathy page.

What makes SI joint pain worse?

It flares when the joint takes weight unevenly. Common triggers:

• Standing on one leg, dressing or putting on socks
• Rolling over in bed
• Getting in and out of a car
• Climbing stairs
• Standing up after sitting
• Sitting a long time on a soft sofa
• Carrying a child or a bag on one hip

The first few steps are often the worst.

Why sacroiliac joint pain gets missed for years

This is one of the more commonly overlooked sources of lower back pain, for three reasons.

It hurts below where anyone looks. Patients and clinicians look for spinal problems in the lower back itself. This joint sits under that area, at and below the belt line, outside the region checked.

No scan proves it. No X-ray, CT or MRI finding confirms mechanical sacroiliac joint pain. Imaging often reads as normal, or shows ordinary age-related disc change that gets blamed instead. Such change is common in people with no pain, so finding it proves nothing.

The pattern copies a disc or nerve problem. One sided low back pain spreading into the buttock and thigh is close to what an irritated lumbar disc or nerve root produces. Without an examination aimed at the joint, the more familiar diagnosis wins.

If a disc has been treated for years with little change and the pain still sits low on one side, reassess the joint.

What causes SI joint dysfunction?

SI joint dysfunction means the joint itself has become the pain source, usually from loading or injury. Common causes:

• Pregnancy and the months after birth, when hormonal changes soften the pelvic ligaments and delivery adds strain
• A fall directly onto the buttock
• A car crash, which twists the joint under load
• Repeated one sided loading at work or in sport
• A leg length difference, loading one side harder for years
• Previous lumbar fusion surgery, which shifts load onto the joint below
• Inflammatory arthritis of the joint, such as ankylosing spondylitis, which needs different treatment

SI joint dysfunction or inflammatory sacroiliitis?

Sacroiliitis is inflammation of the joint driven by an inflammatory condition rather than load. It is managed differently.

Mechanical SI joint pain:

• Usually one side
• Worse with standing, walking and loading
• Better with rest
• Brief stiffness after sitting that eases in minutes

Inflammatory sacroiliitis:

• Often both sides, or alternating
• Worse after rest, and in the second half of the night
• Eases as you move through the morning
• Morning stiffness lasting more than half an hour
• More common in younger adults

If your pattern matches the second list, ask your family doctor about a rheumatology assessment.

How is SI joint pain assessed?

Assessment rests on history and examination, not a scan. Your clinician asks where the pain sits, what sets it off and how it started, then uses a group of provocation tests, loading the joint in several directions to see whether your familiar pain is reproduced. No single test is reliable alone, so several are used together.

Imaging matters, but its job is to rule other things out: a fracture, a significant disc problem, inflammatory change. It does not confirm this joint.

What helps sacroiliac joint pain?

Care works up a ladder, least invasive first.

Movement and load changes. Walking is usually tolerated well. Standing on one leg, deep soft seating and one sided carrying are not. Changing how you turn in bed or leave a car reduces flares.

Physiotherapy. The aim is control of the hip and trunk muscles that steady the pelvis, rather than stretching the joint. Our post on lower back and hip pain is a companion read if your pain covers both.

A pelvic belt. A snug belt worn low across the pelvis helps some people, especially around pregnancy.

Image-guided injection. If those steps fall short, our physicians can place a sacroiliac joint injection under fluoroscopy, a live X-ray showing where the needle sits. It also has diagnostic value: since no scan proves the diagnosis, a well-placed injection that settles the pain is the strongest confirmation available.

Radiofrequency ablation. If injections confirm the joint but relief is short lived, radiofrequency ablation uses heat to interrupt the small nerves carrying pain signals from it. Responses vary, and your physician will advise whether it fits your case.

Warning signs that need urgent care

Most low back and pelvic pain is not dangerous. These signs are different.

Go to an emergency department for:

• Numbness in the groin or saddle area, the part touching a saddle, with new bladder or bowel changes
• Weakness in both legs

See a doctor promptly for:

• Back pain with a fever
• New pain after a significant fall or crash
• Unexplained weight loss
• New severe back pain and a history of cancer
• Night pain that does not change with position

Common questions about SI joint pain

Where is SI joint pain felt?

Low on one side, just below the belt line, over or beside the dimple at the back of the pelvis. It commonly spreads into the buttock, sometimes the groin and sometimes the back of the thigh. It rarely passes the knee.

Is SI joint pain the same as a disc problem?

No, though they feel similar. A disc or nerve root problem more often sends pain below the knee and can bring numbness or weakness in the leg. SI joint pain tends to stay above the knee, in one spot you can point to.

Can sacroiliac joint pain go away on its own?

It often settles, particularly after pregnancy or a single injury, and load changes plus physiotherapy are enough for many people. Pain lasting several months without improving is less likely to settle alone, and that is when an assessment is worth considering.

Does walking make SI joint pain worse?

Steady walking on level ground is usually tolerated well and many people find it helps, though the first steps after sitting can be sore. Stairs, standing on one leg and uneven ground are harder on the joint. Sharp pain is a reason to stop.

This article is general information, not medical advice. It cannot diagnose your pain. If your symptoms are new, worsening, or worrying you, speak with your family doctor or contact us for an assessment.

Talk to us about pain low in your back

If pain low on one side has not settled, an assessment at our North Toronto pain clinic can work out whether the sacroiliac joint is involved. Most services are covered by OHIP with a physician referral, and you need not wait for one: complete our self-referral form, or ask your family doctor to send a physician referral.