SI joint pain: the backside ache that is not quite the spine
Last updated September 3, 2026.
Sacroiliac (SI) joint pain comes from the joints joining the spine's base to the pelvis: a one-sided ache below the beltline, in the buttock and back-of-hip area, worse with sitting, standing on one leg, rolling in bed, and stairs. It accounts for a real share of low back pain (and is famously over- and under-diagnosed in turn), it links to pregnancy, leg-length differences, and the inflammatory arthritis family, and it is mostly fixed by physiotherapy, not surgery.
What does it feel like?
The map: pain below the beltline on one side, centered on the dimple of the buttock (patients point with one finger: the Fortin finger test), sometimes radiating to the groin or the back of the thigh (rarely below the knee). The provokers: long sitting (the classic SI complaint), standing on one leg (the getting-dressed test), rolling over in bed, climbing stairs, getting out of the car, and the sit-to-stand transition. Stiffness after rest, aching after load. It mimics sciatica and hip problems well enough to fool everyone periodically.
Why does it happen?
The SI joints are built for stability with small movements, and they hurt when the load-sharing goes wrong: too much movement (pregnancy's relaxin hormone, hypermobility, after falls onto the buttock) or too little (stiffness with age, after spinal fusion surgery, the inflammatory spondyloarthritis family inflaming the joint: that variant comes with morning stiffness and young onset). Leg-length differences, scoliosis, and uneven gait feed it. Pregnancy and postpartum SI pain is so common it is almost a rite, and it mostly settles after delivery.
What actually helps?
- Physiotherapy first: the evidence-backed core: gluteal and core strengthening, pelvic stability work, and movement retraining; the large majority improve.
- The sitting and sleeping craft: break long sitting, support the sit (cushion, both feet down), sleep with a pillow between the knees, and change sides when rolling.
- Pelvic support belts: for the unstable subtype (especially pregnancy): a snug belt low over the pelvis genuinely offloads the joint.
- Anti-inflammatories and heat: for the flares.
- Injections and beyond: an SI joint steroid injection (diagnostic and therapeutic) for the physio-resistant, radiofrequency ablation for recurring relief, and fusion surgery only for the genuinely refractory few.
- The inflammatory version: morning-stiffness-predominant, young-onset, both-sided SI pain is a spondyloarthritis referral, not a physio case: it gets rheumatology.
When is it an emergency?
SI joint pain is clinic medicine. The bypasses: back or buttock pain with leg weakness, numbness in the saddle area, or bladder or bowel change (cauda equina: emergency), pain with fever (spinal infection), night pain that is constant and unrelenting with weight loss (the sinister pattern), and pain after significant trauma (pelvic fractures hide in the elderly faller). Pregnancy SI pain with vaginal bleeding or contractions is the maternity unit, not the physio. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
What a Pymander AI doctor consult looks like
Illustrative example, not a real member's messages.
Common questions
How is SI joint pain different from sciatica or a hip problem?
The map and the provokers separate them: SI pain centers on the buttock dimple below the beltline (one finger points to it) and is provoked by sitting, one-leg standing, and rolling in bed, with radiation only to the groin or back thigh; sciatica (nerve pain) shoots below the knee to the calf or foot, with tingling, numbness, or weakness, and follows the nerve, not the joint; hip-joint pain lives in the groin and front, and hates rotation (putting on socks, getting into cars the other way). The bodies overlap enough that examination matters (the SI provocation tests, the hip scour, the nerve stretch), and the diagnostic injection (numbing the SI joint and watching the pain vanish) is the tie-breaker when the picture is mixed.
Why does sitting make it worse when sitting is rest?
Because for the SI joint, sitting is load, not rest: the sitting posture parks weight directly through the pelvis onto the SI joints, flattens the lumbar curve that shares load fairly, and (with the wallet-in-pocket, leg-crossed, slumped variants) twists the pelvis asymmetrically, which the irritable joint protests. It also weakens and tightens the muscles (glutes and hip flexors) that stabilize the joint between sits. The desk-worker's craft: stand or walk briefly every 30-45 minutes, sit supported (both feet down, cushion or lumbar support, nothing in the back pocket), and do the glute squeezes at the desk. The joint wants movement snacks, not marathons of stillness.
Is this the pregnancy back pain people talk about?
A large share of it is: pregnancy relaxes the pelvic ligaments (the hormone relaxin, preparing for delivery), the growing bump shifts the load forward, and the SI joints, suddenly hypermobile under new load, ache: classically one-sided buttock pain, worse rolling in bed, on stairs, and on one leg, peaking in the second half. The management is specific: the pelvic support belt (worn low and snug), the physiotherapy stability work, the pillow between the knees, avoiding one-leg activities (dressing seated), and heat. The reassurance: it is mechanical and hormonal, it does not harm the baby, and the large majority settle within months of delivery as the ligaments re-tighten, with persistent cases earning physio and occasionally injection after birth.
What does the injection do, and when is it offered?
The SI joint injection does double duty: as a test (local anesthetic into the joint: if your familiar pain vanishes for the hours it lasts, the joint is confirmed as the source, which settles months of arguing) and as treatment (the steroid joined to it calms the joint for weeks to months, creating the window in which physiotherapy finally gets ahead of it). It is offered when a genuine physio course has not cracked it, done under X-ray or ultrasound guidance, and repeatable within limits. For the recurrent responders, radiofrequency ablation (heating the joint's pain nerves) extends the relief. Fusion surgery exists for the genuinely refractory few, and the path to it is long and gated on purpose.
Could this be the inflammatory kind? What is the difference?
The distinction that changes the specialty: mechanical SI pain (the common kind) comes from overload and instability, worsens with activity and sitting, eases with rest, and starts at any age; inflammatory SI pain (sacroiliitis, the spondyloarthritis family including ankylosing spondylitis) starts young (typically under 40-45), is often both-sided, comes with marked morning stiffness over 30 minutes and second-half-of-night pain, improves with exercise rather than rest, and travels with its cousins (psoriasis, inflammatory bowel disease, uveitis, the family history). The inflammatory kind is a rheumatology diagnosis (blood tests, MRI of the SI joints) with different (and excellent, biologic-era) treatments: worth naming at the appointment if the young-onset, morning-stiffness, exercise-helps pattern fits.
What should I expect from physiotherapy?
The plan, usually 6-12 weeks: assessment first (which subtype: the unstable joint needing stability, or the stiff one needing mobility, because the programs differ), then the core work: glute strengthening (the gluteus medius is the joint's main guard: side-lying work, bridges, band walks progressing to single-leg control), deep core engagement, hip mobility, and movement retraining (how you roll, stand on one leg, and get out of the car), with manual therapy for the stiff joints and taping or the belt for the loose ones. The home program is the actual treatment (the sessions coach it), and the honest expectation: steady weekly improvement rather than a quick fix, with the sitting craft running alongside. Most SI pain answers to exactly this.