Sacroiliac Joint Pain
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Direct answer
The sacroiliac joint accounts for roughly 15-25 percent of chronic low back pain — the Bernard and Kirkaldy-Willis figure of about 22 percent is the quoteable classic — yet it is the diagnosis most often missed because clinicians look only at the lumbar spine. Diagnosis is a cluster, not a single test: three or more positive provocative manoeuvres (distraction, compression, thigh thrust, sacral thrust, Gaenslen) with concordant pain make the joint the likely pain generator, and an image-guided block that relieves half to three-quarters of the pain confirms it. Postpartum laxity heads the causes, inflammatory sacroiliitis and tuberculosis sit at the dangerous end, and treatment runs from belts and physiotherapy through radiofrequency denervation to fusion as the last resort.
What you must remember
- Contribution figure: 15-25 percent of chronic axial low back pain, with the classic Bernard and Kirkaldy-Willis study quoting about 22.5 percent — the number examiners expect.
- Laslett cluster: distraction, compression, thigh thrust, sacral thrust and Gaenslen or FABER; three or more positive tests with pain the patient recognises as their own gives high post-test probability — the cluster beats any single manoeuvre because individual tests have weak specificity.
- Fortin finger sign: the patient points with one finger to pain just below the posterior superior iliac spine — a localising clue, since referred lumbar pain is vague and broad.
- Exclusion frame: inflammatory sacroiliitis (young patient, night pain, morning stiffness, HLA-B27 — ankylosing spondylitis), infection (tuberculosis and brucellosis in India — unilateral SI osteomyelitis with a cold abscess), insufficiency fracture (elderly, or U-shaped sacral fracture in ankylosed spines), tumour.
- Diagnostic block: image-guided intra-articular injection; relief of 50-75 percent or more, confirmed on a second occasion, is the conventional positive — and the same needle delivers the therapeutic steroid.
- Denervation: radiofrequency ablation of the lateral branches of the L4-S3 dorsal rami supplying the posterior joint; evidence is moderate and outcomes imperfect because the joint also receives anterior innervation.
- Surgical fusion: the last resort for block-confirmed recalcitrant pain — minimally invasive triangular titanium implants or open plating with grafting.
- Indian practice reality: pregnancy-related pelvic girdle pain is the commonest presentation in women, and a pelvic belt plus physiotherapy is cheap and effective; in any unilateral SI lesion with an abscess or chronic discharging sinus, TB sacroiliitis is the viva answer.
Proving the joint is the pain source
A 39-year-old woman, two years after her last delivery, points below her right PSIS with one finger and describes pain climbing into the buttock on stairs. Lumbar examination is unremarkable, but distraction, compression and thigh thrust all reproduce her exact pain — three of the cluster. The hip is excluded with a full range of motion and a negative FABER as a hip test (remembering the same manoeuvre also stresses the SI joint, which is why it is weak alone), and neurology is normal, arguing against a radiculopathy. Under fluoroscopy, 1.5 mL of local anaesthetic into the joint abolishes three-quarters of her pain for the afternoon — diagnostic by the Laslett convention. A pelvic belt, core and gluteal strengthening, and a single steroid injection carry many postpartum patients through; when blocks keep working but keep wearing off, radiofrequency denervation is discussed, with honest counselling that the anterior innervation caps the success rate.
The cluster, not the single test
Any single sacroiliac provocation test throws false positives — FABER stresses the hip as much as the joint, compression can hurt a lumbar spine — which is why Laslett's cluster exists and why examiners ask for it by name. The second trap is diagnostic tunnel vision: a young man with night pain and stiffness is not a mechanical SI problem but an inflammatory one, and the morning-stiffness question changes the entire pathway toward HLA-B27 and an rheumatology referral. In Indian practice the third frame matters most: a unilateral destructive SI lesion is tuberculosis until proven otherwise, complete with cold abscess, sinus and the imaging pattern that spares the joint space late. Students who can hold all three frames — mechanical, inflammatory, infective — on one joint score the whole question.
Frequently asked questions
How much chronic low back pain does the sacroiliac joint cause?
Roughly 15-25 percent in pooled series, with the classic Bernard and Kirkaldy-Willis study quoting about 22.5 percent.
What is the Fortin finger sign?
The patient localises pain with one finger just below the posterior superior iliac spine — a useful pointer toward the sacroiliac joint rather than the lumbar spine.
What makes a sacroiliac diagnostic block positive?
Fifty to seventy-five percent or more pain relief following image-guided intra-articular injection, ideally reproduced on a second occasion.
Which nerves are targeted in sacroiliac radiofrequency denervation?
The lateral branches of the L4-S3 dorsal rami that innervate the posterior joint; results are imperfect because the joint also receives an anterior supply.
Why is unilateral sacroiliac destruction suspicious in India?
Tuberculous sacroiliitis — often with a cold abscess or chronic sinus — is the classic unilateral destructive SI lesion, ahead of tumour and pyogenic infection.