Sacroiliitis: the low back pain that often goes undiagnosed

What sacroiliitis is and where it hurts

The sacroiliac joint connects the sacrum (the base of the spine) to the iliac bones of the pelvis, one on each side. It is a joint with very little movement, yet it carries the entire weight of the body and absorbs part of the impact of walking or jumping.

When that joint becomes irritated or wears down — whether from overload, repeated microtrauma, pregnancy, leg-length differences, biomechanical problems or inflammatory disease — pain appears that typically:

• Sits very low in the back, off to one side, just above the line of the buttock.
• May radiate into the buttock or the back of the thigh, but rarely below the knee.
• Gets worse with prolonged sitting, when standing up from a chair, when climbing stairs or with movements that load the weight onto one leg.
• May ease with walking and worsen when staying still in one position.

It is a common cause of chronic low back pain: an estimated 15 to 30% of chronic low back pain cases have a significant sacroiliac component. And yet it remains one of the most frequently overlooked diagnoses.

One important nuance: the diagnosis of sacroiliitis is, above all, clinical. There are specific maneuvers on physical examination that, taken together, point to this joint with considerable accuracy. MRI and X-rays add information in selected cases — especially when an underlying inflammatory disease is suspected — but they can be normal in mechanical sacroiliitis. When suspicion is strong and the pain persists, an image-guided diagnostic block is the most reliable tool for confirming that the pain truly originates in the sacroiliac joint.

Why it is mistaken for sciatica or hip pain

Sacroiliitis has a clinical quirk: its referred pain can mimic very different conditions, which explains why it is so often misdiagnosed.

Mistaken for sciatica: sacroiliac pain can radiate into the buttock and sometimes down to the back of the thigh. When a patient says "it hurts from my back down into my leg", sciatica is the first diagnosis that comes to mind. But there are differences: in sacroiliitis the pain rarely travels below the knee, there is no neurological deficit, and provocative tests (FABER, Gaenslen, compression) are positive.

Mistaken for hip pain: because the joint sits close to the hip and the pain can radiate toward the gluteal fold, many patients are evaluated by orthopedic surgeons looking for a hip problem. The hip X-ray comes back normal, generic physical therapy is prescribed, and the problem continues.

Mistaken for ordinary low back pain: when the pain is mostly low lumbar, it is assumed to be a generic mechanical problem. Anti-inflammatories and non-specific physical therapy fail to resolve it, because the pain generator is the sacroiliac joint — and that is not how it is treated.

How it is diagnosed

The diagnosis of sacroiliitis rests on three pillars:

• A focused history: location of the pain, what makes it better or worse, absence of radiation below the knee, and relevant background (pregnancies, trauma, leg-length differences).
• A physical exam with provocative maneuvers: tests such as FABER, Gaenslen, pelvic compression, distraction and the sacroiliac thrust test. When several of these are positive (3 or more), suspicion is high.
• A diagnostic block: the definitive test is an image-guided intra-articular sacroiliac injection. If the injection relieves the pain for the duration of the anesthetic's effect, the sacroiliac origin is confirmed.

Imaging (MRI, X-rays) is useful for ruling out other conditions or detecting active inflammation in cases associated with rheumatic diseases (spondyloarthritis), but it can be normal in mechanical sacroiliitis.

Effective treatment options

Treatment is stepwise. Most patients respond to well-conducted conservative treatment, and those who do not have interventional options with very good supporting evidence.

• Targeted physical therapy: focused on pelvic stabilization, core strengthening, and correction of leg-length and gait abnormalities. Generic physical therapy for low back pain does not work for sacroiliitis.
• Medication: NSAIDs for acute flares, muscle relaxants for associated muscle spasm. In chronic cases it is worth thinking beyond NSAIDs and considering other analgesics depending on the patient's profile.
• Image-guided intra-articular sacroiliac injection: with local anesthetic and a corticosteroid, this is the most effective option in the short and medium term. It is performed under fluoroscopy or ultrasound to make sure the medication is placed correctly inside the joint.
• Radiofrequency ablation of the sacral lateral branches: when blocks confirm the sacroiliac origin but relief is short-lived, radiofrequency ablation of the lateral branches that supply the joint capsule can provide prolonged relief (6-18 months).
• Regenerative medicine: intra-articular platelet-rich plasma is an option in selected cases, with promising results although the evidence is still limited compared with other indications.

My approach to sacroiliitis starts with ruling out systemic inflammatory forms, which require specific rheumatological management. In mechanical cases, I begin with a physical therapy plan aimed at lumbopelvic stabilization, strengthening of the glutes and core, and correction of postural and movement habits. If the pain persists and limits daily life, I consider image-guided sacroiliac blocks — first as a diagnostic tool, then as treatment. In recurrent, well-confirmed cases, radiofrequency ablation of the posterior branches offers prolonged relief in selected patients.

When to see a doctor

A focused evaluation is worthwhile when:

• You have low back pain, off to one side, that has persisted for more than three months.
• The pain gets worse when sitting or when standing up from a chair.
• You have been treated for "sciatica" or ordinary low back pain and are not improving.
• Your X-rays and MRI scans come back normal or show only mild degenerative changes, yet the pain persists.
• After pregnancy or childbirth you have been left with one-sided low back pain that never fully resolves.

When a patient with sacroiliitis comes back and tells me they can now sit, stand up, drive and sleep without constantly shifting position, I know we reached the right diagnosis and the right plan. Often the most important part is finally putting a name to a pain that spent years mislabeled. Sacroiliitis is a concrete condition with concrete treatment, and recognizing it early spares the patient months of generic treatments that never target the real source of the pain. My job, above all, is to keep that focused perspective from the very first visit.

Dr. Mauricio Casarsa

Anesthesiologist, specialist in Pain Medicine and Interventional Pain Management. Postgraduate training at UBA (Fundación Dolor) and UNLP (CAIDBA). Staff physician at Hospital Alemán, Buenos Aires.

MN 137756 · San Isidro · Villa Urquiza