Diagnostic vs. Therapeutic SI Joint Injections: What Is the Difference?

An injection into the sacroiliac joint can serve two distinct purposes: confirming that the joint is the source of pain, or providing sustained pain relief. These goals are related but not identical, and the type of injection used — and what it contains — differs accordingly.

A diagnostic SI joint injection uses a small volume of local anesthetic (numbing medication) injected directly into the joint under image guidance. The purpose is to test whether the SI joint is the primary pain generator.

How it works: If the patient experiences substantial relief of their usual pain — typically 75% or greater — within the expected duration of the anesthetic, this confirms that the SI joint is contributing significantly to their symptoms. If there is no meaningful relief, the SI joint is less likely to be the primary source, and evaluation of other pain generators is warranted.

What it does not do: A positive diagnostic injection confirms the SI joint as a pain source. It does not treat the underlying condition, and relief lasts only as long as the anesthetic — typically a few hours.

Why it matters: The diagnostic injection is the most reliable tool for confirming SI joint pain before committing to more definitive treatment. Proceeding to radiofrequency ablation or surgical fusion without a confirmed diagnosis significantly reduces the likelihood of a good outcome.

A therapeutic SI joint injection adds corticosteroid (anti-inflammatory medication) to the local anesthetic, "with the goal of providing longer pain relief," as the American Academy of Orthopaedic Surgeons describes.

The corticosteroid reduces inflammation within and around the joint, which may provide relief lasting weeks to months in appropriate patients.

Medical source: American Academy of Orthopaedic Surgeons, OrthoInfo — Sacroiliac Joint Dysfunction. orthoinfo.aaos.org

Patients should understand that responses to SI joint injections vary considerably:

• Excellent response: Substantial, prolonged relief that allows meaningful participation in physical therapy and daily activities
• Partial response: Some improvement, but incomplete or short-lived relief
• No response: No meaningful change in pain — which is itself diagnostically useful, as it suggests the SI joint may not be the primary pain source

A single injection is not a cure. Even patients who respond well may experience gradual return of symptoms over weeks to months. The response to an injection informs subsequent treatment decisions, including whether repeat injections, radiofrequency ablation, or other interventions are appropriate.

There is no universal answer. Most practitioners limit corticosteroid injections to two or three per year to minimize the systemic effects of repeated steroid exposure. When injections provide only temporary benefit and symptoms significantly affect quality of life, other treatment options — including radiofrequency ablation — are discussed.

Dr. Blythe uses image-guided injections both to confirm the diagnosis and to provide therapeutic relief in appropriate patients. The decision to proceed with a therapeutic injection is based on the clinical picture, the results of prior diagnostic evaluation, and the patient's overall treatment goals. Injections are one component of a broader treatment plan, not a standalone solution.

Related articles: How Doctors Confirm That Pain Is Coming From the SI Joint · Do Steroid Injections Help SI Joint Pain? · Radiofrequency Ablation for SI Joint Pain

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Medical review date: July 2026