I'm having some low back pain that's off to the right of center. No one seems sure just what's causing it. One person thinks it's the lumbar vertebrae, another says it's coming from the sacroiliac joint. Is it always this difficult to tell what's going on?
Pain coming from the sacroiliac (SI) joint can be difficult to diagnose. That's because the problem can be inside the joint (intraarticular) or it can be extraarticular (outside of the joint). Extraarticular structures include ligaments and muscles. And the diagnosis is made more difficult by the fact that pain coming from the SI joint can be felt in the buttock, groin, and/or leg -- not just in the area of the sacroiliac joint.
There isn't one single test that can be used as the gold standard in diagnosis. Instead, doctors use a variety of pain provocation tests and/or joint injection with a numbing agent combined with an antiinflammatory medication. Pain provocation tests stretch, compress, or contract tissue structures around the SI joint. Injection numbs or silences the pain-generating structures.
There are diagnostic criteria set out by the International Association for the Study of Pain (IASP) regarding sacroiliac joint pain. The IASP proposes that 1) SI joint pain can be identified by the location of the painful symptoms, 2) SI joint pain will be reproduced by carrying out the provocation tests, 3) And the pain will go away after injection with the numbing agent. But is this really so? There are some experts who say both sets of tests are needed. And even then, the results are not 100 per cent full-proof.
Using the location of pain as a diagnostic tool is not a valid approach. Too many patients with pain around the SI area end up with a problem originating someplace else. And too many patients with true SI joint problems have buttock, leg, or back pain (not SI joint pain).
With that out as a gold standard, two of the pain provocation tests might be helpful but they don't really isolate exactly where the problem is coming from. For example, is it ligamentous? If so, which one(s) is involved? The same goes for muscles -- is there a muscular problem and if so, which one is generating the pain signals and why? The source of pain could be the joint capsule (outside the joint) or the joint articular (cartilage) surface (inside the joint)? None of the compressive tests really sort this out carefully enough.
That leaves us with the intraarticular injection as a potential gold standard diagnostic test. But the numbing agent can leak out of the joint affecting the nearby nerves and soft tissues. Thus, the injection cannot be designated as the single most reliable and valid test for SI problems either.
Until and unless scientists can figure out how to isolate individual structures in and around the SI generating pain, it remains certain that differentiating low back from sacroiliac joint problems will be difficult. One individual diagnostic test just isn't going to be possible. For now, it has been suggested that examiners use the provocation tests best known for their ability to recreate pain from the SI. The presence of a positive thrust test and positive compression test signal the need for further diagnostic workup with an intraarticular injection.
That brings us back to the International Association for the Study of Pain (IASP) and their criteria for the diagnosis of SI joint pain. These make a nice place to start, but there isn't enough evidence to support them as reliable and valid diagnostic guidelines at this time.