Simple schema of imaging findings aids arthritis Dx

Step-by-step checklist should enable differentiation of pathology even on plain film x-ray

Differential diagnosis of arthritic joint conditions and degenerative spinal disorders can be far from straightforward. Leading musculoskeletal radiologists at ECR 2006 showed how careful evaluation of imaging data can improve diagnostic accuracy.

Joint pathology can occur in key areas: the synovium, hyaline cartilage and subchrondral bone, and entheseal insertions. Radiologists seeking to diagnose basic peripheral arthritis should first assess which of these three areas is most affected, said Dr. Iain Watt, a radiologist at the Leiden University Medical Center in the Netherlands. A checklist of possible pathological features within each group should then lead to the most likely diagnosis.

One of the first things to look for in the synovium is x-ray opacity. This may indicate iron deposition from hemophilia or calcium deposits, for instance. Swelling is another important marker and a possible indication of synovitis.

"The more inflamed the joint is and the bigger the swelling, the more ill defined it becomes and the more you should think of infection," he said.

Classification of synovial erosions as either active or inactive and evaluation of new bone formation can further help narrow down the diagnostic options. Ill-defined erosions are suggestive of active disease and most likely to be rheumatoid arthritis. Inert erosions with no soft-tissue swelling are more likely indicative of gout. Observation of new bone formation adjacent to the erosion should trigger suspicion of psoriasis or reactive arthritis.

"The last radiological sign in the synovium is juxta-articular osteopenia," Watt said. "The literature is not certain what it actually represents. Although we do see this in inflammatory synovitis, and particularly in rheumatoid disease, I find it to be unhelpful."

Hyaline cartilage is attracting considerable attention in the radiological literature because of its excellent visualization on MRI, he said. Basic diagnostic signs can also be seen on plain film, however. Thick cartilage is a classic hallmark of acromegaly. Thin cartilage is associated with a variety of pathological conditions but is also a normal sign of aging. Care must be taken not to confuse thinning in elderly patients with pathological cartilage destruction.

Another perfectly normal finding in joints is calcium pyrophosphate. Again, older patients are more likely to have greater areas of calcified hyaline cartilage. Individuals with large depositions of these crystals, a cause of pseudogout, are likely to produce a hypertrophic response to joint disease. Higher-than-average levels of basic calcium phosphate, or hydroxyapatite, signals an inability to mount a robust response to joint insults.

"Hypertrophic patients are usually robust males with normal or high central bone density," Watt said. "Patients who get destructive or atrophic osteoarthritis tend to be little old ladies with osteoporosis on their bone scans. So maybe the way your skeleton responds to load and stress indicates how your joints will behave."

Consideration of enthesis disease concludes the diagnostic evaluation. The most significant question here is whether disease is erosive or nonerosive, according to Watt. Forestier's disease is the most common nonerosive endotheseal disorder. Erosive conditions include ankylosing spondylitis, psoriasis, and Reiter's syndrome.

Another major task for musculoskeletal radiologists is distinguishing the 5% of back pain patients with infection, inflammation, or neoplasm from the 95% of individuals whose discomfort is due to a mechanical cause. While plain-film radiography is of limited value, x-rays can indicate certain signs of disc degeneration: disc space narrowing, sclerosis, and osteocyte formation. But diagnosis of degeneration is best made on MRI, said Prof. Iain McCall, a professor of radiology at the Robert Jones and Agnes Hunt Hospital in Oswestry, U.K.

Another common sign of spinal degeneration is annular bulging, or a circumferential extension of the disc beyond the endplate. Herniation of nuclear material can also occur. The herniations can be graded according to the severity of symptoms, McCall said. The posterior annulus remains intact in protrusions, whereas it is disrupted in extrusions. Sequestration involves migration of material away from the disc.

"If the patient can stand the pain, there is a high probability that sequestrations will resolve spontaneously. Similarly for extrusions," he said. "For the protrusions, it is much more difficult."

Notice should also be taken of the nerve root, which if compressed, can cause sciatica. So-called high-intensity zones on T2-weighted MRI are less significant, contrary to previously held beliefs, McCall said.

"High-intensity zones certainly may be symptomatic and produce quite intense back pain," he said. "But studies have shown that there is a relatively poor positive predictive value of symptoms from high-intensity zones."

Both CT and MRI can be used to examine facet osteoarthritis and produce a grading of 0 to III according to severity. Intraobserver variation appears to be better with CT, though the relevance of a grading scheme is itself questionable.

"A grading system for osteoarthritis may be of value. However, I have to say that no relationship has been shown between facet osteoarthritis and low back pain," McCall said.

Identification of cysts on facet joints, while rare, is an important finding, he said. Sciatica may be due to a facet cyst and not disc herniation. This distinction should be made before the patient reaches the operating table.