The principles of differential diagnosis that apply to reading joint x-rays are no different than the principles of differential diagnosis that apply to the evaluation of joint pain at the bedside except that the input data are different for each situation. At the bedside, morning stiffness that lasts more than a half an hour, the presence of hot, swollen joints, and the increase of joint symptoms in the morning are indicative of an inflammatory process as opposed to those symptoms that reflect a degenerative process. At the x-ray box, the presence of periarticular osteoporosis or sclerosis, the pattern of articular cartilage narrowing, and/or the presence of erosions or bone spurs are all important in trying to decide whether one is dealing primarily with an inflammatory process of the synovium or a degenerative process in the articular cartilage. If the x-ray changes suggest an inflammatory process, then the differential diagnosis would be based on whether one is dealing with a monoarthritis, an oligoarthritis, or a polyarthritis. If one is dealing with a polyarthritis on x-ray, the presence of a symmetric pattern vs. the presence of an asymmetric pattern of joint involvement can be helpful in further limiting the differential diagnosis. The other point to remember is that the prevalence of osteoarthritis in the population increases over the age of forty and there may be baseline x-ray changes of osteoarthritis upon which there are new ‘inflammatory’ changes that reflect the current joint symptoms. When there is a discrepancy between the differential diagnosis that is created at the bedside and the differential diagnosis that is created at the x-ray box, I would follow the bedside impression and watch the patient over time.
In osteoarthritis, the x-ray changes reflect the wearing of the articular cartilaginous cushion and the resultant transmission of the stress of weight bearing to the underlying periarticular bone and its subsequent reaction. When the cartilage wears down in osteoarthritis, it wears down in an asymmetric fashion with ‘focal’ cartilage narrowing.
This can be demonstrated in early osteoarthritis by obtaining weight bearing films. The bone then reacts by displaying bony sclerosis and bone spur formation around the narrowed articular cartilage. Eventually, the entire joint space may be lost; but, there will be the telltale signs of bony sclerosis and bone spur formation. There may be subsequent deformity as well.
In an inflammatory arthritis, such as rheumatoid arthritis, the x-ray changes reflect the active inflammatory changes that are occurring in the synovium, d this leads to a subsequent “meltdown’ of the articular cartilage and digestion of the underlying bone. The early x-ray changes in an inflammatory arthritis include periarticular osteoporosis and global cartilage narrowing. Eventually, one will develop broad based marginal erosions. When the joint space narrows completely, “bone on bone’, one will have an end stage joint without any bone spurs present. In general, the x-ray changes of rheumatoid arthritis and the x-ray changes of a septic arthritis are similar as they are both examples of a primary inflammatory synovial process except that the pattern of rheumatoid arthritis is an additive symmetric polyarthritis and the pattern of a septic arthritis is usually a monoarthritis. If one has the ability to do a joint survey by x-ray, the pattern of inflammatory change can be very helpful. The differential diagnosis of a monoarthritis includes septic arthritis, crystalline arthritis, or traumatic arthritis. The differential diagnosis of an oligoarthritis includes one of the spondyloarthropathies or one of the crystalline arthritis. The differential diagnosis of a polyarthritis can be furthered divided by the pattern of symmetry vs. asymmetry. A symmetric polyarthritis can be seen in patients with rheumatoid arthritis, systemic lupus erythematosus, arthritis that is associated with other connective tissue diseases or arthritis that is seen with one of the vasculitides. An asymmetric polyarthritis can be seen in patients with one of the spondyloarthropathies or in patients with a crystal induced arthritis.
Some patients have x-ray features of both osteoarthritis and an inflammatory synovitis. These patients tend to be the same
patients that have an oligoarthritis such as one of the spondyloarthropathies or patients who have crystal induced arthritis.
Patients with ankylosing spondylitis have an arthritis that primarily involves the spine and pelvis. Men with ankylosing
spondylitis develop bilateral sacro-ileitis which tends to march up the spine. Peripheral arthritis develops around the age of forty and involves the shoulders and the hips with an inflammatory arthritis. Women with ankylosing spondylitis can develop an additive symmetric polyarthritis that can mimic rheumatoid disease and have neck involvement. A hallmark of ankylosing spondylitis is the proclivity to have “bone fusion”. In the spine one can develop calcification of the anterior longitudinal ligament which will fuse the vertebral bodies with bony calcification. This calcification is called a syndesmophyte and it differs from an osteophyte that is found in osteoarthritis because a syndesmophyte grows in a north to south direction and then fuses. An osteophyte grows out in an east to west direction and then may touch the adjacent osteophyte as a so called “kissing osteophyte’. In Reiter’s Disease, one may have unilateral sacro-ileitis, a lower extremity oligoarthritis, and a more spotty involvement of the spine as regards bone fusion and syndesmophytes. In psoriatic arthritis, one can have a variety of x-ray changes. In the spine one can have asymmetric sacro-ileitis and a spotty involvement of the spine as regards bone fusion and syndesmophytes. In the peripheral joints one tends to have an asymmetric polyarthritis that can involve both the small and large joints. Distal interphalangeal joints are characteristically involved. The erosions of psoriatic arthritis are also characterized by a distinctive “pencil
and cup” deformity. In the mutilans variant of psoriatic arthritis one can have striking bone erosion and bone shortening which creates at the bedside the “opera glass hand”.
In crystalline arthritis one has a different x-ray picture for gouty arthritis and calcium pyrophosphate deposition disease (CPPD). In gouty arthritis one can have a monoarthritis, an asymmetric oligoarthritis, or an asymmetric polyarthritis. In gouty arthritis the cartilage is spared until the patient develops end stage arthritis, and this feature is different from rheumatoid arthritis where one has cartilage narrowing in a global fashion early in the natural history of the arthritis. In gout one can develop “rat bite or bird’s beak” erosions where one has an overhanging edge of bone over the erosion. This overhanging edge of bone would be very atypical in rheumatoid arthritis which tends to have broad based erosions. In calcium pyrophosphate deposition disease, which is otherwise known as CPPD or pseudo gout, one can have a monoarthritis, an asymmetric or symmetric oligoarthritis, or an asymmetric or symmetric polyarthritis that tends to involve large joints. In CPPD one can have characteristically a linear calcification of the articular cartilage which is known as “chondrocalcinosis”. In cases where the cartilage is grossly narrowed, one may be unable to see the chondrocalcinosis and the joint may look like any other end stage inflammatory joint. In this situation one would have to look at other joints that are characteristically involved in CPPD for the chondrocalcinosis, such as the knees, the wrists, and the shoulders.
Courtesy: Peter Holt M.D.