Measurement of radiographic joint space in the rheumatoid knee: correlation with obesity, disease duration, and other factors.

A new and highly reproducible method for measuring radiographic joint space in the rheumatoid knee is described. The joint space was measured in radiographs of the knees of 120 patients with rheumatoid arthritis and correlated by computer analysis with age, sex, duration of disease, erosions, degenerative change, therapy and Quetelet's index (an index of obesity). Narrowing of both compartments of the knee correlated significantly with the duration of disease. Narrowing of the lateral, but not medial, compartment showed some degree of correlation with obesity as measured by Quetelet's index, but this did not achieve significance. The presence of erosions was associated with marked narrowing of joint space which frequently occurred early in the course of the disease; three-quarters of the patients showing even moderate erosions had at least one knee compartment narrowed to 2 mm or less and in half of these the changes had occurred in less than 10 years. Joint space narrowing also correlated strongly with degenerative change which was, however, remarkable for its infrequency.