The overhanging margin of bone: a roentgenologic manifestation of gout.

Abstract

Characteristic but not diagnostic of gout are “punched-out lytic lesions” of articular bone. Such erosions, appearing as sharply marginated lucent pockets or as discrete subchondral “pseudocysts” which seem unconnected to the joint, are commonly observed in rheumatoid arthritis and in other chronic joint diseases. There is a type of erosion, however, that is typical of gout and seems to be relatively rare in rheumatoid arthritis (1). The purpose of this communication is to describe this finding and emphasize its diagnostic value. Roentgenologic Description At the periphery of the erosion there is an elevated bone margin which appears to hang over the tophaceous nodule as though it were displaced by it (Figs. 1 and 2). This margin characteristically has a thin shell-like configuration and is continuous with the adjacent bone contour; its close relationship to a tophus is usually evident. This type of lesion is most commonly observed in the hand or foot. Comment In a retrospective review of the available roentgenograms of 78 randomly selected cases of gout with bone erosion, there were 31 patients (40 per cent) in whom this finding was manifest. It was noted in the foot in 17 cases, in the hand in 3 others, and in both the hand and the foot in 11 cases. It tended to occur in patients with severe disease but was also observed in instances where there were only a few bone erosions. Many of the patients in this series had roentgenograms of a limited number of joints. It is possible that in patients with more complete radiologic joint surveys this type of lesion will be found to be more widely prevalent. Certain aspects of the differential diagnosis deserve comment. The outward dis-placement of the overhanging margin away from the bone contour is an important feature. Similar erosions without such displacement are common in rheumatoid arthritis (Fig. 3), as well as in gout. One may postulate that this displacement reflects the gradual increase in size of the urate nidus, which grows by multicentric deposition of urate crystals. As this progresses, bone resorption at the interface with the tophus develops into a discrete erosion and, concomitantly, periosteal bone apposition at the outer aspect of the involved cortex causes the over-hanging margin of bone. The tophus usually appears as a relatively discrete, soft-tissue nodule adjacent to the erosion. In rheumatoid arthritis, on the other hand, the erosions are associated with a diffuse soft-tissue swelling of the entire joint and their localization, apparently influenced by the anatomy of the joint in question, follows a more predictable pattern (2) (Fig. 3). Other radiologic features which serve to differentiate the lesions of gout from those of rheumatoid arthritis have been emphasized elsewhere (1, 3). Department of Radiology University Hospital