Heterotopic ossification presenting as arthritis

We report 4 patients with arthritis as a presenting manifestation of heterotopic ossification. This cause of arthritis has only recently been noted. The synovial fluids from all 4 patients showed low leukocyte counts (90, 670, 167 and 500/mm3) while the protein concentrations were discordantly high in 3 cases (3.1, 5.3, 4.3 and 2.6 g/dl, respectively). The mechanism for the relatively elevated synovial protein concentration is unknown. These findings may be useful for the early diagnosis of heterotopic ossification.