Macrophages in trauma-induced myositis ossificans.

The basic cellular mechanisms of different forms of myositis ossificans are poorly known. In the current experiment the nature of the early (24-168 h) inflammatory cell reaction preceding trauma-induced myositis ossificans was studied. New bone formation was induced in the vastus intermedius region of the rabbit quadriceps muscle by means of immobilization and daily passive mobilization. Before the start of treatment, a cell harvesting device (viscose cellulose sponge in a silastic tube) was implanted in the region of interest. The opposite intermedius muscle and a standardized surgical skin wound served as the control sites. The results showed a significantly prolonged invasion of macrophages into the ossifying intermedius muscle as compared with the control intermedius muscle. It is hypothesized that microinjury and subsequent muscle necrosis cause the invasion of macrophages, and these cells respond to the conditions of the traumatized muscle under passive mobilization by releasing osteogenic growth factors.