The effect of local deep microwave hyperthermia on experimental zymosan-induced arthritis in rabbits

The effect of local deep microwave hyperthermia (LDMWH) on normal and Zymosan-induced arthritis has been evaluated in 12 rabbits (24 joints). LDMWH, four treatments to each joint (twice weekly for a period of 2 wk), was generated by an antenna operating at 915 MHz for 60 min, reaching an intraarticular temperature of 42.5 +/- 0.5 degrees C. A surface cooling system was used with the microwave apparatus. Two weeks after the last treatment, all animals were sacrificed. The application of LDMWH on normal joints induced a limited proliferation of the synovial lining cells with a minimal perivascular infiltration of mononuclear and neutrophil cells. However, no histologic damage to the skin, muscles, bone, cartilage or bone marrow adjacent to the heated joints could be noted. Induction of Zymosan arthritis (2 wk before LDMWH) was characterized by pannus formation and granulomatous reaction accompanied by fibrinoid deposits and disseminated necrotic foci in the synovial intima. The LDMWH treatment on the examined arthritic joints brought about a reduction in the degree of granulomatous reaction concomitant with the appearance of some fibrocytes and fine collagen fibrils. These findings suggest that LDMWH can be safely applied, even repeatedly, without morphologic evidence of damage to any normal mesenchymal tissue. Moreover, it reduces the inflammatory process in experimentally induced synovitis.