Effects of hyperthermia on bone. II. Heating of bone in vivo and stimulation of bone growth.

Previous studies in vitro have shown that it is possible to achieve comparable temperature distribution in bone and the adjacent soft tissues, under appropriate experimental conditions. The objective of the present work was to determine the effects of hyperthermia on bone in vivo. In order to obtain direct temperature measurements in bone, catheters were surgically installed on top of and inside the medullary cavity of the femur of normal rabbits. The thighs were irradiated with 915 MHz microwaves for 45 min, once or twice a week. The temperatures on and inside the bone were maintained between 42.5 and 44.0 degrees C; the resulting temperatures in the muscle were within 1.0 degrees C at depths equidistant from the applicator. After four to six treatments the femora were excised for histopathological examination. New trabecular bone was deposited around the catheters; most bone components including periosteum, osteoid, and fully calcified matrix could be seen. Large numbers of osteoblasts and osteoclasts lined the trabecular surfaces, and numerous cement lines were visible, running in all directions, indicating extensive bone deposition and remodelling. In contrast, control bones (catheters installed--no hyperthermia) showed much less ossification, with many areas of thin incomplete osteoid. Further, bones treated with hyperthermia only (no surgical trauma) showed no such changes. Thus, it appears that following an initial insult, hyperthermia promotes bone deposition.