The mechanical behavior and morphological structure of callus in experimental callotasis.

Experimental callotasis was made in rabbit femurs. In order to compare mechanical behaviors and morphological structure of callus in callotasis, mechanical tests and histological examination were performed. Twenty Japanese white male rabbits were used. The right femurs were osteotomized at the level of the midshaft with a rotary cutter and fixed with a mini-model external fixator. After a 5-day waiting period, bone lengthening was started at the rate of 0.25 mm two times a day. Animals were divided into four groups. Group 1 and Group 2 were continual distraction group, Group 3 and Group 4 discontinued distraction group. The viscoelastic property was demonstrated in continual distraction group irrespective of distraction period, we thought the central undifferentiated connective tissue in callus was mainly responsible for the mechanical behavior. On the other hand, in discontinued distraction group, viscoelastic property shifted to elastic property corresponding to the rest period, we thought this change of mechanical behavior was histologically owing to the replacement of undifferentiated connective tissue by cartilage and the partial union of bone in callus.