Biomechanical effects of intramedullary reaming and nailing on intact femora in rats.

In order to study the effects of intramedullary reaming and nailing on bone mechanics, the left femur of male Wistar rats was reamed after circular periosteal stripping of the middle part as if for osteotomy. Osteotomy was, however, not performed. The rats were allocated into three groups: the first group had no nailing, the second group received flexible intramedullary polyacetal nails, and the third group was nailed with solid stainless-steel nails. Ten animals from the first group were sacrificed on the day of surgery. At four, eight, 16, and 24 weeks after operation, ten animals from each of the three groups were sacrificed. Both femora were tested mechanically and examined histologically. Intramedullary reaming resulted in the following immediate reductions in mechanical properties compared to the unoperated right femur: strength, 15%; stiffness, 20%; toughness, 7%; and resilience, 15%. From four weeks on, strength and energy absorption values were always lowest in the group with steel nails. The differences between groups were significant at four, 16, and 24 weeks. The lower values for mechanical properties of the femora with steel nails is interpreted as a result of stress shielding by the rigid nails. Flexible nails did not interfere with the ultimate restoration of mechanical properties of bones initially weakened by reaming.