Stress-protection after external fixation on the intact rabbit tibia.

Bone loss due to the stress-protecting effect of a metal plate on intact bone is a well-known phenomenon. The purpose of the present study was to find whether a similar effect is caused by external fixation. External mini-fixation was applied on the intact rabbit tibial diaphysis of one leg. The other tibia served as control. The animals were sacrificed after 6 or 12 weeks, and both tibiae biomechanically tested in three-point bending. The bone mineral content was measured by photon absorptiometry. No significant changes in bone strength, stiffness or mineral content were found after 6 weeks. The strength and stiffness were significantly reduced after 12 weeks. The median strength and stiffness at that time were 87 and 88 per cent, respectively, in relation to the control bones. The mineral content in the bone segment which had been stabilized by external fixation was significantly reduced after 12 weeks (median 90 per cent of the values for the control bones). No significant change in the mineral content occurred at a level in the tibial diaphysis distal to the external fixation device. It is concluded that the stress-protecting effect caused by external mini-fixation on the rabbit tibia occurs later, and is less pronounced, than that caused by metal plates.