An easily reproducible and biomechanically standardized model to investigate bone healing in rats, using external fixation.

Abstract We have established a new small animal model to investigate the process of bone regeneration. A total of 42 male Sprague-Dawley rats received an osteotomy of the left femur, stabilized with a custom-made external fixator. The fixation method was chosen to create an easily reproducible, biomechanically well-defined model with minimized interference of the implant with the healing zone. At 14 or 56 days post-operation, the animals were sacrificed and examined biomechanically, histologically and radiologically. Radiologically, the femurs of all animals were anatomically positioned directly post-operation and remained in that position throughout the examination period. At 14 days post-operation, a typical periosteal callus formation could be observed both histologically and radiologically. At 56 days post-operation, the osteotomy was almost completely bridged by periosteal callus and the biomechanical competence of the bones was fully restored. Relative to the intact contralateral femur, the torsional stiffness median was 130.3% (interquartile range 118.9-157.7%) and the maximum torsional failure moment median was 135.6% (interquartile range 69.5-208.7%). As this model provides standardized conditions, it is suitable for a wide range of investigations and is particularly valuable for investigations of locally applied therapies, such as osteoconductive materials or osteoinductive factors.