Modulation of fixation stiffness from flexible to stiff in a rat model of bone healing

Constant fixator stiffness for the duration of healing may not provide suitable mechanical conditions for all stages of bone repair. Therefore, we investigated the influence of stiffening fixation on callus stiffness and morphology in a rat diaphyseal osteotomy model to see if healing time was shortened and callus stiffness increased through modulation of fixation from flexible to stiff. An external unilateral fixator was applied to the osteotimised femur and stiffened by decreasing the offset of the inner fixator bar at 3, 7, 14, and 21 days post-operation. After 5 weeks the rats were killed and healing was evaluated with mechanical, histological and micro-computed tomography methods. Constant fixation stiffness control groups with either stiff or flexible fixation were included for comparison. The callus stiffness of the stiff group and all 4 experimental groups was greater than in the flexible group. The callus of the flexible group was larger but contained a higher proportion of unmineralized tissue and cartilage. The stiff and modulated groups (3, 7, 14, and 21 days) all showed bony bridging at 5 weeks, as well as signs of callus remodeling. Stiffening fixation at 7 and 14 days post-osteotomy produced the highest degree of callus bridging. Bone mineral density in the fracture gap was highest in animals where the fixation was stiffened after 14 days. The predicted benefit of a large robust callus formed through early flexible fixation could not be shown, however the benefits of stabilizing a flexible construct to achieve timely healing was demonstrated at all time points.