In vivo cyclic axial compression affects bone healing in the mouse tibia

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Abstract
Abundant evidence exists that fracture healing can be influenced by mechanical loading. However, the specific loading parameters that are osteogenic remain unknown. We hypothesized that the bone healing response in mouse tibial osteotomies would be different with a short delay before loading compared to immediate load application, as well as with higher and lower load magnitudes applied. Eighty 12‐week‐old mice underwent osteotomy of the left tibia followed by intramedullary nailing. Mice were divided into six groups based on days delayed until application of load (0 days or 4 days) and amplitude of cyclic load (0.5N, 1N, or 2N). Loading regimens were applied at 1 Hz for 100 cycles per day, 5 days per week for 2 weeks, using an external device that applied axial compression to the tibia. Bone healing was assessed by both microcomputed tomography (CT) and four‐point bend testing. A short delay followed by cyclic application of a relatively low load led to improved fracture healing, as determined by increased callus strength, but this enhancement disappeared as load amplitudes increased. Load initiation immediately following fracture inhibited healing, regardless of the magnitude of load applied. MicroCT measurements of calluses in the early healing stage did not predict the mechanical strength of the fractures. These findings confirm that controlled, noninvasive cyclic loading can improve the strength of healing callus. However, application of load immediately after fracture appears to be detrimental to healing. Load magnitude also plays a critical role, and must be taken into account in future studies and clinical applications. As the loading parameters necessary to enhance fracture healing become refined, external compression may be used as a potent stimulus for treating fractures with decreased biological capacity. © 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:1679–1686, 2006