Quantification of mechanical stimuli and bone formation in fracture healing using In vivo time-lapsed imaging

During bone regeneration, mechanical loading is believed to be responsible for provoking bone formation, however previous investigations into tissue level loading have been limited to crosssectional studies and relied upon idealized models for mechanics. By applying in vivo time-lapse micro-computed tomography (microCT) in concert with imaged based micro-finite element (microFE) analysis we have overcome these limitations and have identified an association between tissue loading and bone formation during fracture healing. A femoral defect of 1.24[SD = 0.13] mm was created in five female mice (C57BL/6); the femur was first stabilized with an external fixator (MouseExFix, RISystem, Switzerland). Weekly scans were performed using microCT imaging (vivaCT 40, Scanco Medical, Switzerland) over a period of 6 weeks, resulting in a series of timelapsed images. We determined sites of mineralization by registering and overlaying images from the second and third week. Combining this with microFE (Parosol) simulations based upon images of the second week, we separated strains in volumes where mineralization occurred, from volumes where no change occurred. To assess the efficacy of strain as a predictor of mineralization, receiver operating characteristic analysis was used. The optimum strain level correctly predicted 60[SD= 9] % of the mineralization which occurred, and the final state for 86[SD= 4] % of the entire volume. We have for the first time, quantitatively demonstrated that an association exists between local tissue strain and bone formation during fracture healing. This could be used to determine the optimal stiffness for biomaterials intended to promote bone healing.