Geometric mouse variation: Implications to the axial ulnar loading protocol and animal specific calibration

Large variations in axial ulnar load strain calibration results suggest that animal-specific calibrations may be necessary. However, the optimal set of geometric measures for performing an animal-specific calibration are not known, potentially as a result of confounding effects associated with experimentally introduced variation. The purpose of this study was to characterize the inherent variability of ulnar geometric measures known to influence periosteal midshaft strain during axial ulnar exogenous loading, and to further quantify the relationship between the variance of those geometric measures and periosteal strain during axial loading. Thirty-nine right mouse forelimbs were scanned with microCT. Seven geometric measures that influence periosteal strain resulting from a combined axial and bending loading were computed and used to estimate animal-specific strains on the periosteal midshaft. Animal specific strains were estimated using a theoretical model based on the generalized flexure formula. The predicted mean and standard deviation of the simulated midshaft strain gauge measurement resulting from the inter-animal geometric differences was −985±148 με/N. The complete beam bending term associated with bending about the I_min axis accounted for 89% of the variance and reduced the residual RMSE to 50.4 με. Eccentricity associated with the axial loading contributed a substantial portion of variation to the computed strain suggesting that calibration procedures to account for animal differences should incorporate that variable. The method developed in this study provides a relatively simple procedure for computing animal-specific strains using microCT scan data, without the need of a load/strain calibration study or computationally intensive finite element models.