Effect of load orientation on finite element strain predictions in a rabbit tibial loading model

Mechanical loading plays an important role in the maintenance of bone quantity and quality. Rodents are the most frequently used in vivo loading model for examining the relationship between applied mechanical loads and the bone adaptation response, but they do not naturally exhibit human-like intracortical remodeling. Instead, our group has developed a non-invasive in vivo rabbit tibial loading model. This study aimed to develop and validate statically equivalent computed tomography (CT)-based finite element (FE) models of the rabbit tibia to capture the micro-mechanical environment produced by our in vivo mechanical loading device. We further sought to investigate the strain prediction sensitivity to changes in the assumed force vector orientation. Twenty hindlimbs from New Zealand White Rabbits were cyclically loaded in uniaxial compression with strain gauge rosettes affixed to the tibia. The hindlimbs were then disarticulated at the hip, imaged with CT in replica experimental fixtures, and processed into specimen-specific FE models. A mathematical optimization routine was used to determine the individual force vector orientations that minimized the error between FE predicted and experimentally measured bone strains, which yielded highly accurate strain predictions ($R^2=0.96$) that exhibited a $\mathrm{Y=\; X}$ type of relationship after bias adjustment. This approach resulted in substantially lower strain prediction errors when compared to models using various single assumed orientation techniques. We also found that even slight deviations in the assumed hindlimb orientation substantially affect strain predictions. These findings suggest that experimentally informed approaches may be useful for hindlimb-specific loading orientations. This work serves to enable future studies examining the mechanobiology of bone adaptation using the rabbit animal model.