Trabecular bone recovers from mechanical unloading primarily by restoring its mechanical function rather than its morphology

Upon returning to normal ambulatory activities, the recovery of trabecular bone lost during unloading is limited. Here, using a mouse population that displayed a large range of skeletal susceptibility to unloading and reambulation, we tested the impact of changes in trabecular bone morphology during unloading and reambulation on its simulated mechanical properties. Female adult mice from a double cross of BALB/cByJ and C3H/HeJ strains (n = 352) underwent 3 wk of hindlimb unloading followed by 3 wk of reambulation. Normally ambulating mice served as controls (n = 30). As quantified longitudinally by in vivo μCT, unloading led to an average loss of 43% of trabecular bone volume fraction (BV/TV) in the distal femur. Finite element models of the μCT tomographies showed that deterioration of the trabecular structure raised trabecular peak Von-Mises (PVM) stresses on average by 27%, indicating a significant increase in the risk of mechanical failure compared to baseline. Further, skewness of the Von-Mises stress distributions (SVM) increased by 104% with unloading, indicating that the trabecular structure became inefficient in resisting the applied load. During reambulation, bone of experimental mice recovered on average only 10% of its lost BV/TV. Even though the addition of trabecular tissue was small during reambulation, PVM and SVM as indicators of risk of mechanical failure decreased by 56% and 57%, respectively. Large individual differences in the response of trabecular bone, together with a large sample size, facilitated stratification of experimental mice based on the level of recovery. As a fraction of all mice, 66% of the population showed some degree of recovery in BV/TV while in 89% and 87% of all mice, PVM and SVM decreased during reambulation, respectively. At the end of the reambulation phase, only 8% of the population recovered half of the unloading induced losses in BV/TV while 50% and 49% of the population recovered half of the unloading induced deterioration in PVM and SVM, respectively. The association between morphological and mechanical variables was strong at baseline but progressively decreased during the unloading and reambulation cycles. The preferential recovery of trabecular micromechanical properties over bone volume fraction emphasizes that mechanical demand during reambulation does not, at least initially, seek to restore bone's morphology but its mechanical integrity.