Adaptation of Tibial Structure and Strength to Axial Compression Depends on Loading History in Both C57BL/6 and BALB/c Mice

Tibial compression can increase murine bone mass. However, loading protocols and mouse strains differ between studies, which may contribute to conflicting results. We hypothesized that bone accrual is influenced more by loading history than by mouse strain or animal handling. The right tibiae of 4-month-old C57BL/6 and BALB/c mice were subjected to axial compression (10 N, 3 days/week, 6 weeks). Left tibiae served as contralateral controls to calculate relative changes: (loaded - control)/control. The WashU protocol applied 60 cycles/day, at 2 Hz, with a 10-s rest-insertion between cycles; the Cornell/HSS protocol applied 1,200 cycles/day, at 6.7 Hz, with a 0.1-s rest-insertion. Because sham loading, sedation, and transportation did not affect tibial morphology, unhandled mice served as age-matched controls (AC). Both loading protocols were anabolic for cortical bone, but Cornell/HSS loading elicited a more rapid response that was greater than WashU loading by 13 %. By 6 weeks, cortical bone volume of each loading group was greater than of AC (average + 16 %) and not different from each other. Ultimate displacement and energy to fracture were greater in tibiae loaded by either protocol, and ultimate force was greater with Cornell/HSS loading. At 6 weeks, independent of mouse strain, the WashU protocol produced minimal trabecular bone and the trabecular bone volume fraction of Cornell/HSS tibiae was greater than that of AC by 65 % and that of WashU by 44 %. We concluded that tibial adaptation to loading was more influenced by waveform than mouse strain or animal handling and therefore may have targeted similar osteogenic mechanisms in C57BL/6 and BALB/c mice.