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 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 10 s rest-insertion between cycles; the Cornell/HSS protocol applied 1200 cycles/day, at 6.7 Hz, with 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 (avg. +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 of AC by 65% and 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.

Despite a wealth of data on the effects of spaceflight on tendons and bones, little is known about its effects on the interfacial tissue between these two structures, the enthesis. Mice were sent to space on three separate missions: STS-131, STS-135, and Bion-M1 to determine how spaceflight affects the composition, structure, mechanics, and gene expression of the humerus-supraspinatus and calcaneus-Achilles entheses. At the nanoscale, spaceflight resulted in decreased carbonate levels in the bone, likely due to increased remodeling, as suggested by increased expression of genes related to osteoclastogenesis (CatK, Tnfsf11) and mature osteoblasts (Col1, Osc). Tendons showed a shift in collagen fibril size towards smaller diameters that may have resulted from increased expression of genes related to collagen degradation (Mmp3, Mmp13). These nanoscale changes did not result in micro-and milliscale changes to the structure and mechanics of the enthesis. There were no changes in bone volume, trabecular structure, failure load, or stiffness with spaceflight. This lack of tissue-level change may be anatomy based, as extremities may be less sensitive to spaceflight than central locations such as vertebrae, yet results highlight that the tendon enthesis may be robust against negative effects of spaceflight.

Osteoporosis is characterized by low bone mass and increased fracture risk. High frequency, low-amplitude whole-body vibration (WBV) has been proposed as a treatment for osteoporosis because it can stimulate new bone formation and prevent trabecular bone loss. We developed constrained tibial vibration (CTV) as a method for controlled vibrational loading of the lower leg of a mouse. We first subjected mice to five weeks of daily CTV loading (0.5 G maximum acceleration) with loading parameters chosen to independently investigate the effects of strain magnitude, loading frequency, and cyclic acceleration on the adaptive response to vibration. We hypothesized that mice subjected to the highest magnitude of dynamic strain would have the largest bone formation response. We observed a slight, local benefit of CTV loading on trabecular bone, as BV/TV was 5.2% higher in the loaded vs. non-loaded tibia of mice loaded with the highest bone strain magnitude. However, despite these positive differences, we observed significantly lower measures of trabecular structure in both loaded and non-loaded tibias from CTV loaded mice compared to Sham and Baseline Control animals, indicating a negative systemic effect of CTV on trabecular bone. Based on this evidence, we conducted a follow-up study wherein mice were subjected to CTV or sham loading, and tibias were scanned at the beginning and end of the study period using in vivo microCT. Consistent with the findings of the first study, trabecular BV/TV in both tibias of CTV loaded and Sham mice was, on average, 36% and 31% lower on day 36 than day 0, respectively, compared to 20% lower in Age-Matched Controls over the same time period. Contrary to the first study, there were no differences between loaded and non-loaded tibias in CTV loaded mice, providing no evidence for a local benefit of CTV. In summary, 5 weeks of daily CTV loading of mice was, at best, weakly anabolic for trabecular bone in the proximal tibia, while daily handling and exposure to anesthesia was associated with significant loss of trabecular and cortical bone. We conclude that direct vibrational loading of bone in anesthetized, adult mice is not anabolic.

Background The mechanisms underlying the interaction between the local mechanical environment and fracture healing are not known. We developed a mouse femoral fracture model with implants of different stiffness, and hypothesized that differential fracture healing would result. Methods Femoral shaft fractures were created in 70 mice, and were treated with an intramedullary nail made of either tungsten (Young’s modulus = 410 GPa) or aluminum (Young’s modulus = 70 GPa). Mice were then sacrificed at two weeks or five weeks. Fracture calluses were analyzed using standard microCT, histological, and biomechanical methods. Results At two weeks, callus volume was significantly greater in the aluminum group compared to the tungsten group (61.2 mm3 vs 40.5 mm3; p = 0.016), yet bone volume within the calluses was no different between the groups (13.2 mm3 vs 12.3 mm3). Calluses from the tungsten group were more stiff on mechanical testing (18.7 N/mm vs 9.7 N/mm, p = 0.01). The percent cartilage in the callus was 31.6% in the aluminum group and 22.9% in the tungsten group (p = 0.40). At 5 weeks, there were no differences between any of the healed femora. Conclusions In this study, fracture implants with different stiffnesses led to differential fracture healing in this mouse fracture model. Fractures treated with a stiffer implant had more advanced healing at two weeks, but still healed by callus formation. Although this concept has been well documented previously, this particular model could provide a valuable research tool to study the healing consequences of altered fixation stiffness, which may provide insight into the pathogenesis and ideal treatment of fractures and nonunions.

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