Development of an in vivo rabbit ulnar loading model.

Ulnar and tibial cyclic compression in rats and mice have become the preferred animal models for investigating the effects of mechanical loading on bone modeling/remodeling. Unlike rodents, rabbits provide a larger bone volume and normally exhibit intracortical Haversian remodeling, which may be advantageous for investigating mechanobiology and pharmaceutical interventions in cortical bone. Therefore, the objective of this study was to develop and validate an in vivo rabbit ulnar loading model. Ulnar tissue strains during loading of intact forelimbs were characterized and calibrated to applied loads using strain gauge measurements and specimen-specific finite element models. Periosteal bone formation in response to varying strain levels was measured by dynamic histomorphometry at the location of maximum strain in the ulnar diaphysis. Ulnae loaded at 3000 microstrain did not exhibit periosteal bone formation greater than the contralateral controls. Ulnae loaded at 3500, 4000, and 4500 microstrain exhibited a dose-dependent increase in periosteal mineralizing surface (MS/BS) compared with contralateral controls during the second week of loading. Ulnae loaded at 4500 microstrain exhibited the most robust response with significantly increased MS/BS at multiple time points extending at least 2weeks after loading was ceased. Ulnae loaded at 5250 microstrain exhibited significant woven bone formation. Rabbits required greater strain levels to produce lamellar and woven bone on periosteal surfaces compared with rats and mice, perhaps due to lower basal levels of MS/BS. In summary, bone adaptation during rabbit ulnar loading was tightly controlled and may provide a translatable model for human bone biology in preclinical investigations of metabolic bone disease and pharmacological treatments.