Bone remodeling regulation under unloading conditions: numerical investigations.

The present paper addresses the following question: can a simple regulatory bone remodeling model predict effects of unloading conditions on the trabecular bone morphology? In an attempt to answer this question, rat tail-suspension was chosen as a model that mimics the microgravity environment. Over 23 days, histomorphometric analysis was carried out on cross-sections of tibias of the suspended animals. The slices were digitalized and images discretized to obtain osteocyte distribution and apparent bone density. Based on these experimental data, finite element simulations were conducted to evaluate the bone loss and the change in trabecular architecture similar to those observed after a spaceflight. The numerical model is driven by a remodeling law that takes into account the nonuniform osteocyte distribution that may itself provide mechanoreception. We used the bone density rate of change from the remodeling theory and a time stepping algorithm witch are implemented in a finite element software. This approach takes into account the unloading effects on bone remodeling process and permits to confront experimental and numerical data. We showed that there is a good agreement between these data, particularly at the beginning of the simulated bone mass loss during the rat tail-suspension experiment. Indeed, we obtained a variation of 5.25% at day 7 (D7), 2.09% at day 13 (D13) and finally, 51.03% at day 23 (D23). Despite that last variation, the proposed theoretical model can be suitable to simulate the alteration of bone mineral density under the specific unloading conditions of the rat tail-suspension model.