Architectural changes of trabecular bone caused by the remodeling process
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Architectural changes of trabecular bone caused by the remodeling process Richard Weinkamer1, Markus A. Hartmann1, Yves Brechet2, and Peter Fratzl1,3 1 Max Planck Institute of Colloids and Interfaces, Dept. of Biomaterials, Potsdam, Germany 2 ENSEEG, LTPCM, Domaine Universitaire de St. Martin d´Hères, Cedex, France 3 Ludwig Boltzmann Institute of Osteology at the Hanusch Hospital of WGKK and AUVA Trauma Centre Meidling, 4th Medical Department, Hanusch Hospital, Vienna, Austria ABSTRACT Using a stochastic lattice model we have studied the architectural changes of trabecular bone occurring while the structure is remodeled. Our model considers the mechanical feedback loop, which control the remodeling process. A fast algorithm was employed to solve approximately the mechanical problem. A general feature of the model is that a networklike structure emerges, which further coarsens while the bone volume fraction remains unchanged. Decreasing the mechanical response of the system by either lowering the external load or the internal mechano-sensitivity leads not only to a reduction of the bone volume fraction, but results in topological changes of the trabecular bone architecture, where the loss of horizontal trabeculae is the most obvious effect. INTRODUCTION Living bone gains the ability for adaptation by a mechanobiological remodeling process. Mechanobiology in general deals with the question of how load-bearing tissues are produced, maintained and adaptated by specialized cells actively responding to mechanical stimuli in their environment [1]. Another important example of a mechanobiological process in the human skeletal system beside remodeling is tissue differentiation occurring in bone development and fracture healing. Compact and trabecular bone is continuously remodeled in an interplay of bone resorbing osteoclasts and bone depositing osteoblasts. Being mechanically controlled - an idea dating back to the end of the 19th century – bone remodeling follows the principle that bone is removed where it is mechanically not needed and deposited in highly stressed regions in order to avoid overloading (Wolff-Roux law). Different time scales are active during remodeling: while the response of the cells sensing a mechanical stimulus is thought to be rather instantaneous, the complete resorption of a bone packet takes already a few weeks, the deposition of bone even a few months. The turnover time for the whole trabecular structure is about 4-5 years [2]. In a computational approach of bone remodeling two different problems have to be solved (Fig. 1): given a networklike, trabecular architecture, the structure first has to be mechanically assessed calculating the stresses and strains within the structure. In a second step the result is then fed back to the biological part of the model, where a remodeling rule specifies the local probabilities for bone resorption and bone deposition as a function of the mechanical stimulus. Implementing the Wolff-Roux law in a computer model and using finite element methods (FEM), it
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