The bone structure functionally adapts by remodeling itself in response to alterations in the mechanical environment.
In order to elucidate this mechanism, remodeling phenomena occurring from the cell level to the tissue level have been investigated at each individual level of the hierarchy. However, the details of the coupling effects among the hierarchies are mostly unknown. Thus, in this study, with regard to cancellous bones with complex trabecular structures, the adaptation mechanism from the remodeling of the structural element level to the adaptive changes in the entire structure was evaluated by using computational dynamics.
First, based on the remodeling rule that directly relates the trabecular-level mechanical stimulation with the morphological changes in bones, a large-scale finite-element simulation model of the trabecular remodeling was proposed.
Next, as a result of conducting the remodeling simulation of a proximal portion of femur, structural changes in the trabecula along with the mechanical state of cancellous bones at the macroscopic level caused by the uniformalization of trabecular-level microscopic mechanical stimuli was observed. In addition, this simulation model was applied to the digital image model that enables an accurate representation of the actual 3D trabecular structure (Fig. 1). As a result, changes in morphological feature value in the obtained trabecular structure corresponded well with those in the previously reported experimental result. On the other hand, the results of a micro-three-point bending test revealed that the mechanical property of individual trabecula is not significantly affected by its mechanical environment.
Thus, the structural adaptation process of cancellous bones from the micro- to macro-environment due to the alteration of trabecular morphology was elucidated.
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| Fig. 1: Image-based model of cancellous bones in canine distal femur constructed from X-ray micro-CT image data. |
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| Fig. 2: Trabecular structural changes adjacent to the bone-screw interface: (a) Compressive loading case (Ic); (b) shear loading case (Is). |
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Further, an application of this simulation method in the field of medical engineering was performed in the form of a trabecular remodeling simulation of cancellous bones with the assumption of the proximity to a fixing screw (Fig. 2).
As a result, in response to the difference in the loading environment, either the formation or the loss of characteristic trabecula was observed on the screw thread. Furthermore, when this simulation was applied to the design problem of the artificial hip stem conformation, it was revealed that designing the stem conformation is possible with the consideration of bone remodeling.
Consequently, it implied that the simulation method is advantageous to evaluate and design prosthetics for bones. |
The abovementioned method can be used in combination with the medical image data obtained from CT or MRI, and it becomes possible to comply with the conformational differences in individual patients in a flexible manner (Fig. 3).
Therefore, the method described in this study is expected to provide a practical strategy toward the construction of a computational mechanical model for the conformation design problem of bone implants and general biological tissues. |
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| Fig. 3: Image-based finite-element analysis of human proximal femur using medical CT image combined with large-scale computation. |
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