Computer Simulations

Several studies have used computer models to gain more insight in the relation between bone remodeling and changes in the skeleton with age, during menopause, or in osteoporosis. The first models of cancellous bone treated the cancellous bone as a number of bone packages (22) or used trabeculae with a certain thickness distribution derived from published histomorphometric data (23). In these models, the trabeculae were not connected to form a cancellous architecture.

Later models used two-dimensional networks to simulate the trabecular architecture. In these studies, trabeculae were removed or thinned to investigate the effect of aging and bone loss on strength and stiffness of the architecture (24,25). The effects of thinning of trabeculae have also been investigated three dimensionally (26). Both two-dimensional and three-dimensional studies found that loss of trabeculae has more drastic effects on the mechanical properties of cancellous bone than thinning of trabeculae.

In reality, the bone architecture changes during the remodeling cycle because of over- or underfilling of resorption cavities. If a resorption cavity breaches a trabecula, this trabecula is probably not repaired (15). This last effect is ignored in simulations that mimic aging in cancellous bone by gradually thinning trabeculae.

For a close examination of the effects of bone remodeling on cancellous bone architecture and stiffness, models that simulate the whole remodeling cycle are needed. In these models, creation and refilling of resorption cavities should be mimicked in three dimensional cancellous bone models. Currently, two studies that simulate the bone remodeling cycle in cancellous bone in three dimensions are described in literature.

The first study used an artificial bar-plate model to simulate the cancellous bone (27). In this model, resorption cavities were created in the middle of the bars that simulated the trabeculae. Using this model, the authors determined contributions of the formation deficit and breached trabeculae to the total bone loss. They found that breached trabeculae accounted for 20 to 40 % of the total bone loss, depending on remodeling rate.

We introduced another approach using detailed computer reconstructions made by micro-CT (28). These micro-CT models have a resolution high enough to represent the individual trabeculae in the model. In this simulation model, bone resorption could be initiated everywhere on the surface of the trabeculae, mimicking in vivo bone remodeling. For the bone remodeling parameters such as resorption depth and formation deficit, values determined in bone histology studies were used. This second model is described in detail in this chapter.

These simulation models can be used to determine the contributions of the formation deficit, breached trabeculae, and loose fragments to the total bone loss. Changes in morphology caused by remodeling can be investigated and the effects of changes in remodeling parameters, for example, a larger resorption depth, on the architecture can be examined. The effects of these changes in architecture on the mechanical properties of the specimens can be determined.

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