ABSTRACT
In the field of biomechanics, mathematical models are often used to indirectly compute material properties from test results of intact structures. Traditionally these models have been limited to simple structures such as columns, beams, and tubes. In the last few years the advent of large-scale computer methods in computational mechanics combined with automated voxel-based meshing techniques based on microtomographic datasets have made it possible to expand this approach to the determination of tissue or constituent material properties for large-pore foams such as cancellous bone. The goal of this project is to quantify the accuracy of this approach using a porous mechanical analogue to cancellous bone. The analogues are made of a thermoplastic material which has been treated with an additive to make it x-ray absorptive (commercially available bone cement). Voids were produced by including paraffin beads with the material as it is formed, which are later melted and removed. High resolution microCT scans are made of the analogues, and voxel-based finite element (FE) models prepared from the resulting data. When combined with direct mechanical testing of each sample, the FE models allow the constituent stiffness to be computed indirectly. The predicted stiffness can then be compared with the known stiffness of the void-free plastic as a measure of the accuracy of the method. Furthermore we evaluate the potential impact of testing boundary conditions, element size, and assumed constituent Poisson's ratio on the accuracy of the method. We also compare the bone cement analogues to actual trabecular bone samples in terms of morphology. Although this novel combination of cutting edge technology from two distinct fields (large-scale FE analysis and microCT) holds great promise, this approach is untested and no quantification of its accuracy has been attempted using a material with known constituent properties. When validated, the technique will have an enormous impact in the field of orthopaedics as well as the study of large pore size foams in general. In our laboratory, the ability to indirectly determine cancellous bone tissue stiffness will greatly enhance our efforts to discover more about the mechanisms involved in bone adaptation and age-related osteoporosis.
