Anthony G. Au

A NURBS-based technique for subject-specific construction of knee bone geometry

Subject-specific finite element (FE) models of bones that form the knee joint require rapid and accurate geometry construction. The present study introduces a semi-automatic non-uniform rational B-spline (NURBS) technique to construct knee bone geometries from computed tomography (CT) images using a combination of edge extraction and CAD surface generation. In particular, this technique accurately constructs endosteal surfaces and can accommodate thin cortical bone by estimating the cortical thickness from well-defined surrounding bone. A procedure is also introduced to overcome the bifurcation at the femoral condyles during surface generation by combining transverse and sagittal plane CT data. Available voxel- and NURBS-based subject-specific construction techniques accurately capture periosteal surfaces but are limited in their ability to capture endosteal geometry. In this study, the proposed NURBS-based technique and a typical voxel mesh technique captured periosteal surfaces within an order of magnitude of image resolution. The endosteum of diaphyseal bone was also captured with similar accuracy by both techniques. However, the voxel mesh model failed to accurately capture the metaphyseal and epiphyseal endosteum due to the poor CT contrast of thin cortical bone, resulting in gross overestimation of cortical thickness. The proposed technique considered both the local and global nature of CT images to arrive at a description of cortical bone thickness accurate to within 2 pixel lengths.

Representation of bone heterogeneity in subject-specific finite element models for knee

Properly representing the heterogeneous distribution of bone tissue material properties is a key step in constructing subject-specific finite element (FE) bone models from computed tomography (CT) data. Conventional methods represent heterogeneity by subjectively grouping bone of similar attenuation together. A new technique characterizing the level of heterogeneity with an objective metric is presented. This technique identifies the minimal level of heterogeneity needed for an accurate FE model. Subject-specific models of the distal femur and proximal tibia were used in this study. An innovative application of an image processing technique in the context of material properties modeling was introduced to facilitate an objective grouping strategy, which gathered together bone based not only on density but also on location thus capturing the natural variation of bone density seen in CT images. A fully heterogeneous model containing unique material properties for each finite element was not necessary to generate an appropriate solution. Von Mises stress, strain energy density, and nodal displacements were predicted within 5% accuracy using a simplified FE femur model containing less than half the number of bone groups of the fully heterogeneous model. Each group contained attenuations varying less than 20% from the group mean. A substantial computational time savings of 60% was gained with the application of the new technique to assign bone mechanical properties.

Subject-specific finite element model of knee: experimental validation using composite and bovine specimens

This work describes the experimental validation of lower limb finite element models using a composite femur, tibia and bovine tibia. Experimental and predicted equivalent strains and principal strain magnitudes and directions were compared to provide all the necessary information for validation studies. Inaccurate geometry, loading conditions and material properties are frequent errors occurring in validation studies; a sensitivity analysis was done to investigate how the latter two affect the validation. Results were most affected when the directions of the loads were not correctly implemented due to slight misalignment between test apparatus loading axis and the vertical axis in the FE environment.

The path to developing realistic finite element long bone models

Current computational capabilities allow for rapid construction of finite element (FE) models but do not guarantee representative models. Simplifications to FE models are necessary because of computational limitations and scarcity of physiological data. With proper modelling and validation, FE models can progress from the realm of parametric studies to clinical applicability. It is often unclear, in the preliminary stages of FE model development, what simplifications are suitable without sacrificing solution accuracy and clinical relevance. This paper presents a technique to create proper FE long bone models for those wanting to develop their own studies. It highlights four important parameters (geometry, material properties, loading conditions, validation) that must be carefully considered and presents a number of methods to aid in achieving proper representation of each parameter. Knee bones are used as an example but the technique can be extended to reconstruct different long bones in the human body with some adjustments.

Development of a finite element tool for stress analysis of femur and tibia incorporating anatomically realistic mechanical properties

Publisher Summary Accurate stress analyses of the distal femur and proximal tibia are important for total knee replacements as well as knee reconstructions. Experimental methods often employ the use of strain gauges, which do not provide data for the internal stress distribution. The use of a finite element model of these bones can provide valuable data on their internal stress distribution. This chapter focuses on the development of a practical finite element model that can be used as a tool to study the stress and strain in intact, as well as surgically altered, femur and tibia. It allows incorporating realistic loading conditions, orthotropic bone properties, and accounts for the spatial variations of bone properties. The immediate applications include optimization of post design for total knee replacements and the positioning of femoral or tibial tunnel for ligament reconstructions. Furthermore, this model can serve as a tool that can easily incorporate the different bone properties from different age groups to enable studies of aging and related prosthetic design or ligament reconstruction considerations.