Objective To construct a detailed, 3-dimensional, anatomically accurate finite element (FE) model of lumbar L4-L5 segment from CT data with a new kind of computer aided design (CAD) method. Methods A modified "no-seed region segmentation" was done to extract the interest region in the CT scan images and produce a binary image. "Best cross-section planes" accounting for the preferential direction dictated by lumbar spine were placed on the initial iso-surface model, forming a "non-regular piecewise subspace". This subspace and the embedded iso-surface mode were transformed by local affine transforms to a "regular subspace", in which a surface mesh of high quality was generated quickly. Finally a reverse transform procedure was employed to recover the shape feature of the lumbar surface mesh of lumbar L4-L5 in the original 3-dimensional space, which was then imported into ANSYS for the 3-dimensional FE mesh construction. Results All complicated anatomical features of the L4-L5 segment were explicitly represented in the unprecedented finite element model. The predicted results for compression, flexion and extension correlated well with experimental data under similar loading configurations. Conclusion The presented CAD method containing advanced algorithm implements fast and accurate simulation of such complicated geometry with fine mesh representation for lumbar FE analysis.

Changes in bone stress in the proximal femur following implantation can be estimated with the use of composite beam theory. The aim of this study was to construct the mathematical analytical models for predicting the degree of stress shielding and to test the validity of the predictions using finite element simulation. To define the periprosthetic bone stress values, the proximal femur was divided into eleven equidistant cross sections, then each section was divided into four quadrants corresponding to the anterior, posterior, medial and lateral aspects of the femur. Bone stress values were calculated by both mathematical analytical models and finite element analysis, then linear regression analyses produced slopes and R-values that show numerical and finite element results corresponding well to intact femur and both the types of fixation with/without cement. And the results also showed that femoral bone stress shielding by both the prostheses occurred in most periprosthetic zones. The most serious regions occurred in the proximal medial quadrant. This study has succeeded in creating the mathematical analytical models to predict the bone, cement and prostheses stress values, and thus can help us to evaluate the mechanical behavior of total hip replacement, to further understand the distinction between different fixation, and to make advances in implant design, surgical technique and long-term results.
Arthroplasty, Replacement, Hip ; Biomechanical Phenomena ; Femur ; physiopathology ; Humans ; Models, Theoretical ; Stress, Mechanical ; Tensile Strength