Thomas G. Ribble
Michael H. Santare, Ph. D, Freeman Miller, M.D.
Department of Mechanical Engineering
University of Delaware
Newark, Delaware 19176
INTRODUCTION: It is well known that the mechanical forces which act upon the femur have an important role in its development from infancy through adulthood. At birth, the internal architecture of proximal femur is comprised of various tissues such as cancellous bone, growth plate and epiphyseal cartilage. The neck shaft-angle is about 150 degrees. As it develops the growth plate changes shape, the epiphyseal cartilage ossifies and the neck-shaft angle decreases to about 120 degrees at skeletal maturity. In children with cerebral palsy(CP), damaged brain tissue causes loss of muscle control which can cause spasticity of the muscles. The abnormal force environment in the hip joint resulting from the spastic muscles commonly results in a valgus deformity of the femur. Orthopedic problems may arise from this deformity, such as loss of mobility and dislocation of the hip. The intention of this work is to develop relationships between the abnormal forces in the hip joint, which result from pathologies like cerebral palsy, and the deformed proximal femur. These relationships will be useful to the medical community in evaluation and selection of possible treatment for patients.
MATERIALS AND METHODS: In this investigation finite element models have been constructed to analyze the stresses in the developing femur. The Finite Element Method is an appropriate means of analysis because of the complex shape and internal architecture of the developing femur. In addition, this method allows the analyst to quickly produce results for a number of various loading conditions. PATRAN, a solid modeling software package from PDA Engineering, is used to construct a solid model, including the geometry of internal structures such as the growth plate, from Computed Tomography(CT) data. PATRAN is also used to mesh the solid model. In each model, eight noded isoparametric hexahedral elements are used in the analysis. The material properties for the various tissues, such as the growth plate, cancellous bone, epiphyseal cartilage, cortical bone and articular cartilage are based on values in the literature and are modeled as isotropic and homogeneous. Loading conditions applied to the finite element models are generated from a computer program which calculates the forces in the hip joint. This program was developed at A.I. duPont Institute in conjunction with the present research. The program is used to generate hip contact forces on the femur for various activities for a normal child and a typical cerebral palsy patient.
RESULTS: The models are analyzed using ABAQUS finite element software from Hibbit, Karlsson & Sorenson. Results in the form of contour plots and graphs are generated to illustrate the stress state in the proximal femur and in particular, the growth plate. Figure 1 shows the shear stress along the growth plate for various activities.
Figure 1. Shear stress along growth plate for various activities
DISCUSSION: Previous theories have proposed that endochondral ossification is accelerated by high shear stress. Given this theory, the growth plate should define a region of low shear stress to retard its ossification and continue to provide longitudinal growth of the femur. With stress results for activities in the normal and CP patient, implementation of these proposed concepts can aid in the understanding the growth of the proximal femur.
CONCLUSION: Figure 1 illustrates that the stresses in the growth plate change for the modeled activities. It appears that certain activities produce the highest positive or negative stresses, while the spastic case produces relatively low magnitude stresses. This may provide insight into the mechanisms by which growth patterns differ in the normal and spastic cases. Ongoing work will focus on using the stress results, like those seen in Figure 1, to better understand the growth patterns of the proximal femur.
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