Design Optimization by Finite Elements Model

To diminish wear damage due to the potentially high contact stresses in the articulating components in the new design, FEM was employed to optimize the surface geometry of these components. Finite element models of the artificial joint were created using Algor ver12.04. Linear stress analyses were performed based on load acting across the joint components from the middle phalange towards the head of the proximal phalanx. Depending on the angular position of the middle phalanx with respect to the proximal phalanx, the joint load acting on the latter could be from 0° at full extension to 90° at full flexion. To examine the static behavior of joint components to changes in the loading direction, analysis was performed using a two dimensional model across the sagittal section.

Three different shapes of the proximal component were examined to evaluate the optimum geometry favorable for load transfer with minimal contact stresses and distortion to the articulating surfaces. The modal joint size of 7.5 mm was employed and a value of 10 mm was assigned to the joint thickness in the plane stress condition. Model 1 has a proximal component with a thin profile on the articulating surface based on the requirement of minimal excision to the bone joint. Model 3 has a proximal component with complete solid support underneath the articulating surface. Model 2 has a proximal component with a geometry intermediate between models 1 and 3, with the articulating surface partially supported by solid materials. The articulating surface of the distal component was assumed to be in congruence fit to the proximal component and only a short region of the fixation stem was included in the 2-D model to reduce computational time. Appropriate boundary conditions were applied at the bottom edge of the proximal component to restrain both translational and rotational movements in x, y and z directions. A maximum joint load of 442 N was applied perpendicular to the top edge of the distal component. Isotropic 2-D elasticity element was assigned to the distal and proximal components with Young modulus of 210 GPa and Poisson ratio of 0.3 for CoCr. The two components were meshed in quadrilateral elements with a mesh density of 8000, angular step of 15° and geometry ratio not greater than 1.25.

From the computed displacement fields, deformation happens mainly on the joint surface of model 1 which results in the highest bending stresses at the joint surface. With the aid of solid support underneath the joint surface in model 3, deformation shifts to the junction of the stem region and the induced internal stresses in the proximal component are lower than model 1. Since deformation is shared by both the joint surface and the stem region in model 2, the maximum stress induced in the proximal component in model 2 is the smallest among the three models.

The model with the lowest contact stress on the joint surface of the distal component and the lowest bending stress in the stem junction of the proximal

Fig. 9.11 Stress (left) and displacement patterns for the 3 models taken at 60°. Please note that these are b/w images taken from colour originals and therefore some detail is lost

component, as shown in the examples in Fig. 9.11, was chosen for the new design of the artificial PIP joint. Model 3 was found to be the optimum geometry with the smallest contact and bending stresses in the joint components in all the loading directions.

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