Comparison of one-dimensional and two-dimensional functionally graded materials for the backing shell of the cemented...

Among the factors that have been suggested as contributing to the failure of a total joint replacement are stress shielding and the subsequent bone resorption. Recent studies have shown that when a backing shell made from a Ti alloy is used, high stresses are generated in the cement at the edges of the cup, and low stresses are generated at the dome of the bone in the acetabulum; thus, the bone at the dome suffers stress shielding and the cement edge suffers high stresses. The aim of this study was to investigate the effect of using a functionally graded material (FGM), instead of Ti alloy, for the backing shell (BS) on the stress distribution in the BS-cement-bone system. Finite-element and optimization techniques were used to obtain the optimal distribution of materials in the tangential direction only of the backing (1D FGM) as well as in the tangential and radial directions of the backing (2D FGM). It was found that the stress distribution in the BS-cement-bone system was about the same, regardless of whether the BS was fabricated from a 1D or 2D FGM. The stress-shielding factor in the bone at the dome of the acetabulum and the maximum von Mises stress in cement at the cement interfaces for 1D and 2D FGM were reduced by about 51%, 69%, and 50%, respectively, compared to the case when the shell was fabricated from a Ti alloy. The optimal elastic modulus of the 1D FGM was obtained with the materials graded from HA at the dome of the acetabulum to a Ti alloy at the rim of the shell. The optimal elastic modulus of the 2D FGM was obtained with the materials graded from Ti alloy at the right edge of the rim, to Bioglass 45S5 at the left edge of the rim, and to HA at the dome of the shell.