Improved design of cementless hip stems using two-dimensional functionally graded materials.

Increasingly, it is acknowledged that bone resorption around cementless hip implants may cause future problems. The solution is frequently sought in reducing implant stiffness. However, this confronts the designer with a true design conflict: how to reduce the stiffness without excessively loading the proximal bone/prosthesis interface? The aim of this work is to improve the design of cementless hip stem material, using two-dimensional (2D) functionally graded material (FGM) concept in order to solve the above problems. Two models were used in this analysis, using three materials with different elastic moduli, E(1), E(2), and E(3). In model I, the elastic moduli E(1) and E(2) gradually change along the upper stem surface, while E(3) is maintained constant along all the lower surface of the stem. However, in model II, the elastic moduli E(1) and E(2) gradually change along the lower stem surface, while E(3) is maintained constant all along the upper stem surface. It is found that the recommended model is model I, which has three distinct materials of hydroxyapatite, Bioglass, and collagen. The recommended design of 2D FGM is expected to reduce the stress shielding by 91% and 12%, respectively, compared with titanium stem and model II of FGM. It is found that this new design reduces the maximum interface shear stress at the lateral and medial sides of the femur by about 50%, compared with titanium stem. Furthermore, the maximum interface shear stress is reduced by about 17% and 11% at the lateral and medial sides of the femur, respectively, compared with that of model II of FGM.

Design optimization of cementless metal-backed cup prostheses using the concept of functionally graded material.

Metal backing has been widely used in acetabular cup design. A stiff backing for a polyethylene liner was initially believed to be mechanically favourable. Yet, recent studies of the load transfer around acetabular cups have shown that a stiff backing causes two problems. It generates higher stress peaks around the acetabular rim than those caused by full polyethylene cups and reduces the stresses transferred to the dome of the acetabulum causing stress shielding. The aim of this study is to overcome these two problems by improving the design of cementless metal-backed acetabular cups using the two-dimensional functionally graded material (FGM) concept through finite-element analysis and optimization techniques. It is found that the optimal 2D FGM model must have three bioactive materials of hydroxyapatite, Bioglass and collagen. This optimal material reduces the stress shielding at the dome of the acetabulum by 40% and 37% compared with stainless steel and titanium metal backing shells, respectively. In addition, using the 2D FGM model reduces the maximum interface shear stress in the bone by 31% compared to the titanium metal backing shell.