Production Engineering & Design For Development, PEDD7, Cairo, February 7 – 9, 2006 A NEW DESIGN OF CEMENTLESS HIP STEM USING ONE AND TWO-DIMENSIONAL FUNCTIONALLY GRADED MATERIALS H. S. Hedia * , M.A.N. Shabara + , T.T. El-Midany + , N. Fouda # + Professor, * Associate Professor, # Assistant lecturer Prod. Eng. & M/C Design Dept. Faculty of Engineering, Mansoura University, Mansoura, Egypt. hedia@mans.edu.eg ABSTRACT The designer of a cementless hip stem in total hip replacement must find a balance between two conflicting demands. On one hand, a stiff stem shields the surrounding bone from mechanical loading (stress shielding), which may lead to bone loss, particularly around the proximal part of the stem. Reducing the stem stiffness decreases the amount of stress shielding and hence the amount of bone loss. However, reducing stem stiffness inevitably promotes higher proximal interface shear stresses and thereby increases the risk of proximal interface failure. Therefore, the task of this investigation is to optimize the stem stiffness in order to find the best compromise of this conflict, using functionally graded material (FGM). This study is divided into two main parts; in the first part, the stem is designed as one-dimensional FGM (model I), while in the second part the stem is designed as a two-dimensional FGM. The second part is further divided into two models, (model II and model III). In model I, the elastic modulus of the stem is changed gradually along the vertical direction. In model II, the elastic moduli E1 and E2 gradually change along the upper stem surface, while E3 is maintained constant along all the lower surface of the stem. In model III, the elastic moduli E1 and E2 gradually change along the lower stem surface while E3 is maintained constant along all the upper stem surface. It is found that the optimal material design for model I is to change the elastic modulus gradually from 110GPa (hydroxyapatite) at the top of the stem to 30GPa (Bioglass) at the stem bottom. This optimal gradation is found to decrease stress shielding by 63%, while reduces the maximum interface shear stress at the lateral and medial sides of the femur by 43% and 33%, respectively compared to titanium stem. Regarding the 2-D FGM it is found that the recommended model is model II, which has three distinct materials of hydroxyapatite, Bioglass and collagen. This new design of 2-D FGM is expected to reduce the stress shielding by 91%, while reduces the maximum interface shear stress at the lateral and medial sides of the femur by about 50% compared to titanium stem. Keywords: functionally graded materials, cementless hip stem, von Mises stress, interface shear stress. 838