Continuous Stiffness Graded Metal–Ceramic Femoral Stems: UMAT‐Based Design and Finite Element Assessment

Functionally graded metal–ceramic materials represent a promising strategy for tailoring the stiffness of load‐bearing biomedical implants and mitigating the mechanical mismatch between metallic femoral stems and bone. In this study, a continuous UMAT‐based finite element framework is developed for the material‐driven design of stiffness‐graded femoral stems. In contrast to conventional layer‐wise FGM approximations, the proposed formulation assigns the elastic modulus directly at the integration‐point level, thereby generating a continuous stiffness field without artificial material interfaces. Three metal–ceramic systems, namely Ti–HA, CoCr–HA, and SS316L–HA, are systematically evaluated using power‐law, sigmoid, and exponential gradation profiles with different volume fraction indices. The influence of metallic substrate, gradation law, and volume fraction index on stem stress redistribution, proximal femoral stress transfer, stress shielding, and implant–bone micromotion is assessed. The UMAT implementation is verified against equivalent layer‐wise FGM models, which progressively converge toward the continuous solution. The results demonstrate that low‐index power‐law Ti–HA stems provide the most favorable compromise between reduced stress shielding, enhanced proximal load transfer, and controlled micromotion, offering design guidelines for continuously graded metal–ceramic biomedical implants.