Investigating FGM (PLA/UHMWPE/PVA) as an artificial hip joint

This study proposes a hip implant designed with 6 layers of functionally graded polymers in graduated volumetric ratios, namely, polylactic acid (PLA) and ultra-high-molecular-weight polyethylene (UHMWPE). In addition, polyvinyl alcohol (PVA) was introduced as a porous PLA/UHMWPE/PVA inner layer. This design aims to promote osseointegration and reduce stress variation with age resulting from patient differences. The effective modulus of elasticity and density of each layer were assessed using the blending rule, while the properties of the layers were estimated using the Gibson–Ashby foam model. The force-law function was used to describe the continuous change in the modulus of elasticity across thickness. Changes in body weight, contact forces at the hip joint, and activity level were derived from previous studies. Hip implants made from a functionally graded material exhibited a median stress lifetime (approximately 29% to 34%) and significantly lower standard deviation than those made from homogeneous PLA. One-way analysis of variance (ANOVA) confirmed that the differences in the modulus of elasticity, density, stiffness, and abrasion parameters across the graded layers were statistically significant ( P < 0.05). The effect size was very large (Cohen’s coefficient d = 4.60), indicating a substantial mechanical improvement owing to the stepped structure, with a 95% confidence interval between 3.35 and 5.85. These findings confirm that the proposed functionally graded porous structure offers improved mechanical compatibility with human bone.