A novel, internally structured stainless steel implant with titanium characteristics

Abstract

There are many aspects that have direct effects on total hip replacement performance (THR), such as material properties, applied loads, surgical approach, femur size and quality, prosthesis design, bone-implant interface etc. Bone mechanics and traditional implant materials cause a frequent problem for patients of total hip arthroplasty (THA): the bone becomes shielded from the loading. Bone structure follows what is called “Wolff’s Law”, meaning it has an adaptive structure, which alters its geometry when experiencing forces over its life (Goldstein, 1987); (Pearson & Lieberman, 2004). The improved femoral stems act weakly in transferring stress onto the remnant bone and bone tissue atrophies at the interface, which will result in loosening of the implant, pain and thus, revision surgery will need to take place to correct the issue ( Feldt, 2011). For the current study, an innovative hollow spherical structure is developed for femoral hip stems. The aim is to extract volume in the spherical shape from the stainless steel hip implant stems, in order to focus solely on correlating with titanium behaviour. Internal geometry for the femoral stem is optimised in order to transfer more stress onto the bone. Moreover, the approach involves extracting volume in the spherical shape from internal structure to reduce stress shieling. New novel implant is proposed that demonstrated reduction in stress shielding. A new structure has been developed in this study for biomedical applications, such as implants, with the aid of the rule of mixtures and finite element analysis was applied to various models with different complex internal structures. Firstly, the rule of mixtures was used as finite element could not handle the simulation due to the large number of elements created, and also helped developing the designs analysed in this study. Secondly, computational analysis was applied to simplified finite elements containing hollow spheres in their outer shell. Moreover, a compression test was applied to a solid sample and the experimental case. This approach was adopted to investigate the effects of a hollow structure near the side surface and the bone-implant interface. The same method was applied to samples containing uniformly distributed hollow spheres. In the both scenarios, the specimens were designed differently based on the sphere size, their distance from wall and that from each other. Finally, finite element was applied to actual implant samples containing hollow spheres. The sphered models have a smaller Young’s modulus and strength than the solid stainless steel sample. The spheres in hollowed structures reduce the stress shielding and they transfer more stress onto the bone when compared to the solid stainless steel models. This approach also involves restructuring a hard material, such as stainless steel, to enhance osseointegration. The reduction of the Young’s modulus and stress directly depends on the volume of the hollow spheres in the models; however, there is certain volume that can be extracted from solid.