The influence of Young's modulus of loaded implants on bone remodeling: an experimental and numerical study in the goat knee.

The aim of this study was to examine the influence of the Young's modulus of the implant material on the bone remodeling in a loaded condition. A combined animal experimental and computational study was set up. The animal experimental group comprised of 16 Saanen goats, each receiving one titanium implant (Young's modulus 110 GPa) and one high-density polyethylene (HDPE) implant (Young's modulus 1 GPa) in the left femoral condyle. Both types of implants received a titanium coating of 100 nm thickness. The implants protruded in the knee joint space and were directly weight bearing. The first group of eight goats was sacrificed after 6 weeks of loading and the second group of eight goats after 6 months of loading. The 16 femoral condyles with the 32 implants were prepared for microfocus computed tomography (micro-CT) scanning and histological sectioning. Three-dimensional trabecular bone parameters were calculated on the micro-CT images for the zones neck, middle, and apex of the implant. The percent of bone contact with the implant was measured on longitudinal histological sections. An axisymmetric finite element (FE) model was created to compare peri-implant bone strains and relative motion between a titanium and a HDPE implant for the experimental loading condition, and to assess the influence of different bone-implant interface (contact) conditions. From the statistical analysis of the 3D bone parameters, the difference between the titanium and HDPE implants was not significantly different (p > 0.05) between the zones (neck, middle, and apex) for both groups of goats. The implants could be considered in their entirety. After 6 weeks of loading, the PE implant presented lower connectivity and smaller marrow spaces in the circular region of 0-500 microm. In the region 500-1500 microm more bone volume was present for the PE implant. After 6 months, the PE implants showed more bone volume and thicker trabeculae than the titanium implants for the entire length of the implant. This effect was already present in the smallest region of interest, 0-500 microm. After 6 months more fibrous encapsulation was found around titanium implants. FE results demonstrated a substantial influence of the interface conditions on peri-implant strains and relative motion. For interface conditions that were representative for the early postoperative situation (involving press-fit and friction), differences in peri-implant bone strain distributions between titanium and HDPE could be related to the experimentally observed differences in amounts of bone and fibrous encapsulation. In contrast, differences in relative motion did not seem to play a role. Both the experimental and computational results suggest that implant stiffness can affect the peri-implant tissue response, which may be related to differences in peri-implant strains.