In vitro and in vivo evaluation of the osseointegration capacity of a polycarbonate-urethane zirconium-oxide composite material for application in a focal knee resurfacing implant

Currently available focal knee resurfacing implants (FKRIs) are fully or partially composed of metals, which show a large disparity in elastic modulus relative to bone and cartilage tissue. Although titanium is known for its excellent osseointegration, the application in FKRIs can lead to potential stress-shielding and metal implants can cause degeneration of the opposing articulating cartilage due to the high resulting contact stresses. Furthermore, metal implants do not allow for follow-up using magnetic resonance imaging (MRI).To overcome the drawbacks of using metal based FKRIs, a biomimetic and MRI compatible bi-layered non-resorbable thermoplastic polycarbonate-urethane (PCU)-based FKRI was developed. The objective of this preclinical study was to evaluate the mechanical properties, biocompatibility and osteoconduction of a novel Bionate® 75D - zirconium oxide (B75D-ZrO ) composite material in vitro and the osseointegration of a B75D-ZrO composite stem PCU implant in a caprine animal model. The tensile strength and elastic modulus of the B75D-ZrO composite were characterized through in vitro mechanical tests under ambient and physiological conditions. In vitro biocompatibility and osteoconductivity were evaluated by exposing human mesenchymal stem cells to the B75D-ZrO composite and culturing the cells under osteogenic conditions. Cell activity and mineralization were assessed and compared to Bionate® 75D (B75D) and titanium disks. The in vivo osseointegration of implants containing a B75D-ZrO stem was compared to implants with a B75D stem and titanium stem in a caprine large animal model. After a follow-up of 6 months, bone histomorphometry was performed to assess the bone-to-implant contact area (BIC). Mechanical testing showed that the B75D-ZrO composite material possesses an elastic modulus in the range of the elastic modulus reported for trabecular bone. The B75D-ZrO composite material facilitated cell mediated mineralization to a comparable extent as titanium. A significantly higher bone-to-implant contact (BIC) score was observed in the B75D-ZrO implants compared to the B75D implants. The BIC of B75D-ZrO implants was not significantly different compared to titanium implants. A biocompatible B75D-ZrO composite approximating the elastic modulus of trabecular bone was developed by compounding B75D with zirconium oxide. In vivo evaluation showed an significant increase of osseointegration for B75D-ZrO composite stem implants compared to B75D polymer stem PCU implants. The osseointegration of B75D-ZrO composite stem PCU implants was not significantly different in comparison to analogous titanium stem metal implants.