Survivability of TiO₂ nanotubes on the surface of bone screws

Modifying surface chemistry and/or surface topography is considered to be a traditional way of optimizing bone-implant integration (osseointegration) for improved bone bonding. The previous results have shown enhanced in vitro osteoblast cell density on titania nanotube–covered surfaces compared with bare titanium surfaces. However, for titania nanotubes to be considered as a candidate surface modification for titanium implants, they must survive load-bearing conditions. The authors investigated the structural survivability of nanotubes on the surface of Ti-6Al-4V-ELI (extra low interstitials) cancellous bone screws subjected to insertion and removal using bone simulant. Measuring the torque during insertion in bone simulant was performed to provide input loads to a finite element model to predict the survivability of the titania nanotubes. Scanning electron microscopy was used to investigate the nanotube morphology before and after the insertion and removal tests. A finite element model using experimental insertion torque data estimated the maximum von-Mises stress in the titanium oxide nanotubes. The model predicted that the maximum von-Mises stress in the nanotubes due to combined compression and shear loading is well below the yield failure. Scanning electron microscopy observation confirmed the presence and survivability of the nanotubes after being subjected to multiple insertions and removals of the screws.