Case Study: Wear-Resistant Anti-Bacterial Titanium Surfaces

External fracture fixation is a common orthopaedic procedure that is used increasingly in a variety of trauma settings. Titanium self-drilling/self-tapping Schanz pins offer a one-step insertion where pre-drilling is not required because the self-drilling tip acts like a new, sharp drill bit. However, pin track infection is a common complication in external fixation systems with infection rates as high as 30 %. The critical consequences of infected pin sites are pin loosening, fracture destabilisation and osteomyelitis, thus leading to additional surgical interventions and delayed or non-union. In addition, Ti alloys are characterised by low hardness and low wear resistance. Therefore, the wear of Ti self-drilling tip or cutting edge necessitates an increased insertion force. This will in turn result in increased temperature at bone-pin surface during insertion, which would cause damage to the bone and retard its healing after operation.

IMPaCT student Tatiana Mukinay has teamed up with researchers at University of Birmingham and orthopaedic surgeons from Royal Darby Hospital to address this technical and medical challenge via a surface engineering approach – one of the six themes of IMPaCT. The research team has applied, for the first time, an advanced ceramic conversion surface engineering technology to self-drilling external fixation pins to improve their performance in terms of biomechanical, bio-tribological and antibacterial properties. The properties of the surface engineered pins were evaluated by insertion into high density bone simulation material; and with Staphylococcus aureus bacteria.

The results have demonstrated that the surfaces of the pins were successfully converted into a thin TiO2 rutile layer supported by an oxygen hardened case with very good bonding due to the in-situ conversion nature. The hardness has increased significantly (more than 3 times) providing enhanced wear resistance of the cutting edge of the self-drilling Ti pins following the ceramic conversion treatment (picture above, a-untreated; b-surface treated). The antibacterial tests also revealed that there was a significantly reduced number of bacteria isolated from the ceramic conversion treated pins compared to the untreated pins of around 50 %. These encouraging results could pave the way towards high-performance anti-bacterial titanium external fixation pins with reduced pin-track infection and pin loosing. A technical paper based on the results has been accepted for publication in Journal of Materials Science – Materials in Medicine (DOI: 10.1007/s10856-016-5816-0).