Effect of sterilization on the surface properties of Ti6Al7Nb alloy femoral stems

We investigated thirty retrieved femoral components (cement-less stems made from Ti6Al7Nb alloy) of prematurely failed hip endoprostheses, due to aseptic loosening, infection and low-grade infection and two new stems (after their expiry date) for comparison. The main aim was to find the impact of cleaning and sterilization procedures of new and retrieved stems on the surface properties, i.e., surface wetting, roughness and surface chemistry. Our hypothesis was that the effect of cleaning and sterilization should not affect the meaningful difference of surface properties of retrieved and new stems of hip cement-less endoprostheses. Clean, new and retrieved stems of hip endoprostheses were sterilized using classical steam autoclave AC, procedure protocol (121 °C, 1.25 bar, 20 min). Surface wettability was determined by measuring the static contact angle of a water drop, the roughness was measured by optical 3D system. The surface chemistry was analysed by X-ray Photoelectron Spectroscopy (XPS). In wettability results, the contact angle varied from 70.6° to 91°, and the surface roughness (Sa) varied from 4.2 µm to 4.7 µm for all thirty measured stems. We found that surface properties measured on three different stem positions (neck, middle and root) differ significantly, we can conclude that corundum (Al2O3) grit-blasted surface used to obtain the necessary roughness for better osteointegration is not homogeneous. The XPS results of new and retrieved implants that prematurely failed for different reasons showed minor differences of TiO2 thin oxide native film thickness: 6.2 nm for the new stem and 5.2 nm for retrieved (prematurely failed due to aseptic loosening, infection or low grade infection, consisting primarily of TiO2). The results confirm our hypothesis that the effect of cleaning and sterilization on surface properties of new and retrieved stems is negligible.