The effect of immersion times on the solubility and bonding stress of hydroxyapatite coated Ti6al4v pedicle screws.

PURPOSE Main aim of the present study was to test and show the dissolution of coatings onto Ti6Al4V pedicle screws by both in vitro and in vivo analysis. METHODS Hydroxyapatite (HA) was coated on Ti6Al4V by electrophoretic deposition (EPD). The dissolution of the coatings was tested in vitro. The coating was characterized before and after implantation via pull-out tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The coating materials were immersed in a simulated body fluid (SBF) from 1 day to 6 weeks, in vitro, and the elemental analysis were undertaken in a calf spine, in vivo. Fourteen coated (7) and uncoated (7) Ti6Al4V pedicle screws were implanted in a calf spine for 12 weeks and the elemental concentration was analyzed using the wavelength dispersive X-ray fluorescence (WDXRF) technique. RESULTS The analysis of elemental concentrations by WDXRF are presented and 97% of the dissolved elements were determined as Ca, P, Ti, O and 3% of K, Na, Fe, Ni, Zn, Mg, S in total. The solubility of the coating materials increased with increasing immersion time and, interestingly, it increased substantially for reasonably longer times, for example, 6 weeks. CONCLUSIONS Through the in vivo experiments, on the HA coated and uncoated pedicle screws implanted on a calf vertebra, the elemental concentrations by WDXRF analysis showed the Ca(%) around the pedicle screws increased by 30% after re-implantation process. The results may lead us to reconsider such ceramic coatings to test in vitro and/or in vivo for longer periods before clinical applications.