On the bone response to oxidized titanium implants. The role of microporous structure and chemical composition of the surface oxide in enhanced osseointegration

Abstract: Background: Titanium implants have been widely used clinically for various types of bone-anchored reconstructions. A thin native oxide film, naturally formed on titanium implants contacts with bone tissue and has been considered to be of great importance for successful osseointegration. However, the precise role of surface oxide properties in the osseointegration process is not known in detail. Aims and Hypothesis: The overall aims of this thesis were (i) to develop anodic oxidation methods of titanium implants (paper 1), (ii) to characterize the surface properties of native and anodic oxides (paper 2) and (iii) to investigate if and which surface oxide properties will influence the bone tissue response. In vivo animal studies (papers 3-6) in the present thesis hypothesized that osseointegration would be reinforced by mechanical interlocking and chemical bonding between bone and implant surface. Mechanical interlocking is assumed to be associated with the surface roughness/ pore configurations, while chemical bonding is dependent on surface chemistry. Materials and Methods: Machined-turned commercially pure (c.p) titanium implants were used for controls. Test implants were prepared by electrochemical anodic oxidation at the galvanostatic mode in various electrolytes. We tested implants with enhanced oxide films achieved by micro arc oxidation (MAO) process in acetic acid as electrolyte. Other investigated electrolytes were sulphuric acid (S implants), phosphoric acid (P implants) and a calcium containing mixed electrolyte system (Ca implants). The surface oxide properties were analyzed in terms of the oxide thickness, chemical composition, pore configurations (pore size, pore size distribution, porosity), crystal structure and surface roughness by using different analytical techniques including X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Scanning Electron Microscopy (SEM), thin-film X-ray diffractometry (TF-XRD), Raman spectroscopy and TopScan 3D®. Implants (n = 176) were inserted in the femur and tibia of mature New Zealand white rabbits (n = 22). The follow up time was 6 weeks. Bone tissue responses were evaluated with resonance frequency analysis (RFA), removal torque test (RTQ), qualitative histology, histomorphometrical quantifications and enzyme histochemistry of alkaline (ALP) and acidic phosphatase (ACP). Results: The electrochemical oxide growth behaviour was greatly dependent on the nature of the electrolytes employed, the current density, the electrolyte concentration, the electrolyte temperature, the agitation speed and the chemical composition of the titanium electrode. The MAO method at galvanostatic mode demonstrated systemic changes of surface oxide properties of titanium implants by controlling the mentioned electrochemical parameters. This provides an opportunity to investigate the effects of such oxide properties on the bone tissue response.Oxidized, microporous implants having oxide thicknesses of about 600, 800 and 1000 nm demonstrated significantly stronger bone responses as compared to nonporous implants with oxide thicknesses of 17 and 200 nm. Chemically modified S and P implants demonstrated significantly improved bone responses compared to controls. Calcium deposited, oxidized titanium implants showed the strongest bone responses of all tested implantsConclusions: Our findings indicated that osseointegration occurred from a combination of mechanical interlocking and biochemical bonding at least with respect to two tested implants, namely S and P implants. The faster and stronger osseointegration, particularly found with Ca implants may have clinical applications too.