Surface and tribological characterization of dental biomaterials in presence of artificial saliva

Dental implants are used to replace teeth lost due to decay, trauma or periodontal diseases. The most common design for dental implants involves three different unit, a root (or implant) in direct contact with the bone, an abutment across the gingiva and a crown that bears the chewing loads and provides natural appearance to the implant. Titanium and its alloys are widely used to manufacture implants and abutments due to their low density, excellent biocompatibility, mechanical properties and corrosion resistance. However, titanium abutments can change the appearance of soft tissues, thereby compromising aesthetics. To overcome this limitation, dental implants abutment are usually made of ceramic zirconia because of its white appearance, high strength and biocompatibility. Although zirconia is 10 times harder than titanium, it causes significantly less damage to the titanium implant compared to the titanium abutments. In fact, the inert nature of zirconia resultes in low wear volume loss, which is important in minimizing metal ion liberation. The release of metal ions or material particles, or both, into tissues that surround implanted dental devices can create post-implantation complications. These rare but disturbing events are mainly caused by the mechanical movements of the components of the implant against each other, coupled with the influences of local biochemical and electrochemical factors. The aim of this study was to evaluate the possible different tribological behavior of titanium against zirconia in the presence of two artificial saliva, namely Biotène and BioXtra. In order to simulate the dental implant-abutment contact, a sphere-plane system was used, where the sphere simulates the abutment and the plane simulates the implant. Disks made of commercially pure titanium (Gr2) and Ti6Al4V alloy (Gr5) were tested against zirconia balls using a tribometer (High Frequency Reciprocating Rig TR-282, Ducom) which imposed a reciprocal movement with a stroke length of 2 mm. The tests were conducted under a normal load of 2 N, a frequency of 30 Hz, a temperature of 37°C and a duration of 60 minutes. The volume of lubricant was 2 ml. At the end of each test, the extent of wear on the disks and balls was evaluated by means of an optical microscope. Subsequently the surface electric charge of the disks was evaluated by measuring the zeta potential (ζ), after adsorption of artificial saliva on them. Finally, contact angle measurements were performed on the surface of the same samples used to measure the zeta potential to assess their wettability. The results show that, compared to BioXtra, the Biotène reduces the friction coefficient and therefore the wear between the tested materials (in particular the lower values are reached with the Ti6Al4V alloy) and it allows the formation of a stable adsorbed layer with a highly hydrophilic behavior acting as a good lubricant. The best performances of the Biotène could be due to the presence in it of some surfactants and polymers that act as organic corrosion inhibitors; moreover, even the absence of fluorides in this saliva could positively influence the corrosion resistance of titanium and its alloys.