Physico-chemical and Mechanical Properties of a Novel Coating for Dental Implants

Background: Whilst endosseous dental implants have a good clinical success rate, they can suffer from early or late failures; for the latter, microbial colonisation of the implant surface (peri-implantitis) has been implicated in tipping the balance from osseointegration to failure. This thesis describes the development of a surface engineering solution for the improvement of implant osseointegration by hydrothermal synthesis of biocompatible coatings.
Materials and Methods: Two types of apatite coatings, namely fluorapatite (FA) and hydroxyapatite (HA), were deposited using a mild hydrothermal reaction onto two types of in-house prepared substrate, stainless steel (SS) or commercially pure titanium (Ti). Post-deposition optimisation of the coating systems was undertaken through applying sintering heat treatments.
Physical assessment: Scanning Electron Microscopy (SEM), optical profilometry, contact angle measurement, surface free energy (SFE) quantification and X-ray diffraction (XRD) were used to look at the physical properties of both substrates and coatings.
Chemical assessment: Energy Dispersive Spectroscopy (EDS), Ion Chromatography (IC), Fluoride Ion Selective Electrode (ISE), Fourier-transform infrared spectroscopy (FTIR) and degradation in acidic environment were used to characterise coatings’ chemical properties.
Mechanical assessment: Atomic Force Microscopy (AFM), Nano-indentation (NI), Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM), pull-off tensile test (PoT), scratch test (ST) and Screw-in Bone test (SiB) were used to assess coatings’ mechanical properties; this included a detailed study of the FA coating / Ti substrate interface to ensure mechanical integrity.
Results: Optimally structured and stable FA coatings on Ti were obtained with enhanced chemical and mechanical stability. The surface roughness, Sa, of the deposited FA coating on an optimum substrate was 3-4.5 µm and its thickness 8-10 µm; these were significantly reduced after heat-treatment to 2-3.5 µm and 7-8 µm respectively. FA coating enhanced the surface wettability of Ti changing its WCA of 95-105° to hydrophilic with WCA 12-25°; this was further significantly decreased after heat-treatment to 8-14°. This also resulted in change of Ti total SFE from 40 mJ/m2 before coating to 75-85 mJ/m2 after coating that was increased to 80-90 mJ/m2 after heat-treatment. FA coating resulted in shift in the surface charge of Ti from neutral to negatively charged.
X-ray diffraction confirmed that FA was the only phase present, supported by EDS and FTIR spectra of the coatings that showed typical characteristics of an apatite structure. Negligible F- release from the coating at pH 7.00 showed that the FA coating was stable. The accelerated aging of FA coating for 60 days in pH 4.00 acidic media, presented with high F- daily release that lasted for 14 days. After heat-treatment, the daily release was significantly decreased and lasted for 60 days.
The FA coatings presented high mechanical properties as shown through AFM (viscoelastic modulus of 60-83 GPa that was significantly increased to 100-180 GPa after heat-treatment). Furthermore, nano-indentation revealed a significant increase in hardness from 1.5±0.4 GPa before heat-treatment to a range of 2.1 to 8.1 GPa after heat-treatment. Micro- and nano-analysis of FA / Ti interface showed enhanced interface integrity after heat-treatment, which was also confirmed in the PoT and SiB adhesion tests.
The final in vitro assessment of FA coating adhesion was to simulate a clinical scenario using FA coated SS screws inserted in artificial bone. It confirmed the stability of FA coating on SS screws before and after heat-treatment against delamination. Novel and optimised FA coating procedure and the subsequent heat-treatment protocol achieved in this thesis, successfully developed stable FA coating suitable for dental implant applications.
Clinical implications: The clinical reality of encountering challenging situations for implant success justified the necessity for this research to investigate the factors that can improve and accelerate the integration of the dental implant with the surrounding bone and effectiveness of these dental implant against bacteria associated with peri-implantitis. Successful optimisation of the coating procedure demonstrated for the first time the possibility of creating improved FA-coated Ti implants with potential for osteo-inductive and antimicrobial properties.