Design and Development of a Biomimetic Dental Wear Test System and Characterizing Restorative Dental Materials

Dental wear is a progressive phenomenon that can lead to onset of tooth decay. It is characterized by the degradation of an outer protective tooth enamel followed by its breach and subsequent loss of tooth integrity unless treated. The treatment of dental wear involves the restoration of worn tooth surfaces with restorative biomaterials. However, the restorative biomaterials themselves can contribute to wear of the enamel of the opposing teeth. This wear of the opposing teeth is driven by a number of factors, such as toughness, hardness, surface roughness and wear conditions. Proper selection of restorative biomaterials is important to preserve occlusal harmony and normal functionality of dentition. In vitro studies that are repeatable and controllable can be used to characterize the wear propensity of restorative biomaterials. Replicating in vivo conditions within an in vitro test model can provide important evidence for pre-clinical evaluation, including with antagonist teeth specimens. By accounting for tooth functional anatomy, dental biomechanics, and dynamic oral environment, important insights can be obtained for restorative biomaterials and donor tooth specimen interactions. In this study a biomimetic dental wear test method was designed, built, and used for a range of dental testing experiments. A range of user requirements of two-body dental wear tests were formulated. An actuation system was developed to simulate cyclic chewing force on dental specimens. Test parameters, including load-force, contact-time, and chewing cycle frequency, were studied with data logging and follow-up data analysis. The developed wear test system was used to conduct dental wear test experiments with restorative bioceramics, and human teeth. This was done both to validate the test system itself, and also to test the wear caused by dental restorative systems on an opposing natural tooth. Studies were designed and conducted utilizing the following features: continuous water-flow, with or without water temperature thermal-cycling; simulated saliva as a static (non-flowing) liquid medium. In all test configurations, a human tooth was the antagonist, i.e., it was actuated against a test surface (restorative material or human tooth test surface), using controlled cyclic motion and loading. The wear of the enamel of the antagonist tooth was then quantified, and from this data, a wear hypothesis could be evaluated. Furthermore, qualitative surface topographical analysis was also performed, both on antagonist teeth, and their respective test materials. This study therefore enabled the effects of testing independent variables (thermocycling parameters, lubricating medium, fluid draining), on the dependent variables of wear and surface topography, to be evaluated by quantitative and qualitative analysis. Articulating human teeth (antagonist) were tested against two different surface finishes of zirconia ceramic (both of which are used in contemporary dental clinical practice), as well as enamel controls (human teeth). This enabled zirconia to be characterized based on its contribution to the enamel wear of opposing teeth, in comparison to the wear outcome for the natural tooth-on-tooth system. Wear results were analysed, and assessments were made of the effect of surface roughness or surface finishing procedures of zirconia, on the enamel wear of a natural opposing tooth. The results showed that zirconia ceramic, regardless of surface finish and lubricating media, caused less wear to the enamel of an opposing tooth, in comparison to the natural tooth-on-tooth system. Laboratory-polished lithium disilicate ceramic (IPS e.max: used in contemporary dental clinical practice) was also tested, together with laboratory-polished zirconia, in a wear validation study that involved thermocycling over a 10 to 55oC temperature range, with an instantaneous temperature change every 120 seconds. The results showed no noticeable difference between the vertical wear of enamel against zirconia and lithium disilicate. However, there were differences in surface morphology: a deep ploughed wear appearance was seen on the lithium disilicate, while no significant surface wear effects were seen on the zirconia. Wear of the antagonist human tooth against zirconia was measurably higher with thermocycling than for the comparative room temperature study. Test results from the saliva study, using human teeth antagonist articulating against zirconia test surfaces, showed that artificial saliva as a lubricating medium helped increase wear resistance of dental enamel, compared with water media, in wear simulation tests. Associated issues of verification and validation of the designed dental wear testing system were also addressed. Benchmark testing was conducted in a certified wear test device/chewing simulator (SD Mechatronic, Feldkirchen-Westerham, Germany) with the aim of checking validity of the results produced in the designed dental wear testing system, thereby verifying the system performance. The benchmarking tests confirmed the validity of the dental wear testing system, developed as part of this PhD project. Moreover, the study results from the benchmark testing also helped provide further insights and understanding of the results derived from the designed dental wear testing system. Overall, the wear simulator developed in this research project showed promising results for dental wear testing and demonstrated the potential for use in in-vitro pre-clinical laboratory studies for characterizing dental restorative materials