An investigation into surface texture and in-vitro two-body wear of CAD/CAM dental materials antagonised by acrylic denture teeth

Abstract

Introduction: Wear of teeth and of dental materials is a normal chemical and biomechanical process and is one of the physical and behavioral characteristics of dental restorative materials that need to be understood when planning the incorporation of a dental restoration into the stomatognathic system. The aims for this investigation were to analyse and compare two-dimensional (Ra) and three-dimensional (Sa) methods for surface texture analysis and to apply these methods in a two-body in-vitro wear study using PMMA-acrylic denture teeth as antagonists and a range of commonly used dental restorative materials as test specimens. The material wear characteristics of the antagonist and sample materials were analysed both qualitatively and quantitatively in order to better improve the decisions made by clinicians Methods and Materials: The dental materials analysed are all commercially available CAD/CAM materials for clinical dentistry. They included three monolithic zirconia materials (BruxZir™, Lava Plus™, DenZir™), a lithium disilicate ceramic (eMax™), a nano-ceramic composite resin (Lava Ultimate™), and PMMA-acrylic denture teeth (Gnathostar™, Ivoclar Vivadent). The first investigation involved SEM surface texture analysis of the materials using commonly applied 2-D (Ra) methodology and a novel 3-D (Sa) surface profile generation protocol (Me-X). Qualitative and quantitative information was considered to determine which method gave the most accurate and consistent results for dental material surface topography characterisation. The second part of the investigation involved in-vitro two-body wear investigations using PMMA-acrylic denture teeth as an antagonist and samples from each of the test groups. A purpose-build wear simulator applied a load of 99.5N for 240,000 “chewing” cycles with distilled water lubrication. The total material volume loss for the test materials and PMMA-acrylic antagonist were analysed via 3-D surface profile methods. Qualitative analysis was undertaken via SEM. Results: Ra and Sa surface texture analysis and quantification indicated that the 3-D Sa method was more sensitive to surface topography variation in the materials than the 2-D Ra method. Both the Ra and Sa methods gave the same ranking of the materials in terms of which had more pronounced surface topography variation. The highest values were obtained for the PMMA-acrylic group, followed by Lava Plus™, DenZir™, Lava Ultimate™, eMax™, and BruxZir™ in descending Ra and Sa order. Post two-body wear testing analysis showed qualitative and quantitative changes in the materials. Obvious wear facets were noted for the acrylic antagonist and control groups and the Lava Ultimate™ test material. No wear facets were noted in the zirconia or eMax™ ceramic test samples. The acrylic antagonist material showed the highest volume loss when opposing the nano-ceramic Lava Ultimate™ sample material, followed by the eMax™ group, then the acrylic control group. The Lava Plus™ group showed the least acrylic antagonist material volume loss. The highest overall test material volume loss occurred in the nano-ceramic composite resin (Lava Ultimate™), group followed closely by the acrylic-acrylic control group. Conclusions: The Ra and Sa surface texture analysis and quantification results indicated that either method could be used to analyse the surface roughness of dental materials. The Ra method was quicker and more cost effective than the Sa method. The highest values in descending Ra and Sa order were obtained for the PMMA-acrylic group, followed by Lava Plus™, DenZir™, Lava Ultimate™, eMax™, and BruxZir™. The monolithic zirconia material samples (BruxZir™, Lava Plus™, DenZir™) showed significant intergroup variation both qualitatively and quantitatively. The two-body in-vitro wear analysis showed that the acrylic antagonist cusp had a clear wear facet for each test group. Results showed that the nano-ceramic composite resin material was more abrasive to the acrylic antagonist material than the other materials tested.