Abstract
Objective
To investigate the fundamental wear mechanisms of six resin-based composite (RBC) formulations during short-term in vitro wear testing.
Materials
RBC materials were condensed into rectangular bar-shaped specimens and light irradiated using the ISO 4049 specimen manufacture and irradiation protocol. Wear testing ( n = 10 specimens for each RBC) was performed on a modified pin-on-plate wear test apparatus and wear facets were analysed for wear volume loss using a white light profilometer. The wear tested RBC specimens and their corresponding antagonists were analysed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively to determine the wear mechanism.
Results
Data generated using the profilometer showed variations in the mean total wear volume (mm 3 ) between the RBCs tested ( p < 0.05). Abrasive wear was evident in all RBCs investigated with varying degrees of damage. Material transfer/deposition of the filler particles on the corresponding antagonists was evident in two RBC materials (Filtek Supreme and Kalore) indicative of a further adhesive wear mechanism.
Conclusion
It is proposed that the approach employed to use a combination of measurement and analytical techniques to quantify the wear facet volume (profilometry), wear trough (SEM) and material transfer (EDS) provides more useful information on the wear mechanism and the tribology of the system rather than relying on a simple wear ranking for the RBC materials as is routinely the case in dental research studies.
1
Introduction
The assessment of the wear performance of dental resin-based composite (RBC) restoratives has been determined frequently in the dental literature since the first in vitro studies were published . Today using the identifiable Medical Subject Headings (MeSHs) of ‘dentistry AND wear’, almost 2000 manuscripts have been published in the dental literature in the last 10 years. To complicate wear performance data interpretation, a variety of in vitro wear testing devices have been advocated to replicate the in vivo masticatory process . However, no single in vitro wear simulator available can simulate the masticatory cycle in the oral environment . At best wear simulators can provide an indication of the relative ranking of potential novel dental RBC restorative formulations prior to market launch when compared with commercially successful formulations . Variations in RBC materials arise from different manufacturing processing routes and RBCs often include different monomeric resin matrices, functioning silane coupling agents and filler technologies (filler volume fractions, particle size distribution and filler density) . However, the most robust laboratory RBC wear studies in the literature are conducted on a range of commercial dental products, routinely from different manufacturers in the form of round-robin tests .
Until recently, confusion existed on whether wear depth, area or volume should be reported although the volume of material removed due to the interaction of opposing surfaces was shown to be the parameter of choice for reporting the in vitro wear of RBCs based on Archard’s equation . Too frequently in dentistry, wear depth or wear area are reported but wear in the mouth is dependent upon occlusal factors which change continuously with time and the progression of wear . In addition, authors that claim to assess the wear volume often fail to examine the wear facet sufficiently or ensure the accuracy and precision of the wear measurements reported . From a tribology perspective, there are four fundamental wear mechanisms that can exist, namely abrasion, adhesion, fatigue or corrosion and wear facets are infrequently assessed following testing to elucidate the wear mechanisms operative during testing.
The aim of the current study was to investigate the short-term in vitro wear resistance and wear mechanism operative during testing six RBC formulations. The null hypotheses stated were that there would be no differences in the (1) in vitro mean total wear volume data and (2) wear mechanisms operative, for the commercial RBC formulations investigated.
2
Materials and methods
2.1
Materials
Six commercially available RBC materials with innovative claims in terms of monomer chemistry, filler content, filler type and/or filler size and produced by a range of dental manufacturers, for both anterior and posterior clinical use were selected ( Table 1 ).
| RBC | Description | Manufacturer | Resin | Filler type/size | Content | Special characteristics |
|---|---|---|---|---|---|---|
| Filtek Silorane | Universal Microhybrid | 3M ESPE, USA | Siloxanes and Oxiranes | Quartz, YF 0.1–0.2 μm |
76 wt% 55 vol% |
Ring-opening monomers |
| Filtek Supreme | Universal nanofilled | 3M ESPE, USA | BisGMA, BisEMA 6 UDMA, TEDMA PEGDMA |
ZrO 2 , SiO 2 0.6–1.4 μm |
72 wt% 55 vol% |
“True” nanotechnology unique clusters of nano-sized particles |
| Kalore | Universal nanohybrid | GC America, USA | UDMA DX-511 co-monomers, Dimethacrylate |
F-Al-Si, SiO 2 0.4–0.7 μm |
82 wt% | Does not contain BisGMA, DuPont’s new monomer technology |
| Venus Diamond | Universal nanohybrid | Heraeus Kulzer Hanau, Germany | TCD-DI-HEA, UDMA | Ba-Al-F, SiO 2 0.5 nm to 20 μm |
65 wt% 41 vol% |
New cross linker technology. TCD-urethane cross linker |
| Tetric Ceram HB | Universal nanohybrid | Ivoclar-Vivadent Liechtenstein | BisGMA, UDMA, BisEMA | Ba-F-Al-B-Si mixed oxides, SiO 2 , YbF 3 , PPF 0.4–1 μm |
76 wt% 55 vol% |
Containing BisEMA Monomer |
| Clearfil Majesty Posterior | Universal nanofilled | Kuraray, USA | BisGMA, TEGDMA | Alumina and glass-ceramic 20 nm to 1.5 μm | 92 wt% 82 vol% |
Nano Dispersion Technology High filler content |
2.2
Specimen manufacture
The RBC materials was condensed into rectangular bar-shaped specimens (25.0 ± 0.1 mm length, 10.0 ± 0.1 mm width and 3.0 ± 0.1 mm thickness) using a custom made Perspex holder. A constant excess of uncured RBC was placed into the mould, covered with a cellulose acetate strip and a glass microscope slide and a weight of 1 kg was applied for 20 s to ensure consistent and reproducible packing of the specimens. The weight and microscope slide were removed and the specimen was light irradiated using a light emitting diode (LED) light curing unit (LCU) (Demi Plus, Kerr, Orange Co., CA, USA) at ambient room temperature (23 ± 1 °C) with a spectral range of 450–470 nm and an irradiance of 1200 mW/cm 2 . The irradiance was checked prior to use by employing a checkMARK (Bluelight Analytics Inc., Halifax, Canada). The entire length of each specimen was light irradiated using the ISO 4049 specimen manufacture protocol by placing the tip of the light guide in direct contact with the cellulose acetate strip in the centre of the specimen . Both the top and the lower surface of the specimens were light irradiated to produce six groups of 10 specimens by overlapping the exit window by half the LCU tip diameter along the specimen so that areas received twice the irradiation of adjacent areas using the 8 mm LCU tip diameter.
Following light irradiation, the cellulose acetate strip was discarded, the mould dismantled and the specimen removed and checked for surface imperfections. The specimens were wet ground by hand lapping using P400, P600, P800, P1000 and P1200 grit silicon carbide (SiC) abrasive papers (Struers, Copenhagen, Denmark) under copious water irrigation to remove the oxygen inhibited, resin rich layer and produce a planar surface with a consistent surface topography. The specimens were stored in a light-proof container and placed in a water-bath maintained at 37 ± 1 °C for seven days prior to testing and analysis.
2.3
Wear testing and analyses
To facilitate wear testing of contemporary RBC’s, a newly modified pin-on-plate wear test apparatus developed originally by Harrison and colleagues was used. The schematic in Fig. 1 illustrates a cross section cut through one of the ten wear stations where a custom made antagonist holder was devised that could be attached underneath the vertical rod, to hold the abrader with the aid of locking screws. The modification to the original pin-on-plate wear test apparatus allowed for the choice of antagonist to be selected by the operator while the load used was maintained in line with masticatory forces. Differences between the original and the modified pin-on-plate wear testing apparatus are detailed in Table 2 . The steatite sphere (8 mm diameter) wear antagonist was fixed to the vertically moving pins and a loading force of 4.5 N was used in a neutral buffer solution to approximate the in vivo oral environment . The RBC specimens were confined within a Perspex template attached to a horizontal plate moving at a frequency of 2.14 Hz.
| Variable | Original pin-on-plate wear test apparatus | Modified pin-on-plate wear testing apparatus |
|---|---|---|
| Abrader | Silicon carbide paper held individually in the table | Steatite antagonist, 8 mm diameter but can be modified to fit any diameter |
| Test sample | Cylindrical specimens (4.5 mm diameter) cemented onto pin ends | Rectangular-bar shaped specimens (25.0 × 10.0 × 3.0 mm) held in the table with locking screws |
| Number of test specimens | 10 | 10 |
| Pin plate contact frequency | 70/min | 100/min |
| Pin plate contact time | 0.2 s but can be adjusted for each sample | 0.2 s but can be adjusted for each sample |
| Pin plate vertical lift | 4 mm | 4 mm |
| Pin plate contact distance | 1 mm | 1 mm |
| Stroke frequency | 2.10 Hz | 2.14 Hz |
| Environment | Liquid or slurry abrasive | Liquid or slurry abrasive |
| Measurement of wear | Vertical height loss of specimen using a specially designed bench micrometer | Maximum depth (mm) and/or volume loss (mm 3 ) using a non-contact profilometer |
| Masses used | 50–1000 g | 50–1000 g |
Employing the modified pin-on-plate wear test apparatus, a pilot study was carried out to determine the minimum number of cycles to produce a linear wear rate with measurements following 2000, 3000, 4000 and 5000 wear cycles. Based on the results of the preliminary study which showed a linear wear rate following 2000 cycles ( r 2 = 0.99) ( Fig. 2 ), it was decided that all RBC materials should be tested for 4000 cycles – the equivalent of three months simulation in the oral cavity .
2.4
Profilometry
Wear tested samples displayed characteristic shallow wear tracks and were scanned using a TalySurf CLI 2000 profilometer (Taylor-Hobson Precision, Leicester, England) equipped with a with a 300 μm range chromatic length aberration (CLA) gauge scanning at 2 mm/s. Longitudinal traces were taken at 4 μm intervals ( x -direction) across the wear facet with a measurement recorded at every 40 μm interval ( y -direction) thereby generating a three dimensional (3D) profile ( Fig. 3 ) using the TalyMap Gold analysis software Version 4.2 (Taylor-Hobson Precision, Leicester, England). The unworn areas around the wear track were used as the datum from which it was possible to calculate both the mean maximum wear depth and the mean volume loss (mm 3 ) of the RBC materials investigated .
In line with the profilometic analyses, ten traces were performed across a standard step height of 1.0 mm to determine the accuracy and precision of the wear depth measurements for the scanning conditions (300 μm range CLA, scanning at 2 mm/s with longitudinal traces at 4 μm intervals ( x -direction) and horizontal traces recorded at 40 μm intervals ( y -direction) for a resolution of 0.1 μm ( z -direction). The accuracy was calculated as the mean error from the true value, whilst the precision was quantified as the standard deviation of the errors measured .
2.5
Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)
The wear tested RBC specimens and their corresponding antagonists were mounted on aluminium stubs and sputter coated with approximately 5 nm of gold using an argon sputter coating unit (Agar Scientific, Stansted, UK). The samples were analysed using a Hitachi-S-3400N, variable pressure scanning electron microscope (Hitachi High-Tech Technologies, Tokyo, Japan) under low vacuum at a 5 mm distance to elucidate the wear mechanism operative. Additionally, Energy-Dispersive X-ray Spectroscopy (EDS) (Bruker Inc., Berlin, Germany) analyses were conducted on the steatite antagonists to record the elemental spectral maps to further elucidate the wear mechanism operative.
2.6
Statistical analyses
All data sets were checked for normality using a Kolmogorov–Smirnov test and Shapiro–Wilk test. Data were analysed by Kruskal–Wallis test with Post Hoc Bonferroni with a significance level of p < 0.05.
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