Abstract
Objective
To investigate the short- and long-term in vitro wear resistance of experimental resin-based composites (RBCs) derived from a commercial formulation.
Methods
Six experimental RBCs were manufactured by manipulating the monomeric resin composition and the filler characteristics of Grandio (Voco GmbH, Cuxhaven, Germany). The Oregon Health Sciences University (OHSU) oral wear simulator was used in the presence of a food-like slurry to simulate three-body abrasion and attrition wear for 50,000, 150,000 and 300,000 cycles. A three-dimensional image of each wear facet was created and the total volumetric wear (mm 3 ) and maximum wear depth (μm) were quantified for the RBC and antagonist. Statistical analyses of the total volumetric wear and maximum wear depth data (two- and one-way analyses of variance (ANOVA), with Tukey’s post hoc tests where required) and regression analyses, were conducted at p = 0.05.
Results
Two-way ANOVAs identified a significant effect of RBC material × wear cycles, RBC material and wear cycles (all p < 0.0001). Regression analyses showed significant increases in the total volumetric wear ( p ≤ 0.001) and maximum wear depth data ( p ≤ 0.004) for all RBCs with increasing wear cycles.
Significance
Differences between all RBC materials were evident after ≥150,000 wear cycles and antagonist wear provided valuable information to support the experimental findings. Wear simulating machines can provide an indication of the clinical performance but clinical performance is multi-factorial and wear is only a single facet. Employing experimental RBCs provided by a dental manufacturer rather than using self-manufactured RBCs or dental products provides increased experimental control by limiting the variables involved.
1
Introduction
Chemically cured resin-based composites (RBCs) were patented from the pioneering work of Ralph Bowen in 1958 and were introduced to the dental market in the 1970s. These early generation RBCs were placed in bulk without employing an etchant , however, the use of chemically cured RBCs in Class I and Class II restorations was contraindicated due to concerns regarding the wear resistance . It was not until a ‘method of repairing teeth using a composition which was curable by visible light’ was patented that esthetic restorative dentistry became a realization. The development of ‘total etch’ adhesives in the late 1980s , markedly improved the performance of light irradiated RBCs resulting in materials which were advocated for use in Class I and Class II restorations. Current RBC formulations have embraced low-shrink non-methacrylate monomeric resin formulations and/or nano-filler technology . However, the major disadvantages of modern RBCs include an insufficient depth of cure which requires an incremental filling technique with each increment at a maximum of 2 mm in depth and polymerization shrinkage manifest as shrinkage stress which can compromise the integrity of the tooth–RBC restoration interface . Today, questions remain about the wear resistance of RBC materials when placed in load bearing cavities in clenching or bruxing patients .
The wear of tooth structures and dental restorative materials is a complex phenomenon involving a combination of the four fundamental wear mechanisms (abrasion, adhesion, fatigue or corrosion) . In dentistry, the terms used to describe the wear of natural dentition or dental restoratives are confusing as they describe clinical manifestations of wear namely, attrition (surface loss at sites of occlusal contact), abrasion (surface loss at non-contacting sites) and erosion (surface loss attributed to chemical effects), rather than underlying wear mechanisms .
The current knowledge base of RBCs suggests their mechanical properties, including the wear resistance, are dominated by the filler and not the monomeric resin . It has been shown that increasing the filler volume fraction improves the mechanical properties while reducing the mean filler particle size reduces the filler volume fraction allowable but increases the wear resistance . However, most RBC studies in the dental literature were conducted on commercial dental products or self-manufactured RBCs . In this study, the authors have investigated the wear performance of experimental RBCs provided by a dental manufacturer .
Attempts to mimic the masticatory processes encountered in the mouth led to the development of a variety of testing devices to simulate the in vitro wear of dental restoratives including the Materials Testing and Simulation (MTS) artificial oral environment , the Academisch Centrum for Tandheelkunde Amsterdam (ACTA) and University of Alabama wear machines and the Oregon Health Science University (OHSU) oral wear simulator . While there is no single in vitro wear simulator available that can simulate the range of wear mechanisms operative in the mouth , the OHSU oral wear simulator has previously been shown to decipher differences between RBC materials and produce consistent results in round-robin tests . Routinely, in vitro wear resistance testing of RBCs is performed over a relatively short-testing period of ≤50,000 wear cycles , the equivalent of six months intra-oral wear . However, recent in vitro wear resistance studies have shown that longer-testing periods (100,000 to 500,000 wear cycles) provided more useful information on the clinical performance of RBCs .
The aim of the current study was to investigate the short- and long-term in vitro wear resistance of a series of experimental RBCs derived from a successful commercial RBC formulation (Grandio, Voco GmbH, Cuxhaven, Germany) which was used as a control. The hypothesis of the study was that the wear performance of experimental RBCs provided by a dental manufacturer would provide increased insight into the wear behavior of RBCs rather than relying on RBC studies conducted on commercial dental products or self-manufactured RBCs.
2
Materials and methods
2.1
Materials
The RBC materials tested were Grandio and six experimental RBCs (G 1 –G 6 , Voco GmbH, Cuxhaven, Germany) based on the Grandio formulation by manipulating the monomeric resin composition and the barium-alumina borosilicate filler characteristics ( Table 1 ). All RBC materials contained camphorquinone as the initiator and were produced using the same manufacturing processing route. The major differences between the RBC materials were: G 1 had an increased filler volume fraction (74.5 vol%) compared with Grandio (71.4 vol%); G 2 had a urethane dimethacrylate (UDMA):aliphatic dimethacrylate monomeric blend compared with the BisGMA:TEGDMA monomeric blend of G 1 ; G 3 had an increased mean filler diameter (2.5 μm) and filler volume fraction (76 vol%) compared with Grandio (1.5 μm and 71.4 vol%, respectively); G 4 had an increased filler density (2.7 g/cm 3 ) and decreased filler volume fraction (71 vol%) compared with G 3 (2.4 g/cm 3 and 76 vol%, respectively); the monomeric blend ratio of BisGMA:TEGDMA of G 3 was increased from 3:1 to 3.5:1 for G 5 while the filler volume fraction was decreased from 76 to 73 vol%; and G 6 had an ormocer monomeric blend compared with the BisGMA:TEGDMA monomeric blend of G 5 ( Table 1 ).
| RBC | Resin matrix | Barium-alumina borosilicate filler particles | ||||
|---|---|---|---|---|---|---|
| Composition | Ratio | Density (g/cm 3 ) | Diameter (μm) | Fraction (wt%) | Fraction (vol%) | |
| Grandio | BisGMA:TEGDMA | 3:1 | 2.4 | 1.5 | 87 | 71.4 |
| G 1 | BisGMA:TEGDMA | 3:1 | 2.4 | 1.5 | 89 | 74.5 |
| G 2 | UDMA:Aliphatic dimethacrylates | 1:1 | 2.4 | 1.5 | 89 | 74.5 |
| G 3 | BisGMA:TEGDMA | 3:1 | 2.4 | 2.5 | 91 | 76 |
| G 4 | BisGMA:TEGDMA | 3:1 | 2.7 | 2.5 | 89 | 71 |
| G 5 | BisGMA:TEGDMA | 3.5:1 | 2.4 | 2.5 | 87 | 73 |
| G 6 | Ormocer | 1 | 2.4 | 2.5 | 87 | 73 |
2.2
Specimen preparation
Eight disc-shaped specimens (13.0 ± 0.1 mm diameter and 2.0 ± 0.1 mm thickness) were prepared for each RBC material investigated (Grandio and G 1 –G 6 ) using a nylon ring-mold. The ring-mold was placed on a glass slab, covered with an acetate strip and a consistent mass (g) of uncured resin weighed, using a balance reading to 0.001 g (Sartorius Expert, Sartorius AG, Goettingen, Germany), was placed into the center of the ring-mold. A second acetate strip was placed on top and a 1 kg weight applied to spread the uncured resin within the ring-mold . Each RBC specimen was light irradiated using a hand-held quartz-tungsten halogen (QTH) light curing unit (LCU) (Optilux 501, Kerr, Orange Co., CA, USA) coupled with a 13 mm light tip diameter operating at an output intensity of 540 ± 16 mW/cm 2 . The RBC materials were light irradiated from the top surface only for 20 s, by placing the light tip in direct contact with the top acetate strip. The specimens were carefully removed from the ring-mold following irradiation and stored in 50 mL of distilled water maintained at 37 ± 1 °C under light-proof conditions (Firlabo SP BVEHF, Société Firlabo, Meyzieu, France) for 23 h .
2.3
In vitro wear testing
One hour prior to the commencement of in vitro wear testing, the disc-shaped RBC specimens were mounted in a two part cold-setting acrylic resin (Varidur; Buehler, Lake Bluff, IL, USA). The specimens were positioned in the center of individual cylindrical mounting cups (Metset Cups, Buehler, Lake Bluff, IL, USA) and the acrylic resin, mixed using a powder to liquid mixing ratio of 1 g to 1 mL, was poured to a height of 10.0 ± 1.0 mm. The mounting cups had an internal diameter of 25.0 ± 0.1 mm which were compatible with the chambers of the wear machine . After 45 min, the mounted specimens were removed from the mounting cups and wet ground sequentially with P600 and P1200 silicon carbide (SiC) abrasive paper for 30 s each, respectively under water lubrication on a grinding-polishing machine (Alpha and Beta Grinder-Polisher, Buehler, Lake Bluff, IL, USA) using a force of 10 N per specimen.
The embedded disc-shaped RBC specimens were secured into individual wear chambers 24 h after irradiation and marked with a waterproof alcohol pen for future realignment. The abrasion and attrition forces for each OHSU oral wear simulator chamber ( n = 4) were calibrated prior to the commencement of each wear testing regime using a custom jig (Proto-tech, Portland, OR, USA) containing a 500 N load cell . A food-like slurry consisting of 1.0 g of crushed poppy seeds (Holland and Barrett, Burton-upon-Trent, England), 0.5 g of poly(methyl methacrylate) beads with a mean particle diameter of 50–100 μm (ProBase Cold Polymer; Ivoclar Vivadent, Schaan, Liechtenstein) and 5 mL of distilled water was placed into each wear chamber prior to the commencement of testing. The OHSU oral wear simulator forced a spherical (10.0 ± 0.1 mm diameter) steatite antagonist (Union Process Inc., Akron, OH, USA) into contact with the specimens in the presence of the food-like slurry . To simulate three-body abrasion wear, the antagonist was driven along a 7 mm sliding path imparting a 20 N sliding force and three-body attrition wear was simulated by a 90 N force at the end of the 7 mm sliding path . The steatite antagonist was unloaded from the specimen surface at the end of each wear cycle, returned automatically to the start of the 7 mm sliding path and the wear regime repeated at a frequency of 1 Hz for a designated predetermined number of wear cycles. The RBC specimens were subjected to 50,000 wear cycles before being removed from the chambers of the OHSU oral wear simulator.
The wear facets produced were scanned using a contact diamond stylus profilometer (Talysurf CLI 2000, Taylor-Hobson Precision, Leicester, England) containing a 90° conisphere stylus tip of 2 μm radius which had a resolution of 40 nm in the vertical z -axis when scanning at a speed of 1 mm/s. A series of horizontal traces (perpendicular to the sliding direction of the OHSU oral wear simulator ( y -axis)) were conducted across the wear facet at 10 μm intervals with longitudinal measurements (parallel to the sliding direction of the OHSU oral wear simulator ( x -axis)) recorded every 10 μm to scan an area of 32 mm 2 (8 mm × 4 mm) around the wear facet. The accuracy and precision of the wear depth measurements were determined prior to the analysis of the wear facets using a standard step height of 1.0 mm . A three-dimensional image of each wear facet was created using the TalyMap software package (Taylor-Hobson Precision, Leicester, England) ( Fig. 1 ) and the total volumetric wear (mm 3 ) and maximum wear depth (μm) measurements were quantified using the non-worn areas surrounding the wear facet as the reference plane .
Immediately following the profilometric analysis, the RBC specimens were replaced into the individual wear chambers of the OHSU oral wear simulator, aligned using the waterproof alcohol pen markings and secured. A fresh food-like slurry was added to the chambers and the specimens were subjected to a further 100,000 wear cycles (150,000 wear cycles in total) at 1 Hz. The resultant wear facets were further analysed using the contact diamond stylus profilometric procedure outlined above and the total volumetric wear and maximum wear depth measurements were determined. The RBC specimens were then reinserted into the individual wear chambers of the OHSU oral wear simulator with a fresh food-like slurry and subjected to a further 150,000 wear cycles (300,000 wear cycles in total) at 1 Hz. The wear facets were reanalysed using the contact diamond stylus profilometric procedure and the total volumetric wear and maximum wear depth measurements quantified.
2.4
Antagonist wear
Following 300,000 wear cycles, the spherical surface of the steatite antagonists in contact with the RBC specimens during testing were scanned using the contact diamond stylus profilometer. An area of 12.25 mm 2 (3.5 mm × 3.5 mm) was scanned on each antagonist coincident with the center (apex) of the sphere. Profilometric traces were conducted across the antagonist at 10 μm intervals ( y -axis) with measurements recorded at 10 μm intervals ( x -axis). Extrapolation of the spherical form using the software resulted in a flattened surface and the total volumetric wear (mm 3 ) and maximum wear depth (mm) of the wear facet on each antagonist were quantified using the non-worn areas as the reference points ( Fig. 2 ).
2.5
Statistical analysis
The statistical analyses of the total volumetric wear and maximum wear depth data (two- and one-way analyses of variance (ANOVA), with Tukey’s post hoc tests where required) and regression analyses, were conducted using software (SPSS 12.0.1; SPSS Inc., Chicago, IL, USA) at a significance value of p = 0.05. The normality of the data in each group was assessed using the Shapiro–Wilk test ( p < 0.05). Two, two-way ANOVAs with factors of RBC material × wear cycles were conducted, one for the total volumetric wear and one for the maximum wear depth measured data. To compare the total volumetric wear and maximum wear depth data of the wear facets between the seven RBC groups (Grandio and G 1 –G 6 ), six one-way ANOVAs were performed – one for the total volumetric wear and one for the maximum wear depth for each number of wear cycles investigated (50,000, 150,000 and 300,000, respectively). Paired comparisons were conducted between Grandio and G 1 , G 1 and G 2 , Grandio and G 3 , G 3 and G 4 , G 3 and G 5 and G 5 and G 6 using 36 Independent Student’s t -tests (six for each number of wear cycles examined (50,000, 150,000 and 300,000) for the mean total volumetric wear ( n = 18) and mean maximum wear depth data ( n = 18)). Fourteen regression analyses were conducted, two for each RBC material (one for the mean total volumetric wear data and one for the mean maximum wear depth) to determine if significant trends were evident with increasing wear cycles from 50,000 to 300,000. In addition, two one-way ANOVAs were employed to compare the mean total volumetric wear and the mean maximum wear depth data of the antagonists from the seven RBC groups after 300,000 wear cycles.
2
Materials and methods
2.1
Materials
The RBC materials tested were Grandio and six experimental RBCs (G 1 –G 6 , Voco GmbH, Cuxhaven, Germany) based on the Grandio formulation by manipulating the monomeric resin composition and the barium-alumina borosilicate filler characteristics ( Table 1 ). All RBC materials contained camphorquinone as the initiator and were produced using the same manufacturing processing route. The major differences between the RBC materials were: G 1 had an increased filler volume fraction (74.5 vol%) compared with Grandio (71.4 vol%); G 2 had a urethane dimethacrylate (UDMA):aliphatic dimethacrylate monomeric blend compared with the BisGMA:TEGDMA monomeric blend of G 1 ; G 3 had an increased mean filler diameter (2.5 μm) and filler volume fraction (76 vol%) compared with Grandio (1.5 μm and 71.4 vol%, respectively); G 4 had an increased filler density (2.7 g/cm 3 ) and decreased filler volume fraction (71 vol%) compared with G 3 (2.4 g/cm 3 and 76 vol%, respectively); the monomeric blend ratio of BisGMA:TEGDMA of G 3 was increased from 3:1 to 3.5:1 for G 5 while the filler volume fraction was decreased from 76 to 73 vol%; and G 6 had an ormocer monomeric blend compared with the BisGMA:TEGDMA monomeric blend of G 5 ( Table 1 ).
| RBC | Resin matrix | Barium-alumina borosilicate filler particles | ||||
|---|---|---|---|---|---|---|
| Composition | Ratio | Density (g/cm 3 ) | Diameter (μm) | Fraction (wt%) | Fraction (vol%) | |
| Grandio | BisGMA:TEGDMA | 3:1 | 2.4 | 1.5 | 87 | 71.4 |
| G 1 | BisGMA:TEGDMA | 3:1 | 2.4 | 1.5 | 89 | 74.5 |
| G 2 | UDMA:Aliphatic dimethacrylates | 1:1 | 2.4 | 1.5 | 89 | 74.5 |
| G 3 | BisGMA:TEGDMA | 3:1 | 2.4 | 2.5 | 91 | 76 |
| G 4 | BisGMA:TEGDMA | 3:1 | 2.7 | 2.5 | 89 | 71 |
| G 5 | BisGMA:TEGDMA | 3.5:1 | 2.4 | 2.5 | 87 | 73 |
| G 6 | Ormocer | 1 | 2.4 | 2.5 | 87 | 73 |
2.2
Specimen preparation
Eight disc-shaped specimens (13.0 ± 0.1 mm diameter and 2.0 ± 0.1 mm thickness) were prepared for each RBC material investigated (Grandio and G 1 –G 6 ) using a nylon ring-mold. The ring-mold was placed on a glass slab, covered with an acetate strip and a consistent mass (g) of uncured resin weighed, using a balance reading to 0.001 g (Sartorius Expert, Sartorius AG, Goettingen, Germany), was placed into the center of the ring-mold. A second acetate strip was placed on top and a 1 kg weight applied to spread the uncured resin within the ring-mold . Each RBC specimen was light irradiated using a hand-held quartz-tungsten halogen (QTH) light curing unit (LCU) (Optilux 501, Kerr, Orange Co., CA, USA) coupled with a 13 mm light tip diameter operating at an output intensity of 540 ± 16 mW/cm 2 . The RBC materials were light irradiated from the top surface only for 20 s, by placing the light tip in direct contact with the top acetate strip. The specimens were carefully removed from the ring-mold following irradiation and stored in 50 mL of distilled water maintained at 37 ± 1 °C under light-proof conditions (Firlabo SP BVEHF, Société Firlabo, Meyzieu, France) for 23 h .
2.3
In vitro wear testing
One hour prior to the commencement of in vitro wear testing, the disc-shaped RBC specimens were mounted in a two part cold-setting acrylic resin (Varidur; Buehler, Lake Bluff, IL, USA). The specimens were positioned in the center of individual cylindrical mounting cups (Metset Cups, Buehler, Lake Bluff, IL, USA) and the acrylic resin, mixed using a powder to liquid mixing ratio of 1 g to 1 mL, was poured to a height of 10.0 ± 1.0 mm. The mounting cups had an internal diameter of 25.0 ± 0.1 mm which were compatible with the chambers of the wear machine . After 45 min, the mounted specimens were removed from the mounting cups and wet ground sequentially with P600 and P1200 silicon carbide (SiC) abrasive paper for 30 s each, respectively under water lubrication on a grinding-polishing machine (Alpha and Beta Grinder-Polisher, Buehler, Lake Bluff, IL, USA) using a force of 10 N per specimen.
The embedded disc-shaped RBC specimens were secured into individual wear chambers 24 h after irradiation and marked with a waterproof alcohol pen for future realignment. The abrasion and attrition forces for each OHSU oral wear simulator chamber ( n = 4) were calibrated prior to the commencement of each wear testing regime using a custom jig (Proto-tech, Portland, OR, USA) containing a 500 N load cell . A food-like slurry consisting of 1.0 g of crushed poppy seeds (Holland and Barrett, Burton-upon-Trent, England), 0.5 g of poly(methyl methacrylate) beads with a mean particle diameter of 50–100 μm (ProBase Cold Polymer; Ivoclar Vivadent, Schaan, Liechtenstein) and 5 mL of distilled water was placed into each wear chamber prior to the commencement of testing. The OHSU oral wear simulator forced a spherical (10.0 ± 0.1 mm diameter) steatite antagonist (Union Process Inc., Akron, OH, USA) into contact with the specimens in the presence of the food-like slurry . To simulate three-body abrasion wear, the antagonist was driven along a 7 mm sliding path imparting a 20 N sliding force and three-body attrition wear was simulated by a 90 N force at the end of the 7 mm sliding path . The steatite antagonist was unloaded from the specimen surface at the end of each wear cycle, returned automatically to the start of the 7 mm sliding path and the wear regime repeated at a frequency of 1 Hz for a designated predetermined number of wear cycles. The RBC specimens were subjected to 50,000 wear cycles before being removed from the chambers of the OHSU oral wear simulator.
The wear facets produced were scanned using a contact diamond stylus profilometer (Talysurf CLI 2000, Taylor-Hobson Precision, Leicester, England) containing a 90° conisphere stylus tip of 2 μm radius which had a resolution of 40 nm in the vertical z -axis when scanning at a speed of 1 mm/s. A series of horizontal traces (perpendicular to the sliding direction of the OHSU oral wear simulator ( y -axis)) were conducted across the wear facet at 10 μm intervals with longitudinal measurements (parallel to the sliding direction of the OHSU oral wear simulator ( x -axis)) recorded every 10 μm to scan an area of 32 mm 2 (8 mm × 4 mm) around the wear facet. The accuracy and precision of the wear depth measurements were determined prior to the analysis of the wear facets using a standard step height of 1.0 mm . A three-dimensional image of each wear facet was created using the TalyMap software package (Taylor-Hobson Precision, Leicester, England) ( Fig. 1 ) and the total volumetric wear (mm 3 ) and maximum wear depth (μm) measurements were quantified using the non-worn areas surrounding the wear facet as the reference plane .
