Abstract
Objectives
The purpose of this study was to compare four medium particles currently used for in vitro composite wear testing (glass and PMMA beads and millet and poppy seeds).
Methods
Particles were prepared as described in previous wear studies. Hardness of medium particles was measured with a nano-indentor, particle size was measured with a particle size analyzer, and the particle form was determined with light microscopy and image analysis software. Composite wear was measured using each type of medium and water in the Alabama wear testing device. Four dental composites were compared: a hybrid (Z100), flowable microhybrid (Estelite Flow Quick), micromatrix (Esthet-X), and nano-filled (Filtek Supreme Plus). The test ran for 100,000 cycles at 1.2 Hz with 70 N force by a steel antagonist. Volumetric wear was measured by non-contact profilometry. A two-way analysis of variance (ANOVA) and Tukey’s test was used to compare both materials and media.
Results
Hardness values (GPa) of the particles are (glass, millet, PMMA, and poppy, respectively): 1.310(0.150), 0.279(0.170), 0.279(0.095), and 0.226(0.146). Average particle sizes (μm) are (glass, millet, PMMA, and poppy, respectively): 88.35(8.24), 8.07(4.05), 28.95(8.74), and 14.08(7.20). Glass and PMMA beads were considerably more round than the seeds. During composite wear testing, glass was the only medium that produced more wear than the use of water alone. The rank ordering of the materials varied with each medium, however, the glass and PMMA bead medium allowed better discrimination between materials.
Significance
PMMA beads are a practical and relevant choice for composite wear testing because they demonstrate similar physical properties as seeds but reduce the variability of wear measurements.
1
Introduction
While wear of dental composites is a prolifically studied subject , there is great variability in testing methods . The 2001 International Standards Organization report “Wear by two and or three body contact” describes eight methods for measuring in vitro wear. Among other variables, the report describes three different food-simulating media for three-body wear including: millet seed, PMMA beads, and poppy seed . The adoption of these media particles originated from a 1986 publication from de Gee, who compared wear rates produced with different seeds and polymethyl methacrylate (PMMA) powder in the ACTA wear testing device. He determined that using a mixture of 80% millet seeds and 20% PMMA powder most closely correlated in vivo wear data . Later, Leinfelder and Suzuki used PMMA alone as a third-body medium because it does not degrade like millet and also expedites the wear process . Condon and Ferracane introduced poppy seed as a replacement to millet seed in de Gee’s original mixture . Since that time, additional third body particles have been examined including hydroxyapetite, green carborundum , and calcium diphosphate . Glass microbeads have also been used as a third-body medium in industry protocol to expedite wear testing.
Although the effect of media particle selection on composite wear has not been directly studied, various test methods which incorporate different particles have been compared. Two studies by Heintze et al. compared the ACTA, Alabama and OHSU wear testing methods, which incorporate millet seed, PMMA and poppy seed media, respectively. These studies determined that relative wear ranking of composite materials varied significantly between testing methods . Among those testing methods, there is variation in many other factors (such as the methods of masticatory force application and tooth sliding reproduction) , so it is not possible to attribute the discrepancy in wear ranking to variation in media particles alone. The aim of this study is to compare the wear of four composites in the Alabama wear testing device with four currently used third-body medium particles (millet seed, poppy seed, PMMA beads, and glass microbeads) and water. The null hypothesis is that the ranking of materials will be similar for all medium used.
Measuring the physical properties of the abrasive particles is critical for understanding the wear-producing mechanisms that differentiate each medium. Theoretically, a particle will be more abrasive if: (1) it is harder than the surface it is indenting and (2) the size and form of the particle allow it to penetrate through composite filler particles to the wear-prone resin matrix. The hardness, size and shape of each abrasive medium particle will be measured in this study, as these properties have been identified as critical parameters in tribological testing .
2
Materials and methods
2.1
Media particle preparation
Millet seed was prepared, as described by Nihei et al. , by grinding 50 g of seeds in a rotating blade grinder for 5 s. Poppy seed was prepared as described by Condon and Ferracane by grinding 3 g of poppy seed with 100 strokes of mortar and pestle. PMMA beads (Dentsply Caulk, Milford, DE, USA) and soda lime glass microbeads (Size 270, Unibrite Corporation, Port Washington, NY, USA) were obtained from their manufacturer.
2.2
Nano-hardness measurement
The medium particles were embedded in a 95% methyl methacrylate/5% n -butyl embedding epoxy (Fischer Scientific, Pittsburgh, PA, USA) before testing. The glass and PMMA beads were first stained with methylene blue to aid in their visualization. A thin coat of each type of medium particle was dispersed on the surface of a cup half-filled with set epoxy. The specimens were then covered with a layer of unset epoxy which polymerized under ultraviolet light for 48 h. The surface of the specimens were wet polished with a succession of 320 grit, 800 grit and 1200 grit paper on a surface parallel plane grinder (400CS, Exakt Technologies Inc., Oklahoma City, OK, USA) to reveal a layer of sectioned particles. The nano-hardness of the exposed surfaces of the medium particles was measured with a nano-indentation tester (G200, MTS, Oak Ridge, TN, USA). Indentions were depth controlled to 0.5 μm and performed with a diamond Berkovitch pyramid-shaped stylus (diameter = 40 nm). A 4 × 4 grid of indents (5 μm spacing between indents) was selected on three millet and poppy seeds. Fifteen individual glass and PMMA beads were selected for testing. Indents were examined after testing and hardness values that were obtained from indenting the epoxy were discarded.
Composite specimens were prepared in a silicone mold (1 cm diameter × 4 mm) and light polymerized at 2 mm increments with a Coltolux LED curing light (Coltene/Whaledent, Cuyahoga Falls, OH, USA) (583 mW/cm 2 ). They were then polished using 600 and 1200 grit silicon carbon paper followed by 0.5 μm alumina slurry on a polishing wheel (Metallurgical polisher, Buehler Ltd., Evanston, IL, USA) at 80 rotations/s and 20 N of force. Nano-hardness of the composites was determined by creating a 4 × 4 grid of indents (5 μm spacing between indents) at two locations on the composite surface. The same testing parameters were used as described above.
2.3
Particle size measurement
The medium particles were mixed with distilled water in a 3:1 ratio. A 3 mL sample of each medium was measured in a LASER light diffraction optical particle size analyzer (Microtrac 3500, Microtrac Inc., York, PA, USA) operated between the size range of 24 nm and 2800 μm. Three measurements were taken of each sample, and media were sonicated for 2 min between measurements to prevent agglomeration.
2.4
Particle imaging and shape measurement
Particles were randomly dispersed on a glass slide. The particles were imaged with 1000× optical magnification using digital light microscopy (VHX-600, Keyence Co., Osaka, Japan) as described in Table 1 of ASTM standard F1877-05 . Three images of each medium were collected, and within the images, the perimeter (P) and area (A) of the outline of each particle was recorded with image analysis software (ImageJ, NIH, Bethesda, MD, USA). The form of the particles was determined using the form factor (FF) equation: FF = 4 πA / p 2 . Form factor gives an indication of the roughness or roundness of a particle’s outline; particles with a circular outline have a FF = 1. The average of the form factor of all particles from each media type was reported.
| Material | Classification | Lot no. | Matrix | Filler | Total filler content |
|---|---|---|---|---|---|
| Estelite Flow Quick | Flowable microhybrid | UE 401236 | Bis-EMA, TEGDMA, 1-6bis(methacrylethyloxycarbonylamino)trimethyl hexane | Silica/titania and silica/zirconia particles: 0.04–0.6 μm | 71 (wt%) 53 (vol%) |
| Esthet X | Micromatrix | 060329 | Urethane modified bis-GMA | Bariumalumino fluoroborosilicate glass: 0.02–2.5 μm Silica: 10–20 nm | 77 (wt%) 60 (vol%) |
| Filtek Supreme Plus | Nano-filled | 20060606 | Bis-EMA6, UDMA, Bis-GMA, TEGDMA | Silica: 5–20 nm nanoparticle Zirconia/silica: 0.6–1.4 μm nanocluster | 78.5 (wt%) 57.7 (vol%) |
| Z100 | Hybrid | 6KM | Bis-GMA, TEGDMA, 2-benzotriazolyl-methylphenol | Zirconia/silica: 0.01–3.5 μm | 85 (wt%) 66 (vol%) |
2.5
Wear testing of composite materials
Four commercially available light-cured composites were studied: a hybrid (Z100, 3M Co., St Paul, MN, USA), a flowable microhybrid (Estelite Flow Quick, Tokuyama, Tokyo, Japan), a micromatrix (Esthet-X, Caulk Dentsply, Milford, DE, USA), and a nano-filled (Filtek Supreme Plus, 3M ESPE). The materials were chosen to represent a range of filler concentrations (71–85%) and filler particle sizes (0.005–3.5 μm). Their properties are listed in Table 1 .
Specimens ( n = 8) were fabricated in silicone molds (1 cm diameter × 4 mm depth) and light polymerized at 2 mm increments with a Coltolux LED curing light (Coltene/Whaledent) (583 mW/cm 2 ). After production, the specimens were set in brass holders with acrylic (Dentsply Repair Material, Dentsply Caulk) and polished using 600 and 1200 grit silicon carbon paper followed by 0.5 μm alumina slurry on a polishing wheel (Metallurgical polisher, Buehler Ltd.) at 80 rotations/s and 20 N of force. Specimens were stored in water at 37 °C for 48 h. The spring containing antagonist pistons were load calibrated before testing with a universal testing device (Model 4411, Instron, Norwood, MA, USA) to ensure a maximum force of 70 N. Specimens were placed into the Alabama wear device. which operates by pressing the spring-loaded pistons into the composite specimens followed by a 30° rotation of the antagonist. The antagonists on the pistons contact the specimens for 400 ms applying a maximum force of 70 N and counter-rotate 30° prior to lifting off the specimens ( Fig. 1 ). 2 g of the medium prepared as described above was mixed with 3 mL of distilled water. This ratio was chosen based on previous studies . The medium mixture was stirred and vibrated until the particles fully mixed with the water. Due to the differences in densities of each type of particle, some wells contained more particles than others, however, all specimens were completely covered with medium. The medium was then poured into the individual wells above the composite specimens created by the brass rings ( Fig. 2 ). A fifth group was prepared in which the specimen wells were filled with distilled water. The test was run for 100,000 cycles at 1.2 Hz. A new stainless steel antagonist ball ( R a = 4.7 μm) was used for every test. Following the test, specimens were ultrasonically cleaned for 1 min. The surface of each specimen was scanned with a non-contact optical profilometer (Scantron 2000, Scantron Industrial Products, Tauton, England) with a 20 μm × 20 μm resolution. The scans were analyzed with superimposition software (Pro-Form, Scantron Industrial Products) to determine volumetric wear.
Following testing, representative wear specimens were removed from the brass holders and coated with Au-Pd in a sputter coater (Hummer X, Anatech, Union City, CA, USA). Their surfaces were examined by secondary-electron SEM (Model 40, International Scientific Instruments, Milpitas, CA, USA). Samples of glass and PMMA beads were removed from the wells following testing and examined using digital light microscopy (VHX-600, Keyence Co.).
2.6
Statistical analysis
The study design was a two-way layout, with groups defined by material and medium. The primary analysis technique utilized two-way analysis of variance (ANOVA). Pairwise comparisons among group means were conducted using Tukey’s test. A rank transformation of the data was used, due to significant nonhomogeneity of variance among the groups ( p < 0.0001, Levene’s test), and substantial asymmetry of the sample distributions. Separate one-way ANOVA analyses were conducted for materials within each medium and medium within each material group in order to evaluate the significant interactions.
2
Materials and methods
2.1
Media particle preparation
Millet seed was prepared, as described by Nihei et al. , by grinding 50 g of seeds in a rotating blade grinder for 5 s. Poppy seed was prepared as described by Condon and Ferracane by grinding 3 g of poppy seed with 100 strokes of mortar and pestle. PMMA beads (Dentsply Caulk, Milford, DE, USA) and soda lime glass microbeads (Size 270, Unibrite Corporation, Port Washington, NY, USA) were obtained from their manufacturer.
2.2
Nano-hardness measurement
The medium particles were embedded in a 95% methyl methacrylate/5% n -butyl embedding epoxy (Fischer Scientific, Pittsburgh, PA, USA) before testing. The glass and PMMA beads were first stained with methylene blue to aid in their visualization. A thin coat of each type of medium particle was dispersed on the surface of a cup half-filled with set epoxy. The specimens were then covered with a layer of unset epoxy which polymerized under ultraviolet light for 48 h. The surface of the specimens were wet polished with a succession of 320 grit, 800 grit and 1200 grit paper on a surface parallel plane grinder (400CS, Exakt Technologies Inc., Oklahoma City, OK, USA) to reveal a layer of sectioned particles. The nano-hardness of the exposed surfaces of the medium particles was measured with a nano-indentation tester (G200, MTS, Oak Ridge, TN, USA). Indentions were depth controlled to 0.5 μm and performed with a diamond Berkovitch pyramid-shaped stylus (diameter = 40 nm). A 4 × 4 grid of indents (5 μm spacing between indents) was selected on three millet and poppy seeds. Fifteen individual glass and PMMA beads were selected for testing. Indents were examined after testing and hardness values that were obtained from indenting the epoxy were discarded.
Composite specimens were prepared in a silicone mold (1 cm diameter × 4 mm) and light polymerized at 2 mm increments with a Coltolux LED curing light (Coltene/Whaledent, Cuyahoga Falls, OH, USA) (583 mW/cm 2 ). They were then polished using 600 and 1200 grit silicon carbon paper followed by 0.5 μm alumina slurry on a polishing wheel (Metallurgical polisher, Buehler Ltd., Evanston, IL, USA) at 80 rotations/s and 20 N of force. Nano-hardness of the composites was determined by creating a 4 × 4 grid of indents (5 μm spacing between indents) at two locations on the composite surface. The same testing parameters were used as described above.
2.3
Particle size measurement
The medium particles were mixed with distilled water in a 3:1 ratio. A 3 mL sample of each medium was measured in a LASER light diffraction optical particle size analyzer (Microtrac 3500, Microtrac Inc., York, PA, USA) operated between the size range of 24 nm and 2800 μm. Three measurements were taken of each sample, and media were sonicated for 2 min between measurements to prevent agglomeration.
2.4
Particle imaging and shape measurement
Particles were randomly dispersed on a glass slide. The particles were imaged with 1000× optical magnification using digital light microscopy (VHX-600, Keyence Co., Osaka, Japan) as described in Table 1 of ASTM standard F1877-05 . Three images of each medium were collected, and within the images, the perimeter (P) and area (A) of the outline of each particle was recorded with image analysis software (ImageJ, NIH, Bethesda, MD, USA). The form of the particles was determined using the form factor (FF) equation: FF = 4 πA / p 2 . Form factor gives an indication of the roughness or roundness of a particle’s outline; particles with a circular outline have a FF = 1. The average of the form factor of all particles from each media type was reported.
