Abstract
Objective
To investigate the effects of polishing techniques on the surface roughness of Y-TZP ceramic and on the wear behavior of synthetic hydroxyapatite (HA).
Methods
Thirty-two full-contour Y-TZP (Diazir ® ) sliders ( φ = 2 mm × 1.5 mm in height) were manufactured using CAD/CAM, embedded in acrylic resin using brass holders, and randomly allocated into four groups ( n = 8): according to the finishing/polishing procedure: G1-as-machined, G2-glazed, G3-diamond bur finishing and G4 − G3 + OptraFine ® polishing kit. Thirty-two sintered HA disks ( φ = 13 mm × 2.9 mm in height) were similarly mounted in brass holders. Y-TZP sliders baseline surface roughness values (Ra and Rq, in μm) were recorded using a non-contact profilometer (Proscan 2000). A two-body pin-on-disc wear test was performed. HA height (μm) and volume (mm 3 ) losses were measured. Y-TZP height loss was measured using a digital micrometer. One-way ANOVA was used to determine the effect of the polishing techniques on the surface roughness. Comparisons between groups for differences in antagonist height loss/volume, and slider height loss were performed using one-way ANOVA. Statistical significance was set at α = 0.05.
Results
Roughness measurements showed significant differences ( p = 0.0001) among the surface treatments with G1 (Ra = 0.84, Rq = 1.13 μm) and G3 (Ra = 0.89, Rq = 1.2 μm) being the roughest, and G2 (Ra = 0.42, Rq = 0.63 μm) the smoothest ( p = 0.0001). Y-TZP slider height loss was highest for the glazed group (35.39 μm), and was lowest for the polished group (6.61 μm) ( p = 0.0001). Antagonist volume and height losses for groups (G1–G3) were similar, while the polished group (1.3 mm 3 , 14.7 μm) showed significant lower values ( p = 0.0001).
Significance
Although glazed zirconia provides an initially smooth surface, significantly increased antagonist wear was observed compared to the polished Y-TZP zirconia surface.
1
Introduction
Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has unique properties that are desirable for restorative dentistry, including but not limited to chemical and dimensional stability, high mechanical strength, and high fracture toughness . Due to their lack of translucency, however, zirconia cores and frameworks are generally veneered with porcelain to achieve a more natural appearance . In specific clinical situations, such as when the occlusal or palatal space is limited or in cases where a patient’s parafunctional activity (e.g., bruxism) may contraindicate this veneering application, the use of unveneered zirconia ceramic seems to be an option for all-ceramic restorations . As a result of the recent advancements in zirconia stain and glaze techniques, dental laboratories have started to promote glazed all-zirconia crowns without a veneering porcelain.
Wear of natural dentition is considered one of the important issues when choosing a dental material for restorative purposes due to the fact that it is an irreversible process with many functional and aesthetic complications . Wear is defined as the removal of material from a solid surface as a result of mechanical interaction between two relatively moving surfaces . Tooth wear is a complex process and can be affected by many factors, including abrasive nature of food, parafunctional habits, enamel thickness, enamel hardness, dental structural direction and enamel prism orientation, the presence of opposing restorative materials with different wear behavior compared to tooth structure, as well as pH, viscosity, and flow rate of the saliva .
Abrasive wear is the primary form of wear in dentistry and is further subdivided into two types: two-body and three-body. In two-body wear the surfaces are worn by direct contact between opposing surfaces, while in three-body the surfaces are worn away by the slurry of abrasive particles that intervene between the moving surfaces . A combination of two-body and three-body wear typically occurs between opposing enamel and restorative materials . As a general rule, ceramics cause greater abrasive wear of human enamel than other restorative materials . Surface roughness of a dental restoration has been positively correlated with increased wear behavior of the material. Nonetheless, grinding and different finishing procedures are associated with higher teeth wear as it results in increased surface roughness . Ghazal and Kern and Kalin et al. reported that increasing antagonist surface roughness of zirconia and glass-infiltrated ceramics significantly increases wear of human enamel, composite resin , and hydroxyapatite specimens . Preis et al. and Albashaireh et al. demonstrated that zirconia ceramics have lower wear compared with other types of ceramics. The low wear characteristics of zirconia ceramics in hip joint replacements has been attributed to its smoother articulating surfaces compared to cobalt–chromium . While many studies have compared different surface treatments on the surface roughness of ceramics , particularly between glazed and polished surfaces, limited studies are available on the effects of different surface treatments on both the surface roughness of zirconia ceramic and the associated antagonist wear behavior. The rationale for this study was drawn based on the adjustments commonly made to ceramic restorations in clinical situations (e.g., no adjustment/glazed, bur-finished, unpolished, and polished) and to what extent that could affect zirconia ceramic surface roughness and the related antagonist wear behavior.
Human enamel has been considered the best choice for the in vitro study of wear. However, there are some complications to using natural human enamel including the inability to obtain enamel samples of the proper size, and natural variations in shape and physical properties between specimens . In addition, enamel samples require extensive preparation and standardization, which can be very difficult due to the natural variations . Given that human enamel is composed mainly from hydroxyapatite crystals (HA), we have used synthetic HA as a substitute for human enamel, which has demonstrated to be a reliable substrate in wear studies . The aims of this study were to investigate the effects of different finishing/polishing techniques on the surface roughness of an Y-TZP full-contour zirconia, and to evaluate the influence of zirconia surface roughness on the wear behavior against synthetic HA. The null hypotheses of this study were as follows: (1) the different zirconia surface treatments tested would result in similar surface roughness values, and (2) the different zirconia surface treatments would not affect the wear of antagonist HA specimens.
2
Materials and methods
2.1
Preparation of Y-TZP sliders
Thirty-two yttria-stabilized full-contour zirconia (Lot #P02286, Diazir ® , Ivoclar-Vivadent, Amherst, NY, USA) ceramic specimens; hereafter: zirconia sliders, were manufactured using a CAD/CAM machine (CEREC ® inLab MC XL, Sirona, Charlotte, NC, USA). Subsequently, the specimens were sintered in a high-temperature furnace (Programat ® S1, Ivoclar-Vivadent, Amherst, NY, USA) at 1500 °C for 8 h. The specimens experienced a shrinkage of 19.3% during sintering. The computer software compensates for this shrinkage during milling to produce finished specimens of the desired size ( φ = 2 mm × 1.5 mm in height, connected to a base 6 mm in diameter and 4 mm high. In order to guarantee parallelism between the pin and the disc surface, preventing high localized contact stresses and uneven wear , a 1/4 mm wide chamfer was placed circumferentially around the end of the slider reducing the testing surface to 1.5 mm in diameter.
A custom silicone mold was made for mounting the zirconia sliders. The sliders were embedded in auto-polymerizing acrylic resin (Lot #1006-229-X, Bosworth Fastray, Harry J. Bosworth Co, IL, USA) using brass holders. The zirconia sliders were randomly allocated to four groups ( n = 8) according to the surface finishing/polishing procedure as follows: G1-as-machined, G2-glazed, G3-diamond bur finishing for 10 s using a high-speed hand piece under cooling water (Lot #KK1UX, Fine needle diamond bur, #8392.31.016, Brasseler, GA, USA), and G4-diamond bur finishing followed by polishing with an OptraFine ® polishing kit (Lot #PL1781, Ivoclar-Vivadent, Amherst, NY, USA). Each polishing step was carried out for 30 s using a slow speed hand-piece under water cooling, followed by polishing with the diamond paste (Lot #PL1782, Ivoclar-Vivadent) supplied with the kit for 60 s. All specimens were immersed in distilled water and cleaned for 3 min in an ultrasonic bath. Glazed specimens (G2) were prepared and fired (Programat P500, Ivoclar-Vivadent, Amherst, NY, USA) following the manufacturer instructions using the full-contour zirconia glazing system (Lot #P4VM, Diazir ® , Ivoclar-Vivadent, Amherst, NY, USA) before embedding the specimens in acrylic resin.
Baseline surface roughness parameters (Ra and Rq, in μm) were recorded using a non-contact profilometer (Proscan 2000, Scantron, Taunton, England, UK) and dedicated software (version 2.0.17 Scantron Industrial Products Ltd, Taunton, England, UK). Mean Ra and Rq values were associated with each zirconia experimental group. One additional specimen per group was sputter-coated with gold and imaged at different magnifications under a scanning electron microscope (JEOL JSM-5310LV, Jeol Ltd, Tokyo, Japan) operating at 10 kV to obtain representative and qualitative images of the Y-TZP surfaces after sample preparation.
Zirconia baseline vertical height (μm) was measured using a digital micrometer (Digi-micro, Nikon, Tokyo, Japan). Four readings were taken for each sample from four different pre-established reference points situated on two cross lines marked on the zirconia sample base. The procedure measured the difference between the base height and the tip of the zirconia slider .
2.2
Preparation of hydroxyapatite antagonist specimens
Sintered ( n = 32) disk-shaped ( φ = 13 mm × 2.9 mm in height) synthetic HA (Lot #900001, Orthoblock, Calcitek Inc., Carlsbad, CA, USA) samples were mounted in brass holders using auto-polymerizing acrylic resin in a similar procedure as previously described for the zirconia sliders. HA specimens were then wet-finished with SiC paper (600 through 1200-grit) to obtain a flat and standardized test surface. Specimens were cleaned in an ultrasonic bath in distilled water for 3 min, and stored in distilled water at room temperature until testing. Baseline surface roughness (Ra and Rq, in μm) was recorded using a non-contact profilometer (Proscan 2000, Scantron, Taunton, England, UK) to ensure standardization of surface roughness.
2.3
Wear testing and measurements
To simulate the wear that occurs in the occlusal contact, a two-body pin-on-disk wear test was performed. A two-body rotating pin-on-disk wear testing machine containing four wear stations was used . Each brass holder containing the mounted test specimens has a large screw protruding out for attaching the specimen to the wear machine. The mounted HA antagonists were attached to the upper stationary component of the wear stations, and zirconia sliders were attached to the lower rotating component. The stations rotate at a constant speed with a radius of movement of ca. 3–4 mm. The test was run for 25,000 cycles at 1.2 Hz under a 3-kg load. The wear stations were flushed continuously with water to prevent the effect of debris on the wear test. After testing, specimens were removed and cleaned with distilled water in an ultrasonic bath for 5 min.
2.4
Wear measurements
Zirconia sliders were evaluated for height loss using the digital micrometer. The differences in height before and after testing were recorded as zirconia vertical height loss (μm). The vertical substance loss, i.e., the maximum depth of the wear area (μm) and the volume loss (mm 3 ) of the HA antagonists were measured using the non-contact profilometer (Proscan 2000) by comparing the wear track ( Fig. 1 ) to unworn areas . SEM was done to evaluate HA wear characteristics.
2.5
Statistical analysis
The effects of the finishing/polishing techniques on the Y-TZP slider surface roughness and height loss as well as on the HA antagonist height and volume loss were tested using one-way ANOVA followed by Tukey’s post hoc tests at 5% significance level.
2
Materials and methods
2.1
Preparation of Y-TZP sliders
Thirty-two yttria-stabilized full-contour zirconia (Lot #P02286, Diazir ® , Ivoclar-Vivadent, Amherst, NY, USA) ceramic specimens; hereafter: zirconia sliders, were manufactured using a CAD/CAM machine (CEREC ® inLab MC XL, Sirona, Charlotte, NC, USA). Subsequently, the specimens were sintered in a high-temperature furnace (Programat ® S1, Ivoclar-Vivadent, Amherst, NY, USA) at 1500 °C for 8 h. The specimens experienced a shrinkage of 19.3% during sintering. The computer software compensates for this shrinkage during milling to produce finished specimens of the desired size ( φ = 2 mm × 1.5 mm in height, connected to a base 6 mm in diameter and 4 mm high. In order to guarantee parallelism between the pin and the disc surface, preventing high localized contact stresses and uneven wear , a 1/4 mm wide chamfer was placed circumferentially around the end of the slider reducing the testing surface to 1.5 mm in diameter.
A custom silicone mold was made for mounting the zirconia sliders. The sliders were embedded in auto-polymerizing acrylic resin (Lot #1006-229-X, Bosworth Fastray, Harry J. Bosworth Co, IL, USA) using brass holders. The zirconia sliders were randomly allocated to four groups ( n = 8) according to the surface finishing/polishing procedure as follows: G1-as-machined, G2-glazed, G3-diamond bur finishing for 10 s using a high-speed hand piece under cooling water (Lot #KK1UX, Fine needle diamond bur, #8392.31.016, Brasseler, GA, USA), and G4-diamond bur finishing followed by polishing with an OptraFine ® polishing kit (Lot #PL1781, Ivoclar-Vivadent, Amherst, NY, USA). Each polishing step was carried out for 30 s using a slow speed hand-piece under water cooling, followed by polishing with the diamond paste (Lot #PL1782, Ivoclar-Vivadent) supplied with the kit for 60 s. All specimens were immersed in distilled water and cleaned for 3 min in an ultrasonic bath. Glazed specimens (G2) were prepared and fired (Programat P500, Ivoclar-Vivadent, Amherst, NY, USA) following the manufacturer instructions using the full-contour zirconia glazing system (Lot #P4VM, Diazir ® , Ivoclar-Vivadent, Amherst, NY, USA) before embedding the specimens in acrylic resin.
Baseline surface roughness parameters (Ra and Rq, in μm) were recorded using a non-contact profilometer (Proscan 2000, Scantron, Taunton, England, UK) and dedicated software (version 2.0.17 Scantron Industrial Products Ltd, Taunton, England, UK). Mean Ra and Rq values were associated with each zirconia experimental group. One additional specimen per group was sputter-coated with gold and imaged at different magnifications under a scanning electron microscope (JEOL JSM-5310LV, Jeol Ltd, Tokyo, Japan) operating at 10 kV to obtain representative and qualitative images of the Y-TZP surfaces after sample preparation.
Zirconia baseline vertical height (μm) was measured using a digital micrometer (Digi-micro, Nikon, Tokyo, Japan). Four readings were taken for each sample from four different pre-established reference points situated on two cross lines marked on the zirconia sample base. The procedure measured the difference between the base height and the tip of the zirconia slider .
2.2
Preparation of hydroxyapatite antagonist specimens
Sintered ( n = 32) disk-shaped ( φ = 13 mm × 2.9 mm in height) synthetic HA (Lot #900001, Orthoblock, Calcitek Inc., Carlsbad, CA, USA) samples were mounted in brass holders using auto-polymerizing acrylic resin in a similar procedure as previously described for the zirconia sliders. HA specimens were then wet-finished with SiC paper (600 through 1200-grit) to obtain a flat and standardized test surface. Specimens were cleaned in an ultrasonic bath in distilled water for 3 min, and stored in distilled water at room temperature until testing. Baseline surface roughness (Ra and Rq, in μm) was recorded using a non-contact profilometer (Proscan 2000, Scantron, Taunton, England, UK) to ensure standardization of surface roughness.
2.3
Wear testing and measurements
To simulate the wear that occurs in the occlusal contact, a two-body pin-on-disk wear test was performed. A two-body rotating pin-on-disk wear testing machine containing four wear stations was used . Each brass holder containing the mounted test specimens has a large screw protruding out for attaching the specimen to the wear machine. The mounted HA antagonists were attached to the upper stationary component of the wear stations, and zirconia sliders were attached to the lower rotating component. The stations rotate at a constant speed with a radius of movement of ca. 3–4 mm. The test was run for 25,000 cycles at 1.2 Hz under a 3-kg load. The wear stations were flushed continuously with water to prevent the effect of debris on the wear test. After testing, specimens were removed and cleaned with distilled water in an ultrasonic bath for 5 min.
2.4
Wear measurements
Zirconia sliders were evaluated for height loss using the digital micrometer. The differences in height before and after testing were recorded as zirconia vertical height loss (μm). The vertical substance loss, i.e., the maximum depth of the wear area (μm) and the volume loss (mm 3 ) of the HA antagonists were measured using the non-contact profilometer (Proscan 2000) by comparing the wear track ( Fig. 1 ) to unworn areas . SEM was done to evaluate HA wear characteristics.
