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Newly developed translucent zirconia materials have been used for anterior monolithic complete coverage restorations. Surface treatments can improve adhesion, as well as decrease or increase the strength of ceramics. However, information on the influence of surface treatments on the strength of translucent zirconias is sparse.
The purpose of this in vitro study was to measure and characterize the effects of different surface treatments, including airborne-particle abrasion, on the strength of different translucent 4 mol% and 5 mol% yttria-stabilized zirconia materials.
Materials and methods
Disks (N=160) made from 4 types of translucent yttria-stabilized zirconia materials were surface-treated in 4 ways: Control groups were hand-polished with 2000-grit silicon carbide abrasive paper; as-machined; glass bead airborne-particle abraded; and alumina airborne-particle abraded. The biaxial flexural strength was measured by using a piston-on-3-ball test in a universal testing machine. The simple main effects of material type and surface treatment and their interaction on biaxial flexural strength were evaluated with 2-way ANOVA (α=.05). A priori, 1-way ANOVA and the Tukey multiple comparisons tests were used within material and treatment types (α=.05). Surface morphology was assessed by using scanning electron microscopy. Translucency, absolute transmittance, was measured by using a spectrophotometer.
Two-way ANOVA revealed that the effects of zirconia type, surface treatment, and their interaction all significantly affected biaxial flexural strength (P<.001). One-way ANOVA revealed that the 4Y material was stronger than all 5Y materials, regardless of surface treatment; all 5Y materials were ranked from strongest to weakest as polished; as-machined, or glass bead abraded; and alumina abraded. The 4Y material was stronger when alumina abraded than when glass bead abraded. Scanning electron microscopy showed that as-polished surfaces were smoother than all others; as-machined and glass bead abraded surfaces displayed little difference; alumina abraded was the roughest; and differences among materials were not discerned. The 1-way ANOVA and multiple comparisons testing showed that the 4Y material had less absolute transmittance, approximately 5% less, than all the 5Y materials.
Zirconia material type and surface treatment influenced the strength of translucent zirconia materials; a 4 mol% zirconia material was stronger than 5 mol% zirconia materials for all surface treatments tested; airborne-particle abrasion using alumina had a slight strengthening effect on a 4 mol% zirconia but had a weakening effect on 5 mol% materials; airborne-particle abrasion by using alumina produced the roughest surfaces on all materials; and the 4 mol% material was slightly less translucent than the 5 mol% materials.
A zirconia containing 4 mol% yttria was substantially stronger and only slightly less translucent than 5 mol% materials. Unlike the 5 mol% materials, the 4 mol% material was not weakened by alumina airborne-particle abrasion.
Zirconia first gained attention as an engineering ceramic in the 1970s and began to be used in prosthodontics in the late 1990s.
Zirconia exhibits polymorphism, existing in cubic, tetragonal, and monoclinic forms. A martensitic phase transformation from tetragonal to monoclinic (t→m) zirconia forms is accompanied by a 4% increase in volume. However, a small amount of controlled expansion can decrease crack propagation, thereby making a material tougher through a process called transformation toughening.
Recently, airborne-particle abrasion has been reported to improve the strength of 3Y zirconia materials, likely attributable to the induction of compressive residual stresses leading to phase transformation from tetragonal to monoclinic forms.
The purpose of this in vitro study was to measure and characterize the effects of different surface treatments, including airborne-particle abrasion, on the strength of different translucent 4Y and 5Y zirconia materials. The null hypothesis was that the effects of surface treatment and type of zirconia material would not influence biaxial flexural strength.
Material and methods
Four different 4Y and 5Y zirconia materials were included in this study and are listed in Table 1. Disk-shaped specimens were milled from presintered blanks of each material and sintered (N=160). All the specimens were preshaded (Vita shade A2) and were not additionally colored. The specimens were hand dry-polished with 2000-grit silicon carbide paper.
All specimens were sintered (Zircom; KDF Co, Ltd) according to the manufacturers’ instructions with a 10 °C/min temperature rise and a hold time/dwell time of 2 hours at the high temperature (Tables 1 and 2). The sintering furnace was calibrated to ±4 °C by using process temperature control rings (PTCR; Orton Ceramic Foundation). The final dimensions of the specimens were Ø14 ±2×1.2 ±0.2 mm, measured with a digital micrometer (227-211; Mitutoyo Corp.).
Table 2Mean ±standard deviation biaxial flexure strength (MPa) with 95% confidence limits by surface treatment (ST) and material (M)
Horizontal and vertical lines link similar groups (P >.05).
Four different surface treatments were included. A control group was polished with 2000-grit silicon carbide paper under water. A second group was as received from the milling machine. A third group was airborne-particle abraded with 50-μm alumina particles at a distance of 1.27 cm under 0.2 MPa for 20 seconds.
A fourth group was airborne-particle abraded with 50-μm glass beads at a distance of 1.27 cm and 0.25 MPa for 20 seconds. Each of the 16 material-surface treatment groups contained 10 specimens. Specimens were assigned to surface treatment groups by using a random numbers table.
Biaxial flexural strength was measured by using a universal testing machine (1123; Instron Corp.) at a crosshead speed of 1 mm/min according to the International Organization for Standardization (ISO) 6872:2015.
The specimens were placed so that the treated surfaces were in tension. The biaxial flexure strength was calculated from the following equation:
σ=−0.2387 P (X−Y)/b2, where X=(1+ν) ln(r2/r3)2+[(1−ν)/2](r2/r3)2 and Y=(1+ν) [1+ln(r1/r3)2]+(1−ν)(r1/r3)2, where ν is Poisson ratio, r1 is the radius of the support circle in millimeters, r2 is the radius of the loaded area in millimeters, r3 is the radius of the specimen in millimeters, and b is the specimen thickness at the fracture origin in millimeters.
The means and standard deviations of the material-surface treatment group were calculated and plotted. To elucidate the influence of the simple main effects of material type and surface treatment, as well as their interaction, on biaxial flexural strength, a 2-way ANOVA was performed (α=.05).
A priori, 1-way ANOVA and Tukey multiple pairwise comparison testing were used to determine which of the 4 zirconia types differed from one another for each of the 4 surface treatments (α=1.05). Likewise, 1-way ANOVA and Tukey multiple pairwise comparison testing were used to determine which of the 4 surface treatment types differed from one another for each of the four zirconia types (α=.05). Although ANOVA is robust, it assumes a normal distribution; for this reason, box and whisker plots of all 16 subgroups were reviewed for indications of nonnormal distributions. All distributions were reasonably symmetrical around their means and medians.
Surface morphology was assessed by using scanning electron microscopy in a secondary electron and backscatter electron imaging mode at 20 kV (Quanta FEG 650; FEI) for gold sputter-coated specimens of each material at magnifications ranging from ×100 to ×5 000.
Absolute transmission was measured by using a spectrophotometer (Color-i7; X-Rite) for 4 polished specimens of each material.
Disk specimens, 1.0 ±0.05 mm in thickness, were placed inside a closed chamber; all the light that passed through the specimen—full spectrum from UV through visible to infrared—came from the spectrophotometer. Means and standard deviations were calculated. One-way ANOVA and Tukey multiple pairwise comparison tests were used to determine which of the 4 materials differed from one another (α=.05).
A plot of all 16 groups showed a wide 2-fold range of mean ±standard deviation biaxial strengths from 464 ±43 MPa for an alumina airborne-particle-abraded 5Y zirconia to 975 ±89 MPa for an alumina-abraded 4Y zirconia (Table 2; Fig. 1).
The 2-way ANOVA showed that the simple main effects of zirconia type and surface treatment, as well as their interaction, all influenced biaxial strength (P<.001) (Table 3). Although the interaction between zirconia type and surface treatment was significant, that is, zirconia materials responded unequally to surface treatments, this effect was much less influential than either of the simple main effects.
Table 3Two-way ANOVA for effects of zirconia material type, surface treatment, and their interaction
The four 1-way ANOVAs for the 4 zirconia materials by surface treatment all found significant differences among surface treatments (Table 4). Multiple comparison tests showed that for all 5Y zirconia materials, the polished control groups were the strongest, the as-machined and glass-bead-abraded groups were tied in an intermediate position, and alumina-abraded group was the weakest (P<.05) (Table 2). For the 4Y zirconia material, multiple comparison testing found only one difference among surface treatments, that glass-bead abrasion produced a lower strength than alumina abrasion (P<.05) (Table 2).
Table 4One-way ANOVAs for zirconia materials by surface treatment and surface treatments by material
All four 1-way ANOVAS for surface treatment by zirconia material found significant differences among zirconia materials within each type of surface treatment (Table 4). Multiple comparison tests for all surface treatments ranked the 4Y material as being stronger than the 5Y materials, with no differences being discerned within the 5Y materials (Table 2).
Scanning electron microscopy showed that the as-polished surfaces were smoother than all others; surfaces as-machined and airborne-particle abraded with glass beads displayed little difference; and those airborne-particle abraded with alumina were the roughest (Fig. 2). Differences among materials were not discerned.
The 5Y zirconia materials had similar absolute transmissions of approximately 34%, whereas the 4Y material had a significantly lower transmission of approximately 29% (Fig. 3). The 1-way ANOVA found differences among materials (F ratio 18, P<.001), and multiple comparison testing showed that the 4Y material was less translucent than all the 5Y materials.
The null hypothesis was rejected, as the zirconia type was more influential than surface treatment. The trends displayed in Figure 1 were supported by the statistical analyses described previously. The 4Y material was significantly stronger than the 5Y materials for all surface treatments. If a dentist chooses to prioritize strength, the 4Y material offers a substantial advantage over the 5Y materials, with only a small loss in translucency (Figs. 1 and 3).
For the 4Y material, surface treatment choice had a statistically significant effect; airborne-particle abrasion with alumina produced higher strengths than that with glass beads (Fig. 1). Conversely, for all the 5Y materials, airborne-particle abrasion with alumina produced significantly lower strengths than all other surface treatments. For all the 5Y materials, the control polished surfaces produced the highest strengths; this speaks to the damage produced by machining even when followed by sintering. If a dentist prioritizes strength, the 4Y material should be airborne-particle abraded with alumina, and 5Y materials should be left as machined or abraded with glass beads.
When choosing zirconia materials and their surface treatments for anterior restorations, dentists are faced with competing priorities: restoration strength, restoration translucency, and bond strength or choice of cementation technique. In restorations where the translucency of a 4Y material is sufficient, it will offer substantial improvements in strength over 5Y zirconia materials. Moreover, the results of this study suggest that a 4Y material may have undergone some transformation toughening when airborne-particle abraded with alumina, as has been reported for 3Y materials.
Although the 5Y materials allowed more absolute light transmission than the 4Y material, the difference was small (Fig. 3).
Airborne-particle abrasion with glass beads did not appear to visibly alter the surface topography of machined surfaces (Fig. 2). Alumina abrasion roughened all surfaces, weakening the 5Y materials but strengthening the 4Y material (Figs. 1 and 2). The effects of surface treatment on bond strength were beyond the scope of this article but are also needed to inform clinical decisions. Nonetheless, it is reasonable to assume that rougher surfaces produce higher bond strengths and that the undesirability of alumina airborne-particle abrading of 5Y materials may preclude some clinical applications.
The polished control groups produced data consistent with their manufacturers’ data sheets and brochures; however, the 5Y materials will have lower strengths in their clinically used as-machined or airborne-particle-abraded states because intaglio surfaces cannot be polished for clinical usage.
Based on the findings of this in vitro study, the following conclusions were drawn:
Zirconia material type, surface treatment, and the interaction between material and surface treatment all influenced the strength of translucent zirconia materials.
A 4Y, 4 mol% zirconia material was stronger than several 5Y, 5 mol% zirconia materials for all surface treatments tested.
Airborne-particle abrasion with alumina had a slight strengthening effect over airborne-particle abrasion with glass beads on a 4Y zirconia.
Airborne-particle abrasion with alumina had a weakening effect on 5Y zirconia materials.
Airborne-particle abrasion with alumina produced the roughest surfaces on 4Y and 5Y zirconia materials, whereas airborne-particle abrasion with glass beads did not appear to substantively alter machined surface morphologies.
The 5Y zirconia materials were slightly more translucent than a 4Y material.
The authors thank the manufacturers for providing materials, the University of Alabama at Birmingham High Resolution Imaging Facility for SEM support, and Argen Corporation for CAD-CAM support.
CRediT authorship contribution statement
Edward A. McLaren: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft. Anvita Maharishi: Conceptualization, Data curation, Investigation, Methodology, Project administration, Writing – original draft. Shane N. White: Conceptualization, Formal analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing – original draft.