Influence of base design on the manufacturing accuracy of vat-polymerized diagnostic casts: An in vitro study.

STATEMENT OF PROBLEM: Vat-polymerized casts can be designed with different bases, but the influence of the base design on the accuracy of the casts remains unclear. PURPOSE: The purpose of the present in vitro study was to evaluate the influence of various base designs (solid, honeycombed, and hollow) with 2 different wall thicknesses (1 mm and 2 mm) on the accuracy of vat-polymerized diagnostic casts. MATERIAL AND METHODS: A virtual maxillary cast was obtained and used to create 3 different base designs: solid (S group), honeycombed (HC group), and hollow (H group). The HC and H groups were further divided into 2 subgroups based on the wall thickness of the cast designed: 1 mm (HC-1 and H-1) and 2 mm (HC-2 and H-2) (N=50, n=10). All the specimens were manufactured with a vat-polymerized printer (Nexdent 5100) and a resin material (Nexdent Model Ortho). The linear and 3D discrepancies between the virtual cast and each specimen were measured with a coordinate measuring machine. Trueness was defined as the mean of the average absolute dimensional discrepancy between the virtual cast and the AM specimens and precision as the standard deviation of the dimensional discrepancies between the virtual cast and the AM specimens. The Kolmogorov-Smirnov and Shapiro-Wilk tests revealed that the data were not normally distributed. The data were analyzed with Kruskal-Wallis and Mann-Whitney U pairwise comparison tests (α=.05). RESULTS: The trueness ranged from 63.73 µm to 77.17 µm, and the precision ranged from 44.00 µm to 54.24 µm. The Kruskal-Wallis test revealed significant differences on the x- (P<.001), y- (P=.006), and z-axes (P<.001) and on the 3D discrepancy (P<.001). On the x-axis, the Mann-Whitney test revealed significant differences between the S and H-1 groups (P<.001), S and H-2 groups (P<.001), HC-1 and H-1 groups (P<.001), HC-1 and H-2 groups (P<.001), HC-2 and H-1 groups (P<.001), and HC-2 and H-2 groups (P<.001); on the y-axis, between the S and H-1 groups (P<.001), HC-1 and H-1 groups (P=.001), HC-1 and H-2 groups (P=.02), HC-2 and H-1 groups (P<.001), HC-2 and H-2 groups (P=.003); and on the z-axis, between the S and H-1 groups (P=.003). For the 3D discrepancy analysis, significant differences were found between the S and H-1 groups (P<.001), S and H-2 groups (P=.004), HC-1 and H-1 groups (P=.04), and HC-2 and H-1 groups (P=.002). CONCLUSIONS: The base designs tested influenced the manufacturing accuracy of the diagnostic casts fabricated with a vat-polymerization printer, with the solid and honeycombed bases providing the greatest accuracy. However, all the specimens were clinically acceptable.

Effect of milled and lithography-based additively manufactured zirconia (3Y-TZP) on the biological properties of human osteoblasts.

STATEMENT OF PROBLEM: Lithography-based additively manufactured (AM) zirconia has been used to fabricate dental implants and custom barriers for guided bone regeneration procedures. However, studies on the effect of AM zirconia on the biological properties of human osteoblasts are lacking. PURPOSE: The purpose of this in vitro study was to compare the effect of milled and lithography-based AM zirconia on the biological properties of normal human osteoblasts (NHOsts), as well as to compare the chemical composition between the milled and lithography-based AM 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) zirconia. MATERIAL AND METHODS: Three mol percentage yttria-stabilized tetragonal zirconia polycrystal disks (Ø6×2-mm) were fabricated using subtractive milling (Priti multidisc ZrO2 monochrome) (M group) and lithography-based additive manufacturing methods (LithaCon 3Y 210) (AM group) (n=10). NHOsts were exposed to different material extracts (1:1, 1:2, and 1:4) and cytotoxicity, cell migration, cell morphology, and cell attachment biological tests were completed. Additionally, for morphological and chemical analysis, a scanning electronic microscopy with energy-dispersive X-ray analysis (SEM/EDX) evaluation was completed. Data were analyzed by using 1-way ANOVA and the Dunnett test (α=.05). RESULTS: Substantial cell attachment and spreading were observed in both material surfaces. The presence of zirconium was evident in both groups, although the percentage of zirconium in the AM group (64.7%) was slightly higher than in the M group (52.6%). When NHOsts were cultured in the presence of the different material eluates, the M and AM groups exhibited similar NHOst viability and migration rates when compared with untreated cells; no significant differences were found (P>.05). CONCLUSIONS: The lithography-based AM zirconia tested showed adequate cytocompatibility without differences when compared with the milled zirconia (M group) specimens. Slight chemical element composition differences were found between milled and lithography-based AM zirconia.

Best-Fit Algorithm Influences on Virtual Casts' Alignment Discrepancies.

PURPOSE: To measure the influence of best-fit (BF) algorithms (entire dataset, 3 or 6 points landmark-based, or section-based BF) on virtual casts and their alignment discrepancies. MATERIAL AND METHODS: A mandibular typodont was obtained and digitized by using an industrial scanner (GOM Atos Q 3D 12M). A control mesh was acquired. The typodont was digitized by using an intraoral scanner (TRIOS 4). Based on the alignment procedures, four groups were created: BF of the entire dataset (BF group), landmark-based BF using 3 reference points (LBF-3 group), or 6 reference points (LBF-6 group), and section-based BF (SBF group). The root mean square (RMS) error was calculated. One-way ANOVA and post hoc pairwise multi-comparison Tukey were used to analyze the data (α = 0.05). RESULTS: Significant RMS error mean value differences were found across the groups (p < 0.001). Tukey test revealed significant RMS error mean value differences between the BF and LBF-3 groups (p = 0.022), BF and LBF-6 groups (p < 0.001), LB-3 and LB-6 groups (p < 0.001), LBF-3 and SBF groups (p < 0.001), and LBF-6 and SBF groups (p < 0.001). The LBF-6 group had the lowest trueness, while SBF and BF groups obtained the highest trueness values. Furthermore, significant SD differences were revealed across the groups tested (p < 0.001). Tukey test revealed significant SD differences between the BF and LBF-6 groups (p < 0.001), LBF-3 and LB-6 groups (p < 0.001), LBF-3 and SBF groups (p = 0.004), and LBF-6 and SBF groups (p < 0.001). The BF and SBF groups showed equal and highest precision, while the LBF-6 group had the lowest precision. CONCLUSIONS: The best-fit algorithms tested influenced the virtual casts' alignment discrepancy. Entire dataset or section-based best-fit algorithms obtained the highest virtual casts' alignment trueness and precision compared with the landmark-based method.

Implant-Abutment Discrepancy Before and After Acrylic Resin Veneering of Complete-Arch Titanium Frameworks Manufactured Using Milling and Electron Beam Melting Technologies.

PURPOSE: To assess the implant-abutment discrepancy of complete-arch frameworks manufactured using milling and additive electron beam melting (EBM) technologies, before and after acrylic resin veneering application. MATERIALS AND METHODS: A definitive implant cast with six implant replicas was digitized using a laboratory scanner. A software program was used to design an implant-supported framework which was manufactured using milling (M group) and EBM (EBM group) technologies (n = 10). In the M group, titanium milled specimens were fabricated. In the EBM group, titanium EBM specimens were obtained. A coordinate measurement machine (CMM) was used to assess the implant-abutment discrepancy at x-, y-, and z-axed between the specimens and the implant-abutment replicas of the definitive cast. The implant replicas positioned on the lateral incisor positions were not able to be assessed. The 3D gap discrepancy was calculated: 3 D = x 2 + y 2 + z 2 $3D\ = \sqrt {{x^2} + {y^2} + {z^2}}$ . Acrylic resin veneering procedures were finished and the same CMM measurements were completed. Three-way analysis of variance (ANOVA) test was used to analyze the data (α = 0.05). RESULTS: The manufacturing method (df = 1, F = 7.00, p = 0.009) and implant position (df = 3, F = 129.82, p < 0.001) were significant predictors of the x-axis discrepancy. The veneering procedures (df = 1, F = 21.55, p < 0.001) and implant position (df = 3, F = 95.42, p < 0.001) were significant predictors of the y-axis discrepancy. The manufacturing method (df = 1, F = 11.79, p = 0.001) was a significant predictor of the z-axis discrepancy. Lastly, the manufacturing method (df = 1, F = 5.11, p = 0.026), implant position (df = 3, F = 11.36, p < 0.001), and veneering procedures (df = 1, F = 41.56, p < 0.001) were significant predictors of the 3D gap discrepancy in which the manufacturing method explains the 2.37% of variation in the 3D gap discrepancy, the implant position explains the 15.82% of variation in the 3D gap discrepancy, and veneering procedures explain the 19.29% of variation in the 3D gap discrepancy results. CONCLUSIONS: The manufacturing methods, veneering procedures, and implant position influenced the linear implant-abutment discrepancy. The milled technique tested obtained lower linear implant-abutment discrepancy compared with the EBM method evaluated. The acrylic resin veneering procedures increased the implant-abutment discrepancy.

Chemical Composition and Flexural Strength Discrepancies Between Milled and Lithography-Based Additively Manufactured Zirconia.

PURPOSE: To evaluate the chemical composition, flexural strength, and Weibull characteristics of milled and lithography-based additively manufactured (AM) zirconia. MATERIALS AND METHODS: A virtual design of a bar (25×4×2 mm) was completed using a software program. The standard tessellation language file was used to manufacture all the specimens: 3Y-TZP zirconia (Priti multidisc ZrO2 monochrome) milled (M group) and 3Y-TZP zirconia (LithaCon 3Y 210) lithography-based AM (CeraFab System S65 Medical) (AM group) bar specimens (n = 20). The chemical composition of the specimens was determined by using energy dispersive X-ray (EDAX) elemental analysis in a scanning electron microscope. Flexural strength was measured in all specimens using 3-point bend test according to ISO/CD 6872.2 with a universal testing machine (Instron Model 8501). Two-parameter Weibull distribution values were calculated. The Shapiro-Wilk test revealed that the data were normally distributed (p < 0.05). Flexural strength values were analyzed using independent Student's t-test (α = 0.05). RESULTS: There were no major chemical composition differences observed between M and AM groups. The AM specimens (1518.9 ± 253.9 MPa) exhibited a significantly higher flexural strength mean value compared to the milled (980.5 ± 130.3 MPa) specimens (DF = 13, T-value = -5.97, p < 0.001). The Weibull distribution presented the highest shape for M specimens (11.49) compared to those of AM specimens (6.95). CONCLUSIONS: There was no significant difference in the chemical composition of milled and AM zirconia material tested. AM zirconia tested exhibited significantly higher flexural strength compared with the milled zirconia evaluated.

Multipurpose application of custom, rigid, clear matrix for fabricating trial restorations, tooth preparation guides, and interim veneers.

Interim veneers present an esthetic and functional challenge for restorative dentists. Moreover, inadequate fit or trauma to the soft tissue during their fabrication or subsequently may affect soft-tissue health and stability and lead to bleeding that will compromise the adhesive cementation of the definitive ceramic veneers. This article describes a technique for the atraumatic fabrication of interim veneers with a custom, rigid, clear matrix designed for multipurpose use during the patient's restorative care. Based on a diagnostic waxing, the clear matrix is made and first used for the fabrication of the trial restorations and as a tooth-preparation guide. Subsequently, it is used for the atraumatic fabrication of interim veneers with a photopolymerizing direct restorative material.

Physical characterization of 3 implant systems made of distinct materials with distinct surfaces.

STATEMENT OF PROBLEM: Dental implants undergo various surface treatments. Studies that have characterized their surface and subsurface by using the same methods are scarce. PURPOSE: The purpose of this study is to physically characterize the surface and subsurface of implant systems made of commercially pure (cp) titanium (Ti) grade (gr) 4 and Ti alloy gr 23 and to evaluate whether airborne-particle abrasion and acid etching is an appropriate surface treatment for Ti alloy gr 23. MATERIAL AND METHODS: Implant groups (n=3) were as follows: TG4AO, cp Ti gr 4, treated with anodic oxidation (3.5×8 mm) (NobelReplace Conical; Nobel Biocare); TG23AE, Ti gr 23 (TiAlV ELI) airborne-particle abraded-and-etched (3.9×8 mm) (V3; MIS); and TG4AE, cp Ti gr 4, airborne-particle abraded and etched (3.3×8 mm) (BL; Institut Straumann AG). Surface roughness, surface topography, and elemental and surface composition were investigated with optical profilometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The presence and size of Ti hydride (TiH) needles were determined on metallographic sections. Depth profiling was obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS) to determine possible enrichment of an alloying element at the implant surface. RESULTS: The mean arithmetic deviation roughness (Sa), of TG4AO was 0.80 µm. The Sa of TG4AO was 1.22 µm, and the Sa of TG4AO was 1.59 µm. The difference between the groups was significant (P<.001). TG23AE and TG4AE displayed a macrotexture and microtexture with pores; TG4AO showed a 3-to 12-µm canyon-like structure. The surface and subsurface compositions were as follows: for TG4AO, αTi and phosphorus-rich anatase; for TG23AE, α-Ti matrix with ß-Ti grains; and for TG4AE, α-Ti and δ-TiH2-x. TiH needles were found only on TG4AE; the Ti oxide layer of TG4AO was rough, 3-to 16-µm thick, and porous. The time-of-flight secondary ion mass spectrometry (ToF SIMS) concentration profile of TG23AE did not show enrichment of any alloying element. CONCLUSIONS: The roughness, topography, and composition of the surfaces were different for all implants tested. Airborne-particle abrasion and subsequent etching was an appropriate treatment for Ti gr 23 alloy implants.

Cementation of Zirconia-Based Toothborne Restorations: A Clinical Review.

As the demand for esthetics in dentistry has increased over the years, zirconia-based restorations have been successfully used as alternatives to metal-ceramic restorations. A reliable marginal seal is among the factors that are vital to the clinical success of a dental restoration. One advantage of zirconia-based restorations is that the cementation process is generally simpler and more efficient compared to the delivery/cementation of other all-ceramic systems. This article reviews several categories of cement used for the cementation of zirconia restorations: zinc-phosphate cement, glass-ionomer cement (GIC), resin-modified GIC, and composite-resin cement.

Effect of hydrothermal treatment on light transmission of translucent zirconias.

STATEMENT OF PROBLEM: Studies of the light transmission of translucent zirconias after hydrothermal treatment are limited. PURPOSE: The purpose of this in vitro study was to evaluate the effect of hydrothermal treatment on the light transmission of translucent zirconias for monolithic restorations. MATERIAL AND METHODS: Four commercially available zirconia products, BruxZir Anterior Solid Zirconia (BruxAnt, BA), Lava Plus High Translucency (LPHT), Katana Zirconia Super Translucent (KST), and Katana Zirconia Ultra Translucent (KUT) were assessed and 1 type of lithium disilicate, e.max Press LT (LDLT) was used as a control. Plate specimens, 20×20×1 mm (n=80) for the translucency assessment were sectioned from postsintered zirconia bulk materials and ground with a #400-grit diamond wheel and coolant. The specimens were placed under hydrothermal conditions of 134°C at 0.2 MPa (n=5 per group at 0, 5, 50, and 100 hours). Percentage of total transmittance of light (Tt%) of each specimen was measured using a spectrophotometer with an integrating sphere. X-ray diffraction analyses were used to measure tetragonal-monoclinic phase transformation. Surfaces were examined by scanning electron microscopy and energy dispersive spectrometry. Data were analyzed using 2-way ANOVA followed by the Tukey test (α=.05). RESULTS: The Tt% ranged from 6.5% to 28.3%. Group LDLT obtained significantly higher transmittance than other tested groups, whereas groups KST and KUT had significantly higher Tt% than groups BA and LPHT (P<.05). A statistically significant increase in the amount of monoclinic phase was revealed within all translucent zirconia groups (P<.05), and the increase in group LPHT was significantly higher than those of the other 3 translucent zirconias (P<.05). Minimal changes in the percentages of light transmittance were revealed after 100-hour hydrothermal treatment for all tested translucent zirconias and a lithium disilicate glass-ceramic control. CONCLUSIONS: Hydrothermal treatment had minimal effects on the translucency of translucent zirconias. The tetragonal-monoclinic phase transformation rate of translucent zirconias was found to be low, except in group LPHT.

Prospective cohort clinical study assessing the 5-year survival and success of anterior maxillary zirconia-based crowns with customized zirconia copings.

STATEMENT OF PROBLEM: Studies evaluating anterior zirconia-based crowns are limited. PURPOSE: The purpose of this prospective cohort clinical study was to assess the efficacy of zirconia-based anterior maxillary crowns with 0.3-mm customized copings at the cervical third and anatomical design elsewhere for up to 5 years of service. MATERIAL AND METHODS: Eighteen participants who required an anterior maxillary crown (n=20) and who had signed a consent form approved by the University of Washington Health Sciences Center Human Subjects Division were enrolled. All preparations were standardized and prepared with an occlusal reduction of 1.5 to 2 mm and an axial reduction of 1 to 1.5 mm with 10 degrees of convergence angle. All finish lines were located on the sound tooth structure. Zirconia copings (Lava; 3M ESPE) were custom designed and milled to a 0.3-mm thickness at the cervical third and with selective thickness elsewhere to support the veneering porcelain. All restorations were luted with self-etching self-adhesive composite resin cement. Recall appointments were at 2 weeks, 6 months, and 12 months, and annually thereafter for 5 years. Modified Ryge criteria were used to assess the clinical fracture measurements, esthetics, marginal discoloration, marginal adaptation, radiographic proximal recurrent caries, and periapical pathoses. Descriptive statistics and 95% confidence intervals were used to describe the number and rate of complications and self-reported satisfaction with the crowns. RESULTS: Twenty crowns with a mean follow-up of 58.7 months were evaluated. All crowns were rated as Alfa for fracture measurements (smooth surface, no fracture/chipping). Twelve crowns were rated esthetically as Romeo (no mismatch in color and shade) and 8 as Sierra (mismatch in color and shade within normal range). Twelve crowns were rated as Alfa (no visible evidence of crevice) and 8 as Bravo (visible evidence of crevice, no penetration of explorer) for marginal integrity. Nineteen were rated as Alfa (no discoloration) and 1 as Bravo (superficial discoloration) for marginal discoloration. No proximal caries or periapical pathoses were detected in 5 years. Participants were highly satisfied with their crowns after 5 years (mean ±SD: 9.8 ±0.4 on 0 to 10 scale). CONCLUSIONS: Zirconia-based anterior maxillary crowns with customized copings with 0.3-mm thickness at the cervical third and zirconia margins performed well after 5 years of service.

Assessing the feasibility of yttria-stabilized zirconia in novel designs as mandibular anterior fixed lingual retention after orthodontic treatment.

INTRODUCTION: The purpose of this study is to explore the feasibility of yttria-stabilized zirconia (Y-TZP) in fixed lingual retention as an alternative to stainless steel. METHODS: Exploratory Y-TZP specimens were milled to establish design parameters. Next, the specimens were milled according to ASTM standard C1161-13 and subjected to 4-point flexural tests to determine material properties. Finite element analysis was used to evaluate 9 novel cross-sectional designs, which were compared with stainless steel wire. Each design was analyzed under loading conditions to determine von Mises and bond stresses. The most promising design was fabricated to assess the accuracy and precision of current CAD/CAM milling technology. RESULTS: The superior design had a 1.0 × 0.5 mm semielliptical cross-section and was shown to be fabricated reliably. Overall, the milling indicated a maximum percent standard deviation of 9.3 and maximum percent error of 13.5 with a cost of $30 per specimen. CONCLUSIONS: Y-TZP can be reliably milled to dimensions comparable with currently available metallic retainer wires. Further research is necessary to determine the success of the bonding protocol and the clinical longevity of Y-TZP fixed retainers. Advanced technology is necessary to connect the intraoral scan to an esthetic and patient-specific Y-TZP fixed retainer.

Influence of aging on flexural strength of translucent zirconia for monolithic restorations.

STATEMENT OF PROBLEM: Concern has been raised with regard to the low-temperature degradation (LTD) of translucent yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) for monolithic zirconia restorations. PURPOSE: The purpose of this in vitro study was to assess the LTD behavior of 4 commercially available translucent Y-TZP materials by accelerated aging specimens in steam at 134°C, 0.2 MPa. MATERIAL AND METHODS: Thin bars (22×3×0.2 mm) of Y-TZP, including Katana ML (Kuraray Noritake Dental Inc), Katana HT13 (Kuraray Noritake Dental Inc), Prettau (Zirkonzahn), and BruxZir (Glidewell Laboratories) (n=30 for each group), were machined from sintered blocks. Control specimens were assessed in the nonaged condition. Artificially ageing (n=5 per group at 5, 50, 100, 150, and 200 hours) was conducted in steam at 134°C at 0.2 MPa. The specimens were characterized, tested in 4-point flexure, and the fracture surfaces were analyzed. The monoclinic-to-tetragonal (m/t) peak intensity ratio measured by x-ray diffraction was used to calculate the monoclinic phase fraction and monitor LTD. Linear regression with heteroscedasticity-consistent robust standard errors was used to test for the effect of LTD (aging time) on (σf) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between σf and monoclinic phase fraction (α=.05). RESULTS: Artificial aging resulted in LTD as shown by an increase in the monoclinic phase fraction for all specimens. After aging for 200 hours, the mean ±SD monoclinic phase fraction increased from 2.90 ±0.34% to 76.1 ±0.64% for Prettau, 2.69 ±0.18% to 76.0 ±0.26% for BruxZir, 4.6 ±0.19% to 35.8 ±0.80% for Katana HT13, and 3.57 ±0.35% to 33.2 ±1.1% for Katana ML (all P<.001). Flexural strength changed from a mean ±SD of 1612 ±197 MPa to all fractured during aging for Prettau (P<.001); 1248 ±73.5 MPa to all fractured during aging for BruxZir (P<.001); 1052 ±84.2 to 1099 ±70 MPa ±130 for Katana HT13 (P=.45); and from 875 ±130 to 909 ±70 MPa (P=.82) for Katana ML. The mean flexural strength values of Prettau and BruxZir decreased with an increase in the monoclinic phase with Spearman rank correlation coefficients of -0.80 (P=.001) for Prettau and -0.63 (P=.022) for BruxZir. No significant changes in flexural strength were measured for Katana ML or Katana HT13 (P>.05). CONCLUSIONS: The LTD of Y-TZP resulted in a significant decrease in flexural strength of Prettau and BruxZir, whereas Katana ML and Katana HT13 exhibited less LTD and no significant decrease in flexural strength.

Assessment of reliability of CAD-CAM tooth-colored implant custom abutments.

STATEMENT OF PROBLEM: Information is lacking about the fatigue resistance of computer-aided design and computer-aided manufacturing (CAD-CAM) tooth-colored implant custom abutment materials. PURPOSE: The purpose of this in vitro study was to investigate the reliability of different types of CAD-CAM tooth-colored implant custom abutments. MATERIAL AND METHODS: Zirconia (Lava Plus), lithium disilicate (IPS e.max CAD), and resin-based composite (Lava Ultimate) abutments were fabricated using CAD-CAM technology and bonded to machined titanium-6 aluminum-4 vanadium (Ti-6Al-4V) alloy inserts for conical connection implants (NobelReplace Conical Connection RP 4.3×10 mm; Nobel Biocare). Three groups (n=19) were assessed: group ZR, CAD-CAM zirconia/Ti-6Al-4V bonded abutments; group RC, CAD-CAM resin-based composite/Ti-6Al-4V bonded abutments; and group LD, CAD-CAM lithium disilicate/Ti-6Al-4V bonded abutments. Fifty-seven implant abutments were secured to implants and embedded in autopolymerizing acrylic resin according to ISO standard 14801. Static failure load (n=5) and fatigue failure load (n=14) were tested. Weibull cumulative damage analysis was used to calculate step-stress reliability at 150-N and 200-N loads with 2-sided 90% confidence limits. Representative fractured specimens were examined using stereomicroscopy and scanning electron microscopy to observe fracture patterns. RESULTS: Weibull plots revealed ß values of 2.59 for group ZR, 0.30 for group RC, and 0.58 for group LD, indicating a wear-out or cumulative fatigue pattern for group ZR and load as the failure accelerating factor for groups RC and LD. Fractographic observation disclosed that failures initiated in the interproximal area where the lingual tensile stresses meet the compressive facial stresses for the early failure specimens. Plastic deformation of titanium inserts with fracture was observed for zirconia abutments in fatigue resistance testing. CONCLUSIONS: Significantly higher reliability was found in group ZR, and no significant differences in reliability were determined between groups RC and LD. Differences were found in the failure characteristics of group ZR between static and fatigue loading.

A comparative evaluation of the translucency of zirconias and lithium disilicate for monolithic restorations.

STATEMENT OF PROBLEM: Studies comparing the translucency of zirconias and lithium disilicates are limited. PURPOSE: The purpose of this in vitro study was to measure the translucency of recently developed translucent zirconias and compare them with lithium disilicate. MATERIAL AND METHODS: Five types of zirconia, Prettau Anterior (Zirkonzahn GmbH), BruxZir (Glidewell Laboratories), Katana HT, Katana ST, and Katana UT (Kurary Noritake Dental Inc), and 1 type of lithium disilicate, e.max CAD LT (Ivoclar Vivadent AG), were assessed. Non-colored zirconia test specimens (n=5) were prepared as rectangles with dimensions of 15×10×0.5 and 15×10×1.0 mm. The shade of lithium disilicate was B1. A spectrophotometer (Evolution 300 UV-Vis) with an integrating sphere was used to evaluate the total transmittance of light as a percentage (Tt%) at a wavelength of 555 nm for comparison among groups. The Welch robust test for equality of means was used to compare group means (α=.025) and post hoc pairwise comparisons among groups were performed with the Dunnett T3 method. RESULTS: For the 0.5 mm thickness groups, the Tt% was 31.90 ±0.49 for Prettau Anterior, 28.82 ±0.22 for BruxZir, 28.49 ±0.14 for Katana HT, 31.67 ±0.24 for Katana ST, 33.73 ±0.13 for Katana UT, and 40.32 ±0.25 for e-max CAD LT. Post hoc tests indicated that all groups were significantly different from each other, except for between BruxZir and Katana HT, and between Prettau Anterior and Katana ST. Katana UT was significantly more translucent than all other zirconias, and e-max CAD LT was significantly more translucent than all zirconias. For the 1.0 mm thickness groups, the Tt% was 22.58 ±0.41 for Prettau Anterior, 20.13 ±0.22 for BruxZir, 20.18 ±0.39 for Katana HT, 21.86 ±0.39 for Katana ST, 23.37 ±0.27 for Katana UT, and 27.05 ±0.56 for e-max CAD LT. Post hoc tests indicated that all materials were significantly different from each other, except for between BruxZir and Katana HT, and among Prettau Anterior, Katana ST and Katana UT which were significantly more translucent than all other zirconias and less translucent than e-max CAD LT. CONCLUSION: At a thickness of 0.5 mm, Katana UT was significantly more translucent than all other zirconias, and e-max CAD LT was significantly more translucent than all zirconias. At a thickness of 1.0 mm, Prettau Anterior, Katana ST, and Katana UT were significantly more translucent than all other zirconias and less than e-max CAD LT.

Effect of accelerated aging on the fracture toughness of zirconias.

STATEMENT OF PROBLEM: Low temperature degradation (LTD) of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is of concern. PURPOSE: The purpose of this in vitro study was to assess the effect of accelerated aging on the Vickers hardness and fracture toughness of a newly developed Y-TZP and 2 primary Y-TZPs. MATERIAL AND METHODS: Two primary 3 mol% Y-TZP, Lava (LA), Everest Zirconium Soft (EV), and a new 3 mol% Y-TZP, ZirTough (NZ) were assessed. Specimens (n=30 each brand) of 10 × 10 × 3 mm were hydrothermally treated for accelerated aging to examine LTD. Five conditions were used (n = 5 per condition) as follows: control group (no aging); 5 hours at 134°C/0.2 MPa (5h-134°C); 100 hours at 134°C/0.2 MPa (100 h-134°C); 5 hours at 180°C/1.0 MPa (5 h-180°C); and 20 hours at 180°C/1.0 MPa (20 h-180°C). Fracture toughness was measured by using the indentation fracture (IF) method under a loading of 294 N and calculated from the obtained measurements. To observe differences in particle composition and fracture patterns, mirror-polished test specimens (n=5 each brand) were re-sintered at 1200°C for 1 hour as a thermal etching process, and a Vickers indenter was pressed into the test specimens according to the IF method. Test piece surfaces and cracks were observed with scanning electron microscopy (SEM). One-way ANOVA and the post- hoc (Scheffé test were used to examine) interlevel significant differences (α=.05). RESULTS: The Vickers hardness and fracture toughness were as follows: 1319 HV and 7.36 MPa · m(1/2) for LA, and 1371 HV and 6.76 MPa · m(1/2) for EV in no aging; 1334 HV and 7.02 MPa · m(1/2) for LA, and 1346 HV and 6.07 MPa · m(1/2) for EV in 5h-134°C. No significant differences were found between no aging and 5h-134°C for LA and EV for Vickers hardness and fracture toughness. Measurements could not be made for LA and EV for 100 h-134°C, 5h-180°C, or 20 h-180°C because of fractures in the surface layer. For NZ, Vickers hardness and fracture toughness were as follows: 1261 HV and 15.60 MPa · m(1/2) in no aging; 1217 HV and 14.98 MPa · m(1/2) in 5h-134°C; 1231 HV and 15.13 MPa · m(1/2) in 100 h-134°C; 1252 HV and 15.51 MPa · m(1/2) in 5h-180°C; 1224 HV and 15.01 MPa · m(1/2) in 20 h-180°C. No significant differences were shown in the Vickers hardness and fracture toughness. SEM observations after the thermal etching processing of NZ showed zirconia particles and scattered alumina particles. CONCLUSION: Measurements with LA and EV could only be made for no aging and 5h-134°C, and no significant differences were found in Vickers hardness and fracture toughness. Measurements were made with NZ under all conditions and no significant differences were found in Vickers hardness and fracture toughness.

Clinical efficacy of polyvinyl siloxane impression materials using the one-step two-viscosity impression technique.

STATEMENT OF PROBLEM: Impression making is a challenging clinical procedure for both patients and dentists. PURPOSE: The purpose of this clinical study was to compare a recently introduced fast-setting polyvinyl siloxane (PVS) impression material with heavy body/light body (HB/LB) combination (Imprint 4; 3M ESPE) (experimental group) with a conventional PVS impression material with HB/LB combination (Imprint 3; 3M ESPE) (control group), using the 1-step 2-viscosity impression technique. MATERIAL AND METHODS: Two definitive impressions (1 of each material combination) were made of 20 crown preparations from 20 participants. The quality of impressions was rated by 3 evaluators (clinical evaluator, clinical operator, and dental technician) and by the patients for the level of comfort and taste of the impression materials. The order in which the 2 impressions were made with each material combination was randomized for each crown preparation. A paired t test for paired means and McNemar test for paired proportions were used for statistical comparisons (α=.05). RESULTS: Participants rated the comfort of the impression making with the experimental group significantly higher than that with the control group (P=.001). No significant differences were found in participants' rating for the taste of the impression materials (P=.46). The viscosity for tray material was rated as significantly better for the control group by the clinical operator (P=.004). The readability of the impression and visibility around the finish line were rated as significantly better for the experimental group than for the control group (P<.001). Except for the ease of removal of the stone (RS), the ratings for the 2 groups by the dental technician were similar. The ease of RS was rated as significantly better for the experimental group (P<.001). Eleven dies from the control and 9 from the experimental group were selected for fabrication of the definitive crowns (P=.65). CONCLUSION: Within the limitations of this clinical study, no significant differences were found in the overall clinical performance of the experimental and the control groups. Impressions made with both materials were clinically acceptable. Participants rated the comfort provided by the experimental group significantly better than that of the control group.

In vitro evaluation of accuracy and precision of automated robotic tooth preparation system for porcelain laminate veneers.

STATEMENT OF PROBLEM: Controlling tooth reduction for porcelain laminate veneers (PLVs) in fractions of millimeters is challenging. PURPOSE: The purpose of this study was to assess an automated robotic tooth preparation system for PLVs for accuracy and precision compared with conventional freehand tooth preparation. MATERIAL AND METHODS: Twenty maxillary central incisor tooth models were divided into 2 groups. Ten were assigned to a veneer preparation with a robotic arm according to preoperative preparation design-specific guidelines (experimental group). Ten were assigned to conventional tooth preparation by a clinician (control group). Initially, all tooth models were scanned with a 3- dimensional (3D) laser scanner, and a tooth preparation for PLVs was designed on a 3D image. Each tooth model was attached to a typodont. For the experimental group, an electric high-speed handpiece with a 0.9-mm-diameter round diamond rotary cutting instrument was mounted on the robotic arm. The teeth were prepared automatically according to the designed image. For the control group, several diamond rotary cutting instruments were used to prepare the tooth models according to preoperative preparation design guidelines. All prepared tooth models were scanned. The preoperative preparation design image and scanned postoperative preparation images were superimposed. The dimensional difference between those 2 images was measured on the facial aspect, finish line, and incisal edge. Differences between the experimental and the control groups from the 3D design image were computed. Accuracy and precision were compared for all sites and separately for each tooth surface (facial, finish line, incisal). Statistical analyses were conducted with a permutation test for accuracy and with a modified robust Brown-Forsythe Levene-type test for precision (α=.05). RESULTS: For accuracy for all sites, the mean absolute deviation was 0.112 mm in the control group and 0.133 mm in the experimental group. No significant difference was found between the 2 (P=.15). For precision of all sites, the standard deviation was 0.141 mm in the control group and 0.185 mm in the experimental group. The standard deviation in the control group was significantly lower (P=.030). In terms of accuracy for the finish line, the control group was significantly less accurate (P=.038). For precision, the standard deviation in the control group was significantly higher at the finish line (P=.034). CONCLUSIONS: For the data from all sites, the experimental procedure was able to prepare the tooth model as accurately as the control, and the control procedure was able to prepare the tooth model with better precision. The experimental group showed better accuracy and precision at the finish line.

Effect of Tooth-Colored Restorative Materials on Reliability of Heat-Pressed Lithium Disilicate.

PURPOSE: Restorative material selection in complete mouth rehabilitation is an important factor in long-term management of potential technical complications. The aim of this study was to evaluate in vitro the reliability (fracture resistance) of lithium disilicate fatigued with different restorative materials. MATERIALS AND METHODS: A step-stress accelerated life-testing model was used. Seventy disc specimens were heat-pressed. Five groups of different indenter materials fatigued the lithium-disilicate specimens: group WC (tungsten carbide served as a control), group PR (interpenetrating polymer network [IPN] resin-based denture tooth), group POM (heat-pressed leucite glass-ceramic), group LD (heat-pressed lithium disilicate), and group ZR (zirconium dioxide). Lithium-disilicate specimens were randomly divided into four groups (n = 14). Specimens were fatigued to failure according to three step-stress profiles: light, moderate, and aggressive. Use level probability Weibull plots were generated, and each group's reliability, failure rate, and mean life to failure were calculated. RESULTS: The IPN resin-based denture tooth group had the highest reliability and mean life to failure, and lowest failure rate as compared to lithium disilicate and zirconium dioxide. No significant difference existed between the reliability of the tungsten carbide and leucite glass-ceramic groups and the IPN resin-based denture tooth group. CONCLUSIONS: Lithium-disilicate specimens fatigued with IPN resin-based denture teeth exhibited higher reliability than specimens fatigued with lithium disilicate and zirconium dioxide. There was a difference in fracture characteristics in lithium-disilicate specimens fatigued with tungsten carbide, lithium disilicate, and zirconium dioxide, versus those fatigued with IPN resin-based denture teeth and leucite glass-ceramic material.

Effect of hydrothermal degradation on three types of zirconias for dental application.

STATEMENT OF PROBLEM: Concern has been expressed with regard to hydrothermal aging of yttria-stabilized tetragonal zirconia polycrystalline. PURPOSE: The purpose of this study was to assess the accelerated aging characteristics of a new yttria-stabilized tetragonal zirconia polycrystalline material and 2 commercially available yttria-stabilized tetragonal zirconia polycrystalline materials by exposing specimens to hydrothermal treatments in steam at 134°C, 0.2 MPa, and at 180°C, 1.0 MPa. MATERIAL AND METHODS: Thin bars of zirconia: Prettau, Zirprime, and a new zirconia, ZirTough, n = 55 for each brand (22 × 3 × 0.2 mm) were cut and ground from blocks sintered according to the manufacturer's specifications. The control specimens for each group were evaluated in the nonaged condition, and their chemical composition was measured with energy dispersive spectroscopy. The experimental specimens were artificially aged under standard autoclave sterilization conditions, 134°C at 0.2 MPa (n = 5 per group at 5, 50, 100, 150, and 200 hours), and under standard industrial ceramic aging conditions, 180°C at 1.0 MPa (n = 5 per group at 8, 16, 24, and 48 hours). The tetragonal to monoclinic transformation was measured by using x-ray diffraction for all groups. Flexural strength was measured with a 4-point bend test (ASTM 1161-B) for all the groups, and the fracture surfaces were examined with scanning electron microscopy. The data were analyzed as a function of aging time. To test for an aging effect on the flexural strength and the monoclinic-tetragonal ratio, a 1-way ANOVA (with heteroscedasticity-consistent standard errors) was used to test for a general time effect. For the analyses of the monoclinic-tetragonal ratio, the same specimens were used at 0 hours and after aging, and the data were analyzed with an ANOVA for an incomplete block design. The relationship between flexural strength and monoclinic-tetragonal ratio was assessed with the Spearman rank correlation coefficient based on the average value at each aging. RESULTS: After 200 hours at 134°C and 0.2 MPa, the flexural strength decreased from a mean (standard deviation) of 1328 ± 89.9 MPa to all fractured during aging for Prettau (P < .001); 1041 ± 130 to 779 ± 137 MPa for Zirprime (P = .<.001) and 1436 ± 136 to 1243 ± 101 MPa for ZirTough (P = .017). After 200 hours at 134°C and 0.2 MPa, a portion of the tetragonal crystals transformed to the monoclinic phase in all specimens. The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 78.8% ± 2.0% for Prettau, 1.95% ± 0.48% to 74.8% ± 0.52% for Zirprime, and 12.4% ± 0.60% to 31.4% ± 4.4% for ZirTough (all P < .001). After 16 hours at 180°C and 1.0 MPa, all Prettau specimens had spontaneously fractured during aging. The Zirprime and ZirTough specimens were intact after 48 hours at 180°C and 1.0 MPa, and the mean (standard deviation) flexural strength had decreased from 1041 ± 130 MPa to 595 ± 88.4 MPa for Zirprime and 1436 ± 136 MPa to 1068 ± 76.8 MPa for ZirTough (all P < .001). The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 79.0% ± 0.13% for Prettau, from 1.95% ± 0.48% to 68.1% ± 4.4%, for Zirprime, and from 12.4% ± 0.60% to 39.5% ± 5.56% for ZirTough (all P<.001). The flexural strength decreased with an increase in the monoclinic phase for all the groups (Spearman rank correlation coefficients, -0.71 to -1.0). Scanning electron microscope micrographs revealed a transformed layer on the fracture surfaces. The decrease in flexural strength was related to the increase in monoclinic phase from long-term degradation. CONCLUSION: Hydrothermal aging of zirconia caused a statistically significant decrease in flexural strength of thin bars of zirconia, which was the result of the transformation from tetragonal to monoclinic crystal structure. ZirTough exhibited the least decrease in strength and smallest amount of monoclinic phase after aging.