The effects of various scaling instruments on the surface roughness of monolithic zirconia and lithium disilicate.

Purpose: This in vitro study aims to evaluate the effects of plastic piezoelectric maintenance tips on the surface roughness of monolithic lithium disilicate and zirconia. Materials and methods: Fifty-four lithium disilicate and 54 zirconia disks were prepared with CAD/CAM. On each material, scaling with a stainless-steel curette or with a piezoelectric device using either a steel or plastic tip was conducted. The surface roughness of the materials before and after the instrumentation was measured with a profilometer. The changes in roughness of the materials according to the scaling methods were analyzed with generalized linear models. Mann-Whitney U with Bonferroni correction was used for between-group comparisons. Results: The instruments caused surface alterations on both materials (p=0.001), while the roughness change of lithium disilicate and zirconia specimens did not demonstrate any statistically significant difference with each other (p=0.274). However, the curette was found to cause significantly more (p=0.019) roughness change (0.259 ±0.405) on the specimens than the piezoelectric plastic tip (0.060 ±0.238). Conclusion: Piezoelectric scalers with plastic tips cause less deterioration on monolithic zirconia and lithium disilicate surfaces when compared to stainless-steel hand curettes.

Comparison of wear and fracture resistance of additively and subtractively manufactured screw-retained, implant-supported crowns.

STATEMENT OF PROBLEM: Additively manufactured resins indicated for fixed definitive prostheses have been recently marketed. However, knowledge on their wear and fracture resistance when fabricated as screw-retained, implant-supported crowns and subjected to artificial aging is limited. PURPOSE: The purpose of this in vitro study was to evaluate the volume loss, maximum wear depth, and fracture resistance of screw-retained implant-supported crowns after thermomechanical aging when fabricated using additively and subtractively manufactured materials. MATERIAL AND METHODS: Two additively manufactured composite resins (Crowntec [CT] and VarseoSmile Crown Plus [VS]) and 2 subtractively manufactured materials (1 reinforced composite resin, Brilliant Crios [BC] and 1 polymer-infiltrated ceramic network, Vita Enamic [EN]) were used to fabricate standardized screw-retained, implant-supported crowns. After fabrication, the crowns were cemented on titanium base abutments and then tightened to implants embedded in acrylic resin. A laser scanner with a triangular displacement sensor (LAS-20) was used to digitize the preaging state of the crowns. Then, all crowns were subjected to thermomechanical aging (1.2 million cycles under 50 N) and rescanned. A metrology-grade analysis software program (Geomagic Control X 2020.1) was used to superimpose postaging scans over preaging scans to calculate the volume loss (mm3) and maximum wear depth (mm). Finally, all crowns were subjected to a fracture resistance test. Fracture resistance and volume loss were evaluated by using 1-way analysis of variance and Tukey Honestly significant difference (HSD) tests, whereas the Kruskal-Wallis and Dunn tests were used to analyze maximum wear depth. Chi-square tests were used to evaluate the Weibull modulus and characteristic strength data (α=.05). RESULTS: Material type affected the tested parameters (P<.001). CT and VS had higher volume loss and maximum wear depth than BC and EN (P<.001). EN had the highest fracture resistance among tested materials (P<.001), whereas BC had higher fracture resistance than CT (P=.011). The differences among tested materials were not significant when the Weibull modulus was considered (P=.199); however, VE had the highest characteristic strength (P<.001). CONCLUSIONS: Additively manufactured screw-retained, implant-supported crowns had higher volume loss and maximum wear depth. All materials had fracture resistance values higher than the previously reported masticatory forces of the premolar region; however, the higher characteristic strength of the subtractively manufactured polymer-infiltrated ceramic network may indicate its resistance to mechanical complications.

Load-to-Failure Resistance and Optical Characteristics of Nano-Lithium Disilicate Ceramic after Different Aging Processes.

The aim of this study was to compare the load-to-failure resistance and optical properties of nano-lithium disilicate (NLD) with lithium disilicate (LDS) and zirconia-reinforced lithium silicate (ZLS) in different aging processes. Thirty crowns were milled from NLD, LDS, and ZLS (n = 10). All crowns were subjected to thermomechanical aging and loaded until catastrophic failure. Ten specimens from each material were prepared in two different thicknesses (0.7 mm and 1.5 mm, n = 5), and color coordinates were measured before and after coffee thermocycling. Color differences (ΔE00) and relative translucency parameter (RTP) were calculated. Data were analyzed by using ANOVA and Bonferroni-corrected t-tests (α = 0.05). ZLS had the highest load-to-failure resistance (p ≤ 0.002), while the difference between LDS and NLD was nonsignificant (p = 0.776). The interaction between material type and thickness affected ΔE00 (p < 0.001). Among the 0.7 mm thick specimens, ZLS had the lowest ΔE00 (p < 0.001). Furthermore, 1.5 mm thick ZLS had lower ΔE00 than that of 1.5 mm thick LDS (p = 0.036). Other than ZLS (p = 0.078), 0.7 mm thick specimens had higher ΔE00 (p < 0.001). The interaction between material type, thickness, and thermocycling affected RTP (p < 0.001). Thinner specimens presented higher RTP (p < 0.001). NLD and LDS had higher RTP than ZLS (p ≤ 0.036). However, 0.7 mm thick specimens had similar RTP after coffee thermocycling (p ≥ 0.265). Coffee thermocycling reduced the RTP values of 0.7 mm thick NLD (p = 0.032) and LDS (p = 0.008). NLD may endure the occlusal forces present in the posterior region. However, long-term coffee consumption may impair the esthetics of restorations particularly when thin NLD is used.

Effect of printing orientation on the fracture strength of additively manufactured 3-unit interim fixed dental prostheses after aging.

OBJECTIVES: To evaluate the effect of printing orientation on the fracture strength of 3-unit interim fixed dental prostheses fabricated by using additive manufacturing and to compare with those fabricated by subtractive manufacturing after thermomechanical aging. MATERIALS AND METHODS: A 3-unit fixed dental prosthesis was designed by using a dental design software (exocad DentalCAD 2.2 Valetta) in standard tessellation language (STL) format. This STL file was exported into a nesting software (PreForm) and 3-unit interim fixed dental prostheses with 5 different orientations (0°, 30°, 45°, 90°, and 150°) were printed by using a 3-dimensional (3D) printing interim resin (Temporary CB) (n = 10). The same STL file was also used to mill polymethymethacrylate (DuoCAD) 3-unit interim fixed dental prostheses as the control group (n = 10). All specimens were cemented onto cobalt-chromium test models representing a maxillary first premolar and first molar tooth with a long-term temporary cement (DentoTemp), and subjected to thermomechanical aging (120,000 cycles, 1.6 Hz, 50 N, 5-55 °C). Then, all specimens were loaded until fracture by using a universal tester. The data were analyzed with nonparametric 1-way analysis of variance (Kruskal-Wallis) and Dunn's tests (α = 0.05). RESULTS: Additively manufactured specimens printed with 90° showed the lowest fracture strength values (P≤.048). However, the difference between specimens printed with 45° and 90° was nonsignificant (P>.05). Milled 3-unit interim fixed dental prostheses withstood significantly higher loads than 3-unit interim fixed dental prostheses printed with 45° and 150° (P≤.012). In addition, specimens printed with 0° showed higher fracture strength than the specimens printed with 45° (P=.01). Specimens printed with 0° and 30° presented similar fracture strength values with milled (P≥.057) and 150° printed (P>.05) specimens. CONCLUSIONS: Printing orientation had a significant effect on the fracture strength of 3-unit interim fixed dental prostheses. Among the additively manufactured samples, those printed with 0° showed similar fracture strength values with the subtractively manufactured samples. CLINICAL SIGNIFICANCE: Three-unit interim fixed dental prostheses fabricated with 0° and 30° using the 3D printing interim resin tested may be alternatives to milled PMMA in terms of fracture strength.

Effect of partially stabilized zirconia thickness on the translucency and microhardness of resin cement.

STATEMENT OF PROBLEM: Partially stabilized zirconia has been introduced as a more translucent iteration. However, knowledge of the effect of the thickness of partially stabilized zirconia on the microhardness of resin cement is sparse. PURPOSE: The purpose of this in vitro study was to evaluate the effect of thickness and material type on the translucency of partially stabilized zirconia and the microhardness of the resin cement polymerized beneath. MATERIAL AND METHODS: Specimens were prepared from 2 translucent zirconias with different yttrium content (Ceramil Zolid HT+ [HT] and Ceramil Zolid FX [SHT]) and a lithium disilicate glass-ceramic (e.max CAD [EX]) of different thicknesses (0.5, 1, 1.5, and 2 mm) (n=10). Color coordinates were recorded by using a spectrophotometer, and the relative translucency parameter (RTP) was calculated by using the CIEDE2000 formula. The microhardness of the resin cement polymerized under ceramic specimens was measured. Two-way analysis of variance, the Tukey honestly significant difference, and independent samples t tests were used to analyze the data (α=.05). RESULTS: Material type (P<.001), thickness (P<.001), and their interaction significantly affected the RTP (P=.001) and hardness values (P<.001). Regardless of the thickness, EX had the highest RTP (P≤.027), and the resin cements polymerized under EX showed the highest microhardness (P≤.002). However, the difference between HT and SHT for RTP (P≥.082) and resin cement hardness (P≥.984) was not significant. Specimens of 0.5 mm resulted in higher RTP (P≤.001) and resin cement hardness (P≤.006) than the 1.5- and 2-mm specimens of each ceramic. CONCLUSIONS: Increasing material thickness reduced the translucency of the partially stabilized zirconia and the hardness of the resin cements polymerized beneath.

Fracture Resistance of Single-Unit Implant-Supported Crowns: Effects of Prosthetic Design and Restorative Material.

PURPOSE: To evaluate the fracture resistance and fracture patterns of single implant-supported crowns with different prosthetic designs and materials. MATERIALS AND METHODS: One hundred and forty-four identical crowns were fabricated from zirconia-reinforced lithium silicate (ZLS), leucite-based (LGC), and lithium disilicate (LDS) glass-ceramics, reinforced composite (RC), translucent zirconia (ZR), and ceramic-reinforced polyetheretherketone (P). These crowns were divided into 3 subgroups according to restoration design: cementable crowns on a prefabricated titanium abutment, cement-retained crown on a zirconia-titanium base abutment, and screw-cement crown (n = 8). After adhesive cementation, restorations were subjected to thermal-cycling and loaded until fracture. The fracture patterns were evaluated under a stereomicroscope. Statistical analysis was performed by using 2-way ANOVA/Bonferroni multiple comparison post hoc test (α = 0.05). RESULTS: For each prosthetic design, ZR presented the highest fracture resistance (p ≤ 0.005). Other than the differences with ZLS and RC for screw-cement crowns (p > 0.05) and RC for crowns on zirconia-titanium base abutments (p > 0.05), LGC showed the lowest fracture resistance. P endured higher loads than LDS (p < 0.001), except for the crowns on zirconia-titanium base abutments (p > 0.05). Cementable crowns presented the highest fracture resistance (p < 0.001), other than LGC and LDS. The differences between LGC crowns (p > 0.05) or LDS crowns on prefabricated titanium and zirconia-titanium abutments were nonsignificant (p = 0.133). Fragmented crown fracture was predominant in most of the restorations. Screw and abutment fractures were observed in ZR screw-cement crowns, and all P crowns were separated from the abutments. CONCLUSIONS: Restorative material and restoration design affect the fracture resistance and fracture pattern of implant-supported single-unit restorations. Clinicians may restore single-unit implants in premolar sites with the materials and prosthetic designs tested in the present study.

Effect of prosthetic restorations and root canal fillings on periapical health in a selected patient group.

Aims To examine the effect of the quality of root canal fillings and prosthetic restorations on the frequency of apical periodontitis (AP).Methodology A total of 200 radiographs of 1,098 teeth with indirect restorations were selected. Each case was documented by age, sex, tooth location, tooth type, restoration type and presence of root canal treatment (RCT). Teeth were categorised as healthy or diseased considering periapical health. The quality of RCT and prosthetic restorations was categorised by radiographic and clinical examinations. Data were analysed using chi-squared test and logistic regression.Results Inadequate prosthetic restorations were found to be less healthy than the adequate ones. Statistical significance was found in teeth with RCT, which had a higher rate of AP (15.8%). Root-filled teeth categorised as inadequately treated (24.1%) were significantly unhealthier than the adequately root-filled teeth. Teeth with inadequate prosthetic restorations and RCT had an increase in AP risk of 6.41 and 20.74 times, respectively (p <0.05).Conclusions Results showed that AP risk was increased by both inadequate RCT and prosthetic restorations. Quality of RCT significantly affected periapical health more than the quality of prosthetic restorations. Not only radiographic but also clinical examination of restorations is required for successful evaluation.

Influence of coloring liquid immersion on flexural strength, Vickers hardness, and color of zirconia.

STATEMENT OF PROBLEM: Zirconia is a widely used restorative material, yet its white color does not meet esthetic requirements and coloring is needed. However, the effects of different durations of coloring have not been thoroughly investigated. PURPOSE: The purpose of this in vitro study was to evaluate the effects of coloring liquid immersion of different durations on the flexural strength, Vickers hardness, and color of zirconia. MATERIAL AND METHODS: Fifty bar-shaped and 60 disk-shaped zirconia specimens were milled from nonshaded (Copran Zri) and preshaded (Copran Zri Precoloured Medium) disks by using a computer-aided design and computer-aided manufacturing (CAD-CAM) system. Preshaded specimens were assigned to the group PS and received no further immersion. Nonshaded specimens were divided into 4 subgroups as per immersion time in coloring liquid (Copran Color A2 Shade): control (C; no immersion), G45 (45 seconds of immersion), G90 (90 seconds of immersion), and G135 (135 seconds of immersion). Bar-shaped specimens were subjected to a 3-point flexural strength test by using a universal testing machine (Lloyd LRX), following the International Organization for Standardization (ISO) 6872:2015 specification. Color coordinates of disk-shaped specimens were measured with a spectrophotometer (VITA Easyshade Advance 4.0) over a gray background. Color differences were calculated by using both CIE76 and CIEDE2000 formulas, followed by a comparison of obtained values with perceptibility (CIE76: 3.7, CIEDE2000: 0.8) and acceptability (CIE76: 6.8, CIEDE2000: 1.8) thresholds of each formula. Subsequently, Vickers hardness measurements were conducted. The data were statistically analyzed by using the Shapiro-Wilk test, 1-way ANOVA, and the Tukey HSD test (α=.05). RESULTS: Immersing zirconia specimens in coloring liquid decreased the flexural strength and hardness values, as group C presented the highest values for both parameters. In addition, prolonged duration had a negative effect on mechanical properties. The flexural strength of groups PS and G45 (P=.993) and groups G90 and G135 (P=.999) was statistically similar. For Vickers hardness, group G135 presented the lowest values with the difference between groups G90 and G135 being statistically similar (P=.061). Color differences among groups varied from 2.49 to 27.33 for ΔE∗, whereas ΔE00 values ranged from 0.58 to 8.26. Compared with group C, ΔE∗ values of all shaded specimens exceeded the acceptability threshold of 6.8. The color difference between each group was higher than the acceptability threshold values of both ΔE∗ and ΔE00 except for groups G45 and PS. CONCLUSIONS: Colored zirconia specimens had lower flexural strength and Vickers hardness values than nonshaded specimens, regardless of the immersion time or coloring technique.