The International Journal of Prosthodontics, 04/2022

Purpose: To retrospectively evaluate the survival rate and technical and biologic complications of feather-edge zirconia and metal-ceramic implant restorations cemented on conical titanium abutments. Materials and methods: Patients rehabilitated with implant-supported single crowns or fixed dental prostheses (FDPs) were divided into four groups: ZR-TL = tissue-level implant with a convergent collar and zirconia restoration; ZR-BL = bone-level implant and zirconia restoration; MC-TL = tissue-level implant with a convergent collar and metal-ceramic restoration; MC-BL = bone-level implant and metal-ceramic restoration. All of the restorations were cemented onto conical titanium abutments and had feather-edge margins, following the biologically oriented preparation technique (BOPT). Crown-level survival rates, technical and biologic complications, and bone resorption were registered. Chi-square test was performed to analyze all evaluated parameters among the groups. Nonparametric Mann-Whitney test was performed to analyze changes in bone level. Results: A total of 85 patients (133 implants, 66 single crowns, and 28 FDPs) were included in the present study with a mean follow-up time of 4.98 years. The overall survival rate was 98.2% for zirconia and 97.5% for metal-ceramic restorations at the crown level. No significant differences were found between the zirconia (ZR-BL and ZR-TL) and the metal-ceramic (MC-BL and MC-TL) groups for technical complications (P > .05). Bone resorption was 0.33 (0.37) mm in MC-TL; 0.61 (0.52) mm in MC-BL; 0.39 (0.51) mm in ZR-TL; and 0.77 (0.64) mm in ZR-BL, showing significantly greater bone loss in bone-level implants (P = .011). Conclusion: Zirconia implant restorations with feather-edge margins seem to be a viable alternative in cases of both tissue-level and bone-level implants.

Purpose: To compare the clinical outcomes of single implants with titanium (Ti) or zirconia (Zr) abutments after up to 5 years in function. Materials and Methods: This study was a retrospective analysis on single implants with screw-retained abutments (Zr or Ti) covered with porcelain veneer placed between 2011 and 2013 at one referral specialist clinic. Clinical data from patient records were collected from 132 patients and 174 implants. Technical complications such as fractures, chipping, and abutment screw loosening were registered. Radiographs were analyzed comparing both annual bone loss and accumulated bone loss at 5 years. In 57 patients with 85 implants, the values of accumulated bone loss at 5 years were compared to baseline. Results: Technical complications occurred in 16 (9%) of the implants, most often during the first year. The following complications were found: fracture of the abutment (n = 1); loosening of the abutment screws (n = 5); and chipping of the porcelain veneer (n = 11). Ti abutments had more complications than Zr abutments (79%). Of all the implants, 45% had an annual bone loss ranging between 0.05 and 2.15 mm, and 59% had an accumulated bone loss at 5 years ranging between 0.05 and 4.25 mm. Zr abutments had a statistically significantly higher amount of yearly and accumulated bone loss at 5 years. Conclusion: Abutment material affected marginal bone loss. The Zr group displayed statistically more bone loss both yearly and after 5 years compared to the Ti group. Technical complications were not affected by abutment material but were affected by age of the patient.

Purpose: To compare the esthetic outcomes of anterior implant-supported single crowns as evaluated by patients and clinicians of different specialties. Materials and methods: A total of 133 patients with 138 implant-supported crowns were recalled from the National Dental Centre Singapore Implant Registry to rate the esthetic outcomes of their restorations using a visual analog scale (VAS) questionnaire. Patients with crowns in the maxillary or mandibular canine-to-canine region with a contralateral natural tooth and minimum time in function of 6 months were recruited. Three clinicians trained in prosthodontics rated each restoration using the modified Pink Esthetic Score/White Esthetic Score (PES/WES) index through photographs and models. Two periodontists, two orthodontists, and two general dentists each rated a randomized sample of 40 crowns using the same index for comparison. Statistical analysis was done using a linear mixed model, analysis of variance, intraclass correlation coefficient, kappa score, Spearman rank correlation, and Mann- Whitney U tests. Results: Mean PES, WES, and PES/WES scores for the 138 implants were 4.7, 5.0, and 9.7, respectively. Based on modified PES/WES criteria, 22 (15.9%) of the 138 implants were deemed clinically acceptable. Mean patient VAS scores ranged from 79.3 to 84.4 out of 100. Prosthodontists had significantly lower mean PES, WES, and PES/WES scores (P < .05) than the other specialties. When orthodontists were excluded, papilla scores had the highest agreement of all the variables. Conclusion: Patients were less critical of esthetic outcomes than clinicians. A weak correlation was found between patients' subjective evaluations and the modified PES/WES index as rated by prosthodontists. Prosthodontists were the strictest assessors.

Purpose: To evaluate the effect of sintering speed and polishing or glazing on the failure load (FL) of monolithic zirconia fixed partial dentures (FPDs). Materials and methods: A total of 40 three-unit FPDs extending from the mandibular first premolar to the first molar were evaluated. The prepared typodont teeth were scanned, and the prostheses were designed. Afterwards, the prostheses were milled from monolithic zirconia blanks. The samples were divided into classic and speed sintering groups (n = 20 each). Half of the samples in each group (n = 10) were polished with an electric handpiece according to the manufacturer's instructions, and the other half (n = 10) were glazed. All of the samples were thermocycled for 3,500 cycles between 5ºC and 55ºC in water baths. The FL was calculated in Newtons with the three-point bending test. Results: The mean ± SD FL values were as follows: classic sintering/polished group = 2,026.5 ± 172.8 N; classic sintering/glazed group = 1,917.58 ± 174.45 N; speed sintering/polished group = 1,787.58 ± 145.81 N; and speed sintering/glazed group = 1,719.6 ± 143.9 N. There was a significant difference in the mean FL between the two sintering methods (P < .001), with the classic sintering group exhibiting the highest FL. Conclusion: Classic sintering of the monolithic zirconia FPDs led to the maximum amount of FL and strength.

Purpose: To evaluate the occlusal reproductive trueness of zirconia crowns fabricated using additive manufacturing (AM) and to compare the surface roughness of crowns fabricated using AM and conventional milling (CM). Materials and Methods: Crowns were manufactured using AM and CM on abutments with total occlusal convergence angles of 16 and 20 degrees. Results: The surface roughness of the AM crowns was less than that of the CM crowns. The differences in reproductions of the occlusal morphology of the abutment crown were greater at 16 degrees than at 20 degrees. Conclusion: AM could be an effective method for manufacturing zirconia crowns.

Purpose: To evaluate the effects of airborne-particle abrasion (APA) with alumina particles vs silicamodified alumina particles on the bond strength between zirconia and conventional MDP-based (Ph) or selfadhesive resin cements. Materials and Methods: Five surface treatments were evaluated: (1) C = control (no surface treatment); (2) AB = APA with alumina particles; (3) ABP = APA combined with MDP-based primer; (4) SS = APA with silica-modified alumina particles combined with silane; and (5) SSP = SS protocol combined with MDP-based primer. The surface roughness (Ra) of the APA samples (n = 5) was measured by a contact profilometer. Cylinders of the resin cements tested were bonded to the surface-treated zirconia. The microshear test was performed by the application of a load, with a wire loop, parallel to the adhesive interface until debonding of the resin cement cylinders. The microshear bond strength (μSBS) of the samples was measured before and after being subjected to thermocycling (TC; 5ºC to 55ºC, 60-second dwell time, 3,000 cycles). One-way ANOVA (surface roughness) and two-way ANOVA (μSBS) followed by Tukey post hoc test (α = .05) were used. Results: The SS (Ph: 12.6 MPa; Ph + TC: 6.37 MPa; SA: 11.8 MPa; SA + TC: 9.37 MPa) and SSP (Ph: 10.4 MPa; Ph + TC: 5.82 MPa; SA: 10.4 MPa; SA + TC: 10.0 MPa) surface treatments produced the highest surface roughness values (P < .001). The SS samples achieved the highest immediate bond strength for both resin cements. However, after TC, SA resin cement associated with any surface treatment produced the highest bond strength values. Conclusion: Self-adhesive resin cement promotes higher and more stable bond strength values when associated with a surface roughening method.

Purpose: To compare the marginal fit and internal surface roughness of CAD/CAM zirconia copings milled with three- and five-axis milling devices. Materials and Methods: A total of 40 titanium implant stock abutments (4.8 mm in diameter, 4 mm in height) screwed to dental implants (4.1 mm in diameter) embedded in resin were considered phantoms and included in this in vitro study. All 40 phantoms were scanned with the same intraoral scanner, and virtual wax-ups of zirconia copings were created from these images and exported as STL files. Two copings were milled from each resulting STL file—one using a three-axis milling device and the other using a five-axis milling device. After milling, zirconia copings underwent high-speed sintering before being analyzed for marginal fit (ie, marginal gap measurement) and observation of internal surface roughness with scanning electron microscopy (SEM). Statistical comparisons between groups were assessed with Mann-Whitney U test. Results: Median marginal gap values were 34.80 μm (95% CI: 0.00 to 173.98) for the five-axis milling device group and 141.97 μm (95% CI: 82.13 to 163.46) for the three-axis milling device group. A statistically significant difference in marginal gap was found between both milling device groups (P = .039). In addition, qualitative SEM analysis indicated a higher internal surface roughness for the three-axis milling device group. Conclusion: Within the limitations of this study, the present findings suggest that five-axis milling devices outperform three-axis milling devices for milling CAD/CAM zirconia copings from intraoral scans of implant stock abutments.

Purpose: To measure the retrieval force required to remove 1.5-mm–thick CAD/CAM zirconia copings cemented on zirconia (Zr) and titanium (Ti) stock implant abutments after a single application of erbium-doped yttrium scandium gallium garnet (Er:YSGG) laser. Materials and Methods: A total of 60 monolithic zirconia copings were cemented on Zr and Ti implant abutments with either a resin-modified glass-ionomer (RelyX Luting Plus, 3M; Rx) or a zinc oxide eugenol cement (Temp-Bond, Kerr; Tb). These abutment coping specimens were randomly divided into 12 groups based on laser application (vs control [C]), abutment type (Zr vs Ti), cement (Rx vs Tb), and storage condition (dry [D] vs saline water [W]). Er:YSGG laser was applied at 6 W, 30% water–60% air, and 20 Hz (300 mJ/pulse energy) postcementation following a defined pattern. The force required to remove the cemented copings from their abutments was measured on a universal testing machine. Descriptive statistics, multifactor analysis of variance (ANOVA), and post hoc Tukey honest significant difference tests (α = .05) were performed. Results: The mean peak force values at removal of the Zr abutment groups were 470.3 ± 151.33 N (Zr-Rx-C), 161.7 ± 19.29 N (Zr-Rx-D), 316.03 ± 95.24 N (Zr-Rx-W), 103.27 ± 24.53 N (Zr-Tb-C), 39.33 ± 6.21 N (Zr-Tb-D), and 20.33 ± 6.45 N (Zr-Tb-W); and of the Ti abutment groups were 349.80 ± 106.82 N (Ti-Rx-C), 84.63 ± 14.02 N (Ti-Rx-D), 177 ± 62.57 N (Ti-Rx-W), 54.77 ± 9.10 N (Ti-Tb-C), 22.67 ± 4.32 N (Ti-Tb-D), and 11.57 ± 2.30 N (Ti-Tb-W). Conclusion: Within the limitations of this study, the following conclusions were drawn: Er:YSGG laser allows for easier removal of cemented zirconia copings with lower removal forces, with Ti abutment specimens requiring lower forces than Zr abutment specimens. No significant difference was seen between laser and control groups for Tb compared to Rx. Er:YSGG laser shows great clinical promise for predictable retrievability of cemented monolithic zirconia implant crowns, especially with stronger resin-based cement. With further clinical evidence, this could be very useful for clinicians to manage cement-retained implant crown complications.

Purpose: To assess the influence of substructure (enamel vs dentin) and thickness on the fracture strength of ceramic discs made from lithium disilicate (LDS) vs zirconia (Zi). Materials and Methods: A total of 128 intact maxillary third molars were collected, and standardized enamel and dentin discs were fabricated. LDS (n = 64) and Zi (n = 64) discs with 0.5-mm (n = 32 each) and 1-mm (n = 32 each) thicknesses were produced, and each group was divided into two subgroups (n = 16 each), which were luted to the enamel/dentin discs. Half of the specimens in each subgroup were aged via chewing simulation and thermocycling, and all specimens were loaded until fracture in a universal testing machine. Differences between LDS and Zi with respect to enamel vs dentin as substructure and ceramic thickness were analyzed by use of nonparametric testing (Mann-Whitney U test). Results: In unaged specimens, fracture loads were not significantly (P > .05) different between Zi and LDS specimens for the 1- and 0.5-mm thicknesses. However, fracture loads were significantly higher (P < .001) in specimens supported by enamel independent of the ceramic material. In aged specimens, the fracture loads were significantly (P < .01) higher when supported by enamel; however, in the 0.5-mm groups, Zi achieved significantly higher breaking loads than LDS when luted to dentin. Conclusion: When 0.5-mm ceramic discs were luted to dentin, Zi outperformed LDS with respect to fracture loads.

Purpose: To test the bond strength of one-piece zirconia implants with either standard or reduced height using different luting agents and pretreatments of the inner surfaces of ceramic crowns. Materials and Methods: A total of 20 monolithic zirconia CAD/CAM–fabricated single crowns per luting agent group were cemented onto 20 one-piece zirconia implants with either 5-mm (n = 10) or 4-mm (n = 10) abutment height using 13 different luting agents. After water storage, the crowns were removed using a specially developed test fixture in a universal testing machine. The maximum force (N) was recorded, and the force per area (MPa) was calculated. The statistical evaluation was performed using univariate analysis of variance. Results: A mean of 4.19 MPa (SD 2.90) at 5-mm height and 3.89 MPa (SD 2.85) at 4-mm height was obtained for all luting agents. The highest values were achieved for a resin-modified glass-ionomer cement, with 12.37 MPa (4 mm) and 12.00 MPa (5 mm). The lowest values were shown for a long-term temporary material, with 0.73 MPa (4 mm) and 1.07 MPa (5 mm). Only a polycarboxylate cement (P < .001) and a glassionomer cement (P = .006) showed statistically significant differences in favor of the reduced abutment height. Reduced abutment height did not significantly reduce bond strength for any of the materials examined. Conclusion: Implants with a reduced abutment height are clinically suitable. Pretreatment of the crowns’ inner surfaces with ceramic primer showed to be advantageous.

Purpose: To evaluate the retention of zirconia crowns on polyetheretherketone (PEEK) abutments using different luting agents, with and without PEEK primer. Materials and methods: A total of 100 PEEK abutment replicas were fabricated, airborne particle-abraded with aluminum oxide, and divided into four groups (n = 25). A total of 100 zirconia crowns were fabricated and cemented using either adhesive resin cement or self-adhesive resin cement with and without PEEK primer; thus, the groups were: group AA-P (adhesive cement with primer); group AA-N (adhesive cement without primer); group SA-P (self-adhesive cement with primer); and group SA-N (self-adhesive cement without primer). The specimens were thermocycled and subjected to crown pull-out tests. The values were recorded and analyzed using analysis of variance and post hoc analysis (α = .05). The mode of failure of debonded surfaces was evaluated using scanning electron microscopy. Results: The pull-out force values were 3.13 ± 0.31 MPa for group AA-P, 1.77 ± 0.20 MPa for group AA-N, 2.10 ± 0.12 MPa for group SA-P, and 1.49 ± 0.18 MPa for group SA-N. Statistically significant differences were found between all four groups (P < .001). The specimens with PEEK primer applied showed higher values compared to nonprimed specimens for both cements tested. Scanning electron microscopy analysis showed more mixed failures for adhesive cement and more adhesive failures for self-adhesive resin cement. Conclusion: The maximum pull-out forces were recorded for zirconia crowns bonded to PEEK abutments with adhesive cement. The use of PEEK primer increased the pull-out values for both resin cements.

Purpose: To evaluate the effect of different luting techniques on the shear bond strength (SBS) of ultra-translucent multi-layered zirconia (UTML) after 24 hours and 1 year of water storage and to analyze the influence of primers on the wettability of the UTML MTU-6 surface. Materials and methods: Four commercial resin cements were evaluated: Variolink Esthetic LC (Ivoclar Vivadent); RelyX Ultimate (3M); Estecem II (Tokuyama); and Panavia V5 (Kuraray Noritake). Panavia V5 was tested with and without Panavia V5 Tooth Primer (Kuraray Noritake). The SBS immediately after cement/primer application and after aging, scanning electron microscopy of the surfaces, and the contact angle of the primers on the UTML surfaces were analyzed. Results: At 24 hours, Panavia V5 with Tooth Primer exhibited the highest SBS. After 1 year, the SBS of Variolink Esthetic LC and Panavia V5 with Tooth Primer decreased, and RelyX Ultimate increased. There was no statistical difference between RelyX Ultimate and Panavia V5 with Tooth Primer at 1 year. For all groups, the rate of adhesive failures increased after 1 year. The highest contact angle was observed on the control (no primer) group, and the lowest contact angles were obtained when the YSZ surface was treated with Scotchbond Universal and the combination of Clearfil Ceramic Primer Plus and Tooth Primer. Conclusion: Clinicians should prefer 10-MDP-based cementation systems that can be properly polymerized to achieve a stable long-term bond strength to YSZ restorations. Also, the use of ceramic primers improves the capability of the cement to establish an intimate contact with the intaglio of the restoration.

Purpose: To evaluate the biaxial flexural strength of different porcelain-to-zirconia thickness ratios and bonding strategies of a stress-free bilayer CAD/CAM ceramic system. Materials and methods: A total of 60 zirconia discs (diameter: 15 mm; thickness: 0.3 or 0.5 mm; n = 30 for each thickness) were divided into six groups (n = 10 each) according to porcelain-to-zirconia ratio and bonding strategy: VM/Zr (control): zirconia discs veneered with a feldspathic ceramic (VM 9, Vita) in 0.9-mm and 0.7-mm thicknesses using a conventional hand-layering technique; VB/Zr-SBU: zirconia discs airborne particle-abraded with 50-μm Al2O3 particles followed by an MDP primer application (Single Bond Universal, 3M) and bonded to the porcelain with a resin cement (Panavia F 2.0, Kuraray); and VB/Zr-RC: zirconia discs airborne particle-abraded with 30-μm silica-coated Al2O3 particles and silanized and bonded to the porcelain with the same resin cement. Before cementation, the VB (Vitablocs II) discs were etched with 5% hydrofluoric acid (60 seconds), followed by silane application. The bilayers (thickness = 1.2 mm) were loaded with 750 g while light curing the resin cement. Two porcelain-to-zirconia thickness ratios were evaluated: 0.9: 0.3 mm and 0.7: 0.5 mm. All groups were subjected to 106 mechanical cycles, followed by a biaxial flexural test. Data (MPa) were subjected to two-way analysis of variance (ANOVA), Tukey test (5%), and Weibull analyses. Results: Two-way ANOVA revealed that the factor porcelain-to-zirconia ratio (P = .0556) was not significant; however, the bonding strategy factor was statistically significant. Among the 0.5-mm zirconia groups, the VB/Zr-SBU group presented higher flexural strength (s) than the VM/Zr or VB/Zr-RC groups. Similar results were also found for the 0.3-mm zirconia groups, in which the VB/Zr-SBU group also presented higher strength than the others, which were similar in comparison (Tukey). The Weibull modulus was similar among the groups; however, the characteristic strength was significantly different (P = .000). Conclusion: The zirconia bonding strategy with 50-μm Al2O3 airborne-particle abrasion, followed by a primer application, increases the flexural strength of a stress-free bilayer CAD/CAM ceramic system.

Purpose: To present a sample preparation and test methodology for evaluating the fracture resistance of ceramic materials configured in a way similar to the screw-access holes found in implant-supported ceramic full-arch devices. Materials and methods: A finite-width rectangular plate with a central hole was designed to evaluate the critical stresses of ceramic full-arch screw-access holes. Different load scenarios were investigated by comparing forces applied to BruxZir Esthetic (Glidewell Laboratories) and BruxZir Esthetic enhanced with BruxZir Steel (Glidewell Laboratories) specimens. Results: Using the proposed test method, it was found that the fracture resistance of full-arch prostheses is largely dependent on the location of the screw-access hole and the inherent mechanical properties of the material (P < .001). Conclusion: The finite-width plate model may be suitable for evaluating the fracture resistance of ceramic full-arch prostheses. The fracture resistance of 3D ceramic full-arch prostheses can be greatly improved by improving the mechanical properties of the material and/or increasing the wall thickness of the screw access hole by relocation.

Purpose: To evaluate the effect of different cavity designs and cement types on the fracture resistance of monolithic zirconia inlay-retained fixed dental prostheses (IRFDPs). Materials and methods: Four study models consisting of a second premolar, a missing first molar, and a second molar were used for the different cavity designs. Four different inlay cavity designs were prepared: DO-MO (disto-occlusal-mesio-occlusal cavity), MOD-MOD (mesio-occlusodistal-mesio-occlusodistal cavity), WDO-WMO (DO-MO with additional wings), and WMOD-WMOD (MOD-MOD with additional wings). A total of 64 epoxy resin models were produced and scanned individually. IRFDPs were then fabricated from monolithic zirconia using CAD/ CAM software. The bonding surface of the IRFDPs was airborne particle abraded (50-μm alumina/2 MPa), then cemented onto the epoxy resin models using two cementation protocols (n = 8 per group): (1) P = cemented with Panavia SA Cement Plus Automix; and (2) Z/C = cemented with MDP-containing primer (Z-Prime Plus) combined with Calibra Universal resin cement. All IRFDPs were fatigued through thermal aging (6,000 cycles/5°C to 55°C) and chewing simulations (600,000 cycles × 50-N load, 2.1 Hz). All IRFDPs were then subjected to a fracture resistance test using a universal testing machine with a crosshead speed of 0.2 mm/minute. Data were statistically analyzed using one- and two-way ANOVA and Bonferroni multiple comparisons test (P = .001). Results: The mean fracture load (N) of the designs were as follows: WMODWMOD = 1,111.1; WDO-WMO = 1,057.4; MOD-MOD = 725.6; DO-MO = 682.7. According to two-way ANOVA, the differences among the cavity designs were statistically significant (P < .05). Conclusion: The cavity design of IRFDPs affected the fracture resistance. However, the fracture resistance of monolithic zirconia IRFDPs with any cavity design was enough to withstand expected posterior chewing forces.

Purpose: To investigate the precementation mean marginal gap, fracture resistance, and mode of failure of monolithic zirconia endocrowns cemented to endodontically treated molar teeth with butt joint porcelain margins and added ferrule preparation designs. Materials and methods: A total of 20 mandibular molars were endodontically treated and prepared to receive endocrown restorations. The teeth were randomly divided into two groups: Group B had a butt joint porcelain margin, while Group F received additional axial reduction with a shoulder finish line that added ferrule. Monolithic zirconia endocrowns were milled, and the marginal gaps were measured at four locations prior to cementation with resin cement. All samples were subjected to thermocycling, followed by compressive static loading. The maximum load causing fracture and mode of failure were recorded. Results: The mean load to failure for Group B (5,616 ± 1,503 N) was not significantly different compared to Group F (5,762 ± 1,618 N) (P = .84). Both groups recorded high rates of irreparable fractures (P = .2699, df = 2). The mean marginal gap in Group B (48.20 ± 12.37 μm) was not statistically significantly different compared to Group F (45.14 ± 8.45 μm) (P = .527). Conclusion: The addition of ferrule to the preparation design had no significant effect on the precementation mean marginal gap, fracture resistance, or mode of failure of monolithic zirconia endocrowns cemented to endodontically treated molar teeth. Monolithic zirconia endocrowns failed predominantly in an irreparable manner, but these failures were at high failure loads. The precementation mean marginal gap for both groups was small in comparison to other materials.

Purpose: To determine and compare the mechanical properties of 3D-printed yttriastabilized zirconia to milled isostatic pressed yttria-stabilized zirconia, with the following hypotheses: (1) The flexural strength of 3D-printed yttria-stabilized zirconia is comparable to milled yttria-stabilized isostatic pressed zirconia; and (2) thermocycling and chewing simulation do not affect the flexural strength of 3D-printed yttria-stabilized zirconia. Materials and methods: A total of 30 bars of an experimental 3D-printed 3 mol% yttriastabilized zirconia (LithaCon 3Y 230, Lithoz) and 10 bars of milled isostatic pressed zirconia (Prettau Zirconia, Zirkonzahn) were utilized. The printed zirconia bars were divided into three groups (n = 10 bars per group): (1) untreated (control); (2) thermocycled; and (3) tested after chewing simulation. A flexural strength test was performed on all samples using a three-point bend test in an Instron Universal testing machine. One-way analysis of variance on ranks was used to compare milled to printed zirconia. The effects of thermocycling and load cycling on 3D-printed zirconia were also determined. Results: The flexural strength values for milled and printed zirconia were 936.3 ± 255.0 MPa and 855.4 ± 112.6 MPa, respectively. There was no statistically significant difference in flexural strength between the milled and printed zirconia (P = .178). No statistically significant differences were observed between the control 3D-printed zirconia group and the thermocycled (888.4 ± 59.3 MPa) or load-cycled printed zirconia (789.6 ± 133.8 MPa; P = .119). Conclusion: Printed 3 mol% yttria-stabilized zirconia has comparable flexural strength to milled yttria-stabilized isostatic pressed zirconia. The thermocycling and chewing simulation used in this study did not significantly alter the flexural strength of the printed 3 mol% yttria-stabilized zirconia. These results indicate a promising role for 3D printing in the fabrication of zirconia restorations. Additional studies are needed to explore the full potential of this technology.

Purpose: To assess whether material choice for the prosthetic component of an implant restoration influences the failure mode in case of occlusal overload in monolithic restorations fabricated from high-strength ceramics on titanium implants. Materials and Methods: Within this pilot study, finite element analysis (FEA) was conducted to simulate stress and deformation of implant-supported crowns fabricated from lithium disilicate (LiS2) and zirconia (3Y-TZP). Additionally, an in vitro load-to-failure test was conducted using two specimens per group to evaluate the failure mode and to confirm the findings from the FEA. Results/ Conclusion: FEA revealed stress areas at the palatal cervical areas of the crowns. In the load-to-failure test, both LiS2 hybrid abutment crowns fractured (410 N and 510 N) before plastic deformation of the metal implant components could be detected. The 3Y-TZP monolithic hybrid abutment crowns did not fracture until the tests were interrupted at 646-N and 690-N occlusal force, when plastic deformation of the metal implant components was visually observed.

Purpose: To systematically review all clinical studies focusing on the clinical outcomes of zirconia restorations bonded to different types of tooth or implant abutments and to provide practical clinical guidelines. Materials and methods: A search was performed for English-language articles in dental journals published up to September 2020 using a combination of free-text words and MeSH terms. Studies were identified for review according to certain inclusion and exclusion criteria. Results: A total of 2,856 studies were identified, and 24 studies were finally included. The included studies featured different types of surface pretreatment methods, primers, resin cements, tooth and implant abutments, zirconia restorations, and designs. The relatively limited number of identified studies and the heterogeneity of the extracted data did not allow for meta-analysis. Conclusion: Airborne-particle abrasion (APA) and tribochemical silica (TBS) APA surface pretreatment methods yielded positive clinical outcomes on zirconia/resin bonding to all types of tooth surfaces identified. Chemically activating the APA and the TBS APA zirconia surfaces with an MDP monomer-based primer, along with an MDP monomer or phosphoric ester monomer-based resin cement, seems to be a durable bonding protocol for all types of tooth surfaces identified. Skipping the surface pretreatment method step in the bonding protocol did not affect the clinical outcomes of certain types of zirconia restorations on most types of tooth or implant abutment surfaces identified. Type of resin cement seems to be a less influential factor.

Purpose: To describe the current findings regarding the use of lasers to enhance the bonding properties of zirconia and to enable its debonding. Materials and methods: The PudMed database was searched for literature up to July 2021. The keywords used for zirconia surface treatment were: laser irradiation; zirconia; and bond strength. For ceramic laser debonding, the keywords were: laser irradiation; ceramic; and debonding. Results: A total of 36 studies were included for zirconia surface treatment, and 12 for ceramic laser debonding. Nd:YAG, CO2, Er:YAG, Er,Cr:YSGG, and ultrashort lasers were used for surface treatment, whereas only Er:YAG and Er,Cr:YSGG lasers were applied to debond zirconia structures. Conclusion: The use of laser irradiation to improve the bonding properties of zirconia depends on the type of laser and the parameters used. Among the laser types included in this review, ultrashort lasers have shown the most promising results; however, more studies are needed to prove their superiority over the other lasers for enhancing zirconia bond strength. In terms of debonding, irradiation with different lasers was effective regardless of the ceramic type; however, well-established protocols (ie, laser irradiation time and motion, laser settings) are still needed for different thicknesses and material ceramic types.

Purpose: To evaluate the shear bond strength (SBS) of different digital veneering techniques for zirconia and to critically discuss its suitability for application in single-implant prosthetics. Materials and methods: A total of 112 square-shaped zirconia specimens were provided with four different veneering materials (n = 28 per group): a glass-ceramic (group GLA), a feldspathic ceramic (group FEL), a polymer-infiltrated ceramic network (group PIC), and a resin nanoceramic (group RNC). Discs in group GLA were sintered onto the core material, whereas all other specimens were adhesively connected. In each group, 14 specimens (GLA0, FEL0, PIC0, RNC0) were subjected to SBS testing before thermocycling, and the other 14 (GLA1, FEL1, PIC1, RNC1) were tested after thermocycling (10,000 cycles). Data were analyzed by applying SPSS software (P < .05). The surfaces and fracture patterns of the specimens were examined using scanning electron microscopy (SEM). Results: Mean SBS values ranged from 14.09 ± 3.87 MPa (RNC1) to 40.82 ± 4.91 MPa (GLA0). Group GLA presented higher values than all other groups (P < .001). Groups FEL, PIC, and RNC showed no statistically significant differences between them. SBS decreased after thermocycling, but no significant impact was found. Every group exhibited a characteristic failure mode. Conclusion: All digital veneering techniques sufficed to present clinically acceptable SBS values and might be viable alternatives in implant prosthetics. However, some have yet to demonstrate their long-term clinical suitability. At present, lithium disilicate-veneered zirconia abutments and monolithic lithium disilicate hybrid abutment crowns seem to present a proven and reliable restorative option.

Purpose: To evaluate the biologic, technical, and radiographic outcomes of CAD/CAM-milled bilayer and monolithic zirconia crowns using implants and Ti-base abutments with up to 5 years of follow-up. Materials and methods: A total of 182 implant-supported "screw-mentable" (hybrid screw/cement retention) single crowns were evaluated in 118 patients. Ti-base abutments were selected according to the chimney (3.5 or 5 mm) and gingival (1, 2, or 3 mm) heights in the virtual model. Zirconia crowns were designed using CAD/CAM software and then milled from partially stabilized zirconia blocks. After all crowns were cemented onto Ti-base abutments, they were clinically screwed onto the implants. Both implants and crowns were followed up for up to 5 years, and their clinical, technical, and radiologic results were recorded. Results: A total of 118 patients (86 women and 32 men) who received 182 implant-supported screw-mentable crowns were included in this study. The mean follow-up period was 32 ± 18 months (range: 24 to 60 months) for all implants and crowns. No implant was lost during the follow-up period, yielding a cumulative implant survival rate of 100%. Two technical complications were observed in two bruxer patients, yielding a cumulative restoration survival rate of 98.9%. The marginal bone loss was 0.7 ± 0.5 mm from the baseline radiograph to the radiograph taken at the final recall visit. No implants were diagnosed with peri-implantitis. Conclusion: The outcomes of this study suggest that Ti-base abutments are a feasible and affordable alternative to CAD/CAM abutments and that they can successfully support single zirconia crowns.

Titanium oral implants are still considered "state of the art" in implant dentistry, with well-documented survival rates. However, their grayish color and high prevalence of peri-implant infections have resulted in controversial discussion as to whether tooth-like-colored, metal-free zirconia ceramic implants provide sufficient potential to be considered equal regarding treatment outcomes. The present position paper has been composed upon invitation by the European Association of Osseointegration in order to provide an update on the current level of evidence regarding zirconia implants in clinical trials. To date, most available and scientifically documented zirconia implant systems are one-piece implants that require an experienced surgeon and prosthodontist due to the restricted flexibility in cases of compromised angulation or vertical positioning. Taking this limitation into account, there is evidence of a comparable outcome for one-piece zirconia implants compared to titanium implants for the fixed replacement of one to three missing teeth. In contrast, currently available clinical data evaluating two-piece zirconia implants with an adhesively bonded implant-abutment interface suggest an inferior outcome. Data evaluating the clinical applicability of screw-retained solutions, even if revealing sufficient fracture resistance in laboratory investigations, are still missing. High survival rates were reported for all-ceramic reconstructions supported by zirconia implants, but with increased technical complications; ie, fractures of the ceramic veneer in the case of bilayered restorations. Sufficient clinical evidence for recommending monolithic approaches is limited to single crowns.