Influence of Framework Material and Posterior Implant Angulation in Full-Arch All-on-4 Implant-Supported Prosthesis Stress Concentration

This study evaluated the influence of distal implants angulation and framework material in the stress concentration of an All-on-4 full-arch prosthesis. A full-arch implant-supported prosthesis 3D model was created with different distal implant angulations and cantilever arms (30° with 10-millimeter cantilever; 45° with 10-millimeter cantilever and 45° with 6-millimeter cantilever) and framework materials (Cobalt–chrome [CoCr alloy], Yttria-stabilized tetragonal zirconia polycrystal [Y-TZP] and polyetheretherketone [PEEK]). Each solid was imported to computer-aided engineering software, and tetrahedral elements formed the mesh. Material properties were assigned to each solid with isotropic and homogeneous behavior. The contacts were considered bonded. A vertical load of 200 N was applied in the distal region of the cantilever arm, and stress was evaluated in Von Misses (σVM) for prosthesis components and the Maximum (σMAX) and Minimum (σMIN) Principal Stresses for the bone. Distal implants angled in 45° with a 10-millimeter cantilever arm showed the highest stress concentration for all structures with higher stress magnitudes when the PEEK framework was considered. However, distal implants angled in 45° with a 6-millimeter cantilever arm showed promising mechanical responses with the lowest stress peaks. For the All-on-4 concept, a 45° distal implants angulation is only beneficial if it is possible to reduce the cantilever’s length; otherwise, the use of 30° should be considered. Comparing with PEEK, the YTZP and CoCr concentrated stress in the framework structure, reducing the stress in the prosthetic screw.

Effect of Cooling Rate during Glazing on the Mechanical and Optical Properties of Monolithic Zirconia with 3 mol% Yttria Content

Glazing is the final heat treatment process in the manufacturing of a monolithic zirconia prosthesis. Herein, the effect of cooling rate during zirconia glazing was investigated. A 3 mol% yttria-stabilized tetragonal zirconia polycrystal was glazed at the general cooling rate suggested by the manufacturer, as well as at higher and lower cooling rates, and the differences in flexural strength, hardness, optical properties, and crystal structure were evaluated. A higher cooling rate did not affect the flexural strength, hardness, grain size, optical properties, or crystal structure; however, the Weibull modulus decreased by 1.3. A lower cooling rate did not affect the flexural strength, optical properties, or crystal structure; however, the Weibull characteristic strength increased by 26.7 MPa and the Weibull modulus increased by 0.9. The decrease in hardness and the increase in grain size were statistically significant; however, the numerical differences were negligible. This study revealed that a lower cooling rate provides more reliable flexural strength. Therefore, glazing can proceed at a general cooling rate, which takes 3–4 min; however, glazing at a lower cooling rate will provide a more consistent flexural strength if desired, despite being time-consuming.

Influence of the Connector Area on the Chipping Rate of the VM9 Veneering Ceramic in a Posterior Four-Unit Yttria-Stabilized Tetragonal Zirconia Polycrystal Fixed Dental Prostheses: A Pilot Study

Objective The purpose of this study was to evaluate the influence of the connector area on the chipping rate of the VM9 veneering ceramic in a 4-unit yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) framework, using a novel sonographic technique. Materials and Methods The framework was designed as a 4-unit fixed dental prostheses (FDP) with two abutment teeth at the first mandibular premolar and second mandibular molar. The cross-sectional area of the connector was 7.5, 8, or 9 mm2. The prepared teeth were scanned using CAD/CAM technology, and five frameworks were prepared for each design, making a total of 15. The frameworks were divided according to the veneering ceramics: Y-TZP FDPs using Vita VM9 (12 frameworks), and porcelain fused to metal (PFM) FDPs using Vita VM13 (3 frameworks). The specimens were statically loaded until failure. To capture the initiation of the failure within the specimens, a novel sonographic technique, designed for this research, combining acoustic emission and visual monitoring during live load bearing tests, was used to determine the failure load value. Failed frameworks were inspected using a microscope, and failure patterns were identified. One-way ANOVA and Tukey HSD tests were applied for statistical analysis of the results (p-value ≤ 0.05). Results Despite the fact that the chipping strength is proportional to the cross-sectional area of the connector, the effect of the cross-sectional area on chipping load revealed no statistically significant differences, while PFMs showed superior chipping strength relative to the zirconia frameworks regardless of the connector's size. A high rate of catastrophic fracture occurred while loading the specimens. Conclusion The effect of the cross-sectional area on chipping load revealed no statistically significant differences in Y-TZP frameworks. The novel sonographic technique may be a potential method to study the behavior of dental ceramics, including chipping and fracture.

Effect of Sintering Time on The Marginal and Internal Fit of Monolithic Zirconia Crowns Containing 3-4 Mol% Y2O3

Background Short-time sintering may offer advantages including saving time and energy but there is limited evidence on the effect that altering sintering time has on the accuracy of monolithic zirconia crowns. The purpose of this in vitro study was to investigate the effect of shortened sintering time on the marginal and internal fit of 3Y-TZP (three mol% yttria-stabilized tetragonal zirconia polycrystal) and 4Y-TZP (four mol% yttria-stabilized tetragonal zirconia polycrystal) monolithic crowns. Methods Sixty monolithic zirconia crowns were fabricated for the maxillary first molar tooth on the prefabricated implant abutment. Groups were created according to the material composition: 3Y-TZP Generation 1 (alumina wt; ≤0.5%, < 15% cubic phase), 3Y-TZP Generation 2 (alumina wt; ≤0.05%, < 15% cubic phase) and 4Y-TZP (alumina wt; ≤0.05%, > 25% cubic phase). Two different sintering protocols were performed: same final sintering temperature (1500°C) and various rates of heating (10°C/min and 40°C/min), cooling down speed (-10°C/min and − 40°C/min), holding time (45 and 120 minutes), and total sintering time (approximately 2 and 7 hours, respectively). The marginal and internal fit of the crowns were determined using the silicone replica technique. Comparisons between groups and sintering times were analyzed using two-way ANOVA. Pairwise multiple comparisons were performed using the t-test (p < 0.05). Results The mean marginal gap values of 4Y-TZP zirconia revealed statistically significant increase for the speed sintering protocol (p < 0.0001), while no difference was observed between the sintering protocols for the mean marginal gap values of 3Y-TZP groups. Conclusions Although all groups showed clinically acceptable gap values (<120 µm), altering the sintering time had an effect on the marginal fit of the crowns manufactured from 4Y-TZP zirconia. Shortening the sintering time may lead to differences within clinically acceptable limits. The manufacturer's recommendations according to material composition should be implemented with care.