Fracture Resistance and Fracture Behaviour of Monolithic Multi-Layered Translucent Zirconia Fixed Dental Prostheses with Different Placing Strategies of Connector: An in vitro Study

Flexural Strength of Translucent Zirconia Materials Produced with Different Multilayer Technologies: An In Vitro Study

Objective

To evaluate the flexural strength of two translucent multilayered zirconia materials produced with different multilayer technologies. Methodology. Eighty bar-shaped zirconia specimens were prepared from two different multilayered zirconia materials (IPS e.max® ZirCAD Prime and KATANA™ Multilayered Zirconia HTML) and divided into eight groups (n = 10) based on the materials used and the individual layers of the disc for each material: Dentin Prime, Transition Prime, Translucent Prime, Multilayered Prime, Dentin HTML, Transition HTML, Translucent HTML, and Multilayered HTML. The bar-shaped zirconia specimens were cut to include all the layers from translucent to dentin In Multilayered Prime and Multilayered HTML groups. All specimens were fully sintered after cutting from multilayered zirconia discs and subjected to three-point flexural strength test using the universal testing machine.

Results

The specimens made of HTML zirconia material showed significantly (P < 0.001) higher flexural strength than those made of IPS e.max® ZirCAD Prime material, with no significant difference (P > 0.05) compared to the specimens in the Dentin Prime group. The Dentin Prime specimens had the highest flexural strength (743 ± 116 MPa) compared to those in the Translucent Prime (514 ± 120 MPa), Transition Prime (575 ± 102 MPa), and Multilayered Prime (531 ± 132 MPa) groups. The flexural strength of the specimens from the individual layers of HTML zirconia material was not significantly different (P > 0.05) among the Dentin HTML (763 ± 56 MPa), Translucent HTML (791 ± 106 MPa), Transition HTML (816 ± 85 MPa), and Multilayered HTML (793 ± 102 MPa) groups.

Conclusion

Multilayered zirconia materials produced with different yttria contents by layer have lower flexural strength than those produced with gradient shade technology and the same yttria content for each layer. Therefore, various factors such as the type of prostheses, nesting strategies of prostheses within the zirconia disc, and the desired aesthetical requirements should be considered when selecting the multilayered zirconia materials.

Effect of low-temperature degradation on the fatigue performance of dental strength-gradient multilayered zirconia restorations

OBJECTIVES

Fatigue and low-temperature degradation (LTD) are the main factors contributing to zirconia restoration failure. This study evaluated the effect of LTD on the fatigue performance of the novel "strength & shade-gradient" multilayered zirconia restorations.

METHODS

Discs (15 mm × 1.2 mm) of each yttria content layer from a newly developed strength-gradient multilayered zirconia were fabricated and under accelerated aging in an autoclave at 134℃ for 0 h, 32 h, and 64 h. Then, the phase transformation, microstructure, and mechanical properties after LTD were assessed. In addition, the crown samples, including the multi-Zir, 3Y-Zir, and 5Y-Zir were fabricated, and their monotonic and fatigue load before and after LTD, percentage of fatigue degradation (Sd) and the fracture morphology were investigated. Statistical analyses were performed using paired samples t-test (α' = α/3 = 0.017), one-way ANOVA and Weibull analysis.

RESULTS

After LTD, the phase transformation, surface roughness, depth of transformed zone, and residual stress were increased and inversely associated with the yttria content. The indentation elastic modulus and hardness after LTD decreased; however, there was no significant difference between the different yttria content layers. The monotonic and fatigue load of multi-Zir restorations decreased, but their Weibull modulus increased, and Sd decreased, similar to 3Y-Zir. The crack origin was associated with the cervical region.

CONCLUSION

These results show that although LTD reduces the absolute fatigue strength of strength-gradient multilayered zirconia restorations, it also reduces the effect of cyclic fatigue itself on the strength of zirconia (relative to monotonic strength), which might be due to the increase of residual stress.

CLINICAL SIGNIFICANCE

The novel "strength & shade-gradient" multilayered zirconia restorations show a promising performance during in vitro LTD and fatigue test and their reliability to some extent is comparable to 3Y-Zir. Yet, further in vivo longitudinal studies are warranted to confirm their precise performance.

Retrospective 1- to 8-Year Follow-Up Study of Complete Oral Rehabilitation Using Monolithic Zirconia Restorations with Increased Vertical Dimension of Occlusion in Patients with Bruxism

Aim: The aim of this paper is to perform a retrospective assessment of the clinical performance of the complete oral rehabilitation of patients with bruxism treated with implants and teeth-supported veneered and non-veneered monolithic zirconia restorations with increased occlusal vertical dimension. Methods: In this retrospective follow-up study, 16 bruxer patients, mean age 59.5 ± 14.9 years, were treated with 152 veneered and 229 non-veneered monolithic zirconia and followed for a mean of 58.8 ± 18.8 months (range 1–8 years). The patients were examined clinically and radiographically, annually. Clinical data were extracted from the medical records. In the recall appointments, modified California Dental Association (CDA) criteria were used to evaluate the restorations. Implant and restoration survival and success rates were recorded and analyzed. Results: The cumulative survival rates of implants and restorations were 97.7% and 97.6%, respectively. Nine restorations were replaced: three due to horizontal tooth fractures, two because of implant failure and four had secondary caries. A total of 43 biologic and technical complications were recorded. In the veneered group, the predominant complication was minor veneer chipping (16.4%), which required polishing only (grade 1). In the non-veneered group, the main complication was open proximal contacts between the implant restorations and adjacent teeth (14.5%). Conclusions: The survival rates of restorations and implants in patients with bruxism are excellent, even though veneered zirconia restoration exhibited a high rate of minor veneer chipping, which required polishing only. The biologic complication of fractured single-tooth abutment may occur.