OBJECTIVES To evaluate three-dimensional external gap progression after chewing simulation of high translucency zirconia (HTZ) and zirconia-reinforced lithium silicate (ZLS) applied on endodontically treated teeth with different preparation designs. MATERIALS AND METHOD Endodontically treated molars were prepared with low-retentive (adhesive overlay) and high-retentive (full crown) designs above cementum-enamel junction and restored with HTZ and ZLS. Micro-computed tomography analysis was assessed before and after chewing simulation to evaluate three-dimensionally the external gap progression. Results were statistically analyzed with two-way ANOVA and post-hoc Tukey test. RESULTS High-retentive preparation design had a significantly inferior gap progression compared to the overlay preparation (p < 0.01); ZLS exhibited a significant inferior gap progression compared to HTZ (p < 0.01). CONCLUSIONS High-retentive preparations restored with ZLS seem to better perform in maintaining the sealing of the external margin after cyclic fatigue. CLINICAL SIGNIFICANCE The clinician should pay attention to the proper combination of preparation designs and ceramic material selection for an endodontically treated molar restoration. HTZ seems to perform worse than lithium silicate in terms of marginal sealing, still showing lacks in resistance to cyclic fatigue when adhesive preparations are performed.

External gap progression after cyclic fatigue of adhesive overlays and crowns made with high translucency zirconia or lithium silicate.

STATEMENT OF PROBLEM
The outcomes from load-to-failure tests may not be applicable to clinical situations.


PURPOSE
The purpose of this study was to critically evaluate the efficacy of load-to-failure tests in the investigation of the fracture load and pattern of metal-free crowns.


MATERIAL AND METHODS
Four groups were formed from 128 bovine roots restored with metal posts, resin cores, and feldspathic, leucite, or lithium disilicate ceramic systems or polymer crowns. Each group was divided into 4 (n=8) according to the cement: zinc phosphate, self-adhesive resin, autopolymerizing resin, and glass ionomer. Mean fracture loads from compressive tests were submitted to ANOVA and Tukey HSD test. Finite element and fractographic analyses were performed and associated with the fracture load and pattern.


RESULTS
Significantly higher fracture load values were obtained for the lithium disilicate ceramic, but finite element and fractographic analyses showed that the cement effect could not be determined. The finite element analysis showed the cement likely affected the fracture pattern, confirmed that stresses in the cements were little affected by the crown materials, and found that the stressed conditions were lowest in the lithium disilicate compared with other crowns for all cement combinations. The stressed conditions in the crowns depended more on the adhesive properties than on the elastic modulus of the cement materials. The level of the stressed condition in the crowns at the occlusal surface was about the same or higher than along their cement interface, consistent with the fractography, which indicated fractures starting at the load point. Higher stress levels in the crowns corresponded with a lower number of catastrophic fractures, and higher stresses in the cements seemed to reduce the number of catastrophic fracture patterns. The highest stressed conditions occurred along the occlusal surface for crown materials with a low elastic modulus or in combination with adhesive cements.


CONCLUSIONS
The method used was not appropriate either for investigating the crowns' fracture load and pattern or for stating the role of the cements within the crown-cement-tooth interaction.

Crown Fracture: Failure Load, Stress Distribution, And Fractographic Analysis

PURPOSE To evaluate the flexural strength, elastic modulus, microhardness, and surface roughness of monolithic zirconia, lithium disilicate ceramics, and feldspathic ceramics after being exposed to different acidic solutions. MATERIALS AND METHODS Rectangular specimens (n = 180) were prepared from three different ceramic materials: lithium disilicate, monolithic zirconia, and feldspathic porcelain. Initial Surface roughness of ninety specimens (n = 30/material) was evaluated using an optical noncontact profilometer. Thirty specimens of each material were immersed in one of the following solutions (n = 10/group): citric acid; acidic beverage; and artificial saliva, which served as the control. Post immersion surface roughness, flexural strength, and elastic modulus were determined using an optical noncontact profilometer and three-point bending test. Another thirty specimens of each material were immersed in the aqueous solutions (n = 10/group) following the same protocol and subjected to microhardness test using a Vickers diamond microhardness tester. A scanning electron microscope (SEM) was used to examine the surface characteristics changes. ANOVA and Post-hoc Tukey's Kramer tests were used for data analysis (α = 0.05). RESULTS Immersion in different solutions did not affect the flexural strength and elastic modulus of lithium disilicate or zirconia. Microhardness and surface roughness were significantly affected in all groups (p < 0.05). For feldspathic porcelain groups, the flexural strength and elastic modulus were significantly decreased in the citric acid group (p = 0.045 and p = 0.019). Also, there were significant differences among all feldspathic porcelain groups (p = 0.001) in terms of microhardness and surface roughness values. CONCLUSIONS The tested acidic agents significantly affected the flexural strength, elastic modulus, surface roughness, and microhardness of feldspathic porcelain. However, the flexural strength and elastic modulus of evaluated lithium disilicate and zirconia did not change significantly.

Acid Effects on the Physical Properties of Different CAD/CAM Ceramic Materials: An in Vitro Analysis.

Polyetheretherketone (PEEK) has been widely applied in fixed dental prostheses, comprising crowns, fixed partial dentures, and post-and-core. PEEK’s excellent mechanical properties facilitate better stress distribution than conventional materials, protecting the abutment teeth. However, the stiffness of PEEK is not sufficient, which can be improved via fiber reinforcement. PEEK is biocompatible. It is nonmutagenic, noncytotoxic, and nonallergenic. However, the chemical stability of PEEK is a double-edged sword. On the one hand, PEEK is nondegradable and intraoral corrosion is minimized. On the other hand, the inert surface makes adhesive bonding difficult. Numerous strategies for improving the adhesive properties of PEEK have been explored, including acid etching, plasma treatment, airborne particle abrasion, laser treatment, and adhesive systems.

/pdf/peek-in-fixed-dental-prostheses-application-and-adhesion-3vz8ehl4.pdf

PEEK in Fixed Dental Prostheses: Application and Adhesion Improvement

The effect of food medium on the wear behaviour of veneering porcelain: An in vitro study using the three-body abrasion mode.

Photoelasticity is an effective method for evaluating the stress and its spatial variations within a stressed body. In the present study, a method to determine the stress distribution by means of phase shifting and a modified shear-difference is proposed. First, the orientation of the first principal stress and the retardation between the principal stresses are determined in the full-field through phase shifting. Then, through bicubic interpolation and derivation of a modified shear-difference method, the internal stress is calculated from the point with a free boundary along its normal direction. A method to reduce integration error in the shear difference scheme is proposed and compared to the existing methods; the integration error is reduced when using theoretical photoelastic parameters to calculate the stress component with the same points. Results show that when the value of Δx/Δy approaches one, the error is minimum, and although the interpolation error is inevitable, it has limited influence on the accuracy of the result. Finally, examples are presented for determining the stresses in a circular plate and ring subjected to diametric loading. Results show that the proposed approach provides a complete solution for determining the full-field stresses in photoelastic models.

https://iopscience.iop.org/article/10.1088/1361-6501/aaa7ae/pdf

Full-field stress determination in photoelasticity with phase shifting technique

https://research-information.bris.ac.uk/files/162423377/corrected_submission.pdf

The effect of adhesive failure and defects on the stress distribution in all-ceramic crowns.

The objective of this study was to compare the resistance to failure of three types of all-ceramic crowns under static and cyclic contact loading. Three types of crown specimens were fabricated (N = 14 per group) including: i) sintered bi-layer crowns (SBC) made of IPS e.max CAD veneers fused on Lava zirconia core ceramics, ii) adhesive bi-layer crowns (ABC) made of IPS e.max CAD veneers bonded onto Lava zirconia core ceramics, and iii) monolith ceramics crown (MCC) made of IPS e.max CAD. All were bonded onto a compliant substrate of resin composite with commercial adhesive. Eight specimens were selected from each group for monotonic loading to failure via 6 mm diameter ball to obtain the initial load capacity (Pinitial) of the crowns. The remaining six crown specimens were subjected to cyclic contact loading to 5 million cycles, and then loaded monotonically to fracture to obtain residual load capacity after fatigue (Presidual). A finite element analysis (FEA) was used to analyze the stress distributions within the three types of crowns and estimate the crack origins from the maximum stress. The initial load to failure (Pinitial) for the SBC, ABC, and MCC groups were 1120 ± 170 N, 970 ± 150 N, and 950 ± 60 N, respectively, and significantly different (p = 0.027). The residual load to failure after fatigue (Presidual) for the crown types were 890 ± 240 N, 680 ± 240 N, and 1050 ± 120 N, respectively, and also significantly different (p = 0.012). For SBC and MCC, failure occurred predominantly by bulk fracture, whereas the ABC failed primarily by chipping. The FEA suggested that failure of SBC and ABC would originate in the cement layers, and MCC has a more reasonable stress allocation than that in bi-layer crowns (SBC and ABC), which agreed with the observed failure modes. The SBC crowns had superior load to failure, whereas the bi-layer systems (SBC and ABC) had inferior fatigue resistance compared with the monolith crowns (MCC). The cement interface was the weak-link of the bi-layer systems, especially for ABC. Considering their use in clinical practice, the ranking in resistance to failure is SBC>MCC>ABC.

Yonggang Liu

Papers

Full-field stress determination in photoelasticity with phase shifting technique

The effect of adhesive failure and defects on the stress distribution in all-ceramic crowns.

Bearing capacity of ceramic crowns before and after cyclic loading: An in vitro study