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

The loss of resin cement adhesion to ceramic influences the fatigue behavior of bonded lithium disilicate restorations

When partial and/or non-retentive preparation, such as those for occlusal veneers, is indicated, a proper and stable adhesion is essential. Therefore, the aim of this in vitro study was to evaluate the effect of loss of adhesion in different regions of the bonding interface on the fatigue behavior of simplified lithium disilicate restorations. For this, lithium disilicate (IPS e.max CAD) discs (1 mm thick and Ø = 10 mm) were fabricated, polished with #400-, #600-, #1200-grit silicon carbide (SiC) papers, and crystallized. As substrate, fiber-reinforced resin epoxy discs (2.5 mm thick and Ø = 10 mm) were fabricated and polished with #600-grit SiC paper. The ceramic bonding surface was treated with 5% hydrofluoric acid and a silane-containing primer (Monobond N), while the substrate was etched with 10% hydrofluoric acid followed by the application of the bonding system primers (Primer A + B). A lacquer (nail polish) was used to simulate the loss of adhesion in specific areas according to the study design to compose the testing groups: bonded (control; did not received nail polish application); - non-bonded (loss of adhesion in the whole specimen area); - margin (loss of adhesion in the ceramic margin); - center (loss of adhesion in the ceramic central area). The adhesive area of partially bonded groups was 50% of the adhesive surface. Then, the discs (n = 12) were bonded to the respective substrate using a resin cement (Multilink N), light-cured, water-stored for 90 days, and subjected to thermocycling (25,000 cycles, 5° to 55 °C) before testing. A cyclic fatigue test was run (20 Hz, initial load of 200 N for 5000 cycles, 50 N step size for 10,000 cycles each until specimen failure), and the fatigue failure load and number of cycles for failure were recorded. As complementary analysis, finite element analysis (FEA) and scanning electron microscopy analysis were performed. Kaplan-Meier log-rank (Mantel-Cox) was conducted for survival analysis. The results showed that as the loss of adhesion reaches the central area, the worse is the fatigue behavior and the higher is the stress peak concentration in the ceramic bonding surface. The bonded specimens presented better fatigue behavior and stress distribution compared to the others. In conclusion in a non-retentive preparation situation, proper adhesion is a must for the restoration fatigue behavior even after aging; while the loss of adhesion reaches central areas the mechanical functioning is compromised.

Weak adhesion between ceramic and resin cement impairs the load-bearing capacity under fatigue of lithium disilicate glass-ceramic crowns

OBJECTIVE

To evaluate the fatigue behavior of lithium disilicate crowns with a simplified anatomy against progressive cement/ceramic debonding scenarios.

MATERIALS AND METHODS

Lithium disilicate crowns were fabricated via CAD/CAM and luted onto a dentin analogue material using resin cement following the manufacturer's instructions. Then, the different crown regions were isolated with paraffin oil for the absence of chemical adhesion according to four experimental groups (n = 15): Shoulder; Shoulder + Axial; Fully isolated; and Control (no insulation/fully bonded). Load to failure tests (n = 3) were run to determine cyclic fatigue parameters, and the specimens were subsequently submitted to a cyclic fatigue test (n = 12) (initial load 200 N for 5000 cycles, step 100 N, 15,000 cycles/step, frequency 20 Hz) until cracks were observed, and later fracture. The data were analyzed by Kaplan-Meier + Mantel-Cox post-hoc tests for both outcomes (cracks and fracture). Fractographic, cross-sectional surface, and finite element (FEA) analyzes were performed.

RESULTS

When it comes to crack occurrence when the chemical adhesion to the occlusal surface is compromised, there is worsening (p < 0.05) in fatigue behavior compared to groups where the occlusal portion of the crown is still bonded. Considering fracture occurrence, there was no difference (p > 0.05) among the tested groups. All cracks occurred in the occlusal portion, first as a radial crack at the ceramic intaglio surface, and posteriorly unleashing a Hertzian cone crack at the top surface, resulting in fractures on the frontal walls. The interface analysis showed no interference of the insulating agent. FEA showed that as the isolated areas increased, there was also an increase in both tensile and shear stresses concentration in the crown and in the cement layer.

CONCLUSION

The chemical adhesion between cement and ceramic is essential for better fatigue behavior of lithium disilicate crowns with a simplified anatomy, especially in the occlusal portion, but the restoration performance is impaired when such adhesion is compromised. There is an increase in crown and cement stress concentration with the progressive loss of chemical bonding of the crown's walls.

Does adhesive luting promote improved fatigue performance of lithium disilicate simplified crowns?

The present study aimed to determine the influence of adhesive luting on the fatigue mechanical properties of simplified lithium disilicate crowns luted to dentin analogues preparations, and to compare two different approaches of simulation of a non-adhesive luting procedure. To this end, 30 prosthetic preparations of glass fiber-filled epoxy resin used as dentin analogues were milled and lithium disilicate crowns were pressed (1.5 mm thickness), which were allocated into three different groups: resin cement (RC), resin cement isolated by paraffin oil (RCI) and zinc phosphate cement (ZP). For luting procedures, the ceramic crowns of the RC and RCI groups had their internal surface treated with 5% hydrofluoric acid and universal primer. Afterwards, a thin layer of paraffin oil was applied onto the ceramic crown intaglio surface in the RCI group. The dentin analogues were treated with 10% hydrofluoric acid, followed by primer application and luting as recommended by the manufacturer. No previous surface treatments were performed for the ZP group, and the cement was applied as recommended. The fatigue test was run under cyclic fatigue (load 400-2000 N, step-size 100 N, 15,000 cycles/step, frequency 20 Hz) until failure detection (radial crack) by transillumination. The fatigue failure load (FFL), number of cycles until failure (CFF), and survival rate in each testing step were recorded. The RC group presented a higher FFL, CFF and survival rate compared to the other groups (p < 0.001), which were statistically equal to each other (p > 0.05). Weibull analysis detected no difference between groups for the Weibull module. Therefore, loss of adhesion between the evaluated substrates induces a significant reduction in load-bearing capacity under fatigue of the lithium disilicate crowns, strengthening the strong-importance of long-lasting adhesion for crown's survival. In addition, different methods for the simulation of non-adhesive conditions induce the same performance in the explored outcomes, confirming that both are valid approaches for laboratory studies.

PEEK in Fixed Dental Prostheses: Application and Adhesion Improvement

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.

Investigation of stress distribution within an endodontically treated tooth restored with different restorations

Background/purpose

Recently, metal-free restoration has become the standard in prosthetic treatment. However, it is still unclear which combination is most effective in preventing root fracture and secondary caries. The purpose of this study was to evaluate the influence of different post systems, crown materials, crown thickness and luting agents on the stress distribution around the crown margins, cervical dentin and the tip of the post.

Materials and methods

Ninety-six mandibular first premolar models were developed and analyzed using finite element analysis (FEA). Two designs of crowns, six kinds of crown materials, four types of post and core systems and two kinds of luting agents were included and evaluated for the stress distribution within the abutment teeth. The Von Mises stress magnitudes were compared among all models.

Results

The stress at the tip of the post decreased as the young's modulus of luting agent decreased; The stress concentrated more at the cervical area (dentin and crown), as the physical properties of the crown material increased.

Conclusion

Crowns fabricated using polyetheretherketone (PEEK) can reduce the stress concentration at the cervical area, so it may be possible to reduce the amount of tooth reduction during abutment tooth preparation. The stress distribution around the post tip is affected by the post and core systems and luting agent, regardless of crown materials and thickness. When inserting a post of the higher Young's modulus such as zirconia post, methyl methacrylate luting cement can reduce the stress concentration at the tip of the post.

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

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.

Establishment of optimal variable elastic modulus distribution in the design of full-crown restorations by finite element analysis

To establish optimal elastic modulus distribution throughout the entire all-ceramic crown, aiming at improvement of the mechanical properties of the restoration as well as the adhesive interface, seven 3D models of mandibular first premolars of zirconia monolithic and bilayer crowns and lithium disilicate monolithic and bilayer crowns were constructed. The elastic modulus distribution of 8-layer crown A referred to human enamel, B was calculated by a genetic algorithm (GA) to minimize the principle stresses on the crown, and C minimized the shear stresses at the cementing lines. After applying a static load of 600 N, the maximum principle stresses were calculated and analyzed by finite element analysis (FEA). Group C were found to have the lowest peak shear stress at the cementing line and moderate peak tensile stress in the crown. Introduction of the modified elastic modulus distribution from human enamel into the entire all-ceramic crown reinforces the mechanical properties of the whole restoration as well as the adhesive interface against chipping and debonding.

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