Biaxial flexure strength determination of endodontically accessed ceramic restorations

Traditional versus conservative endodontic access impact on fracture resistance of chairside CAD-CAM lithium disilicate anterior crowns: An in vitro study

PURPOSE

To evaluate the effect of traditional and conservative endodontic access hole preparation on fracture resistance of chairside computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate maxillary right central incisor crowns.

MATERIALS AND METHODS

Fifty-seven milled lithium disilicate maxillary right central incisor crowns were designed and fabricated with a chairside CAD-CAM system (Planmeca Romexis, Planmeca). The abutment preparation had a 1.0 mm incisal reduction and 1.0 mm chamfer finish. The restorations were bonded with resin cement to printed resin dies (n = 19 per group) and were treated and divided into three groups, (1) no endodontic access, (2) traditional triangular endodontic access, and (3) conservative ovoidal endodontic access. The endodontic access of the crowns was sealed with flowable resin composite. Restorations were subjected to 10,000 cycles of thermal cycling between 5° and 55°C. Then, restorations were loaded and exposed to compressive loading force, and the crack initiation (CI) and complete fracture (CF) were recorded. SEM micrographs of broken specimens on the printed dies were captured. ANOVA test and Bonferroni's correction were used for statistical comparison.

RESULTS

The fracture resistance among the three groups varied. Crowns with no endodontic access displayed significantly (p < 0.001) higher resistance [CI: 1025 (121) N; CF 1134 (127) N], followed by crowns with conservative ovoidal endodontic access [CI: 924 (60) N; CF: 1000 (72) N. Crowns with traditional triangular endodontic access showed the significantly (p < 0.001) lowest fracture resistance [CI: 635 (82) N; CF: 709 (75) N].

CONCLUSION

The fracture resistance of chairside CAD-CAM lithium disilicate maxillary anterior crowns is influenced by the type of endodontic access provided. Conservative ovoidal endodontic access provides crowns with higher fracture resistance than traditional triangular endodontic access. Crowns with no endodontic access provided the highest resistance than other types of endodontic access.

Fracture strength of zirconia and lithium disilicate restorations following endodontic access

OBJECTIVES

To compare the fracture load of zirconia and lithium disilicate crowns prepared with endodontic access with fine and coarse diamond instruments.

MATERIALS AND METHODS

0.8 mm (3Y zirconia) or 1 mm (lithium disilicate) crowns were luted to resin composite dies with resin-modified glass ionomer (zirconia) or self-adhesive resin (lithium disilicate) cement. A 2.5 mm endodontic access hole was placed in each crown with fine (8369DF.31.025FOOTBALL) or coarse (6379 DC.31.023FOOTBALL) diamond instruments and restored with composite. A control group was prepared without access holes. Crowns were thermocycled for 10,000 cycles (5-55°C) and tested in compression with a steel indenter until failure (n = 8/group). A one-way ANOVA and Dunnett 2-sided test (alpha = 0.05) compared differences in fracture load between groups.

RESULTS

For zirconia, there was no statistical difference between the control group (2335 ± 160 N) and coarse diamond group (2345 ± 246 N); however, the fine diamond group (2077 ± 216 N) was significantly lower. For lithium disilicate, there was no statistical difference between the control group (2113 ± 183 N) and the fine (2049 ± 105 N) or coarse (2240 ± 118 N) groups.

CONCLUSIONS

3Y zirconia crowns became weaker when accessed with a fine diamond instrument. There was no negative effect of the endodontic access with bonded lithium disilicate crowns.

CLINICAL SIGNIFICANCE

Conservative endodontic access openings in high-strength ceramic restorations do not have a negative effect on their static fracture load. The coarse zirconia-cutting diamond rotary instrument is more efficient and has a less detrimental effect on the strength of the crowns than a fine diamond rotary instrument.

The effect of endodontic access preparation on the failure load resistance of a 3Y-TZP monolithic zirconia crown

The effect of endodontic access preparation on the failure load resistance of 3Y-TZP zirconia crowns was accomplished by preparing human molars and luting monolithic zirconia crowns with a self-adhesive resin cement. Besides the intact control, teeth received endodontic access preparations and then grouped (n = 12) into a positive control (no access repair), dentin core replacement only and complete access repair groups. Specimens were axially tested until failure with results of no significant difference between the failure load of intact controls and the complete access repair group. However, the positive control and dentin replacement only groups failed at significantly lower loads. Under the conditions of this study, there was no significant failure load difference between 3Y-TZP monolithic zirconia crowns with repaired endodontic access preparations to that evidenced by an unprepared control. Although this evidence is encouraging, caution is advised and definitive recommendations cannot be made until verified by clinical studies.

Endodontic Access Effect on Full Contour Zirconia and Lithium Disilicate Failure Resistance

Objectives:

To evaluate the effect of endodontic access on the failure load resistance of both adhesively and conventionally luted, full-contour monolithic yttria-stabilized zirconium dioxide (Y-TZP) and adhesively luted lithium disilicate (LD) crowns cemented on prepared teeth.

Methods and Materials:

Seventy-two human maxillary molars were prepared per respective guidelines for all-ceramic crowns with one group (n=24) restored with LD and the other (n=48) receiving Y-TZP crowns. Preparations were scanned using computer-aided design/computer-aided milling (CAD/CAM) technology, and milled crowns were sintered following manufacturer recommendations. All LD crowns and half (n=24) of the Y-TZP crowns were adhesively cemented, while the remaining Y-TZP specimens were luted using a conventional glass ionomer cement (GIC). One LD group, one Y-TZP adhesive group, and one GIC-luted group (all n=12) then received endodontic access preparations by a board-certified endodontist: the pulp chambers were restored with a dual-cure, two-step, self-etch adhesive and a dual-cure resin composite core material. The access preparations were restored using a nano-hybrid resin composite after appropriate ceramic margin surface preparation. After 24 hours, all specimens were loaded axially until failure; mean failure loads were analyzed using Mann-Whitney U test (α=0.05)

Results:

Endodontic access did not significantly reduce the failure load of adhesively luted LD or Y-TZP crowns, but Y-TZP crowns with GIC cementation demonstrated significantly less failure load.

Conclusions:

These initial findings suggest that endodontic access preparation may not significantly affect failure load resistance of adhesively luted Y-TZP and LD crowns. Definitive recommendations cannot be proposed until fatigue testing and coronal seal evaluations have been accomplished.

Does veneering technique affect the flexural strength or load-to-failure of bilayer Y-TZP? A systematic review and meta-analysis

STATEMENT OF PROBLEM

Causes of failures of bilayer yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) restorations include the processing technique and the properties of the veneer ceramic. The effect of the veneering method on the mechanical behavior of veneered Y-TZP remains unclear.

PURPOSE

The purpose of this systematic review was to assess the effect of the veneering method on the flexural strength and failure load of bilayer Y-TZP.

MATERIAL AND METHODS

This study followed the Preferred Reporting Items for the Systematic Reviews and Meta-Analyses (PRISMA) statement. Searches were performed through August 2017 in PubMed/MEDLINE, Web of Science (Core Collection), Scopus, and Embase, with no year or language limit, targeting in vitro studies evaluating the effect of the veneering technique on the flexural strength and load-to-failure of bilayer Y-TZP immediately or after aging. Statistical analyses were conducted using an appropriate software. Comparisons were drawn with random effect models (α=.05).

RESULTS

From 3242 identified studies, 241 were selected for full-text analysis; from these, 33 studies were included. Manual searching yielded no additional papers. The meta-analysis consisted of 32 studies. Meta-analysis was performed separately for flexural strength and failure load data to compare the hand-layered method (control) with pressed, fused, and cemented veneering techniques. The cemented and fused methods were analyzed using subgroups depending on the veneering material being examined (predominantly glass-ceramics and particle-filled glass-ceramics), and the results were compared with those of the hand-layered method. The pressed group presented flexural strength (7 studies) (P=.150) and failure load (19 studies) (P=.140) values that were similar to those of the hand-layered group. Subgroup analysis revealed that the fused group with particle-filled glass-ceramics (7 studies) produced higher load-to-failure (P=.006) values than the hand-layered group. Subgroup analyses showed a statistical difference that favored the hand-layered over the cemented method, with predominantly glass-ceramic materials (5 studies) (P=.002).

CONCLUSIONS

The fused technique with particle-filled glass-ceramics seems more appropriate for the veneering of Y-TZP, with improved failure load, than the hand-layered method with predominantly glass-ceramic materials. The use of predominantly glass-ceramics for the cemented method is not recommended as failure load was lower than for the hand-layered group. Pressed veneers showed failure load and flexural strength values similar to those of the the hand-layered technique.

Effect of loading configuration on strength values in a highly transformable zirconia-based composite

OBJECTIVES

The aim of this work was to determine mechanical properties of a highly transformable 10Ce-TZP/Al2O3/La2AlO3 composite, currently developed as a biomaterial for dental application, and to investigate the effect of loading configuration on its flexural strength.

METHODS

Fracture toughness is determined by the single-edge-V-notched beam (SEVNB) method. Strength measurements were conducted by four-point bending and biaxial bending tests (piston-on-three balls) according to ISO 6872, dedicated to ceramic materials in dentistry.

RESULTS

Strength obtained by either four-point or biaxial bending are very different, and take the values of 596MPa and 1470MPa respectively. It is demonstrated that the difference in measured strength cannot be attributed to the effect of volume on strength, generally predicted by the standard Weibull analysis, but to different transformation behaviors for the two bending configurations. More extensive transformation occurs in the biaxial configuration, with a lower autocatalytic transformation stress threshold, resulting to substantial compressive residual stresses.

SIGNIFICANCE

The significant influence of the loading configuration on the strength should be integrated when designing a component from a highly transformable ceramic.