Features of fracture of prosthetic tooth-endocrown constructions by means of acoustic emission analysis

Additive manufacturing of ceria and yttria incorporated toughened monolithic zirconia dental ceramic crowns: In vitro simulated aging behavior

STATEMENT OF PROBLEM

The comprehensive characterization of additively manufactured zirconia-based dental prostheses can promote widespread clinical application. However, simulated in vitro analysis of the aging behavior is lacking.

PURPOSE

The purpose of this in vitro study was to assess the simulated in vitro durability of monolithic transformation toughened additively manufactured zirconia-based restorations with different compositions to predict the clinical reliability depending on their ceramic composition.

MATERIAL AND METHODS

Crowns were 3-dimensionally (3D) printed by using a combination of custom-made stereolithography and a laser polymerized digital light processing process with high solid content slurries with suitable photo-interactive polymers. The main characteristics tested for mechanical behavior (structural reliability and flexural strength) were overall toughness and fatigue limits. Combinations of chemical compositions including yttria and ceria additives and processing conditions including pressing and sintering temperatures were applied to transform custom stereolithography and digitally light activated polymerized green parts to high strength and toughened ceramic crowns. The fluctuations in strength and toughness of as-sintered parts before and after physical thermocycling, physiochemical hydrothermal aging, and mechanical mastication simulation were studied via statistical methods (ANOVA) to indicate variable dependencies (α=.05).

RESULTS

Near theoretical density as high as 99.1%, minimum surface porosity as low as 0.25%, medium translucency, and high contrasts were achieved. The high original hardness near 19 GPa, a toughness of 6 to 7 MPa.m1/2, and 1300 MPa flexural strength with 95% confidence interval in as-sintered specimens satisfied the requirements for crowns. The simultaneously yttria and ceria stabilized systems should be able to resist low-temperature degradation aging with decreases as small as 2% in flexural strength and near 25% in fatigue fracture limits. The structure and process dependency of the mechanical properties of flexural strength (P<.020), hardness (P<.030), and modulus of elasticity (P<.020) were statistically significant while the toughness showed significant dependency (P ≤.001).

CONCLUSIONS

The 3D printed posterior crowns with enhanced mechanical properties and augmented simulated in vitro durability can be manufactured by adding tetragonal phase stabilizer oxides (ceria and yttria) to zirconia-based ingredients. The combination of both oxide stabilizers in the additive manufacturing of crowns is a significant approach to improving clinical performance, enhanced toughness, and fatigue limit before and after physicomechanical, mechanochemical, and thermocyclic aging analysis.

Yousief S, Tammam R, M. Galal R, Brakat A, Mohamed El Sayed H, Kamal A, Noushad M, Nassani MZ. Biomechanics of Central Incisor Endocrowns with Different Lengths and Milled Materials after Static and Vertical Loading: A Finite Element Study

Purpose

The aim of this study was to compare and assess the stress distribution and failure possibility of endodontically treated central incisor protected with endocrowns with different heights, with various CAD-CAM blocks such as IPS e.max CAD, Katana Zirconia, and Zolid Fx Zirconia.

Materials and Methods

A root canal-treated central incisor (plastic model) restored with an endocrown was scanned with a laser scanner to prepare a control model with a CAD software and then transferred to an FEA software. Proposed crown heights were 2, 4, and 6 mm. The model that was duplicated and restored with CAD-CAM blocks, IPS e.max CAD, Katana Zirconia, and Zolid Fx Zirconia were tested as endocrown materials. Bone geometry was simplified to be two coaxial cylinders in all models. Stress distributions under 50 N axial and oblique (with 135° angle from the vertical plane) loading were analyzed. Each model was then subjected to two occlusal loading conditions-the lingual slope of the incisal edge and the junction between incisal and middle thirds. Eighteen runs and calculations were performed to determine the endocrown height and material effect.

Results

The results showed a minor or negligible effect of changing the endocrown material. Increasing endocrown height was shown to reduce stresses and deformations on most of the model components (bone, gutta-percha, periodontal ligament, and endocrown), except root and cement. Differences in deformations and stresses between the two models of 4 and 6 mm were relatively smaller (ranged between 1% and 30%) compared to those between the 2 and 4 mm models (ranged between 10% and 400%).

Conclusions

The material used to fabricate endocrowns did not show considerable effect on the underlying structures. However, the endocrown design (2, 4, and 6 mm height) was shown to affect all components of the studied systems. Increasing endocrown height is recommended for bone, periodontal ligaments, and endocrown body, as it reduces stresses and deformations. On the other hand, it dramatically increases stresses on the root and cement layer. Smaller endocrown sizes represent an acceptable treatment option when there is a healthy periodontal state, while using larger sizes will be more suitable when there is a periodontal compromise with bone loss.

Comparative analysis of endocrown fracture resistance and marginal adaptation: CAD/CAM technology using lithium disilicate vs. zirconia-reinforced lithium silicate ceramics

Aims

The present study aimed to compare the fracture resistance and marginal adaptation of endocrowns fabricated using lithium disilicate (LDS) and zirconia-reinforced lithium silicate 9ZLS) ceramics by the computer-aided design and computer-aided manufacturing (CAD/CAM) technology.

Materials and Methods

In this in vitro experimental study, 24 extracted mandibular first molars received standard endocrown preparation and were randomly assigned to two groups (n = 12) for the fabrication of endocrowns with ZLS (Suprinity) and LDS (IPS e.max CAD) ceramics. After scanning the teeth with a CAD scanner, endocrowns were designed by inLab Software version 15 (inLab SW 15) and prepared by an imes-icore 350i milling machine. The vertical marginal gap of endocrowns was measured under a stereomicroscope at three steps before and after cementation and after thermomechanical cycles. The fracture resistance of specimens was then measured by load application at a 45° angle. Mode of failure was also determined as reparable or irreparable. Data were analyzed using Pearson's correlation test and t-test.

Results

The mean fracture resistance of ZLS endocrowns was significantly higher than that of LDS endocrowns (P = 0.000). The reparability of ZLS endocrowns was zero, while that of LDS endocrowns was 83.33 %. The vertical marginal gap was significantly smaller in ZLS than in LDS endocrowns at all three time points (P < 0.05). Also, the marginal gap increased by cementation and thermomechanical cycles in both groups.

Conclusion

ZLS and LDS endocrowns both showed acceptable vertical marginal adaptation. ZLS had superior marginal adaptation and higher fracture resistance.

Comparison of retention and fracture load of endocrowns made from zirconia and zirconium lithium silicate after aging: an in vitro study

BACKGROUND

This study aimed to compare retention and fracture load in endocrowns made from translucent zirconia and zirconium lithium silicate.

METHODS

Fifty-six intact human maxillary molars after being mounted in acrylic resin, were scanned to acquire biogeneric copies. Specimens underwent standard endodontic treatment and were prepared for endocrown up to 2 mm above the cementoenamel junction. The specimens were randomly divided into two groups of 28, and endocrowns were designed using biogeneric copies and milled from high-translucent zirconia disks (Zr) and zirconium lithium silicate blocks (ZLS). After cementation with dual-cure resin cement, all the specimens underwent thermomechanical aging, and pull-out retention test and compressive test were conducted (14 specimens were used for each test in each group, n = 14), and failure modes in both tests were evaluated.

RESULTS

Independent samples t-test showed significant difference between the retention of Zr (271.5 N ± 114.31) and ZLS (654.67 N ± 223.17) groups (p value = 0.012). Compressive test results were also significantly different between Zr (7395.07 N ± 1947.42) and ZLS (1618.3 N ± 585) (p = 0.002). Failure mode of retention test was primarily adhesive failure at the cement-restoration interface in Zr group and cement-tooth interface in ZLS group. Failure modes of fracture test for Zr group were 7 non-restorable fractures and one restorable fracture while 6 specimens resisted compressive loads up to 8500 N without fracture. ZLS group showed 7 restorable and 7 non-restorable failures.

CONCLUSIONS

Zr endocrowns showed significantly lower retention and higher fracture strength. Both materials seem to be suitable for fabrication of endocrown in clinical setup.

Fracture resistance of resin based and lithium disilicate endocrowns. Which is better? - A systematic review of in-vitro studies

Objectives: The primary objective of this systematic review is to compare the fracture resistance of lithium disilicate (LDS)-based endocrowns and resin-based (RB) endocrowns of in-vitro studies, and the secondary objective is to compare their catastrophic failures.

Materials and Methods: The review protocol was registered in the P ROSP ERO database (CRD42020166201). A comprehensive literature search was done in PubMed, Cochrane, EBSCOhost and Google Scholar using key terms. Only in-vitro studies that compared fracture resistance of LDS-based endocrowns and indirect RB endocrowns in molars were included. Data extraction, risk of bias assessment and qualitative analysis of the included studies were performed.

Results: Five studies were included in this systematic review. The overall risk of bias for the included studies was moderate. Under axial loading, RB endocrowns showed similar fracture resistance when compared with LDS endocrowns. However, they showed better fracture resistance when compared with zirconia reinforced lithium silicate (ZLS) endocrowns. Furthermore, RB endocrowns showed fewer catastrophic failures than LDS-based endocrowns.

Conclusions: RB endocrowns have similar or better fracture resistance and fewer catastrophic failures when compared to LDS-based endocrowns.

Fracture resistance of maxillary premolars restored with different endocrown designs and materials after artificial ageing

PURPOSE

To evaluate the effect of three different designs and two monolithic ceramic materials on the durability and fracture resistance of endocrowns on maxillary first premolars, in comparison to post-and-core crowns.

METHODS

Fifty-six maxillary premolars were endodontically treated and shortened to a level of 2 mm from the cervical line, and randomly categorized into six endocrown groups and post-and-core crown control group (n=8); E1; endocrowns with flat occlusal table (without ferrule), E2; endocrowns with 1.5 mm circumferential ferrule, E3; endocrowns with 1.5 mm buccal ferrule preparation. Two materials were used for endocrowns: zirconia (4YSZ; Z), and lithium disilicate (L). The control group was restored with zirconia posts, and lithium disilicate crowns. All restorations were bonded using Panavia V5 and its respective primers and underwent thermo-mechanical fatigue with a 10 kg dynamic load for 1,200,000 cycles and thermocycling between 5 and 55 °C. Thereafter all survived specimens were loaded to fracture. The results were statistically analyzed using ANOVA and T-Test.

RESULTS

None of the specimens showed any signs of debonding or fracture caused by the fatigue test. The PC control group showed no statistically significant difference in comparison to groups ZE1, ZE2 and LE2 ( p > 0.05 ). However, it was significantly different from groups LE1, LE3, and ZE3 ( p ≤ 0.05 ).

CONCLUSIONS

Preparation designs and materials affected the fracture resistance of endocrowns. The results showed a superiority of the post-and-core crowns,zirconia/lithium disilicate endocrowns with 1.5 mm circumferential ferrule, and zirconia endocrowns with the flat occlusal table.

[Effect of occlusal thickness design on the fracture resistance of endocrowns restored with lithium disilicate ceramic and zirconia]

OBJECTIVE

This study aimed to investigate the effect of occlusal thickness design on fracture resistance of endocrowns restored with lithium disilicate ceramic and zirconia.

METHODS

A total of 24 artificial first mandibular molars were randomly divided into four groups with six teeth in each group as follows: group lithium disilicate ceramic-2 mm (lithium disilicate ceramic, with an occlusal thickness of 2 mm and a retainer length of 4 mm); group lithium disilicate ceramic-4 mm (lithium disilicate ceramic, with an occlusal thickness of 4 mm and a retainer length of 2 mm); group zirconia-2 mm (zirconia, with an occlusal thickness of 2 mm and a retainer length of 4 mm); and group zirconia-4 mm (zirconia, with an occlusal thickness of 4 mm and a retainer length of 2 mm). After adhesive cementation (RelyX Ultimate Clicker), all specimens were subjected to thermocycling (10 000 cycles). The specimens were subjected to fracture resistance testing at a 135° angle to the teeth at a crosshead speed of 0.5 mm·min⁻¹ in a universal testing machine. Data were analyzed with ANOVA and Tukey's HSD test by SPSS 15.0. The failure modes were classified.

RESULTS

The fracture resistances of groups lithium disilicate ceramic-
2 mm, lithium disilicate ceramic-4 mm, zirconia-2 mm, and zirconia-4 mm were (890.54±83.41), (2 320.87±728.57), 
(2 258.05±557.66), and (3 847.70±495.99) N respectively. Group zirconia-4 mm had the highest fracture resistance, whereas group lithium disilicate ceramic-2 mm had the lowest.

CONCLUSIONS

The fracture resistance of molar endocrown with zirconia is higher than that with lithium disilicate ceramic. Increasing the occlusal thickness can improve the fracture resistance but increase the risk of fracture of abutment.

Acoustic emission and finite element study on the influence of cusp angles on zirconia dental crowns

OBJECTIVE

The effect of cusp angle on the load-carrying capacity and failure behaviour of BionZ Diamond zirconia crowns is carried out using experimental and numerical investigations.

METHODS

The experimental program using monolithic crowns were divided into three groups (n = 14) for three cusp angles, 60, 80 and 120 degrees and were tested to failure under the static load. A 2-channel acoustic emission (AE) system was used to monitor the failure process while the piezo sensors were attached to the rigid stainless-steel jig for recoding the AE events. Load-displacement and AE response were simultaneously monitored until failure of specimens. Parametric AE analysis was conducted for the factors such as amplitude, energy released, signal duration and cumulative counts, for each AE signal. Fast Fourier transform (FFT) was conducted to assess the frequency at failure. Linear finite element analysis (FEA) was carried out using commercial software Ansys Workbench 19.1 to present the stress distribution and failure modes. Post-failure surface morphology study was carried out using scanning electron microscopy (SEM) and statistical analysis was performed using Weibull analysis.

RESULTS

All the samples in three different groups have failed at the mid-line, splitting the zirconia crowns into two equal pieces. The load to failure was directly proportional to the cusp angle in crowns; 120° group had the highest load-carrying capacity of 2.93 ± 0.26 kN while 60 and 80° groups had a failure load of 2.46 ± 0.53 and 2.52 ± 0.16 kN, respectively. Parametric AE analysis revealed that the failure was instantaneous and 60-degree samples had higher AE signature. FE analysis showed the crack initiation at the occlusal surface of the crown which is in agreement with the SEM images. A close agreement of results for the load and stress distribution from FEA complemented with the experimental study.

SIGNIFICANCE

Optimisation of cusp-angle could help clinicians to accurately design the monolithic zirconia crown focussing on maximum load-carrying capacity, increasing the restoration life.

Mechanical behavior of endocrowns fabricated with different CAD-CAM ceramic systems

STATEMENT OF PROBLEM

The mechanical behavior of ceramic endocrowns is unclear.

PURPOSE

The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic endocrowns made by using different computer-aided design and computer-aided manufacturing (CAD-CAM) systems.

MATERIAL AND METHODS

Sixty mandibular human molars were endodontically treated, prepared for endocrowns, and divided into 4 groups (n=15) according to the following various ceramic systems: leucite-based glass-ceramic (LC group), lithium disilicate-based glass-ceramic (LD group), glass-ceramic based on zirconia-reinforced lithium silicate (LSZ group), and monolithic zirconia (ZR group). After adhesive bonding, the specimens were subjected to thermomechanical loading and then to fracture resistance testing in a universal testing machine. The failure mode of the specimens was qualitatively evaluated. Three-dimensional FEA was performed to evaluate the stress distribution in each group. Data were analyzed by using a 1-way ANOVA and the Tukey HSD test (α=.05).

RESULTS

Statistically significant differences among the groups were observed (P<.05). The outcomes of the LC, LD, and LSZ groups were similar (1178 N, 1935 N, and 1859 N) but different from those of the ZR group (6333 N). The LC and LD groups had a higher ratio of restorable failures, while LSZ and ZR had more nonrestorable failures. Fractographic analysis indicated a regular failure pattern in the ZR group and irregular failure patterns in the other groups. Three-dimensional FEA revealed similar values and stress pattern distributions among the groups.

CONCLUSIONS

The mechanical performance of monolithic zirconia was better than that of the other ceramic endocrowns considered in this research; however, monolithic zirconia presented a higher rate of catastrophic tooth structure failure.

Endocrown: An Alternative Approach for Restoring Endodontically Treated Molars with Large Coronal Destruction

Rehabilitation of endodontically treated molar still remains a challenge. After endodontic treatment, molars lost their mechanical characteristics. In fact, they became fragile and that is in relation with the removal of pulp and surrounding dentin tissues. Endocrown which is a single partial restoration could be considered as a good alternative for restoring molars having large coronal destruction and presenting endodontic treatment difficulties. Through this work, we discuss the indication and use of endocrown to replace single crowns with intraradicular retention and to present a clinical case report of an endocrown-type restoration, fabricated from lithium disilicate ceramic (IPS e.Max CAD) in a mandibular first molar with extensive coronal destruction.