Which dentine analogue material can replace human dentine for crown fatigue test?

The influence of hygroscopic expansion of resin supporting dies on the fracture resistance of ceramic restorations during thermal cycling

OBJECTIVES

To evaluate the hygroscopic expansion characterization of resin composite dies during thermal cycling, and their influence on the fracture resistance of dental ceramic materials as well as the effect of pre-immersion on these measurements.

METHODS

Disc-shaped specimens (φ = 15.0 mm, h = 1.2 mm) and anatomical crown dies of four resin composites (epoxy, Z350, P60, G10) were fabricated. Disc-shaped samples were continuously soaked in distilled water and the volume expansion was measured at different time point by Archimedes method. Disc-shaped samples were pre-immersed for 0, 7, or 30 days, elastic modulus and hardness were measured using Nanoindentation test; thermal cycling (TC) test was performed (5 °C-55 °C, 104 cycles), and volume expansion during TC was measured. Four kinds of resin die with pre-immersion for 0, 7, or 30 days were cemented to 5Y-Z crown, or epoxy dies without pre-immersion were cemented to 5Y-Z, 3Y-Z and lithium disilicate glass (LDG) crowns, and load-to-failure testing was performed before and after TC. Finite element analysis (FEA) and fractography analysis were also conducted.

RESULTS

The hygroscopic expansion was in the order: epoxy > Z350 > P60 > G10. Except for G10, the other three resin composites exhibited different degrees of hygroscopic expansion during TC. Only the elastic modulus and hardness of epoxy decreased after water storage. However, only the fracture loads of 5Y-Z and LDG crowns supported by epoxy dies were significantly decreased after TC. FEA showed a stress concentration at the cervical region of the crown after volume expansion of the die, leading to the increase of the peak stress at the crown during loading.

SIGNIFICANCE

Only the hygroscopic expansion of epoxy dies caused by TC led to the decrease in the fracture resistance of the 5Y-Z and LDG crown, which may be related to the decrease in the elastic modulus of the epoxy die and the tensile stress caused by it.

Erosion Effects of OTC Paediatric Oral Liquids and Traditional Chinese Medicine on Primary Teeth

BACKGROUND

This study compared the in vitro erosive effects of 2 western and 2 Chinese medicinal over-the-counter paediatric medications on primary teeth in a continuous time scale.

METHODS

Twenty enamel blocks obtained from 10 deciduous incisors were randomly assigned into 5 groups (n = 4 each): Group GA and Group GB-paracetamol liquid (Jean-Marie Pharmacal Co. Ltd and Universal Pharmaceutical Laboratories Ltd, respectively); Group GC and Group GD-reconstituted Chinese medicinal granules (Huarun 999 and Guizhou Bailing Group Pharmaceutical Co. Ltd, respectively); and deionised water (control group). The pH levels of these liquids were assessed. The immersion process was repeated for 20 rounds. For each round, each sample was immersed in liquid for 15 seconds and then rinsed with deionised water for 15 seconds. Mean hardness ratio (MHR) at 5, 10, 15, and 20 rounds was evaluated using Vickers microhardness test. The initial and final surface morphology and chemistries were analysed by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS).

RESULTS

All 4 liquid medications have shown acidic pH (4.74-5.76). After 20 rounds of immersion, the MHR20 of the GA (0.886) and GB (0.930) groups decreased significantly from the original MH0. The GC (0.978) and GD (0.985) groups illustrated a similar trend as the deionised water control group (0.985). Despite EDS, chemical analysis showed an increase of Ca/C mole ratio after drug immersion, and no significant difference (P > .05) was found on the ratios Ca/P and Ca/C and the amounts of magnesium or sodium before and after immersion according to the Wilcoxon signed-rank analysis. Distinctive enamel loss with irregular craters, and a corroded surface with fracture lines along the border of prism head were observed in the SEM of groups GA and GB, respectively. However, findings in both Chinese medicine groups were similar, such that erosive changes were barely seen, similar to the control group.

CONCLUSIONS

This study concluded that the pH of liquid medication may not be a significant factor that contributes to enamel softening. Traditional Chinese granule medications may be a safer option for primary teeth.

The Effect of Die Material on the Crown Fracture Strength of Zirconia Crowns

BACKGROUND

Determination of the eligibility of several tooth analog materials for use in crown fracture testing.

METHODS

A standardized premolar crown preparation was replicated into three types of resin dies (C&B, low modulus 3D printed resin; OnX, high modulus 3D printed resin composite; and highest modulus milled resin composite). 0.8 mm zirconia crowns were bonded to the dies and the maximum fracture load of the crowns was tested. Twelve extracted human premolars were prepared to a standardized crown preparation, and duplicate dies of the prepared teeth were 3D printed out of C&B. Zirconia crowns were bonded to both the dies and natural teeth, and their fracture load was tested.

RESULTS

There was no statistical difference between the fracture load of zirconia crowns bonded to standardized dies of C&B (1084.5 ± 134.2 N), OnX (1112.7 ± 109.8 N) or Lava Ultimate (1137.5 ± 88.7 N) (p = 0.580). There was no statistical difference between the fracture load of crowns bonded to dentin dies (1313 ± 240 N) and a 3D-printed resin die (C&B, 1156 ± 163 N) (p = 0.618).

CONCLUSIONS

There was no difference in the static fracture load of zirconia crowns bonded to standardized resin dies with different moduli or between a low modulus resin die and natural dentin die.

Is dentin analogue material a viable substitute for human dentin in fatigue behavior studies?

This study aimed to compare the fatigue performance of a lithium disilicate ceramic cemented on different substrates (human dentin and glass fiber-reinforced epoxy resin - GFRER), treated with different types of conditioning (CTR - without surface conditioning; HF5 - 5% hydrofluoric acid; HF10 - 10% hydrofluoric acid; H3PO4 - phosphoric acid 37%; SAND - sandblasting with aluminum oxide). The occlusal surface of human molars (DENT group) (n = 15) was ground for dentin exposure and the root portion was cut, then the dentin slice (2.0 mm thick) was conditioned with 37% phosphoric acid and a dual-curing dental adhesive was applied. The GFRER in a round-rod format was cut into discs (Ø = 10 mm, 2.0 mm thick). Lithium disilicate glass ceramic blocks (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein) were shaped into a cylinder format and cut, resulting in 90 discs (Ø = 10 mm, 1.5 mm thick). The substrate materials of each group were etched according to the groups and the ceramic was etched with 5% hydrofluoric acid for 30 s. A silane coupling agent was applied over the cementation surface in ceramic and GFRER surfaces and a dual cement was used for cementation (ceramic/GFRER or dentin). The disc/disc sets were submitted to thermocycling (25,000 cycles + storage for 6 months), and then tested in step-wise accelerated cyclic fatigue test. The failure pattern and topography were analyzed and the roughness and contact angle were measured before and after surface treatment. The DENT group presented the lowest load to failure values and number of cycles to failure in fatigue (637.33 N; 118.333), showing no statistical similarity with any of the other tested groups (p < 0.05). The topographic analysis showed that all proposed surface treatments modified the substrate surface when compared to the CTR group. All of the fractographical inspections demonstrated failure by radial crack. Considering the roughness analysis, the post-etched DENT group showed similar roughness to all groups of GFRER materials with their surface treated, except for SAND, which showed greater roughness and statistically different from the other groups. The DENT group (49.5) showed statistically different post-conditioning contact angle values from the HF10 group (96.5) and similar to the other groups. The glass fiber-reinforced epoxy resin was not able to simulate the results presented by the human dentin substrate when cemented to lithium disilicate regarding fatigue failure load and number of cycles for failure, regardless of the surface treatment. Lithium disilicate cemented on dentin analogue overestimates the load values for fatigue failure.

The Effect of Restoration Thickness on the Fracture Resistance of 5 mol% Yttria-Containing Zirconia Crowns

BACKGROUND

To determine what thickness of 5 mol% yttria zirconia (5Y-Z) translucent crowns cemented with different cements and surface treatments would have equivalent fracture resistance as 3 mol% yttria (3Y-Z) crowns.

METHODS

The study included 0.8 mm, 1.0 mm, and 1.2 mm thickness 5Y-Z (Katana UTML) crowns and 0.5 and 1.0 mm thickness 3Y-Z (Katana HT) crowns as controls. The 5Y-Z crowns were divided among three treatment subgroups (n = 10/subgroup): (1) cemented using RMGIC (Rely X Luting Cement), (2) alumina particle-abraded then luted with the same cement, (3) alumina particle-abraded and cemented using a resin cement (Panavia SA Cement Universal). The 3Y-Z controls were alumina particle-abraded then cemented with RMGIC. The specimens were then loaded in compression at 30° until failure.

RESULTS

All 5Y-Z crowns (regardless of thickness or surface treatment) had a similar to or higher fracture force than the 0.5 mm 3Y-Z crowns. Only the 1.2 mm 5Y-Z crowns with resin cement showed significantly similar fracture force to the 1 mm 3Y-Z crowns.

CONCLUSION

In order to achieve a similar fracture resistance to 0.5 mm 3Y-Z crowns cemented with RMGIC, 5Y-Z crowns may be as thin as 0.8 mm. To achieve a similar fracture resistance to 1.0 mm 3Y-Z crowns cemented with RMGIC, 5Y-Z crowns must be 1.2 mm and bonded with resin cement.

Load-bearing capacity under fatigue of bonded-yttria tetragonal zirconia polycrystals and -yttria-stabilized zirconia: Effects of the viscosity of a dual-cured resin cement

This study aimed to evaluate the effect of low and high viscosities of dual-cured resin cement on the mechanical fatigue behavior of yttria tetragonal zirconia polycrystals (3Y-TZP) and yttria-stabilized zirconia (4YSZ) adhesively luted to a dentin analogue (glass fiber-reinforced epoxy resin). Ceramic discs were randomly divided into four groups (n = 20) based on the following study factors: dual-cured resin cement viscosities (low and high) and zirconia microstructure (3Y-TZP and 4YSZ). The discs were treated by air abrasion with aluminum oxide particles (50 μm), followed by the application of primer, and then luted with high or low viscosity resin cement to the dentin analogue. Subsequently, the luted sets underwent a step-stress fatigue test, which involved an initial load of 200 N, step increments of 100 N, 10,000 cycles per step, and a frequency of 20 Hz. Data on fatigue failure load (FFL) and the number of cycles for failure (CFF) were collected and analyzed using survival tests, including Kaplan-Meier and Mantel-Cox analyses, as well as Weibull analysis. Additionally, topography analysis, fractographic features, bonding interface analysis, and Raman spectroscopy were performed. The results revealed that 3Y-TZP exhibited superior fatigue behavior compared to 4YSZ, regardless of the viscosity of the resin cement used for luting. Among all groups, 3Y-Low exhibited the best fatigue performance, while 4YSZ luted with low or high viscosity resin cements yielded the lowest fatigue behavior (FFL). There were no significant differences in Weibull modulus among the groups. After air abrasion, both ceramics showed similar topography. Raman analysis indicated that the surface of 3Y-TZP ceramics prior to sintering had a monoclinic phase, which transitioned predominantly to tetragonal phase peaks after sintering. A similar transition was observed in 4YSZ ceramics. In summary, 3Y-TZP exhibited superior mechanical fatigue behavior compared to 4YSZ. The influence of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. However, the mechanical fatigue behavior of 4YSZ was less affected by the viscosity of the dual-cured resin cement, showing similar results with both low and high viscosities. In conclusion, 3Y-TZP demonstrated superior mechanical fatigue behavior compared to 4YSZ. The impact of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. Conversely, the mechanical fatigue behavior of 4YSZ was less sensitive to the viscosity of the dual-cured resin cement, resulting in similar outcomes with both low and high viscosities.

Mechanical properties of cracked teeth with different dental materials and crown parameters: An in vitro proof-of-concept

OBJECTIVE

This work analyzed and compared the mechanical properties of identical cracked tooth models treated with different materials and crown parameters. Thus, to provide dentists with a more structured way to select materials and geometric parameters and determine the strongest restoration model for cracked teeth.

METHODS

This work used finite element analysis (FEA). We applied 25 restorative models, including five restorative materials, and three preparation parameters. Seven mechanical properties of the cracked tooth preparation were analyzed using correlation analysis.

RESULTS

The highest lifetime of the cracked preparation was obtained for crowns with a 5° of polymerization, width = 0.8 mm, and a length offset of 0.2 mm. The highest lifetime was obtained with ZC crown material, but the least deformation of the cracked tip was obtained with LU material.

SIGNIFICANCE

The results showed that the larger MOE material for the crown and a reasonable increase in the thickness and length of the crown is a favorable method to prevent further cracks to extend. This FEA study, thereby forming a novel basis for clinical guidance as to preparation of dental crowns applicable to cracked teeth.

Fatigue behavior, failure mode, and stress distribution of occlusal veneers: influence of the prosthetic preparation cusp inclinations and the type of restorative material

OBJECTIVE

To evaluate the effects of cusp inclination of the prosthetic preparation's occlusal surface and type of restorative material on the fatigue behavior, failure mode, and stress distribution of occlusal veneers.

MATERIALS AND METHODS

Glass fiber-reinforced epoxy resin prosthetic preparations for occlusal veneers with three different occlusal surface cusp inclination degrees (0°, 15°, and 30°) were produced and assigned into six testing groups (n = 11) according to the cusp inclination (0°, 15°, or 30°) and type of restorative material (lithium disilicate-LD or resin composite-RC). Despite different substrate preparation cusp inclination degrees, the restorations were designed maintaining 30° inclination between the cusps at the occlusal surface and a thickness of 0.7 mm at the central groove region of the restorations to be machined in a CAD/CAM system. After cementation, the specimens were stored for about 7 days (under water at 37 °C), and subsequently submitted to a load to failure test (n = 2) and an intermittent cyclic fatigue test (n = 9) (initial load: 100 N; step size: 50 N; cycles/step: 10,000; loading frequency: 20 Hz; loading piston: 6-mm-diameter stainless steel) until observing cracks. The data were analyzed by two-way ANOVA, Kaplan-Meier, and Mantel-Cox post hoc tests. Finite element analysis (FEA) and fractographic analyses were performed.

RESULTS

The fatigue performance of LD and RC occlusal veneers was evaluated based on different prosthetic preparation cusp inclinations. The 0° inclination showed the best fatigue performance for both materials (LD: 944N, RC: 861N), while the 15° and 30° inclinations had lower values (LD: 800N and 533N, RC: 739N and 717N, respectively). The study also found that for a 0° inclination, LD occlusal veneers performed better than RC ones (LD: 944 N > RC: 861N), while for a 30° inclination, RC occlusal veneers had better fatigue performance than LD ones (LD: 533N < RC: 717N). No significant difference was observed between the materials for a 15° inclination (LD: 800N = RC: 739N). The FEA results showed a higher tensile stress concentration on lithium disilicate than on resin composite occlusal veneers. All lithium disilicate occlusal veneers showed radial crack failures, while resin composite occlusal veneers showed Hertzian cone cracks and radial cracks combined.

CONCLUSION

Considering mechanical perspective only, RC occlusal veneers should be indicated when prosthetic preparation cusps inclinations are 30°. When 0° prosthetic preparation cusps inclinations are observed, LD occlusal veneers will behave mechanically better. When a 15° cusp inclination is preserved, both restorative materials behave similarly.

Fit and fatigue behavior of CAD-CAM lithium disilicate crowns

STATEMENT OF PROBLEM

New computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate glass-ceramics have been marketed. However, information concerning their biomechanical behavior is lacking.

PURPOSE

The purpose of this in vitro study was to compare the fit and fatigue behavior of two recently introduced CAD-CAM lithium disilicate materials with the standard IPS e.max CAD ceramic and to investigate the effect of the thermal treatment for crystallization on crown fit.

MATERIAL AND METHODS

Monolithic crowns (n=15) were milled from 3 CAD-CAM lithium disilicates: IPS e.max CAD (Ivoclar AG), Rosetta SM (Hass), and T-lithium (Shenzhen Upcera Dental Technology). Marginal and internal fit were evaluated using the replica technique before and after crystallization, and the fatigue behavior of the luted crowns was evaluated by the step-stress method. One-way ANOVA and the Tukey test were used to compare fit among the materials. Fatigue failure load was evaluated by the Kaplan-Meier and Mantel-Cox tests. The effect of crystallization on fit was evaluated with the paired t test (α=.05).

RESULTS

Marginal fit was different between IPS e.max CAD (74 μm) and Rosetta SM (63 μm) (P=.02). T-lithium was similar to the other ceramics (68 μm) (P>.05). Occlusal internal space was similar among all materials (P=.69). Fatigue failure loads of Rosetta SM (1160 N) and T-lithium (1063 N) were similar to IPS e.max CAD (1082 N) (P>.05). The fatigue failure load of Rosetta SM was higher than that of T-lithium (P=.04). Crystallization reduced the axial internal space of all materials (P<.05) without significantly affecting marginal fit (P>.05).

CONCLUSIONS

The fit and fatigue behavior of Rosetta SM and T-lithium were similar to that of IPS e.max CAD. Crystallization reduced the internal space of the crowns.