Qualitative SEM analysis of fracture surfaces for dental ceramics and polymers broken by flexural strength testing and crown compression

A novel simplified method for assessing crystal length and crystalline content in dental ceramics

The purpose of this study was to introduce a novel and simple method of evaluating the crystal length and crystalline content of lithium disilicate dental ceramics using images obtained from scanning electron microscopy (SEM) and analyzed with ImageJ (NIH) processing software. Three evaluators with varying experience levels assessed the average crystal length and percentage of crystalline content in four commercial lithium disilicate reinforced glass ceramic materials: IPS e.max (Ivoclar Vivadent), Rosetta SM (Hass), T-Lithium (Talmax), and IRIS CAD (Tianjin). The specimens, prepared from partially crystallized CAD/CAM blocks (3.0 mm3), were fully crystallized and treated with 5% hydrofluoric acid for 20 s prior to SEM analysis. After acquiring the SEM images, ImageJ software was used to evaluate the average crystal length and crystalline content on the surface of the different ceramics. An inter-operator agreement was observed (ICC/p = 0.724), indicating that assessments by the various operators were similar across all ceramic materials tested (p < 0.001). When crystal length and crystalline content were compared, IRIS CAD exhibited significant differences compared to the other materials (p < 0.001), showing a less dense crystalline matrix based on the average length of crystals and the percentage of crystals per unit area. The use of this software facilitated the evaluation of crystalline content and average crystal lengths in dental ceramics using SEM images, and demonstrated very low variability among different operators. RESEARCH HIGHLIGHTS: The described method, using ImageJ open-source software, provides precise and reliable measurements of crystal length and crystalline content in lithium disilicate ceramics, with high inter-operator agreement. The proposed method identified higher crystalline content in IPS e.max CAD compared to Rosetta SM CAD and T-lithium CAD ceramics, while IRIS CAD exhibited significantly lower crystalline content and larger average crystal length. The novel, simplified method for assessing crystal length and crystalline content presented in this study may also be useful for evaluating other dental ceramics.

Mechanical properties of 3D-printed resin denture teeth: An in vitro study

PURPOSE

The purpose of this study was to compare the wear, fracture strength, and mode of failure of various brands of 3D-printed resin denture teeth with prefabricated acrylic resin. Additionally, the study aimed to analyze the different modes of failure exhibited by these teeth.

MATERIALS AND METHODS

The study utilized 90 3D-printed and 30 prefabricated, 3D-printed resin teeth from three brands: L = Optiprint Lumina, A = ASIGA DentaTooth, P = Power resins, along with prefabricated acrylic teeth from M = Major Super Lux. Each of the 30 samples per main group was divided into two subgroups: The first subgroup samples (M1, A1, L1, P1) were subjected to thermal cycling and mechanical loading; M2, A2, L2, and P2 were not aged and tested directly. A scan of a prefabricated acrylic tooth was taken using an intraoral scanner, and then the STL file was printed using an Asiga 3Dprinter. The specimens underwent aging to simulate 5 years of clinical use with 10,000 thermal cycles and 1,200,000 dynamic load cycles on a chewing simulator. Surface roughness parameters (Rz, Ra, Rq) were measured using a 3D Optical Profilometer, fracture resistance was assessed using a universal testing machine, and SEM analysis was performed to observe failure modes. Statistical analysis using T-test, one-way analysis, and two-way analysis processed by the Statistical Package for Social Sciences (SPSS) software version 23.0 (SPSS: Inc., Chicago, IL, USA) was done with a level of significance set at <0.05.

RESULTS

The results showed that the difference in surface roughness parameters (Rz, Ra, Rq) before and after aging for Group M, Group A, Group L, and Group P was statistically significant (p < 0.05). Two-way ANOVA for wear resistance between aging and groups on dependent variable Rz (p = 0.002), Ra (p = 0.001), Rq (p = 0.001) were significant. Multiple comparisons for surface roughness parameters showed Group A and Group L were lower than Group P and Group M (p < 0.05). For fracture strength, One-way ANOVA showed a significant difference between groups for fracture strength either without or after the aging procedure (p < 0.05). Multiple comparisons for fracture strength without aging showed no significant difference between Group M, Group A, and Group L (p > 0.05). After the aging procedure fracture strength for Group M was higher than Group A, Group L, and Group P (p < 0.05).

CONCLUSION

3D-printed resin teeth showed a greater and comparable wear resistance to prefabricated acrylic teeth. Fracture strength was comparable between prefabricated acrylic teeth and 3D-printed resin (Asiga and Lumina) before aging, but after aging 3D-printed resin teeth showed less fracture strength.

A Comparison of Failure Loads for Polycrystalline Zirconia Ceramics with Varying Amounts of Yttria, Glass-Ceramics and Polymers in Two Different Test Conditions

It is unclear how zirconia dental crowns with different yttria compositions will perform clinically, and how they will compare with crowns made of glass-ceramics and polymers. The present objective was to determine failure loads of crowns and discs made of glass ceramics or polymers as compared to yttria-partially stabilized zirconia (Y-PSZ) crowns and discs with varying yttria concentrations. Crowns of zirconia (Cercon XT, Katana UTML, BruxZir Anterior), glass ceramic (Celtra press, IPS e.max press, Lisi press), and polymeric materials (Trilor, Juvora, Pekkton) were fabricated and cemented to epoxy abutments. The total number of specimens was 135 for crowns and 135 for discs (n = 15 specimens per material type and design). A universal testing machine was used to perform compressive loading of crowns/discs to failure with a steel piston along the longitudinal axis of the abutments. Energy dispersive spectroscopy (EDS) was used to identify the yttria concentration for each zirconia brand. The data were analyzed using generalized linear models and regression analyses. The results revealed significant differences (p < 0.05) in mean failure loads for different crown materials: Trilor (6811 ± 960 N) > Juvora (5215 ± 151 N) > Cercon (4260 ± 520 N) = BruxZir (4186 ± 269 N) = e.max (3981 ± 384 N) > Katana (3195 ± 350 N) = Lisi (3173 ± 234 N) = Pekkton (3105 ± 398 N) > Celtra (2696 ± 393 N). The general linear model revealed significant differences (p < 0.05) in mean failure loads when comparing the different materials for the discs, i.e., Trilor (5456 ± 1748 N) > Juvora (4274 ± 869 N) > Pekkton (3771 ± 294 N) > Katana (2859 ± 527 N) > Cercon (2319 ± 342 N) = BuxZir (2250 ± 515 N) = e.max (2303 ± 721 N) = Lisi (2333 ± 535 N) > Celtra (1965 ± 659 N). EDS showed that the zirconia materials contained yttria at different concentrations (BruxZir = 5Y-PSZ, Cercon = 4Y-PSZ, Katana = 3Y-PSZ). The yttria concentration had a significant effect on the failure load of the Katana (3Y-PSZ) crowns, which revealed lower failure loads than the Cercon (4Y-PSZ) and BruxZir (5Y-PSZ) crowns, whose failure loads were comparable or higher than e.max glass ceramic. The failure load of the trilayer disc specimens did not correlate with the failure load of the respective crown specimens for the zirconia, glass-ceramic and polymeric materials.

Relation of Crown Failure Load to Flexural Strength for Three Contemporary Dental Polymers

Polymeric materials show great promise for use in a variety of dental applications. Manufacturers generally provide flexural strength information based on standardized (ISO and ASTM) specimen dimensions and loading conditions. It is not clear, however, if flexural strength data are predictive of the clinical performance of dental crowns. The objectives of this study were, therefore, to determine whether flexural strengths, as measured via three-point bending (3PB), would be predictive of failure loads assessed via crunch-the-crown (CTC) tests. Three brands of polymers (Trilor, Juvora, and Pekkton) were fabricated into rectangular bars and fully contoured crowns (10 specimens of each polymer brand, 30 specimens of each shape). Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and burn off tests were used to characterize/confirm the materials. Bars were tested blindly in 3PB to determine flexural strength, and crowns were CTC-tested to determine failure load after luting to resin abutments. The statistical significance of the test results was evaluated via one-way ANOVA (α = 0.05) and Pearson's correlation coefficient, while regression analysis was used to test for a correlation between 3PB and CTC results. The fracture mechanisms and failure surface characteristics were characterized using scanning electron microscopy (SEM). There were significant differences (p < 0.05) in the mean crown failure loads (Trilor (7033 N) > Juvora (5217 N) > Pekkton (3023 N)) and mean flexural strengths of the bars (Trilor (468 MPa) > Juvora (197 MPa) = Pekkton (192 MPa)). The mode of crown fracture was different between the materials and included deformation (Juvora), ductile-to-brittle fracture (Pekkton), and a combination of cracks and deformation (Trilor). Flexural strengths did not correlate with the corresponding crown failure loads for any of the materials tested. These results suggest that dental practitioners should not rely on the flexural strengths reported from three-point bending tests, as advertised by the manufacturer, to predict the performance of polymeric crowns.