Does glaze firing affect the strength of advanced lithium disilicate after simulated defects?

Effect of different cavity designs and CAD/CAM blocks on fracture resistance of maxillary premolars with MOD cavities

The aim of the study was to evaluate the fracture resistance of maxillary premolars restored by different CAD/CAM blocks with different MOD cavity designs. A total of 56 maxillary premolars were selected and randomly divided into 4 groups. I: intact teeth as a positive control group. Standardized MOD cavities were prepared in the remaining group specimens. II: teeth had MOD cavities but were left unrestored as a negative control group. III: MOD preparations restored with inlays with no cusp reduction. IV: MOD preparations restored with overlays with cusp reduction. Group III and IV were further subdivided into two subgroups according to material used, i.e., either lithium disilicate or composite CAD/CAM blocks. All specimens were subjected to 5000 cycles of thermocycling and then tested for fracture resistance. Failure patterns were also examined. Data were statistically analyzed using Welch one-way ANOVA followed by Games-Howell's post hoc test. The results showed significant differences among the experimental groups (p < 0.001). The highest fracture resistance value was observed in positive control group, followed by overlays restored with lithium disilicate blocks. This was followed by overlays restored with composite blocks, then inlays restored by lithium disilicate blocks, inlays restored with composite blocks, while the lowest fracture resistance value was found in negative control group. Regarding failure modes, there was a significant difference between different groups (p < 0.001). The conclusion was that fracture resistance of maxillary premolars restored by CAD/CAM inlays and overlays are greatly affected by the cavity design and material type.

Mechanisms of Strength Degradation of Dental Zirconia Due to Glazing: Dependence on Glaze Thickness

Glazing is a common method for smoothing the surface of zirconia and imitating the appearance of natural teeth. Several authors have previously reported that glazing reduces the strength of zirconia. However, the dependence of strength on glaze thickness and the mechanism of strength reduction remains unclear. Clarifying these factors is particularly important for improving the reliability of zirconia prostheses. In this study, three types of zirconia were glazed with various thicknesses, and their strength was evaluated. The results showed that the strength of the materials decreased with increasing glaze thickness. The decrease in the fracture load of the glazed test specimen stopped at a load where the stress necessary to fracture the glaze material was applied to the surface of the glaze layer. Furthermore, the strength reduction mechanism was investigated using FEM analysis, fractography, and Raman spectroscopy. The results suggested that the strength reduction due to glazing was a consequence of the crack-tip stress concentration developed upon the preliminary fracture of the glaze layer.

Fit accuracy and fracture resistance evaluation of advanced lithium disilicate crowns (in- vitro study)

BACKGROUND

Increasing demand for durable and aesthetically pleasing dental restorations, including laminates, inlays, onlays, and crowns, has led to advancements in all-ceramic systems, particularly with the development of advanced lithium disilicate materials. However, limited data on the fit accuracy and fracture resistance of these materials restricts their wider application in clinical restorative practices.

AIM OF THE STUDY

This in vitro study aims to compare the marginal and internal fit, assess the fracture resistance, and evaluate the failure modes of crowns fabricated from advanced and conventional lithium disilicate materials.

MATERIALS AND METHODS

Thirty two (n = 32) crowns were fabricated and categorized into two groups based on the material used: Group (CT), where crowns were milled from CEREC Tessera (n = 16), and Group (EM), where crowns were milled from IPS e.max CAD (n = 16) using a CAD/CAM system. The marginal and internal fit were assessed digitally via a triple scan protocol. All samples were subjected to a fracture resistance test with a universal testing machine, followed by an analysis of failure modes under a stereomicroscope.

RESULTS

In the evaluation of marginal, internal and total gaps, CEREC Tessera (CT) showed slightly better fit with lower gap values compared to e.max CAD (EM). However, an independent samples t-test indicated no statistically significant differences between the two groups (p = 0.141, p = 0.471). For fracture resistance (N), the CT group demonstrated higher values than the EM group; however, the independent samples t-test indicated no statistically significant difference (p = 0.053). Additionally, the Chi-squared test with Monte Carlo correction revealed no statistically significant differences in the modes of fracture between the two groups (p = 0.484).

CONCLUSION

Considering the limitations of this study, advanced lithium disilicate crowns demonstrated better results in terms of marginal fit, internal adaptation, and fracture resistance compared to traditional lithium disilicate crowns; however, the differences were not statistically significant. Both materials exhibited comparable fracture patterns.

Effect of Surface Treatment and Resin Cement on the Bond Strength of an Advanced Lithium Disilicate

OBJECTIVES

 The aim of the study was to evaluate the effect of surface treatment and resin cement on the bond strength of conventional and advanced lithium disilicates (ALDs).

MATERIALS AND METHODS

 Ceramic slices (2 × 13 × 15 mm) of conventional lithium disilicate (LD) (IPS e.max CAD) and ALD (CEREC Tessera) were sectioned, polished, and divided into 16 groups (n = 10) according to the factors: ceramic, surface treatment, and resin cement (Panavia V5 and Variolink Esthetic DC). Surface treatments consisted of hydrofluoric acid 4.9% etching for 20 seconds (Hf20) or 30 seconds (Hf30), self-etching ceramic primer (Se), and sandblasting (Sb). Then, a resin cement cylinder (Ø = 2.5 mm) was manufactured on each specimen's surface. The specimens were then submitted to a shear bond strength (SBS) test. Surface roughness was evaluated through a contact profilometer, and surface morphology was evaluated under scanning electron microscopy for qualitative analysis.

STATISTICAL ANALYSIS

 Two-way analysis of variance (ANOVA) was used to analyze the data of SBS and surface roughness. For bond strength, the effects of surface treatment, resin cement, and the interaction were analyzed for each ceramic. For roughness, analyzed factors include ceramic and surface treatment.

RESULTS

 ANOVA revealed that ceramic (p = 0.047) and surface treatment (p < 0.001) factors affected the bond strength, while the cements performed similarly. Both materials showed adequate bond strength (ALD 19.1 ± 7.7 MPa; LD 17.1 ± 7.9 MPa). Sb protocol showed the lowest mean value (9.6 ± 2.9 MPa) compared with Hf20 (22.0 ± 7.1 MPa), Hf30 (21.7 ± 7.4 MPa), and Se (19.3 ± 6.0 MPa).

CONCLUSION

 For both ceramics, the highest performance was obtained after applying Se and Hf20 or Hf30. Therefore, longer etching time is unnecessary. Sb protocol must be avoided.