Resistance to Fracture of Lithium Disilicate Feldspathic Restorations Manufactured Using a CAD/CAM System and Crystallized with Different Thermal Units and Programs

Fabrication trueness and internal fit of different lithium disilicate ceramics according to post-milling firing and material type

OBJECTIVES

To evaluate whether post-milling firing and material type affect the fabrication trueness and internal fit of lithium disilicate crowns.

METHODS

A prefabricated cobalt chromium abutment was digitized to design a mandibular right first molar crown. This design file was used to fabricate crowns from different lithium disilicate ceramics (nano-lithium disilicate (AM), fully crystallized lithium disilicate (IN), advanced lithium disilicate (TS), and lithium disilicate (EX)) (n = 10). Crowns, the abutment, and the crowns when seated on the abutment were digitized by using an intraoral scanner. Fabrication trueness was assessed by using the root mean square method, while the internal fit was evaluated according to the triple scan method. These processes were repeated after the post-milling firing of AM, TS, and EX. Paired samples t-tests were used to analyze the effect of post-milling firing within AM, TS, and EX, while all materials were compared with 1-way analysis of variance and Tukey HSD tests (α = 0.05).

RESULTS

Post-milling firing reduced the surface deviations and internal gap of AM and EX (P ≤ 0.014). AM mostly had higher deviations and internal gaps than other materials (P ≤ 0.030).

CONCLUSIONS

Post-milling firing increased the trueness and internal fit of tested nano-lithium disilicate and lithium disilicate ceramics. Nano-lithium disilicate mostly had lower trueness and higher internal gap; however, the maximum meaningful differences among tested materials were small. Therefore, the adjustment duration and clinical fit of tested crowns may be similar.

CLINICAL SIGNIFICANCE

Tested lithium disilicate ceramics may be suitable alternatives to one another in terms of fabrication trueness and internal fit, considering the small differences in measured deviations and internal gaps.

Optical properties, biaxial flexural strength, and reliability of new-generation lithium disilicate glass-ceramics after thermal cycling

PURPOSE

To investigate the color stability, translucency, biaxial flexural strength (BFS), and reliability of nano-lithium disilicate and fully crystallized lithium disilicate after thermal cycling and to compare with those of a commonly used lithium disilicate.

MATERIALS AND METHODS

Three lithium disilicate glass-ceramics were used to prepare disk-shaped specimens (ø:12 mm, thickness: 1.2 mm) from A2 shaded HT blocks (Amber Mill, AM; Initial LiSi Block, IN; IPS e.max CAD, EX). AM and EX specimens were crystallized, and all specimens were polished with a polishing paste (Diamond Polish Mint). A spectrophotometer (CM-26d) was used to measure color coordinates before and after thermal cycling. BFS test was performed after thermal cycling. Color differences (ΔE00 ) and relative translucency parameter (RTP) values were calculated. One-way analysis of variance (ANOVA) (ΔE00 and BFS), two-way ANOVA followed by Tukey's HSD tests (RTP), and chi-square tests (Weibull modulus and characteristic strength) were used for the statistical analyses (α = 0.05).

RESULTS

No significant differences were observed among the ΔE00 values of tested materials (df = 2, F = 2.933, p = 0.070). RTP values were only affected by material type (p < 0.001) as AM had the highest RTP (p < 0.001), whereas IN and EX had similar values (p ≥ 0.165). BFS values varied among tested materials (df = 2, F = 21.341, p < 0.001). AM and EX had similar BFS values (p = 0.067) that were higher than that of IN (p ≤ 0.001). Weibull moduli of the materials were similar (p = 0.305), whereas EX had the highest and IN had the lowest characteristic strength values (p < 0.001) CONCLUSIONS: Although nano-lithium disilicate had the highest translucency, all materials had imperceptible color and translucency changes after thermal cycling when reported threshold values were considered. Newly introduced lithium disilicate glass-ceramics had adequate flexural strength as compared to the precursor material.

Translucency, color stability, and biaxial flexural strength of advanced lithium disilicate ceramic after coffee thermocycling

OBJECTIVE

To compare the color stability, translucency, and biaxial flexural strength (BFS) of differently glazed advanced lithium disilicate (ALDS) with those of lithium disilicate (LDS) and zirconia-reinforced lithium silicate (ZLS) after coffee thermocycling.

MATERIALS AND METHODS

Forty disk-shaped specimens were prepared from three lithium silicate based materials (CEREC Tessera, ALDS; IPS e.max CAD, LDS; Vita Suprinity, ZLS). ALDS specimens were divided into two subgroups according to glazing procedures (reduced glaze duration, ALDS-S and normal glaze duration, ALDS-N), while LDS and ZLS specimens were crystallized and glazed. Color coordinate measurements were performed before and after coffee thermocycling. Color differences (ΔE₀₀ ) and relative translucency parameters (RTP) were calculated. Specimens were then subjected to BFS test. Statistical analysis was performed by using 1- (ΔE₀₀ and BFS) and 2-way (RTP) ANOVA tests (α = 0.05).

RESULTS

ΔE₀₀ values of tested materials were similar (df = 3, F = 0.150, p = 0.929). Two-way ANOVA showed the significant effect of material type, coffee thermocycling, and the interaction between these parameters on RTP values (p < 0.001). Both before and after thermocycling, LDS had the highest (p ≤ 0.001) and ZLS had the lowest (p < 0.001) RTP values, while ALDS-N had higher RTP than ALDS-S (p ≤ 0.001). Among tested materials, only LDS had similar RTP values before and after thermocycling (p = 0.865) as the other materials had lower RTP values after thermocycling (p < 0.001). ALDS-N had higher BFS values than ALDS-S (p = 0.005), while LDS had similar values to ALDS specimens (p ≥ 0.201). ZLS had the highest BFS (p ≤ 0.007).

CONCLUSIONS

ALDS had comparable values to those of other materials. However, reduced glazing duration resulted in decreased translucency and BFS of ALDS.

CLINICAL SIGNIFICANCE

ALDS may be an appropriate restorative material for those patients with increased coffee consumption considering its color stability and ability to maintain translucency, particularly when glazed by using a conventional porcelain furnace.

Fracture Resistance of Sintered Monolithic Zirconia Dioxide in Different Thermal Units

The purpose of this study was to compare the fracture strength of monolithic zirconium dioxide subjected to a sintering process in two different furnaces: InFire HTC Speed and CEREC SpeedFire. Methods: Forty restorations were designed and machined using a computer aided design / computer aidded machine (CAD/CAM) system. The restorations were randomly assigned to two groups of 20 samples each, Group 1 for the SpeedFire furnace (fast sintering) and Group 2 for the InFire furnace (slow sintering). Each of the crowns was subjected to a maximum compression load recorded in Newtons (N) and a displacement control with a speed of 1 mm/min. Results: Group 1 presented an average of 1222.8 N and a standard deviation of 136.91 N. Group 2 obtained a mean of 1068.5 N and a standard deviation of 316.39 N. Conclusions: There were no significant differences between the two groups, and the mechanical strength of the material was not affected, which would imply a saving of clinical and laboratory time when performing rapid sintering on monolithic translucent zirconium dioxide restorations. However, rapidly sintered restorations have limited reliability.