Effect of thermocycling on flexural strength of dental CAD/CAM ceramics of variable thicknesses and structures: an in vitro study

Introduction

Dental ceramics have evolved to offer restorations with superior esthetic and mechanical characteristics, serving as efficient alternatives to metal-ceramic restorations.¹ These dental ceramics can be categorized based on their fabrication method, composition, firing temperature, and microstructure.² All-ceramic restorations can be fabricated using different methods, including conventional techniques like stacking and sintering, split casting and infusion, and heat- or dry-pressing methods, as well as through computer-aided design and computer-aided manufacturing (CAD/CAM) techniques.^2,3 The homogeneity of ceramic CAD/CAM blocks has notably enhanced the strength of definitive prostheses by reducing crack development and defects compared to conventional ceramic fabrication methods.⁴ Lithium disilicate glass-ceramic (LD) is one of the most common ceramic materials, with its (SiO₂-Li₂O) composition minimizing microcracks and enhancing mechanical properties.^5,6 Previous studies have demonstrated LD’s higher fracture resistance compared to leucite ceramics and improved flexural strength over lithium disilicate-strengthened lithium aluminosilicate glass.^7,8 Leucite glass-ceramic is another option for high-esthetic and translucent all-ceramic restorations, with comparable fracture strength to feldspathic ceramics and resin nano-ceramic.⁹

Lithium silicate CAD/CAM ceramics reinforced with zirconia (ZLS) integrate tetragonal zirconia fillers to improve ceramic strength, making them capable of withstanding occlusal forces.¹⁰ Despite ZLS being challenging to section due to drill blunting,¹¹ its high biaxial flexural strength values have bolstered its utility in fabricating various restorations, including implant-supported molar crowns, occlusal veneers, and endo-crown restorations.¹²

Advanced lithium disilicate (ALD; Li_0.5Al_0.5Si_2.5O₆), comprises lithium disilicate (Li2Si2O5) and virgilite crystals (LiAlSi₂O₆) which form a 0.5-μm-long needle-like shape within a zirconia glass matrix.¹³ Research reporting the mechanical properties of ALD ceramics is scarce.^14,15 One of the published studies showed some positive results regarding the mechanical fatigue behavior of ALD, which is similar to LD but lower than lithium silicate-disilicate and Yttria-stabilized zirconia¹⁴ In contrast, another study reported that ALD had lower fracture toughness¹⁵ as well as lower flexural strengths when compared to LD.¹⁶

The thickness and composition of ceramic restorations have a direct impact on flexural strength and esthetics, where varying restoration thicknesses offer solutions for some clinical challenges.¹⁷ For example, thinner restorations can be used for ceramic veneers of high esthetics and translucency,¹⁸ while thicker ceramics are more suitable for full-coverage restorations.¹⁹ To create a restorative dental material that is highly sustainable, aesthetically pleasing, and safe, all of the material’s qualities must be thoroughly examined and tested.²⁰ Since chewing and biting put occlusal stress on all restorative materials used for tooth restorations, proper flexural strength is considered essential.¹⁶ The maximum stress a material can withstand deformation under load is known as flexural strength.²¹ On the other hand, the minimal and conflicting findings on ALD highlight the need for further studies to comprehensively assess its mechanical properties.^14–16 Additionally, understanding the impact of ceramic thickness and structure can guide the selection of appropriate restoration types for specific dental applications. Thus, this research aimed to evaluate the effect of thermocycling on the flexural strength of four CAD/CAM ceramics of varying thicknesses. According to the null hypothesis, no discernible relationship would be noticed between the flexural strength and the thickness and composition of CAD/CAM ceramics after exposure to thermocycling process.

Methods

Grouping of tested specimens

Four CAD/CAM ceramics of low translucency and A1 shade were examined; advanced lithium disilicate (Cerec Tessera™, Sirona Dentsply, Milford, DE, USA; ALD), zirconia-reinforced lithium silicate (Celtra Duo^®, Sirona Dentsply, Milford, DE, USA; ZLS), lithium disilicate (IPS E.max^® CAD, Ivoclar Vivadent, Schaan, Liechtenstein; LD), and leucite reinforced (IPS Empress^® CAD, Ivoclar Vivadent, Schaan, Liechtenstein; LE) as shown in Figure 1. Each ceramic type included 30 specimens, further categorized into three thicknesses of 0.5-, 1-, and 1.5mm. (n=10 specimens per thickness subgroup). As a result, 120 specimens made up the entire sample size that was evaluated in this study. In order to detect an effect size of 0.42, the total sample was computed using G*Power (Version 3.1.9.4), assuming a 5% alpha error and 80% research power. The least number of specimens required for each group was determined to be 9. However, 10 specimens were included to account for possible problems with the laboratory process.²² As a consequence, the number of subgroups multiplied by the number of members in each subgroup yielded 12 × 10 = 120 specimens as the total sample size.²³

Figure 1. Flow chart representing the study grouping and design.

Result

Table 1 presents the flexural strengths and elastic moduli of the four studied ceramics at different thicknesses. At a thickness of 1.5 mm, ZLS exhibited the greatest flexural strength (mean (SD) = 309.08 (33.49)), while LE showed the most minor flexural strength (mean (SD)= 268.11 (7.48)).

Moreover, at a 1.5 mm thickness, ZLS ceramic required the most significant amount of force to break (mean (SD) = 416.56 (74.55)) in contrast to the same thickness of LE, which required the least amount of force to be broken (mean (SD) = 204.04 (15.66)). Among the 1.5 mm thicknesses, ZLS had the highest Young’s modulus of elasticity (mean (SD) = 82.80 (12.35)), while LE had the lowest (mean (SD) = 82.80 (12.35)).

The 0.5mm thickness across all ceramic groups had the lowest flexural strength, elastic modulus, and forces to break. These values were significantly higher at 1.5 mm thickness when compared to 0.5- and 1-mm thicknesses, ZLS exhibiting the highest values (mean (SD) = 309.08 (33.49), 416.56 (74.55) and 82.80 (12.35)), respectively while the lowest values of 1.5 mm thickness were found among LE samples (mean (SD) = 268.11 (7.48), 204.04 (15.66) and 64.07 (15.81)).

Table 2 and Figure 2 illustrate the association of flexural strength with ceramic type and thickness. The findings indicated significant variations in flexural strength between the materials, with ZLS exhibiting the highest adjusted mean stress (269.49 MPa), followed by LD (260.00 MPa), ALD (242.21 MPa), and LE (230.26 MPa). Additionally, the materials’ thickness was a major factor, with the 1.5 mm thickness demonstrating the highest strength (290.08 MPa) and the 0.5 mm thickness presenting the least strength (205.94 MPa).

Figure 2. Diagram showing the impact of ceramic type and thickness on flexural strength property.

SEM images at ×10000 magnification showed the crystalline structure of ALD, ZLS, LD, and LE specimens (Figure 3). ZLS showed a homogenous crystalline matrix (Figure 3a). At the same time, LD had needle-shaped fine-grained crystals within a glassy matrix (Figure 3b). LE and ALD images showed numerous pores with leucite crystals and lithium aluminum silicate crystals incorporated in a glassy matrix, respectively (Figure 3c and 3d).

Figure 3. SEM representing surface morphology of tested CAD/CAM ceramics at x10000 magnification where (a) ALD; Advanced lithium disilicate (b) ZLS; Zirconia lithium silicate, (c) LD; Lithium disilicate, (d) LE; Leucite reinforced.

Red arrows show the pores among specimens of ceramics.

Discussion

This study assessed the flexural strength of four CAD/CAM ceramic materials of varying thickness and compositions. The results showed that 1.5 mm thickness in the ZLS group was the highest flexural strength. Similar findings were noted in ZLS of 1.5 mm thicknesses in terms of elasticity. The highest strength was noticed among the ZLS specimens, LD, ALD, and LE, respectively, which subsequently required higher force to fracture. Specimens with a thickness of 1.5 mm exhibited significantly greater strength compared to the 0.5 mm samples, which presented the least strength. Thus, the null hypothesis is rejected.

Different thicknesses of ceramic material might yield varying flexural strength values on the ceramic materials. Ceramic materials of less thickness, such as 0.5 and 1mm, are suitable for minimally invasive procedures.³¹ Schweiger J et al.³² investigated three different types of CAD/CAM materials: LE, LD, and 3Y-TCP zirconia at five different thicknesses ranging from 0.4-1.6 mm. The lowest load required to fracture was recorded at the 0.4 mm thickness, while the highest load was required for the 1.6 mm thickness of zirconia, followed by LD and LE ceramics. This is consistent with the current study’s findings, where the 0.5 mm thickness of LE ceramics required the least force to fracture, while the 1.5 mm thickness of ZLS and LD required the highest force to fracture, respectively. The results of the flexural strength test of this study were compared to the readings of the biaxial flexural strength test described by Schweiger J et al.³² due to the lack of similar studies assessing the relation between varying ceramic thickness and the flexural strengths characteristic.

In assessing the relation between the ceramic composition of tested ceramics and flexural strength features, ZLS of all thicknesses exhibited the highest flexural strengths compared to LD and ALD, respectively. Meanwhile, LE ceramics exhibited the least flexural strengths among all tested materials at different thicknesses. This comes in agreement with Attar et al.³³ findings, which reported that zirconia-reinforced lithium silicate ceramics (Vita suprinity) of 2 mm thickness exhibited higher flexural strengths than LD and LE, respectively. Similar findings were stated by Elsaka et al.³⁴ after assessing the flexural strengths and elastic moduli of Vita suprinity and LD of 1.2 mm thickness. The higher strengths and elastic modulus of zirconia-reinforced ceramics are related to the presence of ZrO₂ particles in the glassy matrix as in SEM images, resulting in a higher resistance to crack propagation.^33,34 In contrast, Corade et al.³⁵ showed increased flexural strength of LD specimens of 1.5 mm compared to both ZLS and other zirconia reinforced lithium disilicate ceramics of different manufacturers (Vita suprinity and Rosetta). This might refer to the different methods used in both studies.

The current results showed that LD reported higher flexural strength than ALD and LE ceramics. In agreement with these findings, another study reported that crystalized LD of 1 mm thickness exhibited higher flexural strength compared to ALD after exposure to different firing and glazing protocols.³⁶ Similarly, another study displayed that the highest flexural strength was reported among LD specimens of 3 mm thickness contrasted to ALD.¹⁶ Furthermore, Sonmez et al.³⁷ found that LD specimens of 1.2 mm thickness showed higher flexural strength than those of LE ceramics. These superior properties of LD might be due to the difference in composition, where LD includes a tiny amount of glass phase and lithium disilicate crystals, as shown in SEM.

Strengths of this study included assessing the flexural strengths of the most used CAD/CAM ceramics, varying in thicknesses relevant to fabricating different esthetic restorations such as dental veneers, veneered restorations, and all-ceramic prostheses. Moreover, a thermocycling procedure was applied to simulate an aging process equivalent to 6 months intraorally. Additionally, the study evaluated the flexural strengths and topography of Cerec tessera (ALD) ceramics, a type of ceramic that is relatively new in the CAD/CAM realm and has not been extensively studied in the literature, especially in variable thicknesses.

Despite the strengths, the study has several limitations; one major limitation is that it is an in vitro study, which may not fully represent the complex oral environment. Therefore, further clinical studies are required to assess different restoration designs and a wider range of dental materials to better simulate oral conditions. Moreover, the study only assessed low translucency ceramics, and future studies should consider evaluating different levels of translucency to understand their impact on flexural strengths more comprehensively. Further research is warranted to evaluate the impact of glazing on the mechanical properties of CAD/CAM ceramics following thermocycling-induced aging.

Conclusions

The increase in ceramic thickness significantly impacts flexural strength. A thickness of 1.5 mm was found to be optimum in restoring teeth in the posterior region or subjected to heavy occlusal load. Additionally, the composition of CAD/CAM ceramics has a crucial role in the flexural strength property. Dental ceramics, including zirconia fillers, are more resistant to deformation under masticatory loads than other glass ceramics. This was noted in ZLS ceramics. However, ALD requires further investigations to validate the current findings.