This study was conducted at Riyadh Elm University, and the physical lab of King Saud University. The study proposal was registered and received approval from the Institutional Review Board (IRB) at REU before commencing the experiment, and the IRB approval number is FPGRP/2021/676/714/703.
Prior to conducting the test, the researchers used G power software to calculate the required sample size. It indicated that with alpha = 0.05, 0.3 effect size, and a power of 0.9, a total sample size of 80 was needed (20 for each group).
The researchers prepared a total of 80 specimens, with 40 made of LDS and 40 made of ALDS. These specimens were further divided into four groups, each containing 20 specimens, based on the aging method used. The non-aged groups were named LDS-NA (n = 20) and ALDS-NA (n = 20), while the aged groups were named LDS-TCML (n = 20) and ALDS-TCML (n = 20). The aging process involved subjecting the specimens in LDS-TCML and ALDS-TCML to 1,200,000 cycles of thermomechanical fatigue loading (TCML). Subsequently, all the prepared specimens underwent flexural strength testing using a universal testing machine (Fig. 1).
The specifications of the two ceramic materials being studied is shown in Table 1.
Table 1
Glass ceramic materials used in this study
| Manufacturer | Product name | Material description | Composition | Block Dimensions (mm) | Shade | LOT # |
| Ivoclar Vivadent, Schaan, Liechtenstein | IPS e.max® CAD | lithium dislicate glass ceramic LDS | Li2 Si2 O5 | 14x12x18 (C14) | HT A2 | Z034M8 |
| Dentsply Sirona, Hanau, Germany | CEREC Tessera™ | advanced lithium dislicate glass ceramic ALDS | Li0.5Al0.5Si2.5O 6 | 14x12x18 (C14) | HT A2 | 16013131 |
The study followed the specimen dimension standards set by the International Organization for Standardization (ISO 6872, 2015). Bar-shaped specimens were prepared, measuring 12 mm in length, 3 mm in thickness, and 4 mm in width. The preparation was carried out using an IsoMet 11-1280-250 Buehler low-speed precision saw, with a new disk used for each group.
Each specimen's surface was sequentially polished using up to 600-grit silicon carbide paper under water cooling and cleaned in an ultrasonic bath then dried.
For the LDS specimens, crystalization and glazing were performed using a Programat EP 3010 furnace (Ivoclar Vivadent, Schaan, Liechtenstein). As for the ALDS specimens, they were only glazed.
To mimic the masticatory function, a mastication simulator (CH-4 SD, Mechatronik GMBH, Feldkirchen, Germany) was used to simulate the aging experienced during five years of clinical service.19 The simultaneous thermocycling was set at 10,000 cycles between 5ºC and 55ºC for 50 seconds each, with a water change time of 10 seconds.20 The samples and antagonists were mounted in the thermo-mechanical chewing simulator and subjected to a total of 1,200,000 chewing cycles. The load applied was 49 N of force at an input frequency of 1.67 Hz. The antagonist was a Ø6-mm ceramic ball made of steatite ceramic. The force was applied perpendicularly at the center of the specimen.
After the aging process, flexural strength tests were carried out on a universal testing machine (Instron 5965, Instron Corporation, MN, USA) using a 3-point bending test. Specimens were loaded to failure at a crosshead speed of 1 mm/min over a 10-mm span, and the failure load data were recorded. The flexural strength was then determined using the formula: S = 3 × Fmax × L / (2bd)^2, where Fmax is the failure load, L is the support span length, b is the width, and d is the thickness.
Statistical Package for the Social Sciences (SPSS version 26, IBM Corp, USA) was used for stat entry and analysis. Normality was tested using the Shapiro-Wilk and Kolmogorov-Smirnov tests. Two-way ANOVA and independent sample t-tests with 0.0f significance level were used to assess the differences in flexural strength means and the effect of material and aging on flexural strength.