Resin-based cements play a critical role in the clinical success of indirect restorations, including veneers, inlays, onlays, and ceramic laminate restorations. Their mechanical performance, bond stability, and resistance to intraoral degradation directly influence restoration longevity and long-term clinical outcomes [1,2]. Among the mechanical properties relevant to the clinical behavior of resin cements, flexural strength is particularly important, reflecting a material’s ability to resist tensile and compressive stresses during function and parafunction [3]. In the oral environment, resin cements must withstand complex stress distributions, making the evaluation of their flexural strength essential for predicting clinical performance [4]. Resin cements are available in light-cured, dual-cured, and self-adhesive formulations, each with material-dependent advantages. Light-cured veneer cements offer superior color stability and extended working time, making them suitable for esthetically demanding procedures such as porcelain veneers [5,6]. However, their degree of conversion may be compromised when light transmission is hindered by restoration thickness, ceramic opacity, or limited access for the curing light, potentially reducing mechanical properties [7,8]. Dual-cure resin cements were introduced to address this limitation by combining chemical and light activation to achieve adequate polymerization even in deeper or less translucent regions [9]. Nonetheless, the final mechanical behavior of dual-cure materials still depends on light exposure, with incomplete photoactivation resulting in lower cross-link density and reduced flexural strength [10,11]. Evaluation of flexural strength is commonly performed using three-point bending tests in accordance with ISO 4049, which provides a standardized method for comparing resin-based materials [12]. However, even minor variations in specimen dimensions can influence the calculated flexural strength due to the sensitivity of the bending equation to width and thickness measurements [13]. Some universal testing machines assume idealized specimen dimensions (2 × 2 × 20 mm), which may lead to systematic errors if actual specimen sizes differ. Thus, applying specimen-specific dimensional corrections improves accuracy and ensures valid cross-material comparisons [14,15]. Aging conditions also affect the mechanical behavior of resin cements. Thermocycling is widely used to simulate intraoral temperature fluctuations and induces hydrolytic degradation, plasticization of the resin matrix, and reduction of mechanical properties over time [16,17]. Evaluating flexural strength before and after thermocycling provides a comprehensive understanding of the material’s durability under clinically relevant conditions. The aim of the present study was to evaluate and compare the flexural strength of two light-cured veneer cements and one dual-cure resin cement using standardized three-point bending tests, both before and after thermocycling. Additionally, a correction factor was applied to account for specimen-specific dimensional variability to ensure precise and reliable strength measurements. The null hypothesis tested was that there would be no significant differences in flexural strength among the three resin cements, and that thermocycling would not significantly affect the flexural strength of any material.

Materials and specimen preparation Two light-cured resin cements [Variolink Veneer (Ivoclar Vivadent, Schaan, Liechtenstein) and RelyX Veneer (3M ESPE, Seefeld, Germany)] and one dual-cure resin cement [Variolink II (Ivoclar Vivadent, Schaan, Liechtenstein)] were selected. Bar-shaped specimens (2 mm × 2 mm × 20 mm) were prepared by placing the resin cement into a stainless-steel mold and shaping it between two parallel glass plates covered with transparent Mylar strips before polymerization. Light curing was performed on the top and bottom surfaces of each specimen using three overlapping exposures of 20 s per side. The irradiated sections were aligned so that overlap did not exceed approximately 1 mm of the light-emitting diode (LED) curing tip (Bluephase LED, Ivoclar Vivadent, Schaan, Liechtenstein; 1200 mW/cm²), in order to prevent multiple polymerization of the same area. After removal from the mold, the specimens were finished with silicone carbide abrasive papers of grit sizes 800, 1000, and 1200 (Leco VP 100, Leco Instrumente GmbH, Germany) to remove surface irregularities and edge defects. For each resin cement, 40 specimens were fabricated (total n = 120). All specimens were stored in distilled water at 37°C for 24 h. Following storage, 20 specimens of each cement were tested immediately, while the remaining 20 underwent thermocycling. Thermocycling protocol Thermal aging was performed in a thermocycling device for 5000 cycles between 5°C and 55°C, with a dwell time (e.g., 30 s) and transfer time (e.g., 5–10 s) designed to simulate intraoral temperature fluctuations. This produced six groups (n = 20 each): • Group 1: Variolink Veneer (light-cure) with thermocycling • Group 2: RelyX Veneer (light-cure) with thermocycling • Group 3: Variolink II (dual-cure) with thermocycling • Group 4: Variolink Veneer (light-cure) without thermocycling • Group 5: RelyX Veneer (light-cure) without thermocycling • Group 6: Variolink II (dual-cure) without thermocycling During mechanical testing, all specimens remained immersed in distilled water at room temperature. Flexural strength testing Flexural strength was measured using a three-point bending test in a universal testing machine (MCE 2000ST, Quicktest Prüfpartner GmbH, Langenfeld, Germany), following ISO 4049 recommendations [12]. Each specimen was positioned on two supports with a defined span length, and loaded at the midpoint at a crosshead speed of 0.5 mm/min until fracture. The testing machine software initially calculated flexural strength (σ_machine) assuming a standard bar dimension of 2 × 2 mm. However, since small deviations in specimen width (b) and thickness (d) directly affect the calculated stress, the actual dimensions of each bar were measured with a digital caliper, and a correction factor was applied. Flexural strength in a three-point bending test is defined as: σ=3FL/2bd2, where F is the fracture load (N), L is the support span (mm), b is the specimen width (mm), and d is the specimen thickness (mm). As the testing software used fixed nominal values b₀ = 2.00 mm and d₀ = 2.00 mm, the corrected flexural strength (σ_corrected) was obtained as: σcorrected = σmachine×b0d02/ bd2 = σmachine × 8/ bd2, where σmachine_is the strength value reported by the testing device, and b and d are the individually measured specimen dimensions. This correction ensured that geometric variations among specimens did not bias the mechanical data and allowed for accurate comparisons between groups. Statistical analysis Statistical analyses were performed using IBM SPSS Statistics software (Version 25, IBM Corp., Armonk, NY, USA). The corrected flexural strength values were screened for normality using the Shapiro–Wilk test and for homogeneity of variance using Levene’s test. As the assumptions for parametric testing were satisfied, differences among the six groups (n = 20 per group) were evaluated using one-way analysis of variance (ANOVA). When significant group effects were detected, Tukey’s Honestly Significant Difference (HSD) post-hoc test was applied to identify pairwise differences. Descriptive statistics (mean, standard deviation) were calculated for each group. The level of significance was set at α = 0.05 for all analyses.

The flexural strength values demonstrated clear differences among the six experimental groups. Mean ± standard deviation (SD) values were given in Table 1. Descriptive analysis showed that RelyX Veneer without thermocycling (Group 5) exhibited the highest mean flexural strength (132.90 ± 19.42 MPa), whereas the thermocycled groups of Variolink Veneer (Group 1) and Variolink II (Group 3) showed the lowest mean values (91.08 ± 22.83 MPa and 88.52 ± 24.72 MPa, respectively). A one-way ANOVA revealed a statistically significant difference in flexural strength among the study groups (p < 0.000001). Post-hoc comparisons (Table 2) using Tukey’s HSD test indicated that Groups 1 and 3 were significantly lower than Groups 2, 4, and 5 (all p<0.05). Group 5 also exhibited significantly higher flexural strength than Groups 1, 3, and 6 (all p<0.05). No significant differences were found between Groups 2 and 4 or between Groups 2 and 5 (p>0.05). These results indicate that both the type of resin cement and thermocycling significantly influence flexural strength. Table 1. Descriptive statistics of corrected flexural strength values and overall ANOVA result Group Material / Condition Mean (MPa) SD (MPa) n ANOVA p-value 1 Variolink Veneer + Thermocycling 91.08 22.83 20 p < 0.000001 2 RelyX Veneer + Thermocycling 120.09 23.63 20 3 Variolink II (Dual-cure) + Thermocycling 88.52 24.72 20 4 Variolink Veneer No Thermocycling 118.48 13.00 20 5 RelyX Veneer No Thermocycling 132.90 19.42 20 6 Variolink II (Dual-cure) No Thermocycling 106.29 15.47 Table 2. Full Tukey HSD Pairwise Comparison Matrix (p-values) Groups 1 2 3 4 5 6 1 — 0.0012 0.90 0.0018 0.0001 0.1703 2 0.0012 — 0.0010 0.90 0.3413 0.2802 3 0.90 0.0010 — 0.0009 0.0001 0.0763 4 0.0018 0.90 0.0009 — 0.1826 0.4257 5 0.0001 0.3413 0.0001 0.1826 — 0.0033 6 0.1703 0.2802 0.0763 0.4257 0.0033 — Bold p-values (< 0.05): significant differences; Values > 0.05: no significant difference

This study evaluated the flexural strength of two light-cured veneer cements (Variolink Veneer, RelyX Veneer) and one dual-cure resin cement (Variolink II), with and without thermocycling. The main findings of this study reject the null hypothesis, indicating that both material type and thermal aging significantly affect flexural strength. The pronounced decrease in flexural strength after thermocycling observed in the present study is in line with previous work demonstrating that thermal cycling promotes hydrolytic degradation, microcrack formation, and filler–matrix debonding in resin-based materials, resulting in lower strength and hardness [16,17]. More recent studies have reinforced this concept. Malysa et al. reported that thermocycling significantly reduced the bond strength of self-adhesive resin cements to CAD/CAM ceramics and human dentin, with the magnitude of reduction depending on the cement type [18]. Similarly, Paloco et al. showed that thermocycling decreased microshear bond strength for both light-cured and dual-cure cements luted to lithium disilicate ceramics [19]. Although these studies focused on bond strength rather than flexural strength, they support the notion that thermal aging can compromise the performance of resin cements in multiple mechanical domains. From a material perspective, the superior flexural strength of RelyX Veneer, especially in the non-thermocycled group (Group 5), may be related to its filler system and resin matrix formulation. Recent work has demonstrated that resin composite cements can exhibit enhanced biaxial flexural strength and improved stability when optimized filler loading and curing protocols are used [20]. Benkeser et al. reported that curing mode influenced water sorption, color stability, and biaxial flexural strength of resin composite cements, with light-curing showing beneficial effects on flexural behavior in some formulations [20]. This partially agrees with the present findings, where a light-cured veneer resin cement (RelyX Veneer) exhibited very favorable flexural performance when adequately polymerized. Regarding dual-cure systems, Faria-e-Silva and Pfeifer developed dual-cured resin cements designed for high conversion in the absence of light, reporting good flexural behavior and reduced polymerization stress under optimized formulations [21]. In contrast, in the present study, Variolink II (Groups 3 and 6) showed only intermediate flexural strength values, and its thermocycled group (Group 3) ranked among the lowest. This difference may reflect formulation-dependent variations among dual-cure cements: while some are engineered for robust chemical curing, others remain more dependent on adequate light exposure for optimal network formation [9–11,21]. Thus, our findings are partially in disagreement with the notion that all dual-cure cements uniformly provide superior mechanical performance, and instead support a more material-specific interpretation. The present results also agree with recent work emphasizing the impact of thermocycling on various mechanical properties of resin-based materials. Temizci et al. observed that thermal aging reduced the biaxial flexural strength and hardness of composite-based restorative materials produced by additive and subtractive manufacturing [22]. Alrabeah et al. likewise reported significant reductions in bond strength after thermocycling for different bonding cements, with the degree of degradation depending on the material and surface treatment [23]. Together with these studies, our findings highlight that aging with thermocycling is a critical factor that should be considered when interpreting in-vitro data and extrapolating to clinical performance. In terms of agreement and contrast with similar research, several authors have reported that dual-cure resin cements can show equal or higher flexural strength compared with light-cured systems under some conditions [9,24]. However, other recent studies indicate that light-cured cements may achieve higher degree of conversion and better mechanical properties when sufficient light transmission is available, particularly under thin ceramic veneers [7,24,25]. Our study aligns more closely with the latter, as the light-cured RelyX Veneer, when adequately cured and not thermocycled, provided the highest flexural strength values. In contrast, the dual-cure cement tested (Variolink II) did not outperform the light-cured materials, especially after thermocycling, suggesting that not all dual-cure systems guarantee superior flexural behavior. This in-vitro study has some limitations. First, only one dual-cure and two light-cured veneer cements were evaluated, so the conclusions cannot be generalized to all resin cements. Second, the oral environment includes additional factors such as pH fluctuations, mechanical fatigue, and biofilm activity, which were not simulated here. Third, only one thermocycling regime (5000 cycles, 5–55°C) was used; different cycling numbers or temperature ranges might produce different levels of degradation. Future studies should include a broader range of resin cements and combined aging protocols (thermocycling, mechanical fatigue, pH cycling).

Within the limitations of this in-vitro study, the following conclusions can be drawn: 1. Flexural strength of resin cements is significantly influenced by material type and thermal aging. 2. RelyX Veneer exhibited the highest flexural strength, particularly in the non-thermocycled condition, whereas thermocycled Variolink Veneer and Variolink II showed the lowest values. 3. Thermocycling significantly reduced flexural strength in all tested cements, although the magnitude of reduction was material dependent. 4. The dual-cure resin cement (Variolink II) demonstrated intermediate performance and did not consistently outperform the light-cured veneer cements. Clinical Significance 1. The choice of resin cement can have a substantial impact on the mechanical reliability of indirect restorations. 2. Dual-cure cements should not be assumed to be mechanically superior in all situations; their performance remains highly formulation and curing-protocol dependent.

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