Zirconia and lithium disilicate materials have gained increasing popularity for laminate veneer restorations due to their superior mechanical properties and ease of fabrication using CAD-CAM systems. The null hypothesis of this study posited that there would be no statistically significant differences in VMD and load-to-fracture between CAD-CAM MZLVs and LDLVs manufactured at different thicknesses. However, the study findings reveal a notable distinction in load-to-failure test outcomes between materials, with thickness exerting an influence on this parameter. Regarding VMD, although thickness appears to not affect marginal adaptation, the choice of material significantly influences this aspect. Consequently, the null hypothesis is only partially accepted.
The choice of the maxillary right central incisor was deliberate, as central incisors are among the most prominent teeth in the mouth, often raising significant aesthetic concerns addressed by clinicians [5, 14]. The restoration preparations followed the guidelines provided by manufacturers for veneers as 1.5 mm incisal reduction [13]. The study by Arora et al. [15] and Vaidya et al. [16] concluded based on the result of their studies that the butt joint is the most effective preparation for ceramic veneers if incisal coverage is desired. Consequently, in the present study, a palatal butt joint and labial chamfer finish lines were employed at specified depths. Replica-prepared central incisor teeth were obtained using a 3D printer to ensure standardization, mirroring the methodology employed in the studies conducted by Jurado et al. [5]
The adhesive resin is shielded against repeated exposure to oral fluids through a tight fit between the restorative margins and the tooth structure. This reduction minimizes the occurrence of progressive chemical, mechanical, and physical disintegration, which are known to lead to issues such as recurrent deterioration, microleakage, and the formation of stress concentrations [17, 18]. Ensuring a sufficient marginal fit in laminate veneers holds significant importance [19].
Baig et al. [20], in their systematic review and meta-analysis evaluating the marginal and internal fit of porcelain laminate veneers, included research findings suggesting that zirconia laminate veneers offer improved marginal fit compared to lithium disilicate veneers. The observed effect may be attributed to inherent disparities in the production methods. In the present study, both CAD-CAM materials were fabricated using a 5-axis milling machine, thus eliminating potential discrepancies arising from variations in production methods. In line with the outcomes reported in the studies referenced by Baig et al. [20], the VMD was significantly reduced in MZLVs compared to LDLVs in the present study.
Hasan et al. [21], in their study investigating the marginal chipping of machinable zirconia and lithium disilicate ceramic veneer restorations of various thicknesses (0.3 and 0.5 mm), observed that zirconia veneer restorations exhibited less chipping, resulting in a reduced VMD in comparison to lithium disilicate veneer restorations. Our study yielded similar findings.
The literature describes a broad spectrum of marginal opening values, influenced by factors such as the type of restoration and its location [22]. More precisely, the maximum marginal discrepancy for CAD-CAM restorations has been reported to range between 40 and 90 µm in the literature [23–26]. As indicated by the results of the present study, although the average VMD value is greater in LDLVs compared to MZLVs, it remains within clinically acceptable limits for both materials.
To assess the load to failure of laminate veneers in vitro, various loading angles of 90° [14, 27, 28] and 135° [12] have been suggested. Similar to the study of Saker and Özcan [13], the veneers were positioned at a 90-degree angle to the long axis of the tooth structure to specifically evaluate the horizontal component of the load exerted on the palatal surface of maxillary incisors by mandibular incisors.
Lawson et al. [29] found that the material type significantly influenced crown fracture load in their study, which compared the fracture load of lithium disilicate and zirconia crowns. The lower failure load of LDLV was anticipated due to their lower flexural strength compared to MZLV. Additionally, Yan et al. [30] reported that despite 5Y-Z material exhibiting a higher biaxial flexural strength than lithium disilicate, the fracture load of lithium disilicate bonded to a resin tooth die (18.6 GPa) exceeded that of 5Y-Z.
According to the load-to-failure test results, MZLV showed lower values than LDLV at thicknesses of 0.7 and 1.0 mm, while demonstrating higher values at a thickness of 0.5 mm. While there was no significant difference in load-to-failure values between different thicknesses of MZLVs, LDLVs with a thickness of 0.5 mm exhibited significantly lower results than those with thicknesses of 0.7 mm and 1.0 mm. When comparing the materials, a notable difference in thickness is evident. Although LDLVs with thicknesses of 0.7 mm and 1.0 mm demonstrated higher load-to-failure values, LDLVs with a thickness of 0.5 mm exhibited a significantly smaller difference compared to MDLVs with the same thickness. This variance may be attributed to the increased occurrence of adhesive failure in MDLVs with thicknesses of 0.7 mm and 1.0 mm.
Previous studies [31, 32] have consistently demonstrated a decrease in load-to-failure values as the thickness of LDLVs decreases. The findings of the present study align closely with these established trends in the literature.
The failure mode was impacted by both the type of material and, as expected, the thickness of the material. According to the failure mode analysis in the present study, debonding is identified as the predominant cause of failure for zirconia restorations across all thicknesses, and for lithium disilicate laminate veneers at thicknesses of 0.7 and 1 mm. Conversely, laminate fractures become more prevalent in LDLV at a thickness of 0.5 mm.
A palatal butt-joint finish line was utilized in the present study and no instances of root fracture were observed in any of the materials or thicknesses examined. A meta-analysis of in vitro studies conducted by Da Costa et al. [33] concluded that while there was no statistical difference in ceramic fractures between both preparation types, the butt joint incisal preparation potentially offers greater advantages compared to the palatal chamfer in terms of ceramic fracture incidence and tooth fracture occurrence. The lack of root fracture can be attributed to the findings documented by Da Costa et al. [33].
The authors acknowledge the limitations of correlations between in vitro simulations of intraoral function. Specifically, the simulations conducted in this study did not incorporate thermal or mechanical load cycling. Nevertheless, the data obtained on failure mode and marginal adaptation provide valuable insights into the biomechanical properties of these dental materials. Further, in vivo studies are warranted to ascertain the longevity of zirconia veneers in the oral environment and to investigate their modes of failure.