Resin-based luting agents (RBLAs) with tuned elastic moduli ( E ) were prepared and their influence on the strengthening, reliability, and mode of failure of luted feldspar ceramic was investigated.
RBLAs with low E (2.6 GPa), intermediate E (6.6 GPa), and high E (13.3 GPa) were prepared and used to coat acid-etched ceramic disks. Positive (untreated ceramic) and negative (acid-etched ceramic) control groups were tested. The response variables (n = 30) were biaxial flexural strength ( σ bf , MPa), characteristic strength ( σ 0 , MPa), and Weibull modulus at the ceramic surface (z = 0) and luting agent surface (z = −t 2 ). A 3D finite element analysis simulated the biaxial flexural test. Fractographic analysis and morphology of the bonded interfaces were analyzed using scanning electron microscopy.
The RBLAs improved σ bf and σ 0 at z = 0, particularly those with intermediate and high E , whereas the mechanical reliability was only affected in the negative control. At z = −t 2 , differences between all RBLAs were observed but the structural reliability was independent of the RBLA tested. Increasing E of the RBLA was associated with increased stress concentration at the RBLA and reduced stresses reaching the ceramic. Failures originated on the ceramic surface at the ceramic-cement interface. In the high E group, failure sometimes originated from the RBLA free surface. All RBLAs completely filled the ceramic irregularities.
Increased E of the RBLA reduced the variability of strength, the stress reaching the ceramic structure, and sometimes altered the origin of failure. The use of high E RBLAs seems beneficial for luting feldspar ceramics.
Bonding intrinsically fragile feldspar ceramics to dental structures using resin-based luting agents strengthens the ceramic restorations . The strengthening effect is explained by mechanisms such as ceramic crack healing by polymer infiltration and induction of crack closure stress by polymer shrinkage upon curing . The extent of strengthening is dependent on the formation of a proper interpenetrated polymer-ceramic interphase and has been associated with the Young’s modulus of elasticity ( E ) of the luting agent .
The role of the luting agent in transferring stresses from load-bearing restorations to the underlying tooth structures is still not fully elucidated. It is a general belief that resin-based luting agents should have a similar E to tooth structures. Whereas human dentin is considered moderately resistant to deformation ( E ∼ 13 GPa) , commercially-available resin-based luting agents usually have lower stiffness, with E ranging from 1.2 to 16.5 GPa . Feldspar ceramics, in contrast, are brittle and have higher stiffness ( E ∼ 70 GPa) . Therefore, it is debatable which should be the most appropriate E of the luting agent to allow effective stress transfer from the ceramic to the tooth abutment. This topic is particularly relevant considering that very thin ceramic laminate veneers have been extensively used in restorative dentistry lately.
The few studies assessing the effects of elastic properties of resin-based luting agents on ceramic strengthening indicated that a higher E of the resin-based luting agents is favorable for ceramic strengthening . However, these studies used commercial luting agents with distinct E . The shortcoming of testing commercial materials is that their formulation is not thoroughly known. The content of methacrylate monomers, initiators, shape and composition of filler particles, for instance, is not controlled and may vary between proprietary materials. Therefore, factors other than the E of commercial resin-based luting agents might affect the ceramic strengthening when different materials are tested.
The aim of this study was to use different methacrylate monomers combinations to prepare experimental resin-based luting agents with tuned elastic moduli and investigate their influence on the ceramic strengthening, reliability, and mode of failure of luted feldspar ceramic. The hypothesis tested was that resin-based luting agents with higher E would improve ceramic strengthening.