Influence of curing protocol and ceramic composition on the degree of conversion of resin cement

Abstract Due to increasing of aesthetic demand, ceramic crowns are widely used in different situations. However, to obtain long-term prognosis of restorations, a good conversion of resin cement is necessary. Objective: To evaluate the degree of conversion (DC) of one light-cure and two dual-cure resin cements under a simulated clinical cementation of ceramic crowns. Material and Methods: Prepared teeth were randomly split according to the ceramic's material, resin cement and curing protocol. The crowns were cemented as per manufacturer's directions and photoactivated either from occlusal suface only for 60 s; or from the buccal, occlusal and lingual surfaces, with an exposure time of 20 s on each aspect. After cementation, the specimens were stored in deionized water at 37°C for 7 days. Specimens were transversally sectioned from occlusal to cervical surfaces and the DC was determined along the cement line with three measurements taken and averaged from the buccal, lingual and approximal aspects using micro-Raman spectroscopy (Alpha 300R/WITec®). Data were analyzed by 3-way ANOVA and Tukey test at =5%. Results: Statistical analysis showed significant differences among cements, curing protocols and ceramic type (p<0.001). The curing protocol 3x20 resulted in higher DC for all tested conditions; lower DC was observed for Zr ceramic crowns; Duolink resin cement culminated in higher DC regardless ceramic composition and curing protocol. Conclusion: The DC of resin cement layers was dependent on the curing protocol and type of ceramic.


Introduction
All-ceramic restorations have become popular for excellent aesthetics, color stability, abrasion resistance and biological compatibility 2 . However, to obtain restorations with favorable prognosis, a high degree of conversion (DC) of resin cement should be achieved 1,21 . Incomplete polymerization of the resin cement occurs by decreased energy from the light source through the ceramic material. The amount of this attenuation has been shown to be directly dependent on the composition, thickness, opacity and color of the materials used as restoration 2 .
There are different types of ceramics used in dentistry, with different degrees of translucency cements as a function of light attenuation 21 . The decrease in light intensity can be caused by absorption and scattering of light by physical and structural differences of the restorations 1 . Ceramic restorations are considered optical heterogeneous materials 23 with varying degrees of translucency that can be changed by the thickness, crystal structure, porosity between the layers, possible change in the constitution of between the interface and the ceramic cover 13,14 .
Furthermore, increased thickness more than 2 mm and, consequently, the hardness of resin cements 1,17 . resin cements are related to the energy intensity of the light source, wavelength, time and distance from the energy source in relation to the composite 5,18,30 .
Peutzfeldt and Asmussen 25 (2005) stated that the DC, the extent of crosslinking and the physical and mechanical properties of the composite being cured.
Dual-cure resin cements have been developed under the concept that chemical cure alone could achieve substantial polymerization in areas unreachable by the light source. The light-cured portion should optimize the cure along the exposed cement layer and provide initial stability until the self-curing could mature.
However, it is known that the two curing routes are independent and the rate as to each one cures varies among materials and is rather an inherent aspect of their chemistry. Considering this, we may say that the curing rate of some cements can be more dependent on light exposure than others 20,22,24 .
Among the methods used to determine the DC of composites, the Raman spectroscopy has been shown to be very suitable for being relatively simple, reproducible, noninvasive, and for allowing the use of thin specimens without requiring special preparation 26 .
Some studies have evaluated the DC of resin cements when cured through ceramic slabs 17 . However, there is lack of data on the DC of resin cements when cured through actual crowns, under a simulated clinical setting. This is relevant because under a crown, the cement layer is covered by a varying thickness of ceramic from cervical to occlusal surfaces. Moreover, limited access and the presence of neighboring teeth in the mouth lead clinicians to decide for a curing protocol of the exposed ceramic surfaces, without actually knowing if the DC would be affected. Thus, protocols and different composition of ceramics in the DC of resin cements under a simulated clinical setting.
The following null hypotheses were tested: (1) the curing protocol does not affect the DC of the cements; DC of the cements.

Material and methods
Preparation of the samples

Results
The average results (%) DC and standard deviations of the groups evaluated are shown in   The results showed that the curing protocol 10 on microhardness of resin composites used in class II restorations and showed that the fractional photoactivation promoted superior results than those obtained with single photoactivation. When applying a single, large amount of power as the protocol 1x60, the monomers appear to be activated faster and generate multiple growth centers, which may cause premature polymerization of the composite due to the decrease in mobility of radicals, which, in turn, prevents migration of active free radicals and thus causes a low DC 11,12 .
Conversely, if the reaction process is slower, the resinous material may be capable of remaining in due to the greater mobility that occurs between free Duolink, which also includes Pre-Bond in the bonding procedure.
Because of inherent differences in the formulation and curing mode of the cements, it was not expected that they would have similar DC. However, post hoc test did not show difference between the light-cured (Choice 2) and the dual-cure (Variolink 2) resin cements when used to cement the Zr crowns and cured with the 3x20 s protocol. This can be explained by the rapid crosslinking polymer chain formation of some cements when exposed to light due to the high concentration of photo initiators, which considerably increases the radiation sensitivity, even when this is attenuated by the restorative material 15 . Furthermore, some studies have demonstrated that VL resin cement is very dependent of radiation to enhance optimal DC 15 , and, probably the lower DC could be related to cement. In addition, as previously discussed, the application of Pre-Bond Resin for Choice 2 enhances the formation of free radicals that promotes increased DC 16 .

Conclusions
(1) The curing protocol affected the DC of the resin cements. Superior and more uniform curing was achieved by the 3x20 protocol.
DC of the cements. Higher DC was obtained when the cements were used to cement the lithium dissilicatebased crown.
(3) Duolink always presented higher DC regardless of the ceramic composition and curing protocol.