The degree ofmaturation of resin composite is one of the crucial factors controlling theirfinal shade, as well as their surface hardness. That is why using Vickers orknop indenter can provide an indirect evaluation method for the maturation ofresin composite (9). Off course, curing the resin compositepolymerization will continue in slow rate, sometimes a week (10,21),and that is why in the current study the measurements were taken after oneweek.Different CAD/CAMmaterial thicknesses selected in this study represent the average preparationdepths recommended by the manufacturer in different situations (1,5,16).Dual curingwas introduced to insure full polymerization of resin composite luting cementsin areas where light cannot reach with an efficient intensity. Yet, it wasmentioned in literature that auto cure alone was incapable by itself to deliverfull maturation of the resin composite cement in absence of photo initiatedpolymerization (15).Colordifferences can be evaluated visually or by color measurement devices. Mostcolor measurement devices utilized in dentistry use the ?E from theCommission International d’Eclairage CIE (L*a*b*) color system todetermine color differences or changes.
In this scheme, color is measured inthree coordinate dimensions of L*, which represents lightness (fromwhite to black; similar to value), a*, which corresponds to thegreen-red axis (negative value indicates green; positive value indicates red),and b*, which corresponds to the blue-yellow axis (negative valueindicates blue; positive value indicates yellow). Althoughthe used CAD/CAM materials acts as a light barrier during photo initiation of resin luting cements, yet their translucency may play a major role,allowing sufficient delivery of curing wave length to the under lying maturingmaterial. Therefore, a greater degree of conversion leading to harder resinwith a more stable color after polymerization (22,23, 26,35). The intensityof the photo initiation unit may have played an important role, as the markeddifference in the intensity between both QTH and LED light curing units usedmay have contributed in the results of the study. Visiblelight’s wavelength fall in the range between 4/10 to 7/10 micrometer, and CAD/CAMmaterials and resin cements used, can contain a wide range of filler particlessizes that are larger or smaller than this wavelength(29). Uponfalling of light on the surface of filler particle larger than the wavelengthof light, light diffuse occur as a result of its refraction and reflection infinitelyon the surfaces those particles.
A similar phenomenon occurs as a result of theporosity of CAD/CAM materials. The greater the refractive index differencebetween particle size and matrix of either materials, the greater therefraction and reflection of light, which leads to an opaque effect in CAD/CAM materials(30), and that is why the more translucent material (Vita Enamic)didn’t affect the intensity of the curing light as did the less translucent (28)(Lava Ultimate).Dual curedresin luting cements have a two different amines, one to react with benzoylperoxide and another to react with camphorquinone.
The amine that react withbenzoyl peroxide is usually aromatic and more prone to degradation, whereas thecoinitiator in the light polymerization system is usually aliphatic and morechemically stable, tending to cause less color variation (37). Thecolor instability of Dual cured resin luting cements is attributed to thepresence of unreacted benzoyl peroxide. The reaction between benzoyl peroxideand amine is not very efficient, as it depends on the physical encounter ofthese molecules for chemical interaction during polymerization. The mobility ofthe initiators becomes restricted as the cement matures and become more viscous,leaving unreacted radical in the polymer chain. (38)The quality ofmaturation of a resin luting cement is dependent on the use of a compatiblelight curing unit and with a proper light exposure time and intensity that is dictatedby the thickness as well as the reactivity of the photo-initiator. It was foundthat hardness of a resin luting cement decreases as the thickness of CAD/CAMmaterial increases (16,39,40). In our study by increasing thedistance between the surface of the maturing resin composite luting cement andthe photo initiation source, due to increasing the thickness of the CAD/CAM material a dropin the KHN was noticed.
Dual cure resin luting cement was capableof producing higher KHN though out the study which may be attributed to progress in polymerization and maturation of the cement after thelight initiation time has stopped and till the activity of free radicles iscontrolled by the growing highly cross-linked polymeric network (24).The extent of resincement maturation change considerably among products under various curing conditions(27).Clinicians should optimize the materials choice taking into account the curingcharacteristics of the cements.
Upon comparing the effect of differentthicknesses of the two tested CAD/CAM materials, type of resin luting cement,and type of photo initiating devices on the hardness of the resin lutingcement. A drop in KHN was noticed with increasing the CAD/CAM thickness, thisdrop is consistent with other studies(11,12) and also in agreementwith Koch et al. who stated that the ease at which light flows thoughceramics is proportional with their thickness.(13) The thickness ofCAD/CAM material was considered as the primary factor determining the lighttransmittance available for polymerization(14,15).Resultsof the current study uncovered that increasing the opacity as well as thedistance between the light source and the maturing resin luting cement have anegative effect on its ?E and VHN,and this was in agreement with litrature (36).