In this study, long-term color stability of different amine-free light-cure and dual-cure resin cements was evaluated after aging and long-term color changes of resin cements were observed significantly. The hypothesis was confirmed.
The long-term color stability of resin cement is important for the esthetics of laminate veneers, all-ceramic crowns. The color change of the cement under restorations such as full ceramic and porcelain laminate veneer can be reflected from the restoration, which effects the esthetic appearance of the restoration. Therefore, the cement's color stability is one of the most important clinical factors in the success of such restorations [15,16,17,18].
Adhesive resin cements are used not only for gluing esthetic restorations, but also for the final color of the restoration [17, 18]. Researchers reported that the adhesive resin cement used under ceramic restoration was effective on the final color of the restoration. It should not be considered that only the full ceramic restoration will determine the final color of the restoration to be cemented; It is known that the type and thickness of porcelain used as restorative material on this final color, the color and thickness of the resin cement, as well as the color of the dental tissue under it [19, 20]. Miyasaka et al.  reported that one of the biggest disadvantages of polymeric materials in their studies is that they show color change over time and even restorations need to be renewed due to this change. Terry reported that color discordance is one of the most important reasons for changing the restorations applied to the anterior region .
In resin cements, a low degree of polymer transformation can cause discoloration. Acidic monomers remaining on the tooth surface after etching can react with amines to inhibit the redox reaction of the resin. This reaction depends on the amount of acid and the degree of acidity . Additionally, chemical differences in resin components such as the purity of oligomers and monomers, the concentration and type of activators, initiators, inhibitors, oxidation of unreacted double bonds between carbon and carbon, and fillers can effect the color stability . Differences in color variation between brands may be due to filler particle differences and degradation of polymer material . It is very difficult for the human eye to distinguish small color differences in dental materials. In studies conducted to numerically express the visible color change, there is no exact precision in determining the limit that can be noticed by the human eye [26, 27]. According to some researchers ΔE value that cannot be noticed by the human eye for all-ceramic restorations was 1.6, while Ragain et al. reported the E value as the limit where color difference was not acceptable clinically in their study. ADA (American Dental Association) declared the tolerance limit of ΔE units of color scales as 2 . In the last decade, the detectability and acceptability threshold values for the CIELAB system have been reported as 1.2 and 2.7, respectively, and the detectability and acceptability threshold values for the CIEDE2000 system as 0.8 and 1.8, respectively . For CIEDE2000, the acceptable value of ∆E00 is 1.8, while the noticeability value of ∆E00 is 0.8. The CIEDE2000 formula is better suited for color difference calculations, as it detects even the slightest differences in tooth colors and provides better indications . Janda et al.  showed that the prevalence of coloration depends not only on polymerization time, aging conditions and material, but also on polymerization mode.
The color stability of composites depends on various factors, such as the polymerization mode, polymerization time and the composition of the material. The researchers' results showed that color stability was influenced by polymerization time, aging conditions, and the composition of the material being tested. Despite the same polymerization mode, the same polymerization time application and the use of the same light device, it is thought that the resin cements show a different degree of color change in the study, as researchers pointed out, may be due to the composition of the materials [2, 32].
Turgut and Bagis reported that the color or color coordinate change that occurred after polymerization is related to the color of the material. It was found that the brand and colors of composite resins were significantly effective in color changes during light polymerization. The effects of polymerization time and color tone on color changes of light-curing resin compounds were investigated. Lighter or less chromatic colors showed more color changes than more chromatic or darker colors .
In the studies on color changes of resin composites after polymerization, it has been reported that remarkable color changes occur and the size of the color change varies according to the properties of the material. A statistically significant relationship was found between the colors and brands of composite resins in terms of color change during polymerization with light .
When the studies are examined, it is seen that various aging techniques are used in the evaluation of the color changes of dental materials. These; thermal cycle, water storage, photoaging and accelerated aging . In our study, color change was observed in all resin cement groups after thermal aging. Dual-cure resin cements showed the most color change over time. Magalhães et al. reported that Bis-GMA monomers caused the yellowing of resin cements after exposure to ultraviolet light and heat in the study where cemented porcelain laminate and color change were examined. It has also been stated that, as composite-based materials age, resin monomers can cause changes in color stability due to their water absorption properties .
In some studies, conducted with Variolink II, Variolink Veneer, which were previously produced by the same company and used frequently clinically, with a similar content to Variolink (LC and DC) used in this study, no significant difference was found between LC cements and DC cements in terms of color change [33, 36]. The reason for this difference is that the ratio of light-sensitive molecules in Variolink II resin cement is higher compared to other DC resin cements . DC cements become more reliable in terms of color stability as the proportion of light-sensitive molecules they contain increases. It is also claimed by the manufacturer that the photosensitivity and photo-reactivity of the reaction initiator in the Variolink resin cement used in this study are superior to previously developed systems [12, 37].
In the study where 0.5 mm thick porcelain laminates are cemented with 3 different cements (Rely X Unicem, Rely X Ultimate and Rely X Veneer), the specimens were subjected to accelerated thermal aging and color changes were examined. In the study in which color measurements were made by spectrophotometer 24 h after cementation and after 1000, 2000, 3000 thermal cycles aging, the color difference (∆E) in all specimens increased but there were no statistically significant differences and all of them were in the clinically acceptable range after 3000 thermal cycles (∆E < 3.5) . In this study, a clinically acceptable color change was observed in Rely X (LC and DC) short-term aging (∆E00 < 1.8). Rely X Ultimate (DC) exhibited color change above the clinically acceptable limit while Rely X Veneer (LC) showed a clinically acceptable color change in long term aging (∆E00 < 1.8).
In the study of Kılıc et al. where the porcelain veneers, which were cemented with light cure and dual cure cements, were subjected to rapid aging in the thermal cycle device, examined the color difference that occurred after the light cure cements showed less color changes, found statistically significant differences, but the color change in all cements was clinically accepted. They reported that they were within the limits, therefore, each type of cements was clinically successful . Pissaia et al. investigated the color change of NX3 (LC and DC) resin cement in 6 months and 3 years in their study. They observed that NX3 (LC) cement was above the clinically acceptable limit at 6-months color change, but NX3 (DC) cement was at the clinically acceptable limit. In the long term, they observed that both cements showed high color change (∆E = 3.5) . Parallel results were obtained in this study. It can be attributed to the fact that NX3 (DC) cement contains HEMA, PTU, CHPO that can absorb water in higher ∆E00.
Both intrinsic and extrinsic factors are effective in resin cement coloring. The chemical content of the material as the intrinsic factor, the degree of water absorption, the polymerization type, etc. It is effective. Food, drinks, cigarettes etc. as extrinsic factors. It is effective. Especially esthetically, the coloring of the resin material in the anterior region results in esthetically negative results [2, 24].
Kim et al. reported that color stability is directly related to the resin phase of composite resins . Falkensammer et al. reported that, under changing physicochemical conditions, the color stability of resin composites can be improved by using less water absorption of materials, higher filler-to-resin ratio, reduced particle sizes, and the use of the optimal filler-matrix system .