Patients’ esthetic demands for the highest quality tooth-colored restorative solutions that have the best natural appearance are steadily increasing. At the same time, restorative dental materials and the adhesive cements such as self-adhesive composite resin cements (SACRCs) are subjected to major effects resulting from varying liquids and temperatures during food intake. It is already known that the continuous exposure of the restoration margins to the oral environment reduces the mechanical properties by hydrolytic degradation [1
]. This leads to sorption that may result in swelling of the material, weakening of the polymer network, degradation of the filler matrix composite and, consequently, to secondary caries and hypersensitivity of the teeth at restoration margins [3
]. Apart from the mechanical properties, the embedding of color particles from food may also lead to adverse visual effects with discoloration. Higher sorption appears with increasing storage time, while being unaffected by the storage medium [7
]. When reviewing all fixing composite resins, the interactions with humidity appear to be higher with composite luting materials, such as SACRCs, than for conventional cements [9
]. In spite of this, SACRCs are used very commonly in dental practice to insert restorations due to their positive aspects. The use of SACRCs constitutes a time-saving type of fixing that is less technique sensitive and very simple. SACRCs do not require pretreatment of the hard tooth tissues with adhesive systems [10
]. Compared to conventional composite resins, SACRCs exhibit almost identical compositions in respect of the functional monomers, such as bisphenol-A-diglycidylmethacrylate (Bis-GMA), urethandimethacrylate (UDMA), and hydroxyethylmethacrylate (HEMA), as well as the filler content. However, they contain additional acidic groups (for example, phosphoric acid esters or carboxylate groups). These lead to an improved adhesion to the hard tooth tissues by demineralization of the tooth stump, in contrast to the purely micromechanical adhesion [10
], which is important for a tight marginal seal. A permanent, stable marginal seal without micro-gaps and stability of the color is a key factor for the long-term success of a restoration and is one of the criteria for the currently much used SACRCs. The aforementioned sorption and the potentially resulting micro-gaps, due to liberation of filler particles, may lead to discoloration of the margins, which is, alongside secondary caries, one of the most common causes for a necessary renewal of a restoration [16
]. Soluble color molecules with electrostatic charges consequently move into the lower layers and remain there causing discolorations [3
]. These discolorations can be differentiated as extrinsic and intrinsic discolorations. Extrinsic discolorations are the surface agglomeration of plaque and food particles. They can be fully removed by cleaning and polishing as is performed in oral hygiene treatments [28
]. Intrinsic discoloration, however, is a deeper-reaching discoloration, where dye components enter the deeper layers of polymer-based materials for example, which cannot be removed by polishing [30
]. This leads to permanent discoloration of the materials, additionally increasing with higher surface roughness [31
]. A rate of discoloration (ΔE value) below 3.3, by a spectrophotometer measurement, is generally specified as clinically not detectable [34
]. The medium red wine seems to have the highest discoloration rates after short term storage [35
To date, there is limited information available about the discoloration potential of different SARCs, especially in comparing diverse brands. The aim of this study was to compare the sorption and color stability of 8 SACRCs for one year of storage in the 4 media: red wine, curry solution, cress solution, and distilled water.
The tested null hypotheses were:
all SACRCs showed no impact of storage medium and aging level on sorption,
all SACRCs showed no impact of storage medium and aging level on ∆E, and
∆E/Discoloration rates can be completely removed after final polishing after 365 days storage.
Margin discolorations of restorations may significantly affect the visual appearance in a negative way, particularly for visible restoration margins in the esthetic zone. In these cases, the type and chemical composition of the composite resins plays an important role. In addition, it depends on the restoration accuracy, the cementation technique and the location of the restoration margin. Tooth-colored restorations are mainly used with conventional or self-adhesive composite resins. Due to the acidic carboxyl or phosphate groups, SACRCs generally exhibit more hydrophilic properties than the non-self-adhesive fixing composites [14
]. In these cases, one also needs to review the filler particle size, filler particle content, and the filler type, which all significantly contribute to the sorption and solubility [20
]. In SACRCs the number of organic filler particle matrices decreases in line with the increase in filler particle size. Discolorations by the sorption of Bis-GMA-containing fixing composite resins, like SACRCS, should generally be higher than in materials containing UDMA. The UDMA monomer is said to be less hydrophilic and more resistant to discoloration [22
]. The sorption is consequently higher due to the OH groups in the Bis-GMA monomer, which is also able to transport the dye components into a deeper surface layer [17
]. Few conclusions could be drawn from these results because the exact compositions of some of the SACRCs investigated were not available in detail; the aforementioned statements could not be confirmed with the present material information. BIF, which contains both monomers, exhibited the lowest sorption, followed by CLE with the Bis-GMA monomer. In contrast, the SOC containing UDMA had the highest sorption over the one year storage period. In general, the SACRCs continued to absorb water during the storage period, which can be interpreted from the significant differences between the storage times. Based on the significantly different values between day 180 and day 365, there was evidence of further sorption, even after 180 days of storage. In addition, it was found that the sorption differed significantly with all individual materials, apart from RXU and SET. Presumably, this is due to the small differences in the matrix composition and the difference in the content and size of the filler particles. In this case, further studies that could access the exact filler particle sizes and matrix compositions are necessary. Correlation analyses were not possible due to the lack of some information on the materials. There were also significant differences between the storage media. The difference in polarity and particularly in the pH value of the medium might have been the key factor. In one of the following paragraphs, the pH value will be reviewed in more detail. In this study, the sorption was generally below the permitted ISO standard threshold of 40 µg/mm3
in all SACRCs tested. This value, which is listed in the ISO standard for storage of 14 days, was not reached, even after one year of permanent storage in the aging medium. Importantly, the permanent storage in the liquids does not exactly represent the intraoral situation, considering that during sleep, or in general with patients who breathe a lot through the mouth, the oral cavity often becomes dry. Furthermore, the time of intraoral food and beverages while eating is reduced due to fast swallowing. The first null hypothesis is rejected based on the present results, because both the storage medium and the storage time exhibited a significant effect on the sorption with all SACRCs.
Since the discoloration results from sorption and embedding of dye particles in the matrix, the SACRCs with larger filler particles and higher filler particle numbers should express more noticeable discoloration [19
]. The filler particle type itself also appears to have a significant effect on the discoloration rate. These rates were analyzed in this aforementioned study in composite resin artificial teeth and are not directly comparable [25
]. Furthermore, these results could not be confirmed in this study, because the smallest ΔE values could be analyzed for CLE (66 wt % filler) and BIF (70 wt % filler). In contrast, SMC (69 wt % filler) and SET (65 wt % filler) exhibited the highest rate of discoloration, even though they had similar filler particle sizes. The storage medium itself also had a significant effect on the rate of discoloration of the SACRCs in the present study. This result could also be confirmed for other artificial materials in further studies [25
]. There is also a significant difference between the rates of discoloration of the four media. The highest discoloration in this study was exhibited in red wine, followed by curry solution, cress solution and distilled water. Evidence for the high rate of discoloration for red wine has been found in the past [35
], while none of the studies known to the authors shows a storage period of one year. Compared to a study that investigated discoloration rates of three different veneering composites over 180 days of storage in three discoloring food medium, only the values for red wine can be considered. With ΔE values between 23.02 and 30.30, they were significantly below the ΔE values of 37.53 to 88.04 for the 180 days measurement in this study. This is presumably due to the SACRC materials flow out of the respective cartridges with small bubbles, or the hydrophilic acidic carboxyl and phosphate groups, otherwise they have very similar compositions. The micro pores remain in the material after photo-curing, and presumably the color particles are better able to embed themselves, which leads to higher ΔE values. In this respect, a material that is free of bubbles would be desirable, and would presumably significantly change the results. Compared to the results of the present study, one could also create a potential link to the different pH levels of the medium. Contrary to the ΔE values, red wine showed the lowest pH level, followed by curry solution, cress solution and distilled water. A lower pH level might negatively affect the surface structure and roughness of the materials [5
]. The reduced pH level with red wine is caused by the acids it contains, for example, acetic acid, butyric acid, and lactic acid. Acetic acid can attack polymer material surfaces and thus promote the embedding of color molecules. The alcohol it contains also negatively affects the polymer matrix [37
]. The different pH levels of the storage medium did not influence the ΔE values of the resin materials themselves [38
]. Surface roughness that might be caused by acidic pH levels increases the susceptibility for extrinsic and intrinsic discolorations of dental restorations [31
]. The SACRCs exhibited significantly different ΔE levels, apart from those between SPC and SET. In general, the lowest discoloration was shown with CLE, and the highest with SMC. No correlation could be drawn to the SACRC compositions. All ΔE levels very much depended on the medium and the storage time that was measured. With each measurement time the ΔE levels significantly increased. These results were already confirmed in previous studies [35
]. Therefore, the second null hypothesis is also rejected, because both the storage medium and the storage time exhibited a significant effect on the ΔE levels with all SACRCs.
All measurements were performed using a spectrophotometer in vitro. In Vivo investigations are not possible in this way, because fixing composites resins are subject to constantly changing conditions with the rinsing effect of saliva and varying food products, as well as temperature variations. Moreover, it is not easily possible to remove the materials and automatically analyze them. Only mostly visual or purely subjective measurement methods can be used, which makes it more difficult to compare different materials, and makes it difficult to reproduce. In the present study, the specimens´ surfaces were polished to a high gloss finish. It will not be possible to reproduce this with cervical restoration margins in vivo. The surface is presumably more significantly roughened and could discolor more. In this case, a repolish would be necessary in case of clinically visible discolorations. Clinicians, however, should be aware, that every polishing or repolishing procedure might cause abrasion to the restoration material, the luting materials, as well as the tooth structure itself. Moreover, the specimens were stored for 24 h in the food medium, without any interim cleaning. This does not correspond to the natural conditions, because oral hygiene is generally performed twice daily, which constitutes a type of polishing. This surface cleaning would almost entirely remove extrinsically attached color particles and food components and counteract an intrinsic discoloration [29
], except for less accessible approximal areas. The important esthetic appearance of the margins of inlays, partial crowns and full crowns can be restored by polishing [28
]. The SACRCs should reveal more slowly occurring discolorations under natural conditions than the specimens stored in this study. Restorations may then feature a ΔE value below 3.3 for a longer period, and therefore appear clinically acceptable [34
]. In this study, even the storage in distilled water reached the value of at least 4.4 (SET) for SACRCs before polishing after 365 days, which therefore cannot be regarded as clinically acceptable either. In all materials and storage media, the ΔE levels were between 5.36 (cress) and 32.23 (red wine). Consequently, all specimens showed a clinically unacceptable discoloration after 365 days. These facts are not critical, since storage in a medium up to one year is not realistic. The goal of this study was to extremely age these materials to compare them with respect to the discoloration parameter. The pink color of the polish paste was one limitation of the study. In this case, there was an increase of the ΔE levels between day 365 and after the polish, mainly for the distilled water storage medium. This could be explained by the embedding of the color particles into the small pores of the SACRCs. Further studies should use a neutral polish paste. In general, it was possible to significantly reduce the grade of discoloration in all, but most notably in the seriously discolored specimens with red wine and curry solution. It can be assumed that the discolorations are mostly extrinsic. This could also be shown by a significant formation of plaque on the specimens. However, it was not possible to achieve the ΔE starting levels, because intrinsic discolorations occurred. The storage time was 365 days, which constitutes a very long period and is not comparable to the clinical situation. The third hypothesis, which states that the SACRC discolorations can be fully removed back to the baseline level, can be rejected. The main limitation of the present study was that no control group of conventional composite resin was included and compared. This should be further investigated since the SACRCs could be better classified within all composite resins. With a better knowledge about the discoloration behavior with resultant significant differences between some material brands and storage media, the clinician should carefully select the right SACR for cementation procedure. The margin discolorations of restorations especially for visible margins in the esthetic zone are diet-related. A careful management of a perfect fit of the restoration, the right cementation procedure, as well as continuous check-ups of the restoration margins with timely repolishing of visible staining are essential.