# Surface Gloss, Radiopacity and Shear Bond Strength of Contemporary Universal Composite Resins
## 4 Discussion  
Under clinical conditions, operators may find it difficult to choose one of the commercially available CR alternatives and may be tempted by the different characteristics of CRs. Surface gloss is a feature similar to natural enamel and should be copied in order to achieve natural tooth form and aesthetics. The marketing strategies of the newly introduced universal CR; GA include highlighting its natural fluorescence, invisible filling and long-lasting gloss properties [20]. Surface gloss is a more clinically detectable feature for clinicians and patients. Gloss indicates the smoothness of the restoration surface [21]. The high gloss of the material is a  
Figure 4: The light microscopy images of failure modes (40\(\times\)): (**a**) adhesive failure; (**b**) mixed failure. CR: composite resin; AI: adhesive interface. The scanning electron microscope images (\(\times\)100 magnification) of failure modes: (**c**) adhesive failure; (**d**) mixed failure. AF: adhesive failure; CF: cohesive failure. Figure 3: Distribution of the failure types on dentin. positive feature, but long-term gloss retention is more important and difficult. The long-term condition can be assessed by water storage/thermal cycle or by exposing to coloring foods such as coffee [10; 22]. Therefore, in the present study, the gloss measurements were repeated after the CR specimens were subjected to darkening by coffee solution. The American Dental Association (ADA) has defined 40-60 GUs as a typically desired gloss [23]. In this study, all the CRs exhibited the GUs. However, significant differences were found among the gloss of the tested CRs. Therefore, the first hypothesis was rejected. Moreover, GA showed superior gloss values compared with other universal CRs either immediately after polishing or after darkening. The present results are consistent with previous studies reporting that CRs with different organic matrix and filler sizes exhibit different properties [24; 25]. All CRs tested in this study have nano-sized particles, but the larger particle size (200 nm) of EA compared with other tested CRs may have led to lower gloss values. On the other hand, GA showed the highest GU. This may be due to the filling technology, high filler load and uniform nano-filler distribution stated by the manufacturer [20]. In the literature color changes have been reported when CRs are exposed to coffee solution [24; 26; 27; 28], and the current study also investigated the effect of coffee on the surface gloss of CRs. [PERSON] et al. [29] have reported that many yellow dyes with low polarity in the content of coffee could diffuse into the organic matrix of the restorative material. Moreover, it has been reported that coffee promotes optical changes on the surfaces of CRs [22]. These findings are consistent with the results of this study, as the glosses of all the tested CRs were decreased after immersion in coffee solution compared with baseline measurements. Radiopacity is one of the five necessities a dental material must meet according to the ADA [30; 31]. CR, having a sufficient radiopacity, allows the clinician to distinguish secondary caries, detect open edges, overhangs, and proximity to dental pulp on radiographs [32]. All five universal CRs tested provided higher radiopacity values than dentin (1.96 mm Al) and 1-mm thick Al, and therefore met the ISO 4049 standard [33]. This finding is a positive feature for the tested materials and means that they cannot be misinterpreted as dental caries on radiography. On the other hand, the radiopacity of EA (69.19 \(\pm\) 4.02, 3.63 mm Al) and the radiopacity of enamel (3.29 mm Al) tissue were similar. Some previous studies have reported that the radiopacity of restorative materials should be slightly higher than enamel or equal to enamel in order for clinicians to recognize secondary caries [34; 35].