## 4 Discussion  
This clinical study evaluated the clinical performance and effectiveness of a flowable composite as an intermediate stress-breaking cavity liner in posterior cavities combined with a conventional nano-hybrid composite material. This study was conducted in vivo and in a split-mouth design; consequently, the individual differences with regard to risk factors such as oral hygiene, saliva composition, and dietary habits can be almost eliminated [24; 25; 26; 27; 28]. Furthermore, the study was performed in a randomized-controlled, single-blinded manner, thus increasing internal validity [29; 30]. This study showed a recall rate of 94% after 24 months. In comparison with other studies evaluating the 24-month results, this is an acceptable recall rate as demonstrated in  
Figure 8: Plaque index (Silness and Löe) [22; 23]. Figure 9: Gingival index (Silness and Löe) [22; 23]. similar clinical trials (recall rate: 89.9% [18], recall rate: 95% [31]). The overall cumulative survival rate after 2 years, combining all Alpha and Bravo codes, was 96.8% according to the modified USPHS/Ryge criteria, considering both groups with and without the additional use of a flowable composite together. Significant differences in the cumulative success rate between the control group (100%) and the test group (93.6%) could be detected within the observation period. After 24 months, three restorations out of the test group showed Code Delta because of negative tooth vitality and root canal treatment. One of these teeth received an initial Class I restoration and developed an approximal lesion on the mesial aspect of the tooth within the following 24 months leading to irreversible pulpitis with endodontic treatment needs. The second endodontic-treated tooth of another patient underwent endodontic therapy abroad. The third endodontically treated tooth was diagnosed with irreversible pulpitis within the first 24 months. In this case, the possible impact of temperature changes during polymerization might be an explanation for the loss of vitality. It is known that these temperature changes can cause pulpitis [32]. However, in all three cases, endodontic therapy was necessary due to pulpal inflammation after deep caries excavation prior to restoration. In all three cases, indirect pulp capping using a calcium hydroxide liner (Calcicur(r), VOCO GmbH, Cuxhaven, Germany) was performed. The applied restorations showed no secondary caries or marginal failures. Based on these facts, it can be assumed that these three teeth were correctly restored using both of the composite materials. The fact that all teeth were assigned to the test group must be rated as a random incident. Unfortunately, according to our study protocol and the used USPHS/Ryge criteria, the three teeth had to be rated as failures. Comparing the success rates with another randomized clinical trial of the same conventional and heavy flowable composites Grandio(r)SO, no Code Charlie or Delta was detected as well after 24 months (survival rate 100%) [18]. Another clinical trial on composite filling by [PERSON] et al. [31] showed that 92.8% of composite restorations placed with the additional use of a flowable composite as a liner survived compared to a 94.6% survival rate without the flowable composite. These results are in accordance with our findings. Other authors described further clinical layering approaches, which might help to perform quick, predictable, and natural-looking restorations, reducing the need for occlusal adjustments [33]. This is different from our dentin lining using a flowable composite to increase adaption to the dentin and avoid polymerization stress. Among the 94 teeth, 12 indirect capping procedures with a calcium hydroxide preparation were performed [34; 35]. Seven teeth from the control group and five teeth from the test group were indirectly capped. It is remarkable that nine of twelve teeth showed a positive and regular sensitivity after 24 months of observation. Three teeth of the test group had to receive endodontic treatment (see above).