## 1 Introduction  
Today, resin-based direct composite restorations in the posterior teeth have become a well-established and common procedure in dental practice [1; 2; 3]. These restorations meet the demands of the patient and the practitioner for an aesthetic and minimally invasive therapeutic concept. Marginal adaptation followed by secondary caries is still a well-known problem with composite restorations, in particular on the approximal wall of Class II restorations [4]. Insufficient marginal sealing has been correlated with postoperative sensitivity, restoration failure, and secondary caries [5]. Furthermore, the main intention of using flowable composites as cavity liners is to avoid occlusal and particularly approximal leakage.  
However, the effect of flowable composites applied as cavity liners on the long-term success of composite restorations remain inconclusive and no common consensus exists [4; 6; 7; 8; 9; 10]. Despite the limited scientific evaluation, there is an increased use in the general practice of a flowable resin composite as a cavity lining and stress-relieving gingival increment in Class I and II restorations [11; 12; 13]. The flowable composite (Grandio\({}^{\copyright}\)SOHeavy Flow) used in the present study is a flowable nano-hybrid composite with a higher viscosity (Table 1). Due to these properties, it could be used as a flowable composite lining in the posterior teeth instead of a conventional flowable composite when increased strength is required [14]. While some flowable composites in combination with conventional composite materials have already produced promising in vitro results and good clinical experience [1; 15; 16], clinical data for a flowable composite exhibiting a higher viscosity are scarce. Considering the positive in vitro studies of a high-viscosity composite [17; 18], it was the aim of the present study to test the clinical performance of such a material as a cavity liner.  
Therefore, the objective of the study was to determine whether the additional use of a flowable composite in combination with a nano-hybrid composite is suitable for the restoration of Class I and II occlusion-bearing cavities and that the combination of both increased the performance of the restorations.  
The following main hypothesis was stated:  
1. The main objective of this study was to investigate the impact of an additional use of a flowable composite layer (Grandio(r)SO Heavy Flow) in combination with a nano-hybrid composite (Grandio(r)SO) on the clinical success of Class I and II restorations. The objectives of the study were as follows:  
1. The differences in different criteria (secondary caries, tooth vitality, postoperative sensitivity, filling integrity/fracture, proximal contact, surface roughness, marginal adaption, marginal discoloration, and color match) should be identified.
2. Is the dentin-bonding system Futurabond(r) DC (VOCO GmbH, Cuxhaven, Germany) used in the self-etch modus capable of ensuring a long-lasting seal of the fillings in the enamel and dentin over time? Can Futurabond(r) DC effectively prevent postoperative pain?
3. How does the nano-hybrid composite Grandio(r)SO, which has been on the market since 2010, perform in terms of abrasion, shade stability, and surface roughness in occlusal-loaded Class I and II cavities?