Even though improvements in oral health have been made, 35% of the global population suffers from untreated caries lesions in permanent teeth [1]. Therefore, caries remains a widespread problem within our population, even though its prophylaxis, treatment and awareness have improved over the past years. Modern treatment of caries is achieved by composite fillings, attached by dental adhesives. The development of dental adhesives can be traced back to the 1950s, when a separate etching step was shown to improve retention of acrylic resin to tooth tissue [2]. Dental adhesives have been evolving ever since, especially towards a user-friendlier approach in workaday life. Nowadays, state of the art self-etch adhesives have several advantages: short application time, fewer steps than etch-and-rinse adhesives and lesser post-operative sensitivity [3].

Most fillings show good long-term performances, though secondary caries is the highest reason for failure in composite fillings [4]. While some failure reasons are within patients’ control, others like the collagen degradation by host-derived proteinases [5] and hydrolysis of the bond [6], are not. The organic matrix of dentin consists of nearly 90% collagen type I [7]. In between collagen, trapped endogenous proteinases are activated by lowering the pH-value in the context of etching [8], [9]. Present in dental hard tissue are matrix-metalloproteinases (MMP), more specifically the gelatinases MMP-2 and ‐9 [10], collagenase MMP-8 [11], stromelysin MMP-3 [12], enamelysin MMP-20 [13] as well as cysteine cathepsins [14]. These enzymes are built within the extracellular matrix during tooth development, mainly degrading collagen matrices, proteoglycans and glycoproteins [10], [12]. Since the presence of proteinases cannot be prevented, inhibiting their activity has drawn more attention during the latest development of adhesives.

Plant-derived agents have caught the researchers’ eyes for their potential collagen crosslinking ability [15] as well as MMP inhibiting effect, while showing low toxicity values [16]. While proanthocyanidin is considered a collagen crosslinker [17], catechins work as such MMP inhibitors [18] and collagen stabilizers [19]. Such catechins are part of green tea (Camellia sinensis) and present excellent biocompatibility, being of a natural source [20]. The predominant catechin in green tea is epigallocatechin-3-gallate (EGCG), performing well in laboratory bond-strength trials [21]. Further catechins are epicatechin-3-gallate (ECG), gallocatechin (GC) and epicatechin (EC) [22]. While others besides EGCG have shown lesser effects on MMP inhibition, a synergistic effect of the combination has been explored [23]. Thus, a green tea extract (GTE), containing all the above mentioned catechins, might be able to inhibit the present MMPs more effectively. Trying to find ways to utilize these natural polyphenols has been an important part of recent research, varying in the ways of applying these MMP inhibitors on dentin from adding an extra step beforehand or incorporating polyphenols within existing bonding agents [24]. State of the art appears to be the addition of an extra pretreatment step, either using an aqueous solution [25], an ethanol/water solution [26] or the ethanol wet bonding approach [27], which in our eyes is contrary to the recent development of adhesive dentistry towards user-friendliness, as an extra pretreatment step means more effort and is thus more time consuming. On the other hand, mixing an existing system with other solutions might impede bond strength values [28]. The novelty in the present work therefore lays in creating experimental adhesive formulations that contain the polyphenol from the beginning in order to utilize its MMP inhibiting and collagen strengthening effect, without adding an extra step to the bonding procedure. One of the experimental reference adhesives consists of bisphenol-A-diglycidyl-methacrylate (bis-GMA), triethylene-glycol-dimethacrylate (TEGDMA) and 2-hydroxyethyldimethylacrylate (HEMA), and the other further contains tricalcium phosphate (TCP) and chitosan. Chitosan has caught attention in endodontology and periodontology, working similarly to ethylenediaminetetraacetic acid (EDTA) by binding calcium [29] and further having anti-plaque as well as anti-bacterial properties [30]. TCP offers calcium and phosphate to stabilize bond strength [31]. Both adhesives were compared to their respective controls without GTE to investigate the effect of polyphenol addition on bonding performance, resulting in four experimental groups in total. Since these are experimental formulations, a comparison is drawn to a two-step self-etch adhesive, resembling a gold-standard control. Long-term shear bond strength (SBS), fractographic behavior and cytotoxicity were assessed. The null hypotheses are therefore that the addition of GTE alone or in combination with TCP and chitosan has no effect on a) dentin bonding, b) fracture mode and reliability as well as c) cytotoxicity.