## 1 Introduction  
Direct restorations as fillings in the primary dentition are the main restorative procedure in children's daily practice [1, 2]. Successful care of the child is not only essential to complete treatments, but also to establish a favorable basis for the patient's future dental care acceptance throughout life; however, it is a constant challenge to the skill and experience of the practitioner, and the current trend is to simplify operative procedures, thus shortening treatment times is the aim [2]. New dental adhesives, which decrease the clinical steps, are becoming the strategy of choice in pediatric restorative dentistry [3, 4]. Despite great advances in adhesive technology in the last few decades, one of the unresolved dilemmas is the deterioration of the resin/dentin union with time, leading to a very short clinical longevity of composite resins [3, 4], a problem caused by the activity of the metalloproteinases (MMPs) of the fundamental substance our organism. Adhesion to dentine substrate is significantly more complicated than bonding to enamel [5, 6]. Dentine contains a higher moisture content and a lower inorganic content, properties that make durable bonding difficult [5, 6]. Additional relevant factors are the chemical and microstructural characteristics of the substrates of the deciduous and permanent dentition [5; 6]. The hybrid layers created on primary teeth are thicker than those obtained on permanent teeth with identical etching time [6]. This suggests that the dentine in deciduous teeth is more acid-etch reactive because of its reduced mineral content [6]. The incomplete infiltration of the resin monomers into previously demineralized dentine would expose the underlying collagen fibrils, which are subject to hydrolytic breakdown, leading to hybrid layer softening and possible reduction of bond strength, which could be more pronounced in temporary dentine [5; 6]. Dentine comprises collagenolytic enzymes, matrix MMPs and cysteine cathepsins [5]. MMPs are a set of zinc- and calcium-dependent enzymes which control natural and pathological processes of collagen-based structures [7]. Various studies have shown that endogenous metalloproteinases, such as collagenases (MMP-8) and gelatinases (MMP-2 and -9), have a role in the development of decay and in altering the hybrid layer that forms in the bonding process [8; 9; 10]. Due to the dentine mineralization process in the contingogenesis stage, they are stored latently in the extracellular matrix, but may be reactivated by demineralization of the dentine [5]. As the inorganic structure around the collagen fibrils in dentine is removed by decay and/or acid etching, the collagen fibrils are uncovered and this manifests the MMPs' activity, initiating the breakdown process [7]. Similar to an enzymatic pathway, the abnormal execution of MMPs might be stopped by the utilization of specific inhibitors, preserving the collagen fibers' structural integrity, and potentially slowing down the breakdown of the hybrid layer [7]. Nowadays, the approaches to enhance the longevity of resin-dentin bonds, and to prevent restoration fails, are based on the capacity to suppress the activity of endogenous enzymes in dentine [11; 12]. Several synthetic inhibitors of MMPs such as EDTA, benzalkonium chloride and chlorhexidine have been proposed. Among others, 2% chlorhexidine (CHX) is a broad-spectrum antiseptic and antimicrobial that bonds to amino acids in dentin and continuously inhibits bacteria over many hours [13]. In addition to its known antiseptic and antimicrobial action, CHX also acts on dentin as a potent inhibitor of MMPs [13]. Thus, a 2% concentration of chlorhexidine digluconate, used as a dentine pretreatment, can inhibit 99% of the collagenolytic activity of metalloproteinases [14; 15; 16].