# Influence of Selected Restorative Materials on the Environmental pH: In Vitro Comparative Study
## 1 Introduction  
Acid demineralisation of enamel due to the metabolic activity of bacterial plaque is the first stage of the caries process, leading to the formation of a cavity [1, 2, 3]. The destructed dental tissues are restored with various materials; however, on the contact surface of the tooth and the filling, there is usually a niche that favours the plaque accumulation and may increase the risk of secondary caries. Therefore, it is worth using such restorative materials that may prevent bacterial adhesion or limit the development of bacterial plaque. The pH of the filling seems to be an essential factor influencing the potential bacterial colonisation [4]. Numerous in vitro studies show that at acidic pH, the surface of the composite material is more stable than the surface of glass ionomer cement [4, 5, 6]. Thus far, few researchers have analysed the potential effect of the filling materials on the environmental pH. However, conventional glass ionomer cements (GICs) can buffer lactic acid and release fluoride, which appears to be very beneficial in a clinical approach [7, 8, 9]. It is believed that there are no clear limits to the number of fillings a dentist can perform in one visit. If necessary, it is recommended to perform several fillings in the same area within the conduction anaesthesia. However, other factors can affect the amountof time a patient spends in the dental chair. Most often, apart from the time limitations of the doctor and the patient, the general health condition and ailments related to the temporomandibular joint can pose a limitation. After two hours with an open mouth, it is recognised that there may be irreversible changes in the joint [10]. There are no previous articles on how the pH in the oral cavity changes after tooth restoration depending on the number of fillings. The literature reports the hydrolysis process of fillings under low and high pH conditions in vitro. The carious bacteria fermentation and gastric acid attack are most often simulated by lactic acid and hydrochloric acid, respectively [11; 12]. Both composite materials and glass ionomer cements undergo hydrolysis at acidic pH [4; 13; 14]. However, GICs seem to exhibit more favourable properties than resin-based materials, among others: chemical adhesion to enamel and dentin in the presence of moisture, resistance to microleakage, good marginal integrity, dimensional stability at high humidity, coefficient of thermal expansion like tooth structure, biocompatibility, fluoride release, rechargeability with fluoride, and less shrinkage than resins upon setting with no free monomer being released [15]. In the case of composite materials, the key is proper preparation of the cavity edge, gradual material application, and a meticulous polishing procedure [1; 16]. However, even in vitro, a microleakage can be observed in the most carefully placed composite [17]. Certain limitations can also be seen in the bonding systems. All adhesive systems are somehow unstable and susceptible to hydrolytic degradation, which may be responsible for the partial failure of the material adhesion to the tooth, leading to marginal leakage [18; 19]. The literature shows that resin restoration degradation is a complex process involving the hydrolysis of the resin and the dentin collagen fibril phases within the hybrid layer. Inhibition of the collagenolytic activity, as well as the use of cross-linking agents, are the two main strategies to increase the resistance of the hybrid layer to enzymatic degradation [20; 21]. There is still no answer as to whether the effect of the restorations' pH on the oral environment could be a helpful factor in reducing secondary caries. It is known that despite the continuous modernising of the filling materials, the enamel still has a more significant buffer capacity than the composite or glass ionomer cement [22; 23]. Our study aimed to analyse the potential pH effect of the most used dental materials, i.e., composites and glass ionomers, on the oral environment modelled in vitro.