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Uncompleted polymerization and cytotoxicity of dental restorative materials as potential health risk factors

Department of Orthodontics, Medical University of Warsaw, Poland
Department of Oral Surgery, Medical University of Warsaw, Poland
Department of Transplantology and Central Tissue Bank, Centre of Biostructure Research, Medical University of Warsaw, Poland
Institute of Chemistry, Faculty of Advanced Technologies and Chemistry, Military University of Technology, Warsaw, Poland
Private Practice, Białystok, Poland
Center for Public Health and Health Promotion, Institute of Rural Health in Lublin, Poland
INTRODUCTION Composite materials used in many areas of modern dentistry are made of organic matrix, inorganic filler, coupling agent/ silane and systems of initiators, catalysts and polymerization inhibitors. Matrix of polymer-based composites consists of basic monomers Bis-A, UDMA, and comonomers such as TEDGMA, EDGMA and HEMA [1]. During polymerization, monomer and comonomer molecules merge to form a spatial network connected to an inorganic filler fraction. Polymerization of most restorative materials is initiated by visible light emitted by lamps. Unfortunately, numerous studies indicate that the cross-linking process of composite materials is incomplete [2]. Incompletely polymerized material contains partially unbounded monomers released directly to the external environment [3]. Incompletely polymerized non-homogeneous material is also more susceptible to physical and chemical degradation [4, 5]. To-date, about 30 [6] chemical compounds emitted from dental materials have been identified, including biologically harmful ones such as TEGDMA, UDMA, EDGMA, HEMA or bisphenol A. Released components of composite materials have cytotoxic [7[, mutagenic [8] properties, and they can be potent allergens [9]. Nowadays, research teams pay special attention to bisphenol A, a cytotoxic compound with biological activity of estrogen group hormones. The wide spectrum of its adverse effects on living organisms is confirmed in many studies. BPA may be responsible for impaired spermatogenesis, oogenesis, of causing disorders of nervous system development in fetuses, and the induction of gland cell cancerous hypoplasia. Thus, the safety of polymer-based dental materials releasing BPA raises doubts, particularly in the treatment of pregnant women. The conversion degree of polymerized materials determines their stability and depends on many factors, including the chemical structure of monomers, effectiveness of photoinitiators, filler type, translucency of material, thickness of the irradiated layer, distance between the light source and polymerized material, light intensity and emission time, and the composition of the surrounding atmosphere [10]. The presence of oxygen-containing air in the oral cavity contributes to the creation of an oxygen inhibition layer on the surface of composites, where the polymerization process occurs to a low degree [11]. Laboratory tests offer optimal conditions for polymerization, thus the values of conversion in the laboratory may be higher than those observed in clinical conditions. The literature reports many different methods for assessing the cytotoxic effects of dental materials. They include tests for estimating the amount of ribonucleic acids and the damage to their chains [12], assessing the glutathione level in cells [13], assessing the expression of heat shock proteins [14], or studies evaluating the severity of apoptotic action [15]. Each of these methods has advantages and disadvantages, but provide extra information that cannot be obtained by other methods. The cytotoxicity of dental materials is commonly tested by evaluating the effect of the studied materials’ eluates on cultured cells. These methods allow imitation of oral cavity conditions, where dental composites remain in constant contact with saliva or fluids consumed by patients, which act as media for potentially harmful biological substances. A common method used to assess the cytotoxic activity of dental materials in tissue cultures is the MTT assay [16, 17]. This test utilizes the ability of an enzyme contained in the mitochondria of living cells to catalyze the reduction reaction of thiazolyl blue formazan to insoluble formazan. The reduced compound is red, and its concentration in the culture is expressed by colour change, which is evaluated using a spectrophotometer. The cells most commonly used in this method are gingival fibroblasts, keratinocytes in oral epithelium, and standardized strains of mouse L -929 or 3T3 fibroblasts