PSI - Issue 28

Vesela Hristova et al. / Procedia Structural Integrity 28 (2020) 1002–1009 Vesela Hristova, Tsanka Dikova, Vladimir Panov / Structural Integrity Procedia 00 (2019) 000–000

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Wedge shaped defects represent loss of dental tissue in the cervical region of the tooth close to the enamel-cement border, without the presence of carious process. They have the shape of a wedge, which they acquire after the loss of the HDT - the gingival and coronary walls meet at right angles with tip, directed to the dental pulp. That is why they are also designated as V-shaped defects. The evolution of this pathology leads to the destruction of the tooth crown, pulpopathosis and periodontitis, accompanied by disturbed esthetic. The problems, associated with wedge shaped defects, are many and varied. Their timely treatment is essential for the pathological processes to be stationed (Hristova et al. (2017). Nowadays the repair of the teeth cervical lesions is a common practice (Perez et al. (2012). The reason may be sought in increasing the age of the population, less tooth loss, as well as an increase in some etiological factors (inadequate techniques in cleaning the teeth, corrosive foods and drinks, bruxism, occlusal stresses). Unfortunately, these are one of the least healthy restorations, characterizing with high index of retention loss, marginal excess and secondary caries. Some of the reasons for these problems are related to the difficulty of isolation, contouring and final polishing procedures. In these cases the choice of restorative material and the appropriate technique are very important. Depending on the location of V-shaped defects - coronary and cervical, glass-ionomer cements (GIC), modified glass-ionomer cements and composite materials are mostly used for their repair (Ichim et al. (2007-1), Ichim et al. (2007-2), Onal B. and Pamir T. (2005). It should be pointed out that for successful obturation of V-shaped defects the features of the tooth/filling system, the characteristics of the materials and techniques used as well as the specificity of the setting process must be taken into account (Dikova et al. (2020). GICs are discovered in the late 1960s by Smith and are implemented widely into the dental practice by Wilson and Kent in 1972 (Lohbauer (2010), Sahu et al. (2018), Singla et al. (2012). The chemical adhesive bond between the GIC and the HDT leads to eliminating the need of processing the cavity to create retentive surfaces. Due to their advantages - biocompatibility, good adhesion to the moistened dental tissues and long-term release of fluorine ions, in some cases GIC are preferred for cavity obturation of temporary and permanent teeth. However, these cements are characterized by low mechanical properties, brittleness and moisture sensitivity, which limits their use as restorative materials. In order to overcome the low physical properties of the conventional GIC and their sensitivity to moisturizing, a new group of resin-modified glass-ionomer cements (RMGIC) has been created. The new generations GIC do not require the use of an additional adhesive system for increasing adhesion strength to the HDT Singla et al. (2012). The higher adhesion strength in RMGICs is due to the penetration of the 2-hydroxyethyl methacrylate (HEMA) monomer into the dentin tubules, providing mechanical adhesion in addition to the chemical one (Sahu et al. (2018), Zhao et al. (2017). RMGICs show poorer restoration strength than the latest generation of adhesive systems, but higher than the conventional GIC. Dental composites are a preferred material for restoration of cervical defects because of their high aesthetic properties. The polymerization process, typical for dental composites, is one of the major factors that affect the success of the composite restorations. Polymerization shrinkage can lead to forming gaps and following microleakage along the cavity/obturation boundary (Rizzante et al. (2019), Senawongse et al. (2010). During the polymerization process and subsequent shrinkage of the material, stresses occur due to a decrease in the flowability, volume shrinkage ratio and the composite hardness. To reduce the stresses due to shrinkage, some authors propose an intermediate elastic layer at the tooth/composite boundary to be created that serves as a buffer and absorbs the deformations (Senawongse et al. (2010). Adhesive systems used can play such a role, as last years layers of flowable composites are added. The effectiveness of these layers for deformations absorption and stresses relaxation has been demonstrated by a number of researchers (Kemp-Scholte C.M. and Davidsson, C.L. (1990), Van Meerbeek et al. (1993). As a result, the microleakage at the boundary of the obturation is reduced (Senawongse et al. (2010), Van Meerbeek et al. (1993). The flowable composites are characterized by low modulus of elasticity, low viscosity and high wettability of dental structures (Hirayama et al. (2014). From clinical point of view, due to their low viscosity, the flowable composites adapt well to the cavity walls. They are intended primarily for deep cavity liners or for filling fissures and V-shaped defects. Since 2000 years, the development of the dental composites leads to transformation from mini-filled to nano-filled and nanohybrid types (Hirayama et al. (2014), Papadogiannis et al. (2008), Dikova T. (2015), Objelean et al. (2016), Schmidt C. and Ilie N. (2012). The inorganic filler of the nanohybrid composites consists of particles with sizes in the mini- and nano-range (5-100 nm), which allows a larger amount of filler to be incorporated into the organic matrix. On the other hand, the use of particles of different sizes provides a more homogeneous distribution of the filler in the matrix. The organic matrix of conventional dental composites consists of Bis-GMA (Bisphenol A glycydil dimethacrylate) and TEGDMA (Triethylene glycol dimethacrylate) (Anusavice K.J. (2003), Dikova T. (2015),