HIGH VISCOSITY BULK-FILL GIOMER AND ORMOCER-BASED RESIN COMPOSITES: AN IN-VITRO COMPARISON OF THEIR MECHANICAL BEHAVIOUR
the formation of an inorganic Si-O-Si network by hydrolysis and poly-condensation reactions, while the methacrylate groups are available for photochemical polymerization. 19 The reason beneath introducing Ormocers as dental materials was motivated in their reduced polymerization shrinkage and wear when compared to regular RBCs as well as in the very similar coefficient of thermal expansion compared to the natural tooth structure. 19 However, it was technically not possible for a long time to create commercial RBCs based integrally on ormocers. Therefore, ormocers were used in commercial materials only to replace parts of the regular organic matrix, which diminished the aforementioned positive effects. It might be worth mentioning that the first commercial ormocerbased restorative was Admira( Voco), launched in 1999, followed by the nano-hybrid composite Grandio of the same company. Recently, new technologies allowed creating purely ormocer RBCs, as implemented also in the analysed material Admira Fusion x-tra. With respect to the mechanical properties, the direct comparison of both bulk-fill RBCs identified consistently higher values in X-tra Fil compared to Admira Fusion x-tra, which is mainly attributed to the higher inorganic filler content and the enlarged filler size. The aforementioned drawbacks related to enhanced filler size must however be considered, when trying to reproduce the in-vitro data to a clinical perspective. A further interesting material development was implemented in Beautifil Bulk restorative. The material is denominated as a Giomer, an acronym derived from the main material compounds- Glass ionomer( GIC) and polymer-, since the material category contains as filler a GIC derivate, which is implemented into a resinous matrix. The fillers contained in Beautifil Bulk restorative are named S-PRG, which means surface pre-reacted glass ionomer, and were developed by Roberts et al. 18 in 1999. In S-PRG, a fluoroaluminosilicate glass has been reacted with a polyalkenoic acid in the presence of water, to form a wet siliceous hydrogel. In contrast to GICs 30, the acid-base reaction occurred in S-PRG fillers during manufacturing, thus resulting in a surface modified layer which is described to subsequently protect the glass core from the damaging effects of moisture. 18 S-PRG fillers were proved to release and recharge fluoride but also other ions such as Na +, Sr 2 +, Al 3 +, BO 3
3−
, SiO 3
2-31-38
Besides S-PRG fillers, the material contains also large pre-polymerized filers 4 comprising 87.0 weight % and 74.5 volume %, respectively. The bulk-fill giomer restorative proved in previous in-vitro studies to be adequately cured in increments up to 4-mm 4 and to induce low, yet statistical similar cusp deflection compared to the ormocer based bulk-fill restorative Admira Fusion x-tra, described above.
As for Tetric EvoCeram Bulk Fill, the filler system is comparable with the filler system observed in the regular nanohybrid RBC Tetric EvoCeram. The enhanced depth of cure was realizes not by enlarging the filler size, as observed in the majority of bulk-fill RBCs, but by implementing an additional photo-initiator, Ivocerin, along with the well-known photo-initiator system camphorquinone( CQ)/ amine and an acyl phosphine oxide initiator. Ivocerin is described as a germanium-based photoinitiator with a higher photo-curing activity than CQ, which is attributed to its larger molar extinction coefficient( ελ) when compared to CQ. Its maximal absorption is described at 411 nm wavelength, 17 therefore the initiator may be activated by regular curing units. A further advantage of Ivocerin is that it can be used without the addition of a co-initiator( amines) as well as its ability to form at least two radicals able to initiate the radical polymerization process. Therefore, Ivocerin is considered more efficient when compared to the CQ / amine systems, in which only one radical able to initiate the polymerization reaction is formed. 17 Regarding SonicFill it shows, similarly to x-tra fil, changes focusing primarily on the filler system, particularly on the filler size. 1 A previous study identified for SonicFill specimens of thicknesses up to 6 mm, lower blue light transmittance compared to Tetric EvoCeram Bulk Fill or x-tra fil. The amount of blue light transmitted is therefore comparable for SonicFill with values recorded in regular nanoand micro-hybrid RBCs. 23 Although enlarged fillers were observed also in SonicFill, the material is one of the lesser bulk-fill brands offered in regular shades. Thus, the low light transmittance might be, at least partially, attributed to the pigments’ necessity to adapt the color, which are present in lower amounts in other bulk-fill RBCs, offered mainly in the color“ universal”( Table 1). Compared to regular composites intended for incremental use, the analyzed bulk-fill composites revealed comparable or even superior mechanical properties. 1 Nevertheless, the enhanced filler size as observed in many materials, as well as the reduced amount of pigments, may forfeit the esthetic or the wear behavior of the materials.
5. Conclusions
The analyzed high-viscosity bulk-fill RBCs differ consistently in the properties analyzed, which is directly related to the inorganic filler content. Innovative modifications as the giomers or ormocers were related to materials with moderate mechanical properties, yet comparable to regular RBCs. The changes in chemical composition might however create advantages in terms of polymerization shrinkage stress or a caries protective ability.
60 STOMA. EDUJ( 2016) 3( 1)