My first Magazine | Page 59

HIGH VISCOSITY BULK-FILL GIOMER AND ORMOCER-BASED RESIN COMPOSITES: AN IN-VITRO COMPARISON OF THEIR MECHANICAL BEHAVIOUR

the materials have proved in previous studies to be adequately polymerized in at least 4-mm increments. 4, 5, 23 Therefore, the macro-mechanical properties, which were assessed according to the valid standards 20 on 2-mm thick specimens, may be transferred to larger increments as well. Although low differences in filler amount were identified among the materials analyzed( 81 % to 87 % by weight), the inorganic filler amount was directly reflected in the measured mechanical properties, since the highest filled materials x-tra fil and Quixfil, which contain both 86 weight % fillers, also reached the highest mechanical properties. Table 1 indicates for Beautifil Bulk restorative an even higher filler amount( 87 %). It must be pointed out that the material contains, beside inorganic bulk-filler, also large pre-polymer fillers, thus the total inorganic filler amount is accordingly lower as indicated. A similar conclusion applies for Tetric EvoCeram ® Bulk Fill, in which the pre-polymer fillers amounted to17 % of the total indicated filler amount( Table 1). All materials analyzed fulfill the ISO 4049 criteria( flexural strength ≥80MPa) to be used in loadbearing areas as restorative materials. 20 Yet, the ISO 4049 does not specify a lower limit value for the modulus of elasticity, which represents the relative stiffness of a material and is related to the deformability of a restorative material under masticatory stresses. 24 Consistent larger differences among materials were identified with regard to the flexural modulus, which is directly related to the inorganic filler amount. As for the measured micromechanical properties, the present investigation revealed a strong dependence of all measured parameters on the filler amount. To assess both the elastic and the plastic part of the deformation, a depth sensing hardness measurement device was used in this study. Therefore, a dynamic measuring principle was applied by recording simultaneously the load and the corresponding penetration depth of the indenter. 22, 25 Besides hardness, the indentation modulus is also indicated, since previous studies attested a good correlation between indentation modulus and the modulus of elasticity measured in the more familiar threepoint bending test. 26 This relation was confirmed also by the present study. While flexural modulus and indentation modulus might have attributed a similar clinical interpretation, both measured hardness parameters – Martens Hardness( or universal hardness) and the more familiar Vickers Hardness- are defined as the resistance a material oppose to penetration or indentation, and might be related to properties such as wear resistance or abrasion. 27 The last statement must however be put into perspective for bulk-fill RBCs, since many bulk-fill RBCs contain larger fillers( up to 20 µ m) compared to regular RBCs, 28 which might have a negative impact not only on the wear behaviour of the materials, but also on their aesthetic properties. The materials analyzed in the present study are the result of a large variety of innovative technologies. The concept of bulk-filling, meaning the ability to place and cure a resin-composites in large increments( up to 4 mm) and thus to skip the time consuming layering technique, was first introduced with the high-viscosity bulk-fill RBC QuixFil. Yet, the low-viscosity bulk-fill RBC( SDR flow) of the same company, launched many years later, was the first bulk-fill material achieving a noticeable commercial success and establishing thus the aforementioned concept. This fact triggered a series of products from all other dental companies. As for QuixFil, the enlarge depth of cure was realized by enlarging the filler size and consequently reducing the filler-matrix interface and the scattering along this interface. Whether the refractive indices of the material components were modified in order to reduce differences among fillers and organic matrix, and therefore to reduce light scattering and enhance translucency, 29 is not stated nor analyzed so far. Besides, no remarkable changes in the composition of the organic matrix or photo-initiating system are noted( Table 1). The present study allows comparing two different bulk-fill materials from the same company- X-tra Fil and Admira Fusion x-tra – in which two different concepts in modifying the materials for bulk-fill placement were followed. While X-tra Fil is based on a traditional( methacrylates) organic matrix, containing monomers like Bis-GMA, UDMA, TEGDMA( Table 1), Admira Fusion x-tra is a purely ormocer-based material. As different from regular di-methacrylates, Ormocers are described as 3-dimensionally cross-linked inorganic-organic polymers, synthesized from multi-functional urethane- and thioether( meth) acrylate alkoxysilanes as sol-gel precursors. Alkoxysilyl groups of silane permit