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