Construction of new resin core restoration system using mega-filler

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In order to decrease the polymerization shrinkage stress of resin and reinforce resin core in a direct resin core restoration, we proposed the new resin core build-up system with inserting of mega-filler into core. The silanated alumina (Al_2O_3 93%/SiO_2 5%) balls and stainless steel (SUS304) balls treated with MDP monomer were used as mega-filler (3 or 4 mm φ). The compressive strength(CS), elastic modulus(EM) and yield stress(YS) of cylindrical resin cores (5mmφ×8mm) with or without mega-filler were measured to evaluate the reinforcing effect of mega-filler. Although the CSs of resin cores with mega-filler were almost equal to that of resin core itself, the EMs and YSs of resin cores with mega-filler were significantly higher than that of resin core itself. This means that the resin core with mega-filler is hard to deform comparing to resin core only and it is suggested that the inserting of mega-filler is effective to improve the durability of resin core.The cavities of 5mm in diameter and 5mm in depth were prepared in bovine tooth roots and restored with core resin only or with resin and mega-filler. All restored samples were applied to a thermal cycling test (for 1min each in 4℃ and 55℃ water) for 5,000 or 10,000 times and sectioned in halves. The sectioned surface was observed with a stereo optical microscope and the existence or absence of gap between resin core and cavity wall was examined. For core resin only group, the gap formations between resin and dentin in portions near to cavity floor were found in many samples. For mega-filler group, no gap formation was found in any samples. From these results, it is suggested that the inserting of mega-filler decrease polymerization and thermal shrinkage stresses and improve sealing ability and bonding durability of direct resin core restorations.