[A study on disintegration of posterior composite resins caused by long-term water immersion]

The purpose of this study is to clarify a cause of disintegration for posterior composite resins by long-term immersion in distilled water during the 3-year period. Three kinds of posterior composite resins [Bellfirm P (BP), Clearfil Posterior (CP) and Micro Jar (MJ)] and one conventional anterior composite resin [Clearfil FII (FII)] were prepared as to specimens with 20 mm diameter, and 1 mm in thickness. These specimens were immersed in distilled water at 37 +/- 1 degree C for 3 years. These specimens were observed by scanning electron microscope (SEM). The residuces in distilled water were analyzed by X-ray microdiffractometry, infrared (IR) and nuclear magnetic resonance (NMR) spectrometers. One posterior composite resin (microparticle filled resin) was eluted in water more than the other posterior and anterior composite resins with conventional fillers. In SEM observations, many air bubbles were found inside all sections of the four composite resins, because of all mixing type composite resins. The subsurface layer of all four composite resins showed signs of disintegration, but the center layer of the specimens was not disintegrated. Three cured composite resins except MJ had crystalline X-ray diffraction patterns before water immersion, but the cured MJ alone was amorphous. These peaks of crystals showed fillers of these composite resins respectively. X-ray diffraction patterns of the dissolved substances for three composite resins except BP were also crystalline, but the peaks of the crystals except CP were different from the peaks of the fillers respectively. In IR and 1H-NMR spectra of dissolved substances, an unreacted monomer could be detected in one posterior composite resin (BP), but not in the other composite resins. In 1H-NMR spectra of dissolved substances, new signals not found originally were observed in all four composite resins. Therefore, the progress of disintegration was demonstrated clearly. The dissolved substances of all four composite resins were shown as the disintegrated substances between resin matrixes and silane coupling agents. It is suggested that the disintegration of these composite resins by long-term water immersion is derived from hydrolysis.