Condensable Resin Composite: Instrumental Observations Of Marginal Closure M. Baldi*, M. Frascaria, A. Rosati, M. Giannoni (University Of L'Aquila, Dep. Surg. Sciences, Clin. Dentistry: Dir. Prof. M. Giannoni) The most important factor in durability of condensable resin composite, which are created to replace alloy in posterior sectors, is marginal closure.   An experimental trial was performed to evaluate the modifications between resin composite and dental tissues interface, due to cyclical termical stresses in ‘in vitro’ restoration, on extracted theeth. II class cavities are built with cervical margins on root cement, filled with 5 new different generation resin composites: Alert (Jeneric Pentron), Arabesk Top (Voco) , Ariston pHc (Vivadent), Definite omc (Degussa) and Pyramid (Bisco).   The samples were termocycled in water for 1 min.at 5 C° temperature and for 1 min. at 55 C°, performing 804 cycles.   The observations of marginal closure before and after termocycles were obtained in the same samples using a pression controlled S.E.M. (Philips XL 30 CP) which didn’t need metal-lining of samples.   Starting images showed an optimal and complete marginal closure in the whole cavity perimeter; no differences were observed in the 5 resin composites used with enamel or root cement. Only in a few cases microfractures were observed over enamel surface near the filling side due to after polimerization contraction stress. After the termical cycles marginal closures of every resin composite with enamel show a continuous micro-gap of a few micron, while in root-cement interface the micro-gap resulted less evident and costant.   The termical expansion and contraction coefficients and the aelasticity module more similar between resin composite and root cement in addition to the enamel dentin bonding adhesive action give better resistance to termical stresses.   The resistance of marginal closure in time, depends on different expansion coefficients, on termical contraction aelastometric module of filling material against biological tissues and on the role of enamel dentin adhesive in their interface.