Determining the direction of shrinkage in dental composites by changes in surface contour for different bonding configurations

Purpose: To determine if polymerization shrinkage of a light-cured composit e is directed toward the light source, and if a relationship exists between C-factor and magnitude of deflection. Materials and Methods: Either a self -cured composite or light-cured composite each was placed in an experimenta l model of composite cured in a glass ring. Changing the surface area prepa red for bonding within the cavity preparation produced three different C-fa ctors (the ratio of bonded to unbonded surface areas): 0, 1.9, and 4.8. Pro filometer tracings were obtained, and the largest deviation from the height of the glass ring was reported. An uncured control sample also was evaluat ed and showed a linear tracing with fluctuations of less than 0.5 mum. The specimens were then cut in half and examined by scanning electron microscop e. Results: The top (unbonded) surface exhibited a concave shrinkage profil e with the greatest deflection at the center. The magnitude of deflection a t the center increased as the cavity's C-factor increased from 31 mum (C=0) to 57 mum (C= 4.8). The bottom (unbonded) surface, similarly formed a conc ave surface. When the composite was bonded to the glass, fractures in the g lass wall near the glass-composite interface were observed.