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

To determine if polymerization shrinkage of a light-cured composite is directed toward the light source, and if a relationship exists between C-factor and magnitude of deflection.

Either a self-cured composite or light-cured composite each was placed in an experimental model of composite cured in a glass ring. Changing the surface area prepared for bonding within the cavity preparation produced three different C-factors (the ratio of bonded to unbonded surface areas): 0, 1.9, and 4.8. Profilometer tracings were obtained, and the largest deviation from the height of the glass ring was reported. An uncured control sample also was evaluated and showed a linear tracing with fluctuations of less than 0.5 micron. The specimens were then cut in half and examined by scanning electron microscope.

The top (unbonded) surface exhibited a concave shrinkage profile with the greatest deflection at the center. The magnitude of deflection at the center increased as the cavity's C-factor increased from 31 microns (C = 0) to 57 microns (C = 4.8). The bottom (unbonded) surface, similarly formed a concave surface. When the composite was bonded to the glass, fractures in the glass wall near the glass-composite interface were observed.