The effect of the photopolymerization method on the quality of composite resin samples.

An optimal degree of conversion and minimal polymerization shrinkage are generally antagonistic goals, as increased monomer conversion invariably leads to elevated polymerization shrinkage values. However, both parameters are indispensable for an optimal resin composite restoration. A number of approaches have been used to reduce the stress on the restoration cavity wall interface, such as dentine bonding agents to counteract polymerization shrinkage, stress-absorbing lining materials and low-intensity curing lights to control the flow capacity of the material during polymerization. However, the configuration of the cavity and cohesive fractures of the material and surrounding tooth tissues are still a problem in day-to-day clinical practice. A new photopolymerization light source, pulsed laser, ensures a higher degree of conversion and lower polymerization shrinkage, and differentiates this technique from standard polymerization methods and continuous-wave argon laser polymerization. The coherence and monochromacity of pulsed laser light set at 468 nm and the far greater intensity of laser nanopulses produce a saturation effect in the depths of the composite, thus resulting in higher monomer conversion. The total amount of energy illuminating the sample surface, which is only one-fifth of that of conventional methods, and the cooling and relaxation of the material between nanopulses may be responsible for the reduced net polymerization shrinkage.