Polymerization shrinkage of visible-light-cured composites.

This study investigated the long-term dimensional changes of a conventional and a polyacid-modified composite resin and the effects of hydration on polymerization shrinkage. A strain-monitoring device was used to measure the linear polymerization shrinkage of the composites in the free state when stored in water at 37 degrees C or air at 26 degrees C over a one-month period. Results showed that the polymerization reaction of both conventional and polyacid-modified composite resins was accompanied by a dimensional shrinkage change. The rate of shrinkage for both composites was greatest during the polymerization reaction and continued after removal of the curing light. When stored in water, the greatest shrinkage was noted at one hour for both materials. This was followed by a slow uptake of water and expansion from one day to one month. The polyacid-modified composite had significantly less polymerization shrinkage than the conventional composite after one month of storage in water.