Shrinkage forces due to polymerization of light-cured dental composite resin in cavities

Abstract

A new method has been developed for determining shrinkage forces due to polymerization of light-cured dental composite resin in artificial cylindrical cavities. The cavities were fabricated in stainless steel plates, and shrinkage forces were measured using a load cell inserted from the rear of the plate in place of the floor of the cavity. The cavities were 3 mm in diameter, and depths varied from 0.5 to 3.5 mm to study the effect of light intensity. Cavities were filled with a composite resin after being prepared with a bonding agent and were then irradiated for 20 s with a light-curing unit at a power of 300 mW/cm 2. To study the effects of boundaries, two conditions were employed: a free surface condition without any restriction on the top surface of the cavity, and a constrained surface condition in which the top surface of the resin was bonded to a 1 mm thick transparent polymethyl methacrylate (PMMA) plate. The shrinkage forces were measured as functions of time and the cavity depth, and the following results were obtained. For both conditions, the force increased with time, mostly during the irradiation stage (20 s), and then increased slightly after the irradiation. The force measured at 300 s increased gradually with the cavity depth and exhibited a maximum value before decreasing gradually. The cavities with constrained surfaces yielded much greater shrinkage forces for any given cavity depth than those with free surface conditions.