Effect of stepped light exposure on the volumetric polymerization shrinkage and bulk modulus of dental composites and an unfilled resin.

PURPOSE

To compare the volumetric polymerization shrinkage and dynamic bulk modulus of five resins and an experimental unfilled resin polymerized with a stepped power light and with a conventional one step curing light.

MATERIALS AND METHODS

Parallel sided glass rings 5 mm in diameter and 2 mm high were etched for 5 min with hydrofluoric acid and silanated. The density and volume of the rings were calculated before they were filled with either SureFil, AEliteflo, Pertac II, Z100, or an experimental unfilled resin. When cured, the resin was bonded to the glass ring which produced a mold with a configuration C-factor of approximately 0.8. The resin was cured for 40 s on each side using either a one step curing light or a stepped power curing light. After curing, the specimens were weighed in air and in water to calculate their final density and volumetric shrinkage. Ten samples were made using each light system and for each resin (total of 100 samples). The bulk modulus was determined for each resin using an ultrasonic method, and the percentage filler contents were determined by thermogravimetric analysis.

RESULTS

The mean +/- SD volumetric polymerization shrinkage values ranged from 3.10 +/- 1.19% for SureFil using the one step light to 12.35 +/- 1.38% for the unfilled resin using the stepped power curing light. Student-Newman-Keuls post hoc test separated the 10 different materials into three significant groups at P= 0.05. The unfilled resin exhibited the highest volumetric shrinkage followed by the flowable composite (AEliteflo). For all materials tested, there was no significant difference in the volumetric shrinkage values when the resin was cured with the one step or the stepped power curing light. There was no significant difference in the volumetric shrinkage values for the hybrid composites (Pertac II, Z100, and SureFil). The volumetric shrinkage values were highly sensitive to filler weight (P < 0.001). Except for Z100, there was no difference in the specific gravity values and velocities of the longitudinal and shear waves and bulk modulus when the one step and stepped power curing lights were used. The bulk modulus values ranged from 6.30 +/- 0.19 GPa for unfilled resin, to 17.10 +/- 1.09 GPa for Z100. Curing with the one step or the stepped curing light had no significant effect on the modulus values for any resin with the exception of Z100, P = 0.05. The bulk modulus values were highly sensitive to filler weight, and higher modulus values were associated with increased filler weight (P < 0.001). There was no significant difference between the bulk modulus of Pertac II and SureFil (P = 0.05).