Effect of a light-emitting diode on composite polymerization shrinkage and hardness.

PURPOSE This study evaluated the effect of light-emitting diodes (LEDs) on polymerization shrinkage and bottom-to-top hardness ratios of composites. MATERIALS AND METHODS Six LEDs (Elipar FreeLight, 3M ESPE, St. Paul, MN, USA; Versalux, Centrix, Shelton, CT, USA; Ultra-Lume LED2, Ultradent, South Jordan, UT, USA; Zap LED only, CMS-Dental/Soft-Core Texas, North Richland Hills, TX, USA; Zap dual light; and L.E. Demetron I, Kerr Manufacturing Inc., Orange, CA, USA) and a quartz-tungsten-halogen (QTH) light (Optilux 501, Kerr Manufacturing Inc.) were tested. Ten specimens each of a microhybrid (Point 4, Kerr Manufacturing, Inc.) and a hybrid (Filtek Z250, 3M ESPE, St. Paul, MN, USA) composite, measuring 2 mm thick by 5 mm in diameter, were polymerized using each of the lights. Linear shrinkage was recorded using a contactless displacement instrument. Ten specimens were also polymerized with each light to determine the Knoop hardness number (KHN) at the top and bottom surfaces. RESULTS For the microhybrid, Zap dual light had the least volumetric shrinkage 2.08% (+/- 0.33) and Elipar FreeLight had the highest 3.02% (+/- 0.73). There was no significant difference (p < or = .05) in shrinkage for the LEDs when compared with the Optilux 501. The hybrid showed the least amount of shrinkage with the L.E. Demetron I, 1.42% (+/- 0.12), and the greatest with the Zap dual light, 2.47% (+/- 0.31). The Ultra-Lume LED2 (p < or = .05) and Zap LED and dual light (p < or = .001) had significantly greater shrinkage than did the Optilux 501. Zap LED had the lowest depth of cure with a bottom KHN of 11.46 (+/- 2.71) and 33.62 (+/- 3.57) for the microhybrid and hybrid, respectively. The L.E. Demetron I had the highest bottom hardness value for the microhybrid, with a value of 40.65 (+/- 1.50). The Optilux 501 had the highest bottom hardness value for the hybrid, with a value of 62.03 (+/- 0.82). The Zap LED and dual light and Versalux lights had significantly lower bottom-to-top hardness ratios than did the QTH (p < or = .001) with the microhybrid. CONCLUSIONS There was no statistically significant difference in shrinkage for the microhybrid with any of the lights tested. The hybrid, however, showed significantly less shrinkage with the halogen compared with the Ultra-Lume LED2 and Zap LED and dual light. All LEDs had equal or lower bottom hardness values than did the QTH, except for L.E. Demetron I with the microhybrid.