title: The effect of ceramic restoration shade and thickness on the polymerization of light- and dual-cure resin cements.
abstract: OBJECTIVE
  Inadequately polymerized resin cements may negatively affect the clinical performance of cemented all-ceramic restorations. The purpose of this in vitro study was to evaluate the effect of ceramic thickness and shade on the microhardness of various light-cured (LC) and dual-cured (DC) resin cements. The amount of light transmission through the restoration was also evaluated to correlate the results.
  
  
  METHODS
  Three different brands of resin cements (Appeal/Ivoclar; Calibra/Dentsply; Nexus 2/Kerr) were used in LC and DC forms to prepare disk-shaped samples (0.5-mm thickness × 5-mm diameter) (n=15). Study group samples were light-cured for 40 seconds (Flashlite 1401/Discus Dental) through four shades (ETC1, ETC2, ETC3, ETC4) and four thicknesses (1 mm, 2 mm, 3 mm, 4 mm) of all-ceramic ingot discs (IPS Empress Esthetic/Ivoclar). Control samples were directly cured without the presence of ceramic. The light transmission through various shades and thicknesses of ceramics was measured using a hand-held radiometer (Demetron, Kerr). Vickers microhardness measurements were performed (Micromet/Buehler) at 24 hours following dark storage in 37°C water. Correlation between ceramic shade, thickness, and light intensity readings (mW/cm2) with respect to microhardness was statistically evaluated using analysis of variance (p=0.05).
  
  
  RESULTS
  Ceramic thickness of 3 mm and/or above significantly decreased the microhardness values in all LC and DC groups (p<0.0001). Ceramic shade had a significant effect only on Calibra in both LC and DC groups (p<0.0001). Microhardness values of LC groups were significantly lower than DC groups (p<0.0001). Control groups had significantly higher hardness values in all cement groups (p<0.0001). There was a significant correlation between the amount of light transmitted and hardness (p=0.000).
  
  
  CONCLUSIONS
  The ceramic thickness has a more intense effect on polymerization compared to the ceramic shade. Overlying ceramic thickness of 3 mm and above was found to adversely affect the polymerization of LC and DC resin cements and therefore a 3-mm thickness was considered the critical threshold.
authors.affiliation: Nova Southeastern University College of Dental Medicine, Cariology and Restorative Dentistry, Fort Lauderdale, FL, USA.
authors.forename: E
authors.initials: E
authors.lastname: Kilinc
authors.forename: S A
authors.initials: SA
authors.lastname: Antonson
authors.forename: P C
authors.initials: PC
authors.lastname: Hardigan
authors.forename: A
authors.initials: A
authors.lastname: Kesercioglu
mesh_terms: D003116:Color; D003776:Dental Porcelain / Q000737:chemistry*; D003793:Dental Restoration, Permanent / Q000379:methods; D006244:Hardness; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D058105:Polymerization; D019279:Resin Cements / Q000737:chemistry* / Q000528:radiation effects*; D055116:Self-Curing of Dental Resins
chemical_list: D019279:Resin Cements; D003776:Dental Porcelain