title: Comparison of PAC and QTH light sources on polymerization of resin composites.
abstract: OBJECTIVE
  To evaluate the polymerization shrinkage, Knoop hardness number (KHN) and compressive strength and to suggest a suitable time for irradiating resin composite restorations, when using a high-intensity light source.
  
  
  METHODS
  Two restorative resin composites, UniFil F, and Clearfil AP-X were employed. A high-intensity light unit (more than 1000mW/cm2) with a xenon discharge lamp (Apollo 95E), which is generally called a plasma arc light-curing unit (PAC), was compared with a conventional light-curing unit fitted with a quartz-tungsten-halogen lamp (QTH) (GC Newlight VL2). The resin composites were exposed to the light in four ways. For QTH, the irradiation time was for 40 seconds (QTH 40 seconds). For PAC, 3 seconds (PAC 3 seconds), 3+3 seconds (PAC 3+3 seconds) and 3+3+3 seconds (PAC 3+3+3 seconds) was used. Polymerization shrinkage using the bonded disk technique developed by Watts, Knoop hardness number (KHN), and compressive strength were then determined.
  
  
  RESULTS
  Two-way ANOVA revealed that the two materials for PAC 3+3 seconds and PAC 3+3+3 seconds made no difference in polymerization shrinkage compared to QTH 40 seconds (P < 0.001). The polymerization shrinkage of the materials cured by PAC for 3 seconds was significantly lower than those cured by QTH for 40 seconds in a range from 61% to 72%, by the PAC for 3 + 3 seconds in a range from 65% to 88%, and those by PAC 3+3+3 seconds in a range of 61% to 72% (P < 0.001). With regard to microhardness, the composites in PAC 3+3+3 seconds exposure made no difference in hardness compared with QTH 40 seconds (P < 0.001). PAC 3+3 seconds exposure gave hardness at less than 3.0 mm depth equivalent to that of the QTH 40 seconds. PAC 3 seconds at 2.0 mm depth produced inferior hardness compared with the QTH 40 seconds. The compressive strength for the PAC 3 seconds exposure was significantly lower than that of PAC 3+3 seconds, PAC 3+3+3 seconds and QTH 40 seconds for each material.
  
  
  CONCLUSIONS
  Irradiation by the high-intensity light source for 3 seconds provided significantly lower microhardness and compressive strength for light-cured resin composites. The 3+3+3 seconds repeated irradiation with the PAC unit and QTH 40 second values were not significantly different.
authors.affiliation: Toranomon Hospital Dental Clinic 2-2-2, Toranomon, Minato-ku, Tokyo, 105-8470, Japan. katahira0623@theia.ocn.ne.jp
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Katahira
authors.forename: Richard M
authors.initials: RM
authors.lastname: Foxton
authors.forename: Norimichi
authors.initials: N
authors.lastname: Inai
authors.forename: Masayuki
authors.initials: M
authors.lastname: Otsuki
authors.forename: Junji
authors.initials: J
authors.lastname: Tagami
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000528:radiation effects*; D019245:Compressive Strength; D006219:Halogens; D006244:Hardness; D008027:Light; D008422:Materials Testing; D008689:Methacrylates / Q000528:radiation effects*; D015394:Molecular Structure; D044367:Phase Transition; D011791:Quartz; D014978:Xenon
chemical_list: C112828:Clearfil AP-X; D003188:Composite Resins; D006219:Halogens; D008689:Methacrylates; C487361:Unifil F; D011791:Quartz; D014978:Xenon