title: Time reduction of light curing: Influence on conversion degree and microhardness of orthodontic composites.
abstract: BACKGROUND
  The aim of this study was to assess the influence of curing time and power on the degree of conversion and surface microhardness of 3 orthodontic composites.
  
  
  METHODS
  One hundred eighty discs, 6 mm in diameter, were divided into 3 groups of 60 samples according to the composite used-Transbond XT (3M Unitek, Monrovia, Calif), Opal Bond MV (Ultradent, South Jordan, Utah), and Transbond Plus Color Change (3M Unitek)- and each group was further divided into 3 subgroups (n = 20). Five samples were used to measure conversion, and 15 were used to measure microhardness. A light-emitting diode curing unit with multiwavelength emission of broad light was used for curing at 3 power levels (530, 760, and 1520 mW) and 3 times (8.5, 6, and 3 seconds), always totaling 4.56 joules. Five specimens from each subgroup were ground and mixed with potassium bromide to produce 8-mm tablets to be compared with 5 others made similarly with the respective noncured composite. These were placed into a spectrometer, and software was used for analysis. A microhardness tester was used to take Knoop hardness (KHN) measurements in 15 discs of each subgroup. The data were analyzed with 2 analysis of variance tests at 2 levels.
  
  
  RESULTS
  Differences were found in the conversion degree of the composites cured at different times and powers (P <0.01). The composites showed similar degrees of conversion when light cured at 8.5 seconds (80.7%) and 6 seconds (79.0%), but not at 3 seconds (75.0%). The conversion degrees of the composites were different, with group 3 (87.2%) higher than group 2 (83.5%), which was higher than group 1 (64.0%). Differences in microhardness were also found (P <0.01), with lower microhardness at 8.5 seconds (35.2 KHN), but no difference was observed between 6 seconds (41.6 KHN) and 3 seconds (42.8 KHN). Group 3 had the highest surface microhardness (35.9 KHN) compared with group 2 (33.7 KHN) and group 1 (30.0 KHN).
  
  
  CONCLUSIONS
  Curing time can be reduced up to 6 seconds by increasing the power, with a slight decrease in the degree of conversion at 3 seconds; the decrease has a positive effect on the surface microhardness.
authors: Private practice, Ribeirão Preto, São Paulo, Brazil.
authors: Patrícia Alves Ferreira
authors: PA
authors: Amato
authors: Invited professor, Orthodontic Program, Araraquara School of Dentistry, São Paulo State University, Araraquara, São Paulo, Brazil; private practice, Araraquara, São Paulo, Brazil. Electronic address: dr_renatopmartins@hotmail.com.
authors: Renato Parsekian
authors: RP
authors: Martins
authors: Associate professor, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University, Araraquara, São Paulo, Brazil.
authors: Carlos Alberto
authors: CA
authors: dos Santos Cruz
authors: Assistant professor, Department of Physical Chemistry, Araraquara Institute of Chemistry, São Paulo State University, Araraquara, São Paulo, Brazil.
authors: Marisa Veiga
authors: MV
authors: Capella
authors: Chairman, Department of Orthodontics, Araraquara School of Dentistry, São Paulo State University, Araraquara, São Paulo, Brazil.
authors: Lídia Parsekian
authors: LP
authors: Martins
mesh_terms: D001965:Bromides / Q000737:chemistry; D003188:Composite Resins / Q000737:chemistry*; D055117:Curing Lights, Dental / Q000145:classification; D006244:Hardness; D006245:Hardness Tests; D006801:Humans; D055115:Light-Curing of Dental Adhesives / Q000295:instrumentation / Q000379:methods; D008422:Materials Testing; D009971:Orthodontics, Corrective; D058105:Polymerization; D017680:Potassium Compounds / Q000737:chemistry; D011829:Radiation Dosage; D019279:Resin Cements / Q000737:chemistry*; D013057:Spectrum Analysis / Q000295:instrumentation; D013997:Time Factors
chemical_list: D001965:Bromides; D003188:Composite Resins; D017680:Potassium Compounds; D019279:Resin Cements; C477790:Transbond XT; C039004:potassium bromide