title: Time dependence of composite shrinkage using halogen and LED light curing.
abstract: OBJECTIVE
  The polymerization shrinkage of light cured dental composites presents the major drawback for these aesthetically adaptable restorative materials. LED based light curing technology has recently become commercially available. Therefore, the aim of the present study was to investigate if there was a statistically significant difference in linear and volumetric composite shrinkage strain if a LED LCU is used for the light curing process rather than a conventional halogen LCU.
  
  
  METHODS
  The volumetric shrinkage strain was determined using the Archimedes buoyancy principle after 5, 10, 20, 40 s of light curing and after 120 s following the 40 s light curing time period. The linear shrinkage strain was determined with a dynamic mechanical analyzer for the composites Z100, Spectrum, Solitaire2 and Definite polymerized with the LCUs Trilight (halogen), Freelight I (LED) and LED63 (LED LCU prototype). The changes in irradiance and spectra of the LCUs were measured after 0, 312 and 360 min of duty time.
  
  
  RESULTS
  In general there was no considerable difference in shrinkage of the composites Z100, Spectrum or Solitaire2 when the LED63 was used instead of the Trilight. There was, however, a statistically significant difference in shrinkage strain when the composite Definite was polymerized with the LED63 instead of the Trilight. The spectrum of the Trilight changed during the experiment considerably whereas the LED63 showed an almost constant light output. The Freelight I dropped considerably in irradiance and had to be withdrawn from the study because of technical problems.
  
  
  CONCLUSIONS
  The composites containing only the photoinitiator camphorquinone showed similar shrinkage strain behaviour when a LED or halogen LCU is used for the polymerization. The irradiance of some LED LCUs can also decrease over time and should therefore be checked on a regular basis.
authors.affiliation: Friedrich Schiller University Jena, Institute of Materials Science and Technology (IMT), Löbdergraben 32, D-07743 Jena, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Uhl
authors.forename: Robin W
authors.initials: RW
authors.lastname: Mills
authors.forename: Angelika E
authors.initials: AE
authors.lastname: Rzanny
authors.forename: Klaus D
authors.initials: KD
authors.lastname: Jandt
mesh_terms: D000704:Analysis of Variance; D002516:Ceramics; D003188:Composite Resins / Q000528:radiation effects*; D003748:Dental Equipment; D018772:Dental Marginal Adaptation; D003799:Dental Stress Analysis; D006219:Halogens; D006244:Hardness; D008027:Light; D008422:Materials Testing; D063225:Organically Modified Ceramics; D044367:Phase Transition; D012666:Semiconductors; D012821:Silanes; D012822:Silicon Dioxide / Q000528:radiation effects; D012833:Siloxanes / Q000528:radiation effects; D013729:Terpenes / Q000528:radiation effects; D013997:Time Factors; D015040:Zirconium / Q000528:radiation effects
chemical_list: D003188:Composite Resins; C421785:Definite composite resin; D006219:Halogens; D063225:Organically Modified Ceramics; D012821:Silanes; D012833:Siloxanes; C469658:Solitaire 2 composite resin; C467914:TPH spectrum; D013729:Terpenes; C083568:Z100 composite resin; D012822:Silicon Dioxide; D015040:Zirconium