title: Specimen aspect ratio and light transmission in photoactive dental resins.
abstract: OBJECTIVE
  To test the influence of specimen dimensions on light transmission and shrinkage strain properties of curing dental resins.
  
  
  METHODS
  Photocurable resin specimens (Bis-GMA/TEGDMA) with aspect ratios (AR) of 2 (4mm×2mm); 4 (4mm×1mm and 8mm×2mm); 8 (8mm×1mm); 12 (AR: 12mm×1mm); and 24 (12mm×0.5mm) were light cured. Light transmission and shrinkage-strain data were recorded throughout, and upper and lower surface hardness measurements were performed following cure.
  
  
  RESULTS
  Light transmission was significantly affected by the specimen aspect ratio even at similar thicknesses (p<0.05). By comparing light transmission through a negative control resin without photoinitiator, the lowest AR specimens showed a relative increase in transmission above 100% throughout curing, which was caused by specimen constraint. The extent of lower surface cure (as assessed by increasing hardness) was principally affected by cavity height and decreased for thicker specimens (p<0.05). Only the 2mm thick specimens showed a significantly greater lower to upper hardness ratio with increasing cavity diameter (p<0.05). The highest AR specimen showed the greatest lower to upper hardness percentage (p<0.05), and was expected since this AR was obtained by reducing the sample thickness to 0.5mm. Generally, total shrinkage strain increased and shrinkage strain per unit mass decreased with increasing AR.
  
  
  CONCLUSIONS
  Specimen constraint in low AR cavities may compromise light transmission as unexpected light intensity variations may occur for low configuration factors, which ultimately affect polymer conversion of light-cured resin-based restorations through depth.
authors.affiliation: Biomaterials Unit, University of Birmingham, School of Dentistry, College of Medical and Dental Sciences, St. Chads Queensway, Birmingham B4 6NN, United Kingdom. HadisM@bham.ac.uk
authors.forename: Mohammed A
authors.initials: MA
authors.lastname: Hadis
authors.forename: Adrian C
authors.initials: AC
authors.lastname: Shortall
authors.forename: William M
authors.initials: WM
authors.lastname: Palin
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D018772:Dental Marginal Adaptation; D003799:Dental Stress Analysis; D006244:Hardness; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D058105:Polymerization; D013314:Stress, Mechanical; D013499:Surface Properties
chemical_list: D003188:Composite Resins