title: Reduced polymerization stress of MAPO-containing resin composites with increased curing speed, degree of conversion and mechanical properties.
abstract: OBJECTIVE
  The degree and rate of photopolymerization in resin-based dental composites will significantly affect polymer network formation and resultant material properties that may determine their clinical success. This study investigates the mechanical properties, the generation of stress from polymerization, tooth cusp deflection and marginal integrity of experimental resin composites that contain different photoinitiators.
  
  
  METHODS
  Experimental light-activated resin composites (60vol% particulate filled in 50/50mass% bis-GMA/TEGDMA) were formulated using a monoacylphosphine oxide (MAPO) photoinitiator and compared with a conventional camphoroquinone (CQ)-based system. Similar radiant exposure was used (18Jcm(-2)) for polymerization of each material although the curing protocol was varied (400mWcm(-2) for 45s, 1500mWcm(-2) for 12s and 3000mWcm(-2) for 6s). Degree and rate of polymerization was calculated in real-time by near infrared spectroscopy and the generation of stress throughout polymerization measured using a cantilever beam method. Flexural strength and modulus were acquired by three-point bend tests. Standardized cavities in extract pre-molar teeth were restored with each material, the total cuspal deflection measured and post-placement marginal integrity between the tooth and restoration recorded.
  
  
  RESULTS
  Generally, MAPO- exhibited a significantly higher degree of conversion (72±0.8 to 82±0.5%) compared with CQ-based materials (39±0.7 to 65±1.6%) regardless of curing protocol (p<0.05) and MAPO-based materials exhibited less difference in conversion between curing protocols. CQ-based materials exhibited between ∼85 and 95% of the maximum rate of polymerization at <15% conversion, whereas MAPO-based RBCs did not approach the maximum rate until >50% conversion. Higher irradiance polymerization had a significant deleterious effect on the mechanical properties of CQ-based materials (p<0.05) whereas MAPO-based materials exhibited increased strength and modulus and were less affected by the curing method. Total cuspal deflection in restored extracted teeth was higher for CQ- compared with MAPO-based materials cured at the lowest irradiance curing protocol (12.9±4.0 and 8.3±1.5μm) and similar at 3000mWcm(-1) for 6s (10.1±3.5 and 9.0±1.5μm). A significant decrease in marginal integrity was observed for CQ-based RBCs cured at high irradiance for short exposure time compared with that of the MAPO-based RBC cured using a similar protocol (p=0.037).
  
  
  CONCLUSIONS
  Polymer network formation dictates the final properties of the set composite and the use MAPO photoinitiators may provide an effective restorative material that exhibits higher curing speeds, increased degree of conversion, strength and modulus without compromise in terms of polymerization stress and marginal integrity between tooth and restoration.
authors.affiliation: Biomaterials Unit, University of Birmingham, College of Medical and Dental Sciences, School of Dentistry, St Chad's Queensway, Birmingham B4 6NN, UK. Electronic address: w.m.palin@bham.ac.uk.
authors.forename: W M
authors.initials: WM
authors.lastname: Palin
authors.affiliation: Biomaterials Unit, University of Birmingham, College of Medical and Dental Sciences, School of Dentistry, St Chad's Queensway, Birmingham B4 6NN, UK.
authors.forename: M A
authors.initials: MA
authors.lastname: Hadis
authors.affiliation: Institute of Condensed Matter and Nanoscience, Bio- and Soft-Matter, Université catholique de Louvain, Louvain-la-Neuve, Belgium; School of Dentistry and Stomatology, Université catholique de Louvain, Brussels, Belgium; CRIBIO (Center of Research and Engineering of Biomaterials), Brussels, Belgium.
authors.forename: J G
authors.initials: JG
authors.lastname: Leprince
authors.affiliation: Institute of Condensed Matter and Nanoscience, Bio- and Soft-Matter, Université catholique de Louvain, Louvain-la-Neuve, Belgium; School of Dentistry and Stomatology, Université catholique de Louvain, Brussels, Belgium; CRIBIO (Center of Research and Engineering of Biomaterials), Brussels, Belgium.
authors.forename: G
authors.initials: G
authors.lastname: Leloup
authors.affiliation: Materials Science Unit, Division of Oral Biosciences, Dublin Dental University Hospital, Lincoln Place, Trinity College Dublin, Ireland.
authors.forename: L
authors.initials: L
authors.lastname: Boland
authors.affiliation: Materials Science Unit, Division of Oral Biosciences, Dublin Dental University Hospital, Lincoln Place, Trinity College Dublin, Ireland.
authors.forename: G J P
authors.initials: GJ
authors.lastname: Fleming
authors.affiliation: Department of Periodontology, Endodontology and Cariology, University of Basel, Switzerland, Hebelstrasse 3, CH-4056 Basel, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Krastl
authors.affiliation: Biomaterials Research Group, School of Dentistry, University of Manchester, Manchester, UK.
authors.forename: D C
authors.initials: DC
authors.lastname: Watts
mesh_terms: D003188:Composite Resins; D055117:Curing Lights, Dental; D008422:Materials Testing; D058105:Polymerization
chemical_list: D003188:Composite Resins
keywords: Conversion; Cuspal deflection; Flexural strength; Irradiance; Modulus; Photoinitiator; Photopolymerization; Resin-based composites; Shrinkage; Stress