title: System compliance dictates the effect of composite filler content on polymerization shrinkage stress.
abstract: OBJECTIVE
  The effect of filler content in dental restorative composites on the polymerization shrinkage stress (PS) is not straightforward and has caused much debate in the literature. Our objective in this study was to clarify the PS/filler content relationship based on analytical and experimental approaches, so that guidelines for materials comparison in terms of PS and clinical selection of dental composites with various filler content can be provided.
  
  
  METHODS
  Analytically, a simplified model based on linear elasticity was utilized to predict PS as a function of filler content under various compliances of the testing system, a cantilever beam-based instrument used in this study. The predictions were validated by measuring PS of composites synthesized using 50/50 mixtures of two common dimethacrylate resins with a variety of filler contents.
  
  
  RESULTS
  Both experiments and predictions indicated that the influence of filler content on the PS highly depended on the compliance of the testing system. Within the clinic-relevant range of compliances and for the specific sample configuration tested, the PS increased with increasing filler content at low compliance of instrument, while increasing the compliance caused the effect of filler content on the PS to gradually diminish. Eventually, at high compliance, the PS inverted and decreased with increasing filler content.
  
  
  CONCLUSIONS
  This compliance-dependent effect of filler content on PS suggests: (1) for materials comparison in terms of PS, the specific compliance at which the comparison being done should always be reported and (2) clinically, composites with relatively lower filler content could be selected for such cavities with relatively lower compliance (e.g. a Class-I cavity with thick tooth walls or the basal part in a cavity) and vice versa in order to reduce the final PS.
authors.affiliation: Biosystems and Biomaterials Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, United States.
authors.forename: Zhengzhi
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Biosystems and Biomaterials Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, United States. Electronic address: martin.chiang@nist.gov.
authors.forename: Martin Y M
authors.initials: MY
authors.lastname: Chiang
mesh_terms: D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry*; D002164:Camphor / Q000031:analogs & derivatives / Q000737:chemistry; D003188:Composite Resins / Q000138:chemical synthesis / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation*; D004548:Elasticity; D008422:Materials Testing / Q000295:instrumentation*; D011092:Polyethylene Glycols / Q000737:chemistry*; D058105:Polymerization; D011109:Polymethacrylic Acids / Q000737:chemistry*; D013499:Surface Properties; D062366:para-Aminobenzoates / Q000737:chemistry
chemical_list: D003188:Composite Resins; D011109:Polymethacrylic Acids; C113300:TPH hybrid; D062366:para-Aminobenzoates; C020946:triethylene glycol dimethacrylate; D011092:Polyethylene Glycols; D017438:Bisphenol A-Glycidyl Methacrylate; D002164:Camphor; C030747:ethyl 4-dimethylaminobenzoate; C553149:camphorquinone
keywords: Compliance of constraint; Curing kinetics; Dental composites; Filler content; Polymerization shrinkage stress