title: Shrinkage stress compensation in composite-restored teeth: relaxation or hygroscopic expansion?
abstract: OBJECTIVE
  Polymerization of composite restorations causes shrinkage, which deforms and thus stresses restored teeth. This shrinkage deformation, however, has been shown to decrease over time. The objective was to investigate whether this reduction was caused by hygroscopic expansion or stress relaxation of the composite/tooth complex.
  
  
  METHODS
  Extracted molars were mounted in rigid stainless steel rings with four spherical reference areas. Twelve molars were prepared with large mesioocclusodistal slots, etched, bonded, and restored with a composite material (Filtek Supreme, 3M ESPE) in two horizontal layers. Ten intact molars were the controls. The teeth were stored either in deionized water or silicone oil. They were scanned after preparation (baseline), restoration (0-week), and after 1, 2, and 4 weeks storage. Scanned tooth surfaces were aligned with the baseline using the unchanged reference areas. Cuspal flexure was calculated from lingual and buccal surface deformation. To verify that the restorations had remained bonded, dye penetration at the interfaces was assessed using basic fuchsin dye. Statistical assessment was done by ANOVA followed by Student-Newman-Keuls post hoc test (p=0.05).
  
  
  RESULTS
  Substantial cuspal contraction was found for restored teeth after the composite was cured (13-14 μm cuspal flexure). After 4 weeks cuspal contraction decreased significantly for restored teeth stored in water (7.3 ± 3.2) but not for those stored in silicone oil (11.4 ± 5.0). Dye penetration of the occlusal interface was minimal in both groups (106 ± 87 and 21 ± 28 μm in water and silicone oil, respectively).
  
  
  CONCLUSIONS
  The results suggest that hygroscopic expansion was the main mechanism for shrinkage stress compensation.
authors.affiliation: College of Dentistry, University of Tennessee Health Science Center, Memphis, TN, USA.
authors.forename: Laurel A
authors.initials: LA
authors.lastname: Meriwether
authors.forename: Bernard J
authors.initials: BJ
authors.lastname: Blen
authors.forename: Jarred H
authors.initials: JH
authors.lastname: Benson
authors.forename: Robert H
authors.initials: RH
authors.lastname: Hatch
authors.forename: Daranee
authors.initials: D
authors.lastname: Tantbirojn
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry*; D001840:Dental Bonding / Q000379:methods*; D003793:Dental Restoration, Permanent / Q000379:methods*; D006801:Humans; D008963:Molar; D058105:Polymerization; D012827:Silicone Oils / Q000737:chemistry*; D017155:Wettability
chemical_list: D003188:Composite Resins; D012827:Silicone Oils