title: Marginal accuracy and fracture strength of ceromer/fiber-reinforced composite crowns: effect of variations in preparation design.
abstract: BACKGROUND
  Targis/Vectris restorations provide excellent esthetics and clinical success; however, the relationship of their marginal accuracy and fracture strength to the tooth preparation design requires further investigation.
  
  
  OBJECTIVE
  The aim of this study was to evaluate the effect of variations in tooth preparation design on the marginal accuracy before and after cementation and on the fracture strength of the ceromer/fiber reinforced composite crown.
  
  
  METHODS
  Three metal dies with varying total occlusal convergence angles (6 degrees, 10 degrees, 15 degrees) were prepared. A total of 30 (10 for each angle) Targis/Vectris crowns were fabricated. The restorations were evaluated at 48 points on the entire circumferential margin with a stereomicroscope measuring in micrometers for margin adaptation before and after cementation. The specimens then were compressively loaded to failure in a universal testing machine. Marginal adaptation was analyzed with Kruskal-Wallis test and post-hoc Dunnett test (alpha=0.05). The fracture strength was analyzed with analysis of variance and the Scheffe adjustment at the 95% significance level. Fracture surfaces of the crowns were examined with a scanning electron microscope to determine the mode of fracture.
  
  
  RESULTS
  The smallest marginal gap was recorded in angled crowns with a 6-degree convergence (47 microm mean). The marginal gap of most (95.6%) of the crowns was within a clinically acceptable level (established as </=100 microm). The mean marginal gap increased significantly after cementation, with the largest increase in the 6 degree group (76 microm mean). The 6-degree angled crowns had a significantly higher fracture strength (1,543 N) than the more convergent crowns (1,366 N) (P<.001). Mean fracture strength of all the crowns was 1,390 N. Scanning electron microscope observation showed that all crowns exhibited a 2-phase fracture pattern: a crack and chipping of the Targis layer followed by adhesive failure.
  
  
  CONCLUSIONS
  Within the limitations of this experimental design, decreasing the axial convergence angle of the ceromer/fiber-reinforced composite crowns diminished their marginal gap and increased their fracture strength.
authors.affiliation: College of Dentistry, Kangnung National University, Kangnung, Korea.
authors.forename: LeeRa
authors.initials: L
authors.lastname: Cho
authors.forename: HoYong
authors.initials: H
authors.lastname: Song
authors.forename: JaiYoung
authors.initials: J
authors.lastname: Koak
authors.forename: SeongJoo
authors.initials: S
authors.lastname: Heo
mesh_terms: D000704:Analysis of Variance; D002484:Cementation; D002516:Ceramics; D003188:Composite Resins; D003442:Crowns; D018772:Dental Marginal Adaptation; D003764:Dental Materials; D017267:Dental Prosthesis Design; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D004548:Elasticity; D005898:Glass; D005899:Glass Ionomer Cements; D006801:Humans; D007180:Incisor; D008422:Materials Testing; D003765:Models, Dental; D012823:Silicate Cement; D018709:Statistics, Nonparametric; D019231:Tooth Preparation, Prosthodontic / Q000379:methods*
chemical_list: D003188:Composite Resins; D003764:Dental Materials; D005899:Glass Ionomer Cements; C111841:ceromer; C111282:dental polyglass; C009753:fiberglass; D012823:Silicate Cement