title: Influence of the resin cement thickness on the fatigue failure loads of CAD/CAM feldspathic crowns.
abstract: OBJECTIVE
  to evaluate the influence of the occlusal resin cement thickness on the cyclic loads-to-failure of feldspathic crowns and to compare the results to data from monotonic tests. A large range of cement thickness (50μm and 500μm) was tested, in order to better measure the influence of this variable.
  
  
  METHODS
  Feldspathic ceramic crowns (Vita Mark II blocks, Vita Zahnfabrik) were bonded to dentin analog dies (G10 (NEMA grade G10, International Paper), with occlusal resin cement thicknesses of 50μm and 500μm (Multilink Automix, Ivoclar). The dies were prepared with microchannels for water transport to the cement layer. After 96-h water storage, the specimens (n=20) were submitted to cyclic loads (500,000 cycles at 20Hz; initial maximum load=40% of monotonic load, from previous data) following a staircase sensitivity design (step size=25N). Failure loads at 500,000 cycles were compared to monotonic failure loads (from a previous study with specimens produced by the same author, using the same materials, specimen configuration and cementation protocol).
  
  
  RESULTS
  Crowns with an occlusal cement layer of 50μm were more resistant than those cemented with 500μm (246.4±22.9N vs. 158.9±22.9N), under wet cyclic testing conditions (p<0.001). The fatigue failure loads were reduced compared to monotonic loads: to 37% of monotonic for 50μm; to 53% of monotonic for 500μm.
  
  
  CONCLUSIONS
  An occlusal cement thickness of 50μm was more favorable for the structural performance of feldspathic crowns than was 500μm. Cyclic fatigue reduced failure loads well below those found under monotonic loading.
authors.affiliation: Restorative Dentistry Department, Federal University of Santa Maria, Santa Maria, RS, Brazil. Electronic address: liligmay@gmail.com.
authors.forename: Liliana
authors.initials: L
authors.lastname: Gressler May
authors.affiliation: Reconstructive Sciences, University of Connecticut Health Center, Farmington, CT, USA.
authors.forename: J Robert
authors.initials: JR
authors.lastname: Kelly
authors.affiliation: Department of Dental Materials and Prosthodontics, São Paulo State University, São José dos Campos, Brazil.
authors.forename: Marco Antonio
authors.initials: MA
authors.lastname: Bottino
authors.affiliation: Ivoclar/Vivadent Inc., Division for Research, Amherst, NY, USA.
authors.forename: Tom
authors.initials: T
authors.lastname: Hill
mesh_terms: D019245:Compressive Strength; D017076:Computer-Aided Design; D003442:Crowns; D003776:Dental Porcelain / Q000737:chemistry*; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D008422:Materials Testing; D019279:Resin Cements / Q000737:chemistry*
chemical_list: D019279:Resin Cements; C082340:Vita Mark II; D003776:Dental Porcelain
keywords: Cement thickness; Ceramic; Crowns; Failure load; Fatigue