title: The influence of incisal veneering porcelain thickness of two metal ceramic crown systems on failure resistance after cyclic loading.
abstract: BACKGROUND
  In some clinical situations, the length of either a prepared tooth or an implant abutment is shorter than ideal, and the occlusal clearance to be restored by a porcelain crown is large. Incisal thickness of the coping and the veneering porcelain should be considered to prevent mechanical failure of the crown.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the influence of incisal veneering porcelain thickness of metal ceramic systems on failure resistance after cyclic loading.
  
  
  METHODS
  With a standardized technique, 60 metal ceramic anterior crowns with 2 different incisal thicknesses of porcelain veneer (2.0 and 4.0 mm) and 2 different metal alloys (Pisces Plus (B), base metal, and Leo (N), high noble metal) were fabricated to fit a custom abutment and divided into 4 groups (B20, B40, N20, N40) (n=15). The crowns were then cemented using resin cement (PANAVIA 21) and thermal cycled and mechanically cycle loaded (49-N load) for 2,000,000 cycles. The specimens were evaluated for cracks and/or bulk fracture with an optical stereomicroscope (x10) and assigned a score of success, survival, or failure. The specimens without bulk fracture after cyclic loading were loaded along the long axis of the tooth, on the incisal edge, in a universal testing machine at a crosshead speed of 1.5 mm/min until fracture. The fracture strength value (N) was recorded. The exact logistic regression and Fisher's exact test were used to study the effect of different alloys and porcelain incisal thicknesses on the success and survival rates after cycle loading. The forces at failure (fracture) of different groups were compared using the rank transform-based nonparametric 2-way ANOVA (alpha=.05).
  
  
  RESULTS
  According to exact logistic regression, crowns of the high noble alloy group showed significantly higher success rates (P=.002) than those of the base metal group after cyclic loading. The success rate of the crowns with 2 mm of incisal veneering porcelain was significantly higher (P<.001) than that of the crowns with 4 mm of incisal veneering porcelain. For crowns with a 4-mm incisal thickness, N40 showed a significantly higher success rate than B40 (P=.04, Fisher's exact test). As for survival rate, the exact logistic regression indicated that neither metal type (P=.11) nor thickness (P=.60) had a significant effect. The 2-way ANOVA showed a significant effect of thickness (P=.001) and no significant effect of metal type (P=.08) on the failure load.
  
  
  CONCLUSIONS
  The high noble metal ceramic crowns demonstrated a significantly higher success rate after cyclic loading than the base metal ceramic crowns. Crowns with a porcelain incisal thickness of 2 mm showed, for both metal ceramic systems, a greater success rate than those with a 4-mm incisal thickness.
authors.affiliation: Advanced Education in General Dentistry Program, University of Rochester Eastman Dental Center, Rochester, NY, USA.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Geminiani
authors.forename: Heeje
authors.initials: H
authors.lastname: Lee
authors.forename: Changyong
authors.initials: C
authors.lastname: Feng
authors.forename: Carlo
authors.initials: C
authors.lastname: Ercoli
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry; D002484:Cementation / Q000379:methods; D003442:Crowns; D000044:Dental Abutments; D003776:Dental Porcelain / Q000737:chemistry*; D017267:Dental Prosthesis Design; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000295:instrumentation; D003801:Dental Veneers; D006801:Humans; D008422:Materials Testing; D016876:Metal Ceramic Alloys / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry; D017680:Potassium Compounds / Q000737:chemistry; D019279:Resin Cements / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature
chemical_list: D000538:Aluminum Silicates; D016876:Metal Ceramic Alloys; C093719:Panavia TPN-S; D010710:Phosphates; D017680:Potassium Compounds; D019279:Resin Cements; D003776:Dental Porcelain; C016447:feldspar