title: The effect of core material, veneering porcelain, and fabrication technique on the biaxial flexural strength and weibull analysis of selected dental ceramics.
abstract: OBJECTIVE
  The objective of this study was to compare the effect of veneering porcelain (monolithic or bilayer specimens) and core fabrication technique (heat-pressed or CAD/CAM) on the biaxial flexural strength and Weibull modulus of leucite-reinforced and lithium-disilicate glass ceramics. In addition, the effect of veneering technique (heat-pressed or powder/liquid layering) for zirconia ceramics on the biaxial flexural strength and Weibull modulus was studied.
  
  
  METHODS
  Five ceramic core materials (IPS Empress Esthetic, IPS Empress CAD, IPS e.max Press, IPS e.max CAD, IPS e.max ZirCAD) and three corresponding veneering porcelains (IPS Empress Esthetic Veneer, IPS e.max Ceram, IPS e.max ZirPress) were selected for this study. Each core material group contained three subgroups based on the core material thickness and the presence of corresponding veneering porcelain as follows: 1.5 mm core material only (subgroup 1.5C), 0.8 mm core material only (subgroup 0.8C), and 1.5 mm core/veneer group: 0.8 mm core with 0.7 mm corresponding veneering porcelain with a powder/liquid layering technique (subgroup 0.8C-0.7VL). The ZirCAD group had one additional 1.5 mm core/veneer subgroup with 0.7 mm heat-pressed veneering porcelain (subgroup 0.8C-0.7VP). The biaxial flexural strengths were compared for each subgroup (n = 10) according to ISO standard 6872:2008 with ANOVA and Tukey's post hoc multiple comparison test (p≤ 0.05). The reliability of strength was analyzed with the Weibull distribution.
  
  
  RESULTS
  For all core materials, the 1.5 mm core/veneer subgroups (0.8C-0.7VL, 0.8C-0.7VP) had significantly lower mean biaxial flexural strengths (p < 0.0001) than the other two subgroups (subgroups 1.5C and 0.8C). For the ZirCAD group, the 0.8C-0.7VL subgroup had significantly lower flexural strength (p= 0.004) than subgroup 0.8C-0.7VP. Nonetheless, both veneered ZirCAD groups showed greater flexural strength than the monolithic Empress and e.max groups, regardless of core thickness and fabrication techniques. Comparing fabrication techniques, Empress Esthetic/CAD, e.max Press/CAD had similar biaxial flexural strength (p= 0.28 for Empress pair; p= 0.87 for e.max pair); however, e.max CAD/Press groups had significantly higher flexural strength (p < 0.0001) than Empress Esthetic/CAD groups. Monolithic core specimens presented with higher Weibull modulus with all selected core materials. For the ZirCAD group, although the bilayer 0.8C-0.7VL subgroup exhibited significantly lower flexural strength, it had highest Weibull modulus than the 0.8C-0.7VP subgroup.
  
  
  CONCLUSIONS
  The present study suggests that veneering porcelain onto a ceramic core material diminishes the flexural strength and the reliability of the bilayer specimens. Leucite-reinforced glass-ceramic cores have lower flexural strength than lithium-disilicate ones, while fabrication techniques (heat-pressed or CAD/CAM) and specimen thicknesses do not affect the flexural strength of all glass ceramics. Compared with the heat-pressed veneering technique, the powder/liquid veneering technique exhibited lower flexural strength but increased reliability with a higher Weibull modulus for zirconia bilayer specimens. Zirconia-veneered ceramics exhibited greater flexural strength than monolithic leucite-reinforced and lithium-disilicate ceramics regardless of zirconia veneering techniques (heat-pressed or powder/liquid technique).
authors.affiliation: Department of Oral Health and Rehabilitation, University of Louisville, School of Dentistry, Louisville, KY, USA. weishao.lin@louisville.edu
authors.forename: Wei-Shao
authors.initials: WS
authors.lastname: Lin
authors.forename: Carlo
authors.initials: C
authors.lastname: Ercoli
authors.forename: Changyong
authors.initials: C
authors.lastname: Feng
authors.forename: Dean
authors.initials: D
authors.lastname: Morton
mesh_terms: D000465:Algorithms; D000538:Aluminum Silicates / Q000737:chemistry; D001031:Apatites / Q000737:chemistry; D002516:Ceramics / Q000737:chemistry; D017076:Computer-Aided Design; D003460:Crystallization; D003776:Dental Porcelain / Q000737:chemistry*; D017267:Dental Prosthesis Design; D003801:Dental Veneers; D055119:Elastic Modulus; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D018583:Pliability; D011176:Post and Core Technique; D017680:Potassium Compounds / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D015019:Yttrium / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry
chemical_list: D000538:Aluminum Silicates; D001031:Apatites; C520133:IPS e.max CAD LT; C508050:IPS e.max Press; C075815:IPS-Empress ceramic; D017680:Potassium Compounds; C493120:lithia disilicate; C499362:yttria stabilized tetragonal zirconia; D003776:Dental Porcelain; C016447:feldspar; C078519:leucite; D015019:Yttrium; C023506:Glass ceramics; D015040:Zirconium; C025105:fluorapatite; C028541:zirconium oxide