title: A comparison of the porcelain fracture resistance of screw-retained and cement-retained implant-supported metal-ceramic crowns.
abstract: BACKGROUND
  The presence of a screw opening on the occlusal surface of implant-supported metal-ceramic crowns may decrease the porcelain fracture resistance and shorten the longevity of the crown.
  
  
  OBJECTIVE
  The purpose of this study was to compare the porcelain fracture resistance between screw-retained and cement-retained implant-supported metal-ceramic crowns and to assess whether narrowing the occlusal table or offsetting the screw-access opening affects fracture resistance.
  
  
  METHODS
  Forty standardized maxillary premolar metal copings were fabricated with a Pd-Ga alloy (Protocol) on an implant abutment. Copings were divided into 4 groups (n=10): Group 1 (Screw-retained; occlusal surface buccolingual width=5 mm), screw access opening placed in the center of the occlusal surface; Group 2 (Screw-retained; occlusal surface buccolingual width=5 mm), screw access opening positioned 1 mm offset from the center of the occlusal surface toward the buccal cusp; Group 3 (Cement-retained; occlusal surface buccolingual width=5 mm), copings were not altered; and Group 4 (Cement-retained; occlusal surface buccolingual width=4 mm), copings designed to have a reduced occlusal surface width. All castings were finished with aluminum oxide stones and airborne-particle abraded. Two layers of opaque and dentin porcelain were applied, respectively, on all specimens, which were then glazed. The crown specimens were positioned in a custom testing apparatus and vertically loaded on the middle of the occlusal surface with a universal testing machine at a crosshead speed of 0.5 mm/min until fracture. Mean values of load at fracture (Kgf) were calculated in each group and compared with a 1-way analysis of variance and Tukey's Studentized test (alpha=.05).
  
  
  RESULTS
  Mean values of loads required to fracture the crowns were as follow: Group 1: 95.01+/-46.6 Kgf; Group 2: 108.61+/-57.9 Kgf; Group 3: 390.94+/-151.3 Kgf; Group 4: 380.04+/-211.8 Kgf. Groups 1 and 2 required a significantly lower force to fracture the crowns compared with Groups 3 and 4 (P=.0001). Comparing Group 1 with 2 (P=.9) and Groups 3 with 4 (P=.6), no significant differences were noted.
  
  
  CONCLUSIONS
  Screw-retained implant-supported metal-ceramic crowns demonstrated a significantly lower porcelain fracture resistance than cement-retained crowns. Placing the screw access opening 1 mm offset from the center of the occlusal surface did not result in lower fracture resistance. Cement-retained crowns with 4- or 5-mm buccolingual width of the occlusal surface showed similar porcelain fracture resistance.
authors.affiliation: Division of Prosthodontics, University of Rochester Eastman Dental Center, NY 14620, USA.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Torrado
authors.forename: Carlo
authors.initials: C
authors.lastname: Ercoli
authors.forename: Majd
authors.initials: M
authors.lastname: Al Mardini
authors.forename: Gerald N
authors.initials: GN
authors.lastname: Graser
authors.forename: Ross H
authors.initials: RH
authors.lastname: Tallents
authors.forename: Luca
authors.initials: L
authors.lastname: Cordaro
mesh_terms: D000704:Analysis of Variance; D003442:Crowns; D000044:Dental Abutments; D003738:Dental Cements / Q000737:chemistry*; D015921:Dental Implants; D003776:Dental Porcelain / Q000737:chemistry*; D017268:Dental Prosthesis Retention; D019094:Dental Prosthesis, Implant-Supported; D019232:Dental Restoration Failure; D005708:Gallium / Q000737:chemistry; D006801:Humans; D008422:Materials Testing; D016876:Metal Ceramic Alloys / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry; D013314:Stress, Mechanical
chemical_list: D003738:Dental Cements; D015921:Dental Implants; D016876:Metal Ceramic Alloys; D003776:Dental Porcelain; D010165:Palladium; D005708:Gallium