title: Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement.
abstract: OBJECTIVE
  To determine, by means of static fracture testing the effect of the tooth preparation design and the elastic modulus of the cement on the structural integrity of the cemented machined ceramic crown-tooth complex.
  
  
  METHODS
  Human maxillary extracted premolar teeth were prepared for all-ceramic crowns using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita MK II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3). The machined all-ceramic crowns were resin bonded to the tooth structure using one of three cements with different elastic moduli: Super-Bond C&B, Rely X Unicem and Panavia F 2.0. The specimens were subjected to compressive force through a 4mm diameter steel ball at a crosshead speed of 1mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the fracture point was recorded for each specimen in Newtons (N). These values were compared to a control group of unprepared/unrestored teeth.
  
  
  RESULTS
  There was a significant difference between the control group, with higher fracture strength, and the cemented samples regardless of the occlusal design and the type of resin cement. There was no significant difference in mean fracture load between the two designs of occlusal preparation using Super-Bond C&B. For the Rely X Unicem and Panavia F 2.0 cements, the proposed preparation design with a flat occlusal morphology provides a system with increased fracture strength.
  
  
  CONCLUSIONS
  The proposed novel flat design showed less dependency on the resin cement selection in relation to the fracture strength of the restored tooth. The choice of the cement resin, with respect to its modulus of elasticity, is more important in the anatomic design than in the flat design.
authors.affiliation: The University of Sheffield, Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, UK. Electronic address: s.shahrbaf@sheffield.ac.uk.
authors.forename: Shirin
authors.initials: S
authors.lastname: Shahrbaf
authors.affiliation: The University of Sheffield, Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: van Noort
authors.affiliation: Tabriz University of Medical Sciences, Tabriz Dental School, Orthodontic Department, Tabriz, Iran.
authors.forename: Behnam
authors.initials: B
authors.lastname: Mirzakouchaki
authors.affiliation: School of Mechanical and Aerospace Engineering, Queen's University of Belfast, Ashby Building, Room 3020, Stranmillis Road, Belfast, BT9 5AH, UK.
authors.forename: Elaheh
authors.initials: E
authors.lastname: Ghassemieh
authors.affiliation: The University of Sheffield, Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, UK.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Martin
mesh_terms: D001641:Bicuspid; D002516:Ceramics; D003442:Crowns; D003738:Dental Cements; D004548:Elasticity; D006801:Humans; D008422:Materials Testing
chemical_list: D003738:Dental Cements
keywords: Anatomic design; CAD/CAM; Elastic modulus; Flat design; Maxillary premolar; Mechanical test; Resin luting agent; Vita MK II