title: Fracture resistance of posterior metal-free polymer crowns.
abstract: BACKGROUND
  Improved mechanical properties of contemporary composites has resulted in the extended use of composites for the restoration of posterior teeth. Although the indication of polymers was extended to metal-free individual crowns, the influence of tooth preparation design and cementation methods on the stability of these artificial crowns remains unknown.
  
  
  OBJECTIVE
  This in vitro study evaluated the effect of axial tooth preparation design, occlusal dimension, and cementation technique on the fracture resistance of metal-free posterior Artglass crowns.
  
  
  METHODS
  Seventy-two extracted human third molars, assigned to experimental groups by size, received standardized tooth preparation. Axial tooth preparation included an invasive approach with 1-mm deep shoulder and a less invasive 0.5-mm chamfer preparation, whereas occlusal reduction was either 0.5 mm or 1.3 mm. Artglass crowns that restored the original tooth contour were cemented with 3 cements: zinc phosphate cement (ZnP), glass ionomer cement (GIC), or a resinous cement in combination with a dentinal bonding agent. After 10,000 thermal cycles between 5 degrees C and 55 degrees C, artificial crowns were vertically loaded until compression to failure. Significant differences of fracture loads between experimental groups were assessed by paired Mann-Whitney U tests.
  
  
  RESULTS
  Minimal fracture resistance for all combinations excluded 500 N. However, 9 of 24 Artglass crowns cemented with ZnP loosened after thermocycling. Adhesive cementation resulted in a significantly greater fracture resistance compared with GIC and ZnP (P=.02). Increased occlusal thickness (0.5 to 1.3 mm) resulted in greater stability, whereas a 1-mm deep shoulder tooth preparation did not improve durability compared with a 0.5-mm chamfer finishing line.
  
  
  CONCLUSIONS
  A minimally invasive 0.5-mm axial chamfer tooth preparation combined with sufficient occlusal reduction and adhesive cementation recorded the greatest stability for posterior metal-free Artglass crowns.
authors.affiliation: Ludwig-Maximilians-University, Munich, Germany. pram@dent.med.uni-muenchen.de
authors.forename: P
authors.initials: P
authors.lastname: Rammelsberg
authors.forename: G
authors.initials: G
authors.lastname: Eickemeyer
authors.forename: K
authors.initials: K
authors.lastname: Erdelt
authors.forename: P
authors.initials: P
authors.lastname: Pospiech
mesh_terms: D002484:Cementation / Q000379:methods; D019245:Compressive Strength; D003442:Crowns; D001840:Dental Bonding; D003776:Dental Porcelain; D017220:Dentin-Bonding Agents; D005899:Glass Ionomer Cements; D006801:Humans; D008422:Materials Testing; D008963:Molar; D019279:Resin Cements; D012823:Silicate Cement; D018709:Statistics, Nonparametric; D019231:Tooth Preparation, Prosthodontic; D015036:Zinc Phosphate Cement
chemical_list: D017220:Dentin-Bonding Agents; D005899:Glass Ionomer Cements; D019279:Resin Cements; C111282:dental polyglass; D003776:Dental Porcelain; D012823:Silicate Cement; D015036:Zinc Phosphate Cement