Influence of substance loss and restoration materials on the fracture resistance of 1-piece endodontic crowns: An in vitro study

How different restorative materials designed for computer-aided design and computer-aided manufacturing (CAD-CAM) and substance loss affect the fracture resistance of endodontically treated maxillary first premolars restored with 1-piece endodontic crowns is unclear. The purpose of this in vitro study was to assess the impact of various CAD-CAM restorative materials and residual tooth structure on the fracture resistance of endodontically treated maxillary first premolars restored with 1-piece endodontic crowns. Sixty-four maxillary first premolars were endodontically treated and divided into 4 main groups (n=16) according to the restorative material: ZP: Zirconia 1-piece endodontic crowns group (IPS e.max ZirCAD Prime; Ivoclar AG); RM: Resin-modified 1-piece endodontic ceramic crowns group (Katana Avencia; Kuraray Noritake Dental Inc); LD: Lithium disilicate 1-piece endodontic crowns group (IPS e.max CAD; Ivoclar AG); and CG: A control group restored with zirconia posts, composite resin cores, and lithium disilicate crowns (IPS e.max CAD; Ivoclar AG). Teeth in subgroups had either 1 or 2 residual coronal walls (n=8). All specimens underwent dynamic loading for 1 200 000 loading cycles in a masticatory simulator. A universal testing machine was then used to quasi-statically load the specimens at 30 degrees until fracture. For the statistical test, the Generalized Linear Model (GLM) with a gamma distribution and log link function was chosen (α=.05). None of the specimens showed any signs of debonding or fracture during the fatigue test. Mean ±standard deviation fracture loads ranged from 247.6 ±70.2 N (for group RM1) to 1211.5 ±243.2 N (for group RM2). A statistically significant increase in fracture resistance was observed with the increasing number of walls (P<.001). However, different CAD-CAM restorative materials did not affect the fracture resistance of endodontically treated maxillary first premolars restored with 1-piece endodontic crowns (P<.05). In this in vitro study, increasing the number of remaining walls dramatically enhanced the fracture resistance of endodontically treated maxillary first premolars restored with 1-piece endodontic crowns and subjected to thermomechanical fatigue, irrespective of the type of restorative material used.