In vitro fracture strength of teeth restored with different all-ceramic crown systems

There is insufficient knowledge of the strength of all-ceramic crowns bonded to natural teeth to warrant the use of all-ceramic crowns in place of metal-ceramic crowns. The purpose of this study was to evaluate and compare fracture resistance of crowns made of 3 different types of 2 all-ceramic crown systems - 0.4-mm and 0.6-mm aluminum oxide coping crowns and zirconia ceramic coping crowns - and metal-ceramic crowns. Forty intact, noncarious human maxillary central incisors were divided into 4 groups (n=10): Group MCC (control), metal-ceramic crown (JRVT High Noble Alloy); Group AC4, crown with 0.4-mm aluminum oxide coping (Procera AllCeram); Group AC6, crown with 0.6-mm aluminum oxide coping (Procera AllCeram); and Group ZC6, crown with 0.6-mm zirconia ceramic coping (Procera AllZirkon). Teeth were prepared for complete-coverage all-ceramic crowns so that a final dimension of 5.5 ± 0.5 mm was achieved incisocervically, mesiodistally, and faciolingually. A 1.0-mm deep shoulder finish line was used with a rounded internal line angle. All restorations were treated with bonding agent (Clearfil SE Bond) and luted with phosphate-monomer-modified adhesive cement (Panavia 21). Fracture strength was tested with a universal testing machine at a crosshead speed of 2 mm per minute with an angle of 30 degrees to the long axis of the tooth after restorations were stored in 100% relative humidity of a normal saline solution for 7 days. The mode of fracture was examined visually. Means were calculated and analyzed with 1-way ANOVA and Tukey's HSD (α=.05). The means of fracture strength were: Group MCC, 405 ± 130 N; Group AC4, 447 ± 123 N; Group AC6, 476 ± 174 N; and Group ZC6, 381 ± 166 N. There was no significant difference between groups (P=.501). The mode of failure for all specimens was fracture of the natural tooth. There was no significant difference in the fracture strength of the teeth restored with all-ceramic crowns with 0.4- and 0.6-mm aluminum oxide copings, 0.6-mm zirconia ceramic copings, and metal ceramic crowns.