Objective: Determination of fracture strength of different all-ceramic dental crown systems is not straight forward. Most in vitrotest methods induce contact damage at the loading point (crunch the crown-tests). Clinical failures, on the other hand, usually originate from cracks in the cervical margins. The aim of this study was to compare three all-ceramic systems using a test method that mimic clinical failure modes.
Method: 30 core-veneer crowns were produced for an upper central incisor; 10 with alumina cores (Vita In-Ceram AL for inLab), 10 with zirconia cores (Starceram Z-Al-Mes HD) and 10 with lithium disilacte cores (IPS e-max). The crowns were cemented with zinc phosphate cement on epoxy models of the prepared abutment and exposed to soft loading at the incisal edge until fracture. The soft loading induces a lateral expansion in the middle of the abutment model causing tension at the cervical margin of the crown. The fractographic features were evaluated by light microscopy and compared to a reference group of crowns failed during clinical use.
Result: There crowns with zirconia cores fractured at statistically significant higher loads than the other two materials (Kruskall-Wallis<0.001). No statistical significant difference was found between crowns with alumina and lithium disilicate cores (Mann-Whitney U-test) (Table 1). All crowns fractured in a manner similar to the clinical references. Fracture initiation was in the core material, cervically in the approximal areas.
Conclusion: Clinically relevant test methods should be further developed for assessment of all-ceramic crown failures. Table 1 Lithium disilicate (n=10) Zirconia (n=10) Alumina (n=10) Newton (SD) 815 (136) 1572 (233) 870 (101)


Clinically relevant evaluation of fracture of all-ceramic crowns
Øilo M, Kvam K, Gjerdet NR.