Effect Of Interface Geometry On The Integrity Of Ceramic Crowns

Method: A typodont maxillary first premolar was prepared for an all-ceramic crown in accordance with the manufacturer’s guidelines for monolithic ceramic crowns (IPS e.max®; Ivoclar-Vivadent, Liechtenstein). 60 dies were duplicated in a polymer with a Young’s Modulus closely matched to dentine (Alpha die, Schütz GmbH). Three different crown fabrication techniques were used (n=20): (i) Manually applied wax spacer and pressed-crown; (ii) digitally scanned preparation, CAD-printed wax-pattern (D76PLUS, Solidscape Inc.) and pressed-crown; (iii) digitally scanned preparation and machined-crown (CEREC-inLab® v3.6 Sirona GmbH). A resin-based cement (Variolink-II, Ivoclar-Vivadent, Liechtenstein) was employed with a standardised mechanised cementation technique to apply a controlled axial cementation pressure [Universal testing machine (Lloyd LRX®, Lloyd Materials Testing Inc)]. The samples were subjected to fatigue life testing with a cyclic impact load of 453N (Fatigue limit minus 100N) for 1.25x10⁶cycles at 37C⁰ and 1Hz frequency until the point of fracture or survival.

Result: There was a significant difference (One-way ANOVA) in the resistance to fatigue loading between the three groups. Weibull probability analysis and the α and β Weibull parameters indicate that the teeth restored with a ‘Manually-applied wax spacer and pressed-crown’ are best able to resist cyclic fatigue loading. They also have the most uniform interface geometry. 

Conclusion: Teeth restored with IPS e.max® crowns constructed by manually applied wax spacer and pressing have a more uniform interface and a greater structural integrity than wax CAD-printed patterns or CAD-CAM crowns.