IADR Abstract Archives
Margin Design Determines Fracture Strength of Zirconia Crown Analogues
Objectives: We demonstrate a systematic approach on zirconia dome-like structures simulating the build-up and effect of radial hoop stress on crown marginal design and fracture. The experiments are combined with numerical simulations, elucidating the distribution of maximum principal stress.
Methods: CAD models of dome-like half-spheres were generated and implemented in the CAM device (PrimeMill, DentsplySirona). The specimens were 10 mm in diameter and 7 mm in height with a wall thickness of 1.0 mm after sintering. A rounded shoulder with 0.5 mm radius was designed and compared with a chamfer and a tapered preparation design. The domes (n=30 per group) were milled using a 4/5Y-PSZ zirconia multilayer chairside block (e.max ZirCAD MT Multi; C17; Ivoclar) and speed sintered in the CS6 Programat (Ivoclar). A tapered (7°) and hardened steel plunger was used to fracture the domes by inducing radial hoop stress in a universal testing machine (Zwick, Germany). FE modelling served to calculate the maximum principal stress distribution at margins. Fractographic analysis was performed to confirm the fracture origins. Weibull statistics were applied on the hoop strength data. The data were statistically analyzed using ANOVA (p=.05).
Results: Characteristic fracture forces and (Weibull moduli) were 744.3 N (8.8) for shoulder, 651.1 N (9.0) for chamfer, and 577.4 N (7.3) for tapered marginal designs. Fracture forces translate into maximum principal stress at fracture of 1588.8/ 1616.5/ 1468.8 MPa (shoulder/ chamfer/ taper) at the inner contact to the metal plunger and 1225.6/ 1328.1/ 1290.1 MPa at the outer margins. Fractography showed fracture initiation at dome margins with prominent machining damage at thin margins.
Conclusions: While the influence of margin design on fracture strength is insignificant when comparing shoulder and chamfer preparations, the influence of fairly tapered marginal design is significant in terms of reduced maximum principal stress at fracture. Fractographic observations especially at tapered margins revealed a high incidence of margin fractures, resulting in reduction of Weibullmodulus.
Methods: CAD models of dome-like half-spheres were generated and implemented in the CAM device (PrimeMill, DentsplySirona). The specimens were 10 mm in diameter and 7 mm in height with a wall thickness of 1.0 mm after sintering. A rounded shoulder with 0.5 mm radius was designed and compared with a chamfer and a tapered preparation design. The domes (n=30 per group) were milled using a 4/5Y-PSZ zirconia multilayer chairside block (e.max ZirCAD MT Multi; C17; Ivoclar) and speed sintered in the CS6 Programat (Ivoclar). A tapered (7°) and hardened steel plunger was used to fracture the domes by inducing radial hoop stress in a universal testing machine (Zwick, Germany). FE modelling served to calculate the maximum principal stress distribution at margins. Fractographic analysis was performed to confirm the fracture origins. Weibull statistics were applied on the hoop strength data. The data were statistically analyzed using ANOVA (p=.05).
Results: Characteristic fracture forces and (Weibull moduli) were 744.3 N (8.8) for shoulder, 651.1 N (9.0) for chamfer, and 577.4 N (7.3) for tapered marginal designs. Fracture forces translate into maximum principal stress at fracture of 1588.8/ 1616.5/ 1468.8 MPa (shoulder/ chamfer/ taper) at the inner contact to the metal plunger and 1225.6/ 1328.1/ 1290.1 MPa at the outer margins. Fractography showed fracture initiation at dome margins with prominent machining damage at thin margins.
Conclusions: While the influence of margin design on fracture strength is insignificant when comparing shoulder and chamfer preparations, the influence of fairly tapered marginal design is significant in terms of reduced maximum principal stress at fracture. Fractographic observations especially at tapered margins revealed a high incidence of margin fractures, resulting in reduction of Weibullmodulus.