The in vitro failure of all-ceramic crowns and the connector area of fixed partial dentures using bilayered ceramic specimens: the influence of core to dentin thickness ratio.

AIMS: To assess the effect of core to dentine thickness ratio on the bi-axial flexure strength and fracture mode and failure origin using bilayered ceramic specimens as an in vitro assessment for all-ceramic crowns and the connector area of fixed partial dentures (FPDs). METHODS: Sets of 30 bilayered composite discs, with a core layer thickness of 1mm and with core to dentine thickness ratios of 2:1, 1:1 and 1:2, were tested in bi-axial flexure with both the reinforcing core and veneering dentine loaded in tension. Mean flexure strengths, standard deviations and associated Weibull moduli (m) were determined. Optical microscopy was employed for identification of the fracture mode and failure origin for the failure all-ceramic crowns and the connector area of FPDs. RESULTS: For a core thickness of 1mm the core to dentine thickness ratio failed to influence the bi-axial flexure strength data when both the reinforcing core and veneering dentine porcelain were tested in tension. The number of fracture fragments, frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc-shaped specimens were dependent on the core to dentine thickness ratio and the surface loaded in tension. SIGNIFICANCE: The fracture mode and failure origin in bilayered ceramics tested to represent the failure mode of all-ceramic crowns and FPDs was dependent upon the core to dentine thickness ratio employed. However, the conventional wisdom regarding bilayered ceramic specimens with core thicknesses greater than 1mm are not followed when the core thickness was reduced to 1mm since the fracture resistance was not dependent on the core to dentine thickness ratio.

The effect of core:dentin thickness ratio on the bi-axial flexure strength and fracture mode and origin of bilayered dental ceramic composites.

OBJECTIVES: The aim of the current study was to assess the effect of core:dentin thickness ratio on the flexure strength, fracture mode and origin of bilayered dental ceramic composite disc specimens. METHODS: Sets of 30 bilayered composite discs with core:dentin thickness ratio of 2:1, 1:1 and 1:2 were tested in bi-axial flexure with both the reinforcing core and veneering dentin loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. A combination of optical and scanning electron microscopy was employed for identification of the fracture mode and origin. RESULTS: The core:dentin ratio influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentin porcelain were tested in tension. The strength and reliability was increased for a core:dentin thickness of 2:1. The number of fracture fragments, the frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc shaped specimens was also dependent on the core/dentin ratio and the surface loaded in tension. CONCLUSIONS: Core:dentin thickness ratio influences the bi-axial flexure strength and fracture mode and origin in bilayered dental ceramic composite specimens.

The in-vitro clinical failure of all-ceramic crowns and the connector area of fixed partial dentures: the influence of interfacial surface roughness.

OBJECTIVES: To assess the effect of interfacial surface roughness on the flexure strength and fracture mode and origin utilizing an in-vitro assessment of the clinical failure conditions expected for all-ceramic crowns and the connector area of fixed partial dentures (FPDs) using bilayered ceramic specimens tested in bi-axial flexure. METHODS: Sets of 20 bilayered composite discs, with core:dentine thickness ratio of 2:1 and interfacial surface roughnesses determined by alumina abrasion with different alumina particle sizes, were tested in bi-axial flexure with both the reinforcing core and veneering dentine loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. Optical microscopy was employed for identification of the fracture mode and origin for the failure all-ceramic crowns and the connector area of FPDs. RESULTS: The interfacial surface roughness influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentine porcelain were tested in tension. The number of fracture fragments, frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc-shaped specimens was also dependent on the interfacial surface roughness and the surface loaded in tension. CONCLUSIONS: The fracture resistance, failure mode and failure origin in bilayered ceramics tested to represent the clinical failure mode of all-ceramic crowns and FPDs are dependent upon the interfacial surface roughness and the modulus of the material in tension.

An in-vitro investigation of the accuracy of fit of Procera and Empress crowns.

The current study aimed to investigate the accuracy of fit and the reproducibility of inner crown profile for two types of high strength ceramics, IPS Empress and Procera. Procera and Empress crowns with four different morphologies were cemented to dies using zinc phosphate dental cement. Vertical and horizontal sections were made through each of the crown/die preparations and images of the vertical sections were compared for curvature reproduction by alignment using image processing. Measurements were made on horizontal sections to determine cement layer thickness. Alignment of the crowns using image analysis identified quantifiable variations in the inner surface profile compared with the outer surface of the die. The largest differences occurred from the cusp tips to the occlusal adaptation area and differences in surface profile were less pronounced for Procera than Empress crowns. Marginal gap varied independently of ceramic or internal crown shape from 7-529 microm for Procera and 26-548 microm for Empress. IPS Empress has a superior ability to reproduce the inner surface profile of the crown morphologies investigated compared with Procera. The reduced reproduction of surface profile was associated with an increased cement thickness at the occlusal contact area that may inadvertently lead to failure of the crowns functional characteristics.

The influence of interfacial surface roughness on bilayered ceramic specimen performance.

OBJECTIVES: The aim of the current study was to assess the influence of interfacial surface roughness on the performance of bilayered ceramic composite disc-shaped specimens. METHODS: Specific surface flaw distributions were introduced to a series of standard aluminous core porcelain disc specimens (12 mm diameter, 1.7 mm thickness) prior to bilayering with dentin porcelain. Mean flexure strengths, standard deviations and associated Weibull moduli (m) were determined using bi-axial flexure (ball-on-ring) for each series of 30 bilayered specimens. RESULTS: The mean bi-axial fracture strengths of bilayered specimens resulted in significant differences between specimen groups with the smoothest interfacial roughness recording the highest strength and fracture strength reliability. Specimen delamination was identified in five of the bilayered group with the smoothest interfacial roughness compared with no specimen delaminations in the rougher interface groups. However, fracture occurred in the weaker dentin layer rather than along the interface between the dentin and core porcelain for specimens in these rougher interface groups. CONCLUSIONS: A smoother interfacial surface roughness resulted in increased performance, namely increased strength and reliability amongst the bilayered dental ceramic composite specimens examined in the current investigation. The incidence of complete or partial delamination of the reinforcing core and aesthetic dentin was increased for these specimens since the reduced tortuosity of the interface between porcelain layers fails to restrict propagation of a crack front along the interface.