Residual Stresses in Bilayer Dental Ceramics
Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, FL 32610, USA. Biomaterials
(Impact Factor: 8.56).
07/2005; 26(16):3235-41. DOI: 10.1016/j.biomaterials.2004.08.025
It is clinically observed that lithia-disilicate-based all-ceramic fixed partial dentures (FPD) can fail because of the fragmentation of the veneering material. The hypothesis of this study is that the global residual stresses within the surface of those veneered FPDs may be responsible for partial fragmentation of the veneering ceramic. Bilayer and monolithic ceramic composites were prepared using a lithia disilicate based (Li2OSiO2) glass-ceramic core and a glass veneer. A four-step fracture mechanics approach was used to analyze residual stress in bilayered all-ceramic FPDs. We found a statistically significant increase in the mean flexural strengths of bilayer specimens compared with monolithic glass specimens (p < or = 0.05). There was a statistically significant difference between the mean longitudinal and transverse indentation-induced crack sizes in bilayer specimens (p < or = 0.05), which indicates the existence of residual stress. Global residual stresses in the veneer layer, calculated using a fracture mechanics equation, were determined to be responsible for the increased strength and observed chipping, i.e., spallation in bilayer ceramic composites.
Available from: Mutlu Özcan
- "  Residual stresses generated due to differential cooling should not be neglected. The mismatch of CTE between the framework and the veneer ceramic yields to increased interfacial failures as a result of increased tensile stresses at the interface and compressive stresses within the veneer ceramic. It has been also reported that the mismatch of CTE causes high tensile pre-stress at the framework–veneer interface. "
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ABSTRACT: This study investigated the adhesion between zirconia framework and four veneering ceramic (VC) materials with varying coefficients of thermal expansions (CTE). Zirconia rods (N = 40) (ICE Zirkon) (diameter: 4 mm, height: 20 mm) were milled and sintered. After firing, the zirconia rods were air-abraded and cleaned. They were randomly assigned to receive four VCs (N = 10/group), namely (a) Vita VM9 (VZ; 9–9.2 × 10−6 K−1), (b) Cerabien ZR (CZ; 9.1 × 10−6 K−1), (c) Matchmaker ZR (MM; 9.4 × 10−6 K−1), and (d) Ice Zirconia Ceramic (IZ; 9.6 × 10−6 K−1). The VCs were then fired onto zirconia rods (height: 2 mm, thickness: 2 mm) circumferentially and were thermocycled for 6000 times (5/55 °C, dwell time: 30 s). Specimens were loaded from the top of the zirconia rods (0.5 mm/min) in a universal testing machine until debonding. Shell–Nielsen bond strength values were calculated (MPa). Failure types were evaluated under SEM. The data were statistically analyzed (one-way ANOVA, Tukey’s; α = 0.05). Weibull distribution values including the Weibull modulus (m) (0.05) was calculated. The highest mean bond strength (MPa) was obtained for CZ (42.08 ± 4.08), followed by VZ (41.77 ± 4.92), MM (40.7 ± 3.64), and IZ (40.05 ± 5.78). While mean bond strength for VZ, MM, and IZ were not significantly different (p > 0.05), CZ was significantly higher than that of IZ (p m = 16.94) and the highest for MM (m = 20.16). Mainly, adhesive failures followed by mixed failures were observed. VCs with a greater mismatch of CTE with the zirconia framework exhibited similar Shell–Nielsen bond strength to those with fewer mismatches. CTE mismatch did not affect the results of CZ (9.1 × 10−6 K−1) and IZ (9.6 × 10−6 K−1).
Available from: Chuchai Anunmana
- "The remarkable increase in strength of the bilayered over monolithic glass veneer was explained by some rationales, including dissimilarity of thermal expansion coefficient between veneer and core materials, and the difference in elastic behaviors of the two materials. These lead to generation of global residual compressive stress in the veneer layer.18,19 Although porcelains designed for application to ceramic frameworks may have appropriate coefficient of thermal expansion (CTE), considerable internal stresses can still develop.20 "
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To investigate the microtensile bond strength between two all-ceramic systems; lithium disilicate glass ceramic and zirconia core ceramics bonded with their corresponding glass veneers.
MATERIALS AND METHODS
Blocks of core ceramics (IPS e.max® Press and Lava™ Frame) were fabricated and veneered with their corresponding glass veneers. The bilayered blocks were cut into microbars; 8 mm in length and 1 mm2 in cross-sectional area (n = 30/group). Additionally, monolithic microbars of these two veneers (IPS e.max® Ceram and Lava™ Ceram; n = 30/group) were also prepared. The obtained microbars were tested in tension until fracture, and the fracture surfaces of the microbars were examined with fluorescent black light and scanning electron microscope (SEM) to identify the mode of failure. One-way ANOVA and the Dunnett's T3 test were performed to determine significant differences of the mean microtensile bond strength at a significance level of 0.05.
The mean microtensile bond strength of IPS e.max® Press/IPS e.max® Ceram (43.40 ± 5.51 MPa) was significantly greater than that of Lava™ Frame/Lava™ Ceram (31.71 ± 7.03 MPa)(P<.001). Fluorescent black light and SEM analysis showed that most of the tested microbars failed cohesively in the veneer layer. Furthermore, the bond strength of Lava™ Frame/Lava™ Ceram was comparable to the tensile strength of monolithic glass veneer of Lava™ Ceram, while the bond strength of bilayered IPS e.max® Press/IPS e.max® Ceram was significantly greater than tensile strength of monolithic IPS e.max® Ceram.
Because fracture site occurred mostly in the glass veneer and most failures were away from the interfacial zone, microtensile bond test may not be a suitable test for bonding integrity. Fracture mechanics approach such as fracture toughness of the interface may be more appropriate to represent the bonding quality between two materials.
Available from: Ke Zhao
- "Based on clinical long-term data LDG bilayered single crowns can be considered as a reliable and promising treatment option that can be recommended in the anterior and posterior region . However, when a core ceramic layer is veneered with porcelain, the resulting bilayered ceramic composite has a significantly lower strength and higher failure rate compared to the monolithic core ceramic   . The following four explanations have been suggested to rationalize these observations: (1) veneering porcelain was susceptible to failure at lower loads, because of lower fracture strength compared to core material ; (2) the use of veneering porcelains to improve esthetics often required a reduction of core thickness that could limit mechanical properties ; (3) the conventional manual layering technique involved laborious manual technique-sensitivity with the potential of defects within the veneer or at the core–veneer interface, that possibly affect the final quality of the restorations ; (4) the resultant multilayer structure increased the complexity of stress distribution within the restorations . "
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ABSTRACT: This in vitro study was designed to investigate the influence of the veneer and cyclic loading on the failure behavior of lithium disilicate glass-ceramic (LDG) crowns on maxillary first molar.
Sixty-four LDG crowns were divided into 4 groups (n=16). Thirty-two monolithic crowns were fabricated from IPS e.max Press (M), and the remaining bilayered crowns using cut-back technique and conventional manual layering technique from IPS e.max Press/Ceram (B). Monolithic or bilayered crowns were subjected to single-load-to-fracture (SLF) testing using a universal testing machine, before (M1 and B1) and after exposure to sliding-contact fatigue (SCF) testing (M2 and B2), consisting of 1,200,000 mechanical cycles (Fmax=98N). Data were statistically analyzed using two-by-two factorial design ANOVA. Fractographic analysis was performed to determine the fracture modes of the failed specimens.
The mean fracture load values (N±S.D.) for M1, B1, M2 and B2 were 2686±628N, 1443±327N, 2133±578N and 1464±419N, respectively. Significant differences were found between the failure loads of all groups (P<0.001), except between groups B1 and B2. Bulk fracture initiating from the occlusal surface is the primary failure mode of monolithic and veneered LDG crowns. Cracking that initiated from core-veneer interfacial defects and ultimately resulted in bulk fracture is another major failure origin of veneered all-ceramic crowns.
Veneer application resulted in significantly lower fracture load values compared to monolithic LDG crowns. Cyclic loading is an accelerating factor contributing to fracture for monolithic LDG crowns but not for bilayered ones.
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