Lifetime prediction of veneered versus monolithic lithium disilicate crowns loaded on marginal ridges.

OBJECTIVE: To evaluate the probability of survival of monolithic and porcelain veneered lithium disilicate crowns comprised by a conventional or modified core when loaded on marginal ridges. METHODS: Lithium disilicate molar crowns (n=30) were fabricated to be tested at mesial and distal marginal ridges and were divided as follows: (1) bilayered crowns with even-thickness 0.5mm framework (Bi-EV); (2) bilayered crowns with modified core design (Bi-M-lingual collar connected to proximal struts), and: (3) monolithic crowns (MON). After adhesively cemented onto composite-resin prepared replicas, mesial and distal marginal ridges of each crown (n=20) were individually cyclic loaded in water (30-300N) with a ceramic indenter at 2Hz until fracture. The 2-parameter Weibull was used to calculate the probability of survival (reliability) (90% 2-sided confidence bounds) at 1, 2, and 3 million cycles and mean life. RESULTS: The reliability at 1 and 2 million cycles was significantly higher for MON (47% and 19%) compared to Bi-EV (20% and 4%) and Bi-M (17% and 2%). No statistical difference was found between bilayered groups. Only the MON group presented crown survival (7%) at 3 million cycles. The mean life was highest for MON (1.73E+06), lowest for Bi-M (573,384) and intermediate for Bi-E (619,774). Fractographic analysis showed that the fracture originated at the occlusal surface. The highest reliability was found for MON crowns. The modified framework design did not improve the fatigue life of crowns. SIGNIFICANCE: Monolithic lithium disilicate crowns presented higher probability of survival and mean life than bilayered crowns with modified framework design when loaded at marginal ridges.

Effect of aging and testing method on bond strength of CAD/CAM fiber-reinforced composite to dentin.

OBJECTIVES: To evaluate and compare the outcomes of shear (S) and microtensile (µT) bond strength tests of CAD/CAM fiber-reinforced composite (FRC) to dentin. Aging with either fatigue or thermocycling were conducted for comparison with baseline microtensile group. METHODS: CAD/CAM FRC (Trinia, Bicon LLC, Boston, USA) blocks were milled to 3-mm diameter cylinders for shear and to blocks (5×5×5mm) for µT. Sixty extracted human molars were flattened to obtain dentin surfaces and randomly divided in four groups (n=15): (1) SC: samples tested in shear 24h after bonding; (2) µTC: samples tested in µT 24h after bonding); (3) µTF: samples submitted to mechanical fatigue prior to µT test, and; (4) µTT: thermocycling prior to µT test. Bonding system was applied onto the FRC material (Cera-Resin Bond, CRB, Shofu Dental, Kyoto, Japan). A conventional three-step adhesive system (All-bond 3, Bisco, Schaumburg, USA) was use with a self-cure resin cement (C&B resin cement, Bisco, Schaumburg, USA). Bond strength tests were conducted at 0.75mm/min and data analyzed using Weibull distribution (p<0.05). RESULTS: Weibull contour plots showed a significantly lower characteristic strength (η) and Weibull modulus (m) for SC (η=6.9MPa and m=1.4) compared to µTC (η=20.9MPa and m=4.5). Fatigued and thermocycled µT groups presented significantly reduced characteristic strength (η=3.1MPa and η=4.1MPa, respectively) compared to µTC. Weibull modulus was significantly reduced only for SC and µTF groups compared µTC. Failure predominantly occurred at the cement/FRC interface. SIGNIFICANCE: FRC bonded to dentin samples presented lower Weibull modulus and characteristic bond strength when immediately tested in shear compared to microtensile. Aging through thermocycling or mechanical fatigue significantly reduced the characteristic strength in microtensile testing, with the majority of failures emerging between restoration material and cement interface.

Lifetime prediction of zirconia and metal ceramic crowns loaded on marginal ridges.

OBJECTIVE: To evaluate the fatigue life of zirconia-veneered and metal-ceramic crowns comprised by an even thickness or a modified framework design when loaded on marginal ridges. METHODS: Eighty marginal ridges were present after fabrication of forty molar crowns cemented onto composite-resin replicas and divided (n=20/each), in the following groups: metal-ceramic with even thickness (MCev) or with a modified framework design (MCm, lingual collar with proximal struts); porcelain-fused to zirconia with even thickness (PFZev) or with the modified framework design (PFZm). Each marginal ridge (mesial and distal) was subjected to cyclic loading separately with a lithium disilicate indenter for 106 cycles or until fracture. Kruskal-Wallis and Wilcoxon matched pair test (p<0.05) evaluated both marginal ridges. Every 125,000 cycles, the test was interrupted for damage inspection. Weibull distribution (90% confidence bounds) determined the probability of survival (reliability). RESULTS: Weibull 2-parameter contour-plot showed significantly higher fatigue life for PFZev compared to MC, and comparable with PFZm. A significant decrease in reliability was observed between groups from 625,000 until 106 cycles. Metal-ceramic groups presented significantly lower probability of survival at 106 cycles (MCev=0.66% and MCm=4.73%) compared to PFZm (23.41%) and PFZev (36.68%). Fractographic marks showed a consistent fracture origin and direction of crack propagation. Reliability was higher for porcelain-fused to zirconia than for metal ceramic crowns, regardless of framework design. SIGNIFICANCE: Zirconia-veneered crowns presented decreased fracture rates compared to metal ceramics, even when loaded at marginal ridges, regardless of framework design.