Date of Completion
John R. Agar, DDS, MA, Patchanee Rungruanganunt, DDS MSD
Field of Study
Master of Dental Science
The purpose of this study was to examine the effect of core-veneer thickness ratios on the fatigue strength of a bi-layered ceramic system bonded to a compliant substrate (like dentin). Our hypothesis was that it is actually the total thickness of a bi-layered ceramic, rather than the thickness of the high-strength core ceramic, that influences the fatigue life of a bi-layered ceramic.
Tabs of veneered lithium di-silicate (IPS e.max CAD, Ivoclar Vivadent) were fabricated from CAD/CAM blocks to the following core/veneer thickness ratios (1) 0.5/1.0; (2) 0.75/0.75; (3) 1.0/0.5; and, (4) 1.5/0.0. (n = varies per group; 29, 42, 40, 48, respectively). IPS e.max Ceram veneering porcelain was used as directed. Thicknesses of both un-veneered cores and veneered specimens were controlled by a ceramic machinist (BOMAS Machine Specialties). All specimens were adhesively cemented with Multilink Automix (Ivoclar) to bases of an epoxy-glass fiber material (NEMA grade G-10, Accurate Plastics), which presents elastic behavior similar to hydrated dentin. Channels were cut into the NEMA G-10 bases to allow deionized water to hydrate the cement during storage and testing. Specimens were tested following 2 weeks of storage in deionized water at 37° C. Each specimen was cyclically loaded by a 2 mm diameter piston made of NEMA G10 using a servohydraulic machine (858 Mini Bionix II;MTS) under load control. Sinusoidal cyclic loading was applied to ceramic specimens at a frequency of 20 HZ from 10 N to the target load for 500,000 cycles. Initially, four to five specimens were tested at a step size of 50 N to discover the 50% probability of failure load. Following completion of cyclic loading, the specimens were examined for subsurface crack formation by transillumination. If cracked, the next specimen was cycled at a lower load. If the specimen did not fail the subsequent specimen was cycled at a higher load. This step size of 25 N was used for formal testing to determine means and standard deviations.
The mean fatigue strength and standard deviations for the different core thicknesses were as follows, 0.5mm core samples 610.94N +/-130.11, 0.75mm core samples 600.0N +/- 132.80, 1.0mm core samples = 537.50N +/- 41.67, and the 1.5mm core samples = 501.14N +/- 70.12. All veneered groups were significantly stronger than the full thickness group (ANOVA p < 0.0005; 95% post-hoc). Limited cone cracking was only observed in the two thinner core groups (χ2, p < 0.05) possibly indicating residual stress in veneer.
Our results indicate that the addition of veneering ceramic to lithium di-silicate cores increases the fatigue strength of the bi-ceramic system. Furthermore, our results suggest that for this system, increasing the thickness of the veneering ceramic, rather than the core ceramic, may have a positive impact on overall fatigue strength.
Dibner, Aurora C., "Fatigue Strength of Bi-layered Ceramics Under Cyclic Loading as a Function of Core Veneer Thickness Ratios" (2014). Master's Theses. 623.
J. Robert Kelly, DDS, MS, DMedSc