Effect of thermo-mechanical cycling on the shear bond strength of porcelain to milled and cast cobalt-chromium alloys

Statement of problem: Most of the failures in metal-ceramic restorations occurs at the interface between the two materials. For long term clinical performance, these restorations should have adequate bond strength between metal and ceramic. Few data is available in regards to the metal-ceramic bond strength of Cobalt Chromium (Co-Cr) alloys fabricated by computer aided designing design/computer aided manufacturing CAD/CAM technology.
Purpose of the study: The purpose of this in vitro study was to explore the influence of Co-Cr fabrication techniques on shear bond strength of porcelain to milled and cast Co-Cr alloys and to evaluate failure mode with & without thermo-mechanical cycling.
Materials & Methods: A total of 20 disc samples (10 mm diameter x 4 mm thickness) were fabricated of Co-Cr alloy and divided into two groups (10 discs each) according to their fabrication technique: Group I: fabricated by CAD/CAM milling and Group II: fabricated by conventional casting. Then porcelain build up for all discs (10 mm diameter x 2mm thickness) was done using feldspathic porcelain. Each group was subdivided according to discs’ exposure to thermo-mechanical cycling into two sub-groups (5 discs each). Sub-group A : without thermo-mechanical cycling and Sub-group B: with thermo-mechanical cycling. All the disc samples were subjected to the shear bond strength (SBS) test and the failure mode was investigated under digital microscope at 40 X. Data were statistically analyzed using Independent sample t-test and Two-way ANOVA test (P ≤ 0.05).
Results: The results showed that the fabrication techniques were statistically insignificantly affecting the SBS of both CAD/CAM milled and conventional casting groups. Regarding the thermo-mechanical cycling, the CAD/CAM milled and conventional casting groups without thermo-mechanical cycling showed a statistically significantly higher SBS than those with thermo-mechanical cycling. Failure mode was mixed type for all disc samples.
Conclusions: Fabrication technique of Co-Cr alloy had no significant effect on SBS. Thermo-mechanical cycling of milled and cast Co-Cr samples significantly affecting SBS with the superiority of the non-thermo-mechanical cycled samples. All the tested samples showed SBS values within the clinically acceptable levels. CAD/ CAM fabricated Co -Cr may be considered a promising alternative to conventional cast Co-Cr for metal ceramic prosthesis in terms of SBS.