10-12,15,16 The findings of the present study showed that there was significant reduction in the metal-ceramic bond strength for each of the Ni-Cr (664.63N) and Co-Cr alloys (497.41N) with the addition of recast alloy to the fresh alloy. This reduction in the bond Ivacaftor CFTR inhibitor strength can be attributed to an increase in the frequency of interfacial voids as the percentage of recast metal is increased. Another possible reason for the failure of recast alloy may be the compositional change that occurs after multiple castings.17 This results in a decreased bond between the metal and the ceramic, since the chemical bond is affected by these elements.

Tucillo reported that the thickness of the adherent oxide layer formed at the metal surface might decline due to multiple castings, and result in decreased bond strength.18 Several tests were reported in the literature which evaluated the metal-ceramic bond strength, such as twist, shear, tension, flexural mode or the combination of flexural and twist modes all showing advantages and disadvantages.7 The shear test is considered by some authors as the most adequate method to measure bond between two materials.10,19-22 The dominant stress in the shear bond test is shear stress, whereas in the 3-point bending test; tensile stress predominates.19 Therefore, in the present study shear bond was tested using the

universal testing machine to evaluate the bond at the metal ceramic interface. In this study, a custom made apparatus was devised so as to concentrate the load predominantly on the metal-ceramic interface and achieve accurate interfacial shear bond results. When high gold alloys are used, the addition of up to 50% remnant alloy from previous castings is acceptable. However, results from the current study suggest that the addition of previously cast alloy should be avoided if base metal dental alloy is selected for metal ceramic restorations.

A significant decrease in the metal ceramic bond after the addition of recast alloy is confirmed with the shear bond test citing to be of the reasons for the clinical fractures observed on metal Anacetrapib ceramic restorations. Considering the decreased cost of base metal dental alloys, when compared to noble and high noble alloy alternatives, the addition of previously used alloy is not crucial and should be avoided. Further studies should be conducted to evaluate the effect of multiple firings on the metal-ceramic bond strength and the variation of bond strength with thermocycling of the samples. The coefficient of thermal expansion (CTE) of the dental alloys used is also altered after multiple castings.23 Additional researches is needed to evaluate the changes in CTE after multiple castings and cytotoxicity of base metal alloys.24 Also, other key structural factors such as grain size, porosity, and oxide thickness need further studies using scanning electron microscopy.