A critique of bond strength measurements.

The lack of consistent values for dentine bond strengths in shear or in tension from what are superficially identical experimental procedures has led to ambiguities in the interpretation of the data. These variations in bond strength are usually considered to be related to different adhesive procedures. However so far little attention has been paid to the detail of the test procedures used. In this study the sensitivity of bond strengths to changes in testing conditions has been calculated using finite element stress analysis. It is shown that tensile and shear bond strengths are highly dependent on the geometry of the test arrangement and the materials involved. It is concluded that the concept of 'average stress' for the measurement of bond strength does not stand up to close examination. The measurement does not provide a material property as its value is dependent on local conditions and the actual stresses have little relationship to the average stress value. This demonstrates that there is a need for the standardization of test procedures for the measurement of bond strengths so that a universally valid comparison between different bonding agents can be performed.