Limitations of push-out test in bond strength measurement

The push-out test has been widely performed to measure the bond strength of intracanal materials in dentistry. However, it is difficult to compare equitably the bond strengths from different testing specimens. The aim of this study was to investigate how a specimen's geometric parameters and the elastic moduli of dentin and intracanal filling materials may affect the bond strength measurement.

Finite element analysis was used to simulate a push-out test. A base model was established, and 3 parameters were modified: the diameter of the pin, the specimen's thickness, and the elastic modulus of the intracanal filler. The analytic stress results and the calculated bond strengths derived from the original formula for the push-out test were compared at the interfaces.

Specifically, the following observations were made: the interfacial stress distributions are mostly unaffected when the ratio of the pin diameter to the specimen's diameter is less than 0.85, and the ratio of the specimen's thickness to the specimen's diameter is greater than 0.6. Two correction factors were suggested for fillers with diverse elastic moduli with respect to the dentin modulus. Two modified formulas for the push-out bond strength test for the test specimens using different bonded composite materials were proposed.

The results showed that geometric parameters and materials have certain effects on the push-out bond strength. A more rigorous standard for the push-out test can be established for future applications.