Push-out bond strength and TEM evaluation of luted fiber posts

Objectives: to measure with the push-out test the bond strength of fiber posts (FRC Postec, Ivoclar-Vivadent) luted with different resin cements, and to assess with TEM the quality of the adhesion achieved on intraradicular dentin by the luting agents. Methods: The tested adhesive cements were: Variolink II/ExciteDSC/37% phosphoric acid (Ivoclar-Vivadent, V), Panavia 21/ED Primer (Kuraray Co., P), and RelyX Unicem (3M ESPE, U). For push-out bond strength measurements, seven roots per group were tested. Each posted root was cut horizontally into 5-6 1mm-thick slices. On every slice the post was loaded by means of an adequately sized punch, that pushed the post segment in an apical-coronal direction until the post-root bond failed. This was manifested by the extrusion of the post segment from the root slice, and the load recorded at this time was divided by the area of the post-root interface, in order to express the bond strength in MPa. Two teeth per group were destined for TEM observations. Results: The bond strength achieved by V (10.18±2.89 MPa) was significantly higher than that of P (5.04±2.81 MPa) [P<0.05], and that of U (5.01±2.63 MPa), which were comparable to each other. TEM micrographs from V specimens exhibited a well developed hybrid layer with resin tags. Conversely, at the interface created when using P or U, the smear layer was only partially removed, smear plugs were retained, and the hybrid layer, where detectable, was often interrupted by gaps. Conclusions: bond strength data and microscopic findings were in agreement in indicating that the bonding mechanism of the luting agent utilizing 37% phosphoric acid for etching was more effective. It is speculated that the acidic resin monomers responsible for substrate conditioning in P and U are unable to efficiently remove the thick smear layer created on root dentin during post space preparation.