Statement of problem: Poor chemical bonding of acrylic resins to metal alloys can result in microleakage and failure of the bond. Metal primers have been shown to be effective in improving the bond strength of acrylic resins to metal alloys. However, there is insufficient information about their effects on bonding to different types of metals.
Purpose: The purpose of this study was to evaluate the effect of metal primers on the shear bond strength of acrylic resins to 3 different types of metals.
Material and methods: A total of 432 disk-shaped wax patterns (10 mm in diameter and 2 mm thick) were cast in a Ti alloy (Tritan), base metal (Co-Cr alloy, Wironit), or noble metal (Au-Ag-Pt alloy, Mainbond EH). After casting, the disk surfaces were finished with abrasive paper under water. The noble alloy was airborne-particle abraded with 50-microm aluminum oxide; the other alloys were airborne-particle abraded with 110-mum aluminum oxide for 10 seconds. Specimens of each metal were divided into 3 groups (n=48) and received 1 of the following acrylic resins: (1) heat polymerized (Meliodent), (2) autopolymerized (Meliodent), or (3) microwave polymerized (Acron MC). The specimens were then divided into 4 subgroups (n=12) which received 1 of the following metal primers: (1) Metal Primer, (2) Alloy Primer, (3) Meta Fast, or (4) no primer (control). All specimens were stored in distilled water at 37 degrees C for 24 hours after polymerization and then thermal cycled (5000 cycles at 5-55 degrees C with a 30-second dwell time). After thermal cycling, the specimens were tested in a universal testing machine at a crosshead speed of 0.5 mm/min in shear mode. Data (MPa) were analyzed using 3-way ANOVA and the post hoc Tukey HSD test (alpha=.05).
Results: The 3-way ANOVA indicated that shear bond strength (SBS) values varied according to the metal type, metal primer, and acrylic resin used (P<.001). The SBS between base metal and heat-polymerized resin with Metal Primer was the highest (3.6 +/-0.67 MPa), and the SBS between noble metal and acrylic resin, for all control groups, was the lowest (0.4 +/-0.07 MPa) (P<.001). For titanium, the highest SBS was observed for Meta Fast and the lowest for the control group. For base metal, the highest SBS was recorded for Metal Primer and the lowest for the control group, while for noble metal, the highest SBS was observed for Alloy Primer and the lowest for the control group (P<.001).
Conclusions: The metal primers were associated with an increase in the adhesive bonding of acrylic resins to metal alloys. The SBS of the acrylic resin to the base metal alloy was significantly higher than the SBS to the noble and titanium alloys.