An in vitro comparison of tensile bond strengths of noble and base metal alloys to enamel

Statement of problem. Many different surface treatments have been used to increase the bond strength of noble and base metal alloys to enamel, but only a few have been studied. Purpose. The purpose of this in vitro study was to compare the tensile bond strength of a tin-plated noble alloy, an Alloy Primer-treated noble alloy, and an airborne particle-abraded base metal alloy, all bonded to enamel with a phosphate-methacrylate resin luting agent. Material and Methods. Seventy noncarious molar teeth were extracted, cleaned, and embedded in autopolymerizing acrylic resin with the buccal surface of the teeth exposed. Seventy wax patterns (4-mm diameter x 2-mm thickness) were waxed, invested, and cast-50 with a noble alloy (Argedent 52) and 20 with a base metal alloy (Argeloy N.P.). Twenty of the noble alloy specimens were tin-plated (TP), 20 noble alloy specimens were treated with Alloy Primer (AP), and 20 base metal alloy specimens were airborne particle abraded (AA). All specimens were luted with a phosphate-methacrylate resin luting agent (Panavia F) and stored in 100% humidity at 37degreesC, half for 24 hours and half for 7 days. Ten noble alloy specimens were tin-plated and stored in water for 48 hours (aged) before cementation and then stored in water for 24 hours after cementation. These specimens were used to test whether there is an advantage to aging the tin-plated surface in water before cementation. All specimens were thermocycled (5degrees to 55degreesC) for 500 cycles and then tested for tensile bond strength (TBS), measured in MPa, with a universal testing machine at a crosshead speed of 0.5mm/min. Various castings (n=6 per test group) were randomly selected from each group and inspected under a scanning electronic microscope to determine mode of failure. The mean values and standard deviations of all specimens were calculated for each group. A 2-way analysis of variance (ANOVA) was performed, and multiple pairwise comparisons were then completed with post hoc Tukey test (alpha=.05). Results. The TBS of the tin-plated noble alloy specimens bonded to enamel (24 hours: 9.33+/-1.31 MPa; 7 days: 11.65+/-1.55 MPa) was significantly greater than the Alloy Primer noble alloy specimens (24 hours: 6.11+/-1.01 MPa; 7 days: 5.45+/-1.22 MPa) (P<.001). The Alloy Primer noble alloy group showed the lowest TBS compared with the tin-plated noble alloy and airborne particle-abraded base metal alloy group (24 hours: 10.61+/-1.41 MPa; 7 days: 6.94+/-1.40 MPa). The tin-plated noble alloy specimens showed greater TBS after storage for 7 days in distilled water compared with storage for 24 hours (24 hours: 9.33+/-1.31 MPa; 7 days: 11.65+/-1.55 MPa). Aging the tin-plated noble alloy for 48 hours in 37degreesC (9.17+/-1.68 MPa) prior to cementation did not increase the TBS to enamel. The airborne particic-abraded base metal alloy showed significantly lower TBS at the 7-day storage time compared to the 24-hour storage time (24 hours: 10.61+/-1.41 MPa; 7 days: 6.94+/-1.40 MPa) (P<.001). SEM examination of the debonded metal and enamel surfaces showed mixed (adhesive and cohesive) failures for all groups. Conclusion. Tin-plating a noble alloy produced the highest bond strength to enamel. Storing the tin-plated noble alloy in 37degreesC distilled water for 48 hours before cementation did not result in a change in TBS. Using an Alloy Primer with a noble alloy resulted in statistically significant lower TBS than tin-plating.