Zirconia is a popular dental restorative material due to its excellent material properties and esthetics. Chemical inertness enables zirconia restorations to withstand the harsh chemical conditions that occur in the mouth. However, the chemical inertness of zirconia also presents a challenge for bonding zirconia to zirconia via traditional resin cements. Unique methods are indicated when bonding a zirconia abutment to a zirconia crown.1 With the large variety of surface treatments, primers and luting agents available today, it is difficult to determine without laboratory testing which combination of materials will produce the most stable bond.
Chemical inertness enables zirconia restorations to withstand the harsh chemical conditions that occur in the mouth. However, the chemical inertness of zirconia also presents a challenge for bonding zirconia to zirconia via traditional resin cements.
Materials and Methods
A total of nine groups of BruxZir® Solid Zirconia implant crowns and Inclusive® Zirconia Custom Abutments were evaluated to determine which cementation protocol produces the strongest bond at the abutment-crown interface. Each group included 10 specimens. The abutments and crowns were treated with different surface modifications, primers and luting agents prior to undergoing force and stress tests (Fig. 1). The surface area of the custom abutments included in the tests was measured.
Groups 1, 2 and 3 were treated with a silica solution (1 μm) before sintering and abraded with Al2O3 (50 μm) after sintering. Groups 4, 5 and 6 were abraded with Al2O3 (50 μm). Groups 7, 8 and 9 were abraded with Rocatec™ Soft (30 μm) (3M™ ESPE™; St. Paul, Minn.).
Primers and luting agents were applied following manufacturers’ instructions. After applying the luting agent, 4-pound weights were placed on the cemented crowns during setting, and excess cement was removed. The margins were light-cured for 20 seconds each. Then specimens were allowed to further set in deionized water at 37 degrees Celsius for 24 hours before loading and de-bonding tests were performed.
Figure 1: BruxZir crowns and Inclusive Zirconia Custom Abutments were treated, primed and luted according to manufacturers’ instructions prior to force and stress testing.
Specimens were placed in the Instron® Model 5564 testing machine (Instron; Norwood, Mass.) with a load cell of 2,000 N. Tensile force was applied at a constant speed of 0.5 mm/min until de-bonding occurred. The force (N) and stress (MPa) at the time of de-bonding were recorded for each group, and means and standard deviations were calculated (Figs. 2–4).
Figure 2: Force at the point of detachment was recorded for each group.
Figure 3: Stress (force per unit area) was calculated for each group.
Figure 4: Means and standard deviations of force measurements and stress calculations. Note: Values with the same letter are not significantly different from each other.
Force and stress values were tested to determine statistical significance of the results. A one-way analysis of variance (ANOVA) showed a p-value less than 0.05 (α= 0.029), which indicates that the differences between the groups were statistically significant. A post hoc Tukey-Kramer test also showed that the test results from Group 8 are statistically significant and due to more than just chance.
Group 8 withstood the highest force and the most stress. The abutment-crown interface for the specimens that were blasted with Rocatec Soft, primed with Monobond Plus, and cemented with Multilink Automix tolerated the greatest force and stress prior to de-bonding. For this reason, when BruxZir implant crowns are prescribed over Inclusive Zirconia Custom Abutments, both are treated with Rocatec Soft. The practitioner can then apply primer and self-adhesive cement when delivering the restoration, yielding the strongest possible bond between zirconia and zirconia.