Effects of Material Thickness and Pretreatment on the Interfacial Gap of Translucent Zirconia Restorations with Self-adhesive Resin Cement
The first objective was to determine if the dual-curing of self-adhesive resin cement (SAC) with reduced light penetrating through zirconia had an effect on interfacial gap of zirconia restorations. The second purpose was to examine whether pretreatment methods for universal adhesive affected interfacial gap. The last aim was to compare the microhardness of SAC polymerized under different zirconia thicknesses. This study evaluated self-adhesive resin cement (RelyX U200, 3M ESPE) after different pretreatment with universal adhesive (Single Bond Universal, 3M ESPE) under different polymerization conditions. CAD/CAM inlay cavities were prepared on extracted third molars. Translucent zirconia restorations were milled using Katana UTML (Kuraray). The teeth were divided into three groups: Groups I, II, and III in which the restoration thicknesses were 1, 2, and 3 mm. Each Group had three subgroups according to different pretreatment methods. For subgroup-1, no pretreatment was done on the prepared cavity. For subgroup-2, universal adhesive was applied and light-cured before cement placement (precure method). For subgroup-3, universal adhesive was applied; however, light-curing was done after cement placement (cocure method). After thermo-cycling, the interfacial gap at the restoration-tooth interface was investigated using swept-source optical coherence tomography imaging. Finally, microhardness was measured for SAC under different zirconia thicknesses. For statistical analysis, the interfacial gap was analyzed using two-way analysis of variance (ANOVA) to test the effect of cavity depth and pretreatment. In terms of each cavity depth and pretreatment, the interfacial gap was compared using one-way ANOVA and Scheffe’s test. One-way ANOVA was also performed for comparison of the Vickers hardness results. Different thicknesses of the restoration resulted in differences in interfacial gaps except between the precure method of Groups I and II (p<0.05). The effect of universal adhesive pretreatment was different depending on the restoration thickness with exceptions in Groups I and III (p<0.05). Vickers hardness number decreased as the low radiant exposure of light was applied (p<0.05). Interfacial gap of zirconia restorations can differ depending on the material thickness, pretreatment, and activation mode. Reduced light intensity penetrating through zirconia may lead to higher interfacial gap percentage and lower microhardness of the self-adhesive resin cement. Application of a universal adhesive showed similar or reduced interfacial gaps in the cement space.SUMMARY
Purpose
Methods and Materials
Results
Conclusion
Experimental procedure for the evaluation of interfacial gap.
SS-OCT images of upside-down specimens. (a) A negative control specimen (without a cement): vertical cross-sectional image. (b) A negative control image of cement space: horizontal cross-sectional image. (c) Lateral cross-sectional image of a specimen in Group III. (d) The first horizontal cross-sectional image in the cement space after cementation with RXU and thermo-cycling. White circle represents the cavity border. The first image was taken parallel to the cavity floor at the level of 5 μm down from the cavity base. (e) The second image in the cement space. The second image was taken parallel at the level of 15 μm down from the first image. (f), (g), (h), (i) and (j) The 3rd, 4th, 5th, 6th, and 7th images, respectively, were taken parallel at the level of 15 μm down from the previous image. (k) The same image as in (j) processed by GapAnalyzer. The white dots on the image (k) are the pixels which have a higher signal intensity than the threshold, indicating a microgap. On the image (k), the area of white dots was measured and then divided by the area of the circle to calculate the interfacial gap (HB%). On the image of (k), the HB% was calculated to be 23.4%.
Vickers hardness test procedure for the dual-cure group.
Interfacial horizontal-sectional images of each group at the cement space. The first column presents the images of Group I including subgroups 1, 2, and 3. The second and third column shows the images of Groups II and III including the same subgroups. The fourth column represents the images of self-cure groups (upper: SC1; lower: SC2).
(a) represents the interfacial gap (HB%) as a function of the radiant exposure measured under each thickness of the zirconia. NoPretx, Precure, Cocure, SC1, and SC2 indicate the HB% of the no pretreatment, precure, cocure, self-cure 1, and self-cure 2 methods. Radiant exposure (J/cm2) is the product of the radiant exitance reduced by each zirconia thickness (mW/cm2) and exposure time (s). (b) represents the Vickers hardness number (VHN) as a function of the radiant exposure reduced by the zirconia. The line of best fit (regression line) is presented on a logarithmic regression analysis.
Contributor Notes
Clinical Relevance
For cementation of a zirconia restoration, dual-curing resin cement is preferred to self-curing even though the intensity of the curing light is reduced through the restoration. Applying universal adhesive can produce a similar or lower interfacial gap when self-adhesive cement is dual-cured.