Effect of Attenuated Light Through Translucent Zirconia on the Interfacial Adaptation and Polymerization of Resin Cements

Abstract

Purpose: The first objective was to determine if dual-curing of resin cement with reduced light could affect interfacial adaptations of zirconia restoration. The second objective was to examine whether cement type and pretreatment method of universal adhesive affected interfacial adaptation. The final objective was to compare the polymerization degree of cement under different reduced-light conditions.

Materials and methods: Inlay cavities were prepared on extracted third molars. Translucent zirconia restorations were milled using Katana UTML (Kuraray Noritake) in three groups with restoration thicknesses of 1, 2, and 3 mm, respectively. Each group had three subgroups using different cementation methods. For subgroup 1, restorations were cemented with self-adhesive cement. For subgroup 2, universal adhesive was applied and light cured. After the restoration was seated with conventional resin cement, light curing was performed. For subgroup 3, after adhesive was applied, the restoration was seated with conventional resin cement. Light curing was performed for the adhesive and cement simultaneously. After thermocycling, interfacial adaptation at the restoration-tooth interface was investigated using swept-source optical coherence tomography imaging. Finally, polymerization shrinkage of the cement was measured using a linometer and compared under the conditions of different zirconia thicknesses and light-curing durations.

Results: Interfacial adaptation varied signficantly depending on the zirconia thickness, pretreatment, polymerization mode and cements used (p < 0.05). The effects of the adhesive and polymerization shrinkage differed signficantly, depending on the reduced light under the zirconia (p < 0.05).

Conclusion: Lower curing-light irradiance may lead to inferior adaptation and lower polymerization of the cement. Polymerization of resin cement can differ depending on the light irradiance and exposure duration.

Keywords: exposure reciprocity; interfacial adaptation; polymerization shrinkage strain; radiant exposure window; resin cement; translucent zirconia.

Fig 1

Experimental procedure for the evaluation of restoration–tooth interfacial adaptation.

Fig 2

Representative SS-OCT images for interfacial adaptation evaluation of upside down specimens. (a) Negative control image (without cement): vertical cross-section. (b) Negative control image: horizontal cross-section. (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. The white circle represents the border of a prepared cavity. The first image was taken parallel to the cavity floor at a level of 5 µm down from the cavity base. (e) The second image in the cement space. The second image was taken parallel at a 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 a 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 brighter pixels which have a higher signal intensity than the threshold to indicate a microgap. On image (k), the interfacial adaptation (HB%) was calculated. The area of white dots was measured and then divided by the circle area. On the image of (k), the HB% was measured to be 24.7%. Procedural modifications were made from a previous study because the specimen was placed upside down.

Fig 3

Representative cross-sectional images at the restoration–tooth interface. All the images were taken after thermocycling of the specimens. The first column shows the representative images of Group I (1 mm depth) including subgroups 1, 2, and 3. For subgroup 1, RU2 cement was used with no pretreatment. For subgroups 2 and 3, RXU cement was used with the pre-cure and co-cure methods. The second, third, and fourth column show the images of Group II (2 mm), Group III (3 mm), and self-cure (SC) including the same subgroups.

Fig 4

(a) represents the polymerization shrinkage strain (PSS) percentage as a function of the radiant exposure measured with self-adhesive resin cement (RU2). Each “thickness” dot (5–0.5 mm) indicates the mean PSS measured by each 20 s light duration and different levels of reduced light using various zirconia thicknesses. Each “duration” dot (7–99 s) indicates the mean PSS measured by one reduced light under 3 mm zirconia and different equivalent light duration. Each line of best fit (regression line) is presented in a logarithmic regression analysis. (b) Represents the PSS as a function of the radiant exposure measured with the conventional resin cement (RXU). Thickness, duration, and lines indicate the same as in (a).