2.1

Experimental design of this study

The experimental design of this study is presented in Fig. 1 . The first step is to measure interfacial adaptation of nanoceramic resin inlay prepared with different pretreatment and curing methods, using SS-OCT. The second step is to measure the radiant exitance for a light curing unit under normal and reduced-light conditions. The final step is to measure the polymerization shrinkage strain for self-adhesive cement under different activation modes.

Experimental design of this study.

2.2

Specimen preparation

The set-up employed in this study is illustrated in Fig. 2 . Extracted human third molars, free of cracks, caries and restorations, were selected and stored in a 0.5% chloramine solution at 4 °C and used within 3 months of extraction. The protocol for use of the teeth was approved by the Institutional Review Board (approval number VC19TOSI0015). One hundred thirty-two teeth with a bucco-lingual dimension of 11 (±1) mm were chosen. The occlusal surface of each tooth was flattened with a trimmer and 320-grit SiC abrasive paper. Round Class I cavities were prepared on the occlusal surface with a flat-end straight diamond bur (Komet Brasseler, Lemgo, Germany). Prepared cavities were 3.5 (±0.2) mm in diameter and 2 (±0.1) mm in depth.

Experimental procedure for interfacial adaptation evaluation.

2.3

CAD/CAM inlay fabrication

Prepared cavity was optically scanned (Medit identica hybrid scanner, Medit, Seoul, Korea), and the virtual inlay was created with 100 μm of cement space (Milling software excad v 2.0.0.3). Lava Ultimate (A2 HT, 3 M, Neuss, Germany) resin nanoceramic block (21 × 19 × 14 mm) was used to fabricate the inlay. Resin inlays of each tooth cavity were milled using a Roland Inlab milling machine (DWX-51D, Roland DG, Harmamatsu, Japan).

2.4

Groups and restorative procedure

For the dual-cure control (DC-control) group (PV, n = 6), a resin nanoceramic inlay was cemented with Panavia V5 (Kuraray Noritake, Tokyo, Japan) by a dual-curing method. Internal surface of the indirect inlay was air-abraded using 50 μm Al ₂ O ₃ particles (Hi Aluminas, Renfert, Germany) at a distance of 10 mm from the surface using 2 bars (30 psi) of pressure until the entire bonding surface appeared matte. After surface treatment, particles were removed with alcohol, and then samples were cleaned ultrasonically for 3 min in distilled water and air-dried. Ceramic Primer Plus was applied and dried following the manufacturer’s instructions. For tooth treatment, the Tooth primer in the Panavia V5 kit was applied to the dentin surface of the prepared cavity. Following the manufacturer’s instructions, the cavities were dried with a gentle stream of dry air after Tooth primer application. The base and catalyst of Panavia V5 were mixed using an auto-mix tip and then placed into the tooth cavity. After positioning the fabricated inlay into the cavity, a load of 0.5 kg was applied to the inlay surface to allow extrusion of excess cement. After a transparent celluloid strip was placed on top of the inlay, light-curing was done using a light curing unit (Elipar S10, 3 M ESPE, St. Paul, MN, USA) for 40 s.

For the self-cure control (SC-control) group (PV, n = 6), resin nanoceramic inlay was cemented with Panavia V5 using a self-curing method. After the same treatment of the inlay surface as that described for the DC-control, Tooth primer was applied to the prepared cavity for 20 s and dried with mild air. The base and catalyst of Panavia V5 were mixed using an auto-mix tip and then placed into the tooth cavity. After placement of the inlay, the cement was allowed to set without light-curing.

For the experimental groups, teeth were randomly divided into five Groups (Group I, II, III, IV, and V). Each group included four subgroups depending on the SACs. For Group I (n = 24), the fabricated and surface-treated inlay was cemented without any pretreatment of the prepared dentin surface. The prepared teeth and fabricated inlay were randomly assigned into four subgroups (n = 6 per subgroup) according to the luting material. The prepared cavity was cleaned with a moist cotton pellet. SAC was mixed using an auto-mix tip and placed into the cavity. For subgroup RU, Relyx U200 (3 M) was used. For subgroup GL, G-cem LinkAce (GC, Tokyo, Japan) was used. For SC and MS, SmartCem2 (Dentsply Caulk, Milford, DE, USA) and Multilink Speed (Ivoclar vivadent, Schaan, Liechtenstein) were used, respectively. After the luting material was applied, the fabricated inlay was placed. Then a load of 0.5 kg was applied to allow extrusion of excess cement. After a transparent celluloid strip was placed onto the inlay, light-curing was done by the same light curing unit described above (Elipar S10, 3 M) for 40 s.

For Group II, the inlay was cemented after 10% polyacrylic acid treatment of the tooth cavity. After tooth preparation, Dentin Conditioner (GC, Tokyo, Japan) was applied to the cavity for 10 s. After rinsing with water, the cavity was swabbed with a moist cotton pellet. Then, SAC was mixed and placed into the cavity. The same seating and light-curing procedures were applied in all subgroups.

For Group III, one step self-etch universal adhesive was applied and light-cured before placement of luting material (pre-cure method). Clearfil Universal Bond Quick (Kuraray Noritake, Tokyo, Japan) was applied as the universal dentin adhesive into the prepared cavity with a rubbing motion. The adhesive was gently air-blown until the adhesive did not move. Care was taken not to let the adhesive pool at the line or point angles of the cavity. Then, light-curing of the adhesive was done for 10 s. SAC was mixed and placed into the cavity. After seating of the inlay, light-curing was done as described above.

For Group IV, the inlay was cemented with a co-cure method. Clearfil Universal Bond Quick was applied to the cavity dentin with a rubbing motion. The adhesive was gently air-blown until the adhesive did not move. No light-curing was done before luting medium placement. SAC was mixed and placed into the cavity. After placement of the inlay, light-curing was performed. Note that Group I and IV specimens were created for a previous study by our research group using the same experimental protocol at the same research institution and the results reported for these groups were thus presented in a previous study.

For Group V, the inlay was cemented with SAC without any pretreatment and polymerized by a self-curing method. The prepared cavity was cleaned with a moist cotton pellet. SAC was mixed and placed into the cavity. After positioning of the inlay as described above, the cement was allowed to set without light exposure.

2.5

Thermo-cycling procedure

All specimens were stored in water at room temperature for 24 h before thermo-cycling. Then, specimens were fatigued with 10,000 thermo-cycles, which has been estimated to represent approximately one year of clinical function [ ]. Teeth were placed in baths between 5 °C and 55 °C for a dwell time of 30 s and a transfer time of 5 s. After thermo-cycling, specimens were stored in water at room temperature.

2.6

SS-OCT system

A swept-source OCT system (SS-OCT, IVS-2000, Santec Co., Komaki, Japan) was used in this study. Backscattered light carrying information about the microstructure of the sample was collected, digitized in a time scale, and then analyzed in the Fourier domain to obtain information at each location on the x-axis or depth (A-scan). The combination of a series of A-scans along a scan path can be used to create a B-scan. By transforming the B-scan raw data into grayscale, a cross-sectional image can be created. By serial B-scan acquisition over an area, a 3D image can be created (horizontal or enface cross-section). The axial resolution of the OCT system we used is 11 μm in air, which is equivalent to 7 μm in oral hard tissues and resin composites, assuming a refractive index of about n = 1.5.

2.7

SS-OCT image-taking and analysis

The specimen was positioned on a metal platform of the OCT system. A hand-held scanning probe connected to the SS-OCT was placed over the surface of the specimen. For vertical cross-sectional image taking, the first SS-OCT image was taken at the center of the restoration. An SS-OCT image of the restoration was taken parallel to the bucco-lingual plane of the cavity at the center of the restoration ( Fig. 2 ). The vertical cross-sectional image was used as the reference and was not used in statistical analysis. To take a horizontal cross-sectional image, the first SS-OCT image of the restoration was taken parallel to the cavity floor 5 μm below the inlay base. After the first image was taken, the platform holding the specimen was moved 15 μm up, followed by acquisition of the second image. A total of seven images were taken for each specimen at 15-μm intervals in the cement space ( Fig. 2 ).

To evaluate the internal adaptation of the resin-tooth interface, OCT raw data were imported into image analysis software (ImageJ ver. 1.48, National Institutes of Health, Bethesda, MD, USA). In the presence of air or water at the tooth-restoration interface, part of the light reflected from the interface can be visualized as bright spots or areas on the image ( Fig. 3 ). The high brightness (HB%) parameter was calculated to assess the presence of microgaps or non-adapted areas in the cement space. HB% was defined as the percentage of brighter pixels with a signal intensity greater than the threshold value in the signal intensity profile [ ]. To calculate the percentage of brighter pixels than the threshold, images were processed using a plug-in for the ImageJ program as described previously [ , ]. Seven horizontal-cut cross sectional images were obtained from each specimen, and the mean percent of high brightness (HB%) per sample was calculated. A higher HB% was considered to indicate inferior interfacial adaptation in the cement space.

SS-OCT images for interfacial adaptation measurement. (a) Representative OCT image of the negative control without luting material: vertical cross-sectional image. (b) Image of the cement space of the negative control: horizontal cross-sectional image. (c) First horizontal-cut image of luting material (PV) in the cement space. Luting material had a similar intensity value to that of dentin. The first image was taken parallel to the cavity floor 5 μm down from the inlay base. (d) Second image in the cement space. The second image was taken parallel to the first image but 15 μm lower. (e), (f), (g), (h) and (i) are the 3 ^rd , 4 ^th , 5 ^th , 6 ^th and 7 ^th images, respectively, which were taken at levels parallel to the first image but 15 μm lower each time. (j) The same image as the fourth image shown in (f) with a circle that indicates the border of a prepared cavity. (k) The same image as shown in (j) with white dots. The white dots are brighter pixels with a signal intensity greater than the threshold value in the signal intensity profile, and represent microgaps in the cement space. The area of the white dots was measured and then divided by the area of the circle to calculate interfacial adaptation (HB%). On the image shown in (k), HB% was calculated to be 13.0%.

2.8

Measurement of radiant exitance with or without the resin nanoceramic block

Radiant exitance (irradiance) of the light curing unit, Elipar S10, was measured under two conditions: normal and reduced light conditions under a LAVA Ultimate block. A spectral radiometer (USB4000, Ocean Optics, Largo, FL, USA) and a general purpose sphere (Labsphere, North Sutton, NH, USA) were used to measure radiant exitance. When the radiant flux of Elipar S10 was measured between 350 and 600 nm, it was found to be 0.53 W (average of five measurements), which is equivalent to 1109 mmW/cm ² of radiant exitance for the normal condition. The radiant exitance of the light source was also measured when a Lava Ultimate resin block (width, length and height: 10 × 10 × 2 mm) was positioned between the integrating sphere aperture and the light curing tip, and the average of five measurements was calculated. The radiant exitance through the 2-mm-thick block was 313.9 mmW/cm ² .

2.9

Measurement of polymerization shrinkage strain

2.9.1

Polymerization shrinkage strain under dual-curing condition (SS-DC)

Polymerization shrinkage strain (SS) of the cement was measured using a custom-made linometer (R&B Inc., Daejon, Korea) which was similar to the device that AJ de Gee had made [ ]. Before placing the cement, the disk and glass slide were coated with separating grease (high vacuum grease, Dow Corning, Midland, MI, USA). Resin cement mixed with dimensions of 8 mm in diameter and 1 mm in height (0.09 g) was placed onto an aluminum disk and covered with a glass slide. Measurement was started at the time of light-curing, which was 90 s after the start of cement mixing. The light curing unit (Elipar S10) was placed 2 mm above the glass slide, using a transparent mold with 8-mm diameter hole (2-mm thickness, Drufosoft Clear, Dentamid, Dortmund, Germany), and the cement material was light-cured. Light was applied for 40 s. As the resin cement under the glass slide polymerized, the aluminum disk moved upward ( Fig. 4 ). During and after light-curing, the amount of disk displacement (ΔL) was measured using an eddy current sensor every 1 s for a period of 30 min at room temperature (23 ± 1)°C. Then, volumetric shrinkage (SS) was calculated by the following method [ ].

Measurement of polymerization shrinkage strain. Shrinkage strain (SS) was measured in three ways. First, SS was measured under the dual-curing condition with light-curing (SS-DC). Curing light was applied, using a transparent mold of 2-mm thickness, without a resin nanoceramic (Lava Ultimate) block. As the cement polymerized, the aluminum disk moved upward and the sensor recorded the amount of displacement for shrinkage strain calculation. Second, SS was measured under the dual-curing condition with reduced light (SS-DCr). A 2-mm-thick Lava Ultimate was placed on the glass slide and curing light was applied through the resin block. SS was measured as described for the SS-DC specimen. Finally, SS was measured under the self-curing condition (SS-SC). No light curing was applied and the glass slide was covered to block light to ensure that only self-curing occurred.

First, linear shrinkage strain (Lin%) was calculated by:

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