Universal adhesive may improve interfacial adaptation for self-adhesive cement.
Interfacial adaptation by pre-cure method was not different from that by co-cure.
Interfacial adaptation by self-curing differed from that by dual-curing.
Initial polymerization shrinkage strain differed depending on self-adhesive cement.
Self-adhesive cement tends to have either self-adhesive or self-curing potential.
The first objective of this study was to determine if the luting material used for resin nanoceramic inlay affects interfacial adaptation. The second was to investigate whether pretreatment and the adhesive curing method before cementation affects interfacial adaptation. The final objective was to compare activation modes of luting material.
Class I cavities were prepared on extracted human third molars. Resin nanoceramic inlays were fabricated using Lava Ultimate CAD/CAM block (3 M). For the control groups, inlays were cemented using Panavia V5 (Kuraray Noritake). For the experimental groups, teeth were randomly divided into five experimental groups with four subgroups using different self-adhesive cements (SACs). Cement in Group I was dual-cured without pretreatment. In Group II, the cement was dual-cured after polyacrylic acid treatment of the tooth cavity. In Groups III and IV, the cement was dual-cured after universal dentin adhesive treatment with pre-cure and co-cure methods. In Group V, the inlay was cemented in self-cure mode. After thermocycling, interfacial adaptation at the inlay-tooth interface was measured using swept-source optical coherence tomography (SS-OCT) imaging. Finally, polymerization shrinkage strain of the luting material was measured and compared.
Interfacial adaptation differed depending on the luting material. After application of a universal adhesive, some subgroups showed improved interfacial adaptation. Interfacial adaptation and polymerization shrinkage strain differed significantly depending on activation mode.
Interfacial adaptation for a resin nanoceramic inlay can differ according to the type of SAC and activation mode. For some SACs, application of a universal adhesive before cementation improves interfacial adaptation.
The use of adhesive resin cements can improve adaptation and retention of indirect restorations [ , ]. Self-adhesive cements (SACs) that do not require any pretreatment of the tooth surface have gained popularity. In cases without pretreatment of the prepared tooth, cement is applied directly over the tooth surface’s smear layer [ ]. For better adhesion, mild acids such as polyacrylic acid can be used to remove or modify the smear layer [ ]. Diverse results regarding the effects of mild acids have been reported. While some studies reported higher bond strength after polyacrylic acid treatment [ , ], another study did not find higher bond strength after polyacrylic acid treatment [ ].
For pretreatment of the prepared cavity, a universal dentin adhesive can be applied. A one-step, self-etch universal adhesive can be applied to the cavity simply and quickly. However, some problems with the use of one-step universal adhesives for composite restorations have been reported [ ]. Moreover, incompatibility problems between one step self-etch systems and self-/dual-cure resin composites have been found [ ]. Tertiary amines, which are used as the accelerator in self-/dual-cure composites, can be neutralized by the acidic functional monomers of the adhesive [ ]. Despite potential incompatibility issues, some manufacturers recommend application of dentin adhesive before self-adhesive cement placement.
If a universal dentin adhesive is applied, there are two ways to polymerize it with light; namely pre-cure and co-cure methods. In pre-cure, the adhesive is light-cured prior to application of the luting medium. Co-cure means that the dentin adhesive is not light-cured before placement of the luting medium, but after the cement is placed with the restoration, they are light-cured simultaneously. Some studies found that pre-curing of dentin adhesive increased bond strength and improved marginal adaptation [ ]. However, another study found no difference in outcomes in terms of microleakage when co-curing was performed instead of pre-curing [ ]. If the pre-cure method is used in clinical practice, care should be taken so that the restoration can be accurately fitted to the preparation. Some studies have found that pre-curing the adhesive can affect the fit of the restoration and have suggested no pre-curing of the adhesive to avoid the risk of seating interference [ , , ]. Therefore, it is wondered under what condition the co-cure method can be used as effectively as the pre-cure method.
Most SACs are known to have dual-curing capability of polymerization: light-curing and self-curing. If there is not sufficient light, the polymerization performance of dual-cure resin cement is more dependent on self-curing. However, several studies reported that many dual-cured resin cements are largely based on photo-polymerization and that self-curing leads to a low degree of conversion, and consequently, compromised mechanical properties [ ]. In addition, SACs were reported to need more time to achieve the maximum degree of conversion than did the conventional resin cement [ ]. The self-curing performance of various SACs can be different.
Polymerization shrinkage strain is an important factor of polymerization shrinkage kinetics. Polymerization shrinkage creates detrimental stress at the interface between prepared cavity and a restoration, which may cause adhesion failure. Shrinkage strain has been reported to vary linearly with degree of conversion [ , ]. Among various methods for investigating polymerization kinetics, the magnitude of shrinkage strain is relatively simple to measure [ ]. When shrinkage strain is measured in real time, the polymerization reaction rate or speed of resin can be compared.
Interfacial adaptation can be assessed by visualizing microgaps at the interface between tooth and restoration. Microfocus-computed tomography (micro-CT) and optical coherence tomography (OCT) are non-destructive imaging methods that can be used to assess microgaps [ ]. OCT is based on low-coherence interferometry [ ]. When light passes the interface between two media with different refractive indices, a portion of light is reflected. If there is a microgap between the tooth and the restoration, there can be air or water at the interface [ ]. When light transverses the air at the interface, a different proportion of light is reflected and the OCT system shows a higher signal intensity. Using OCT, the integrity of the tooth restoration interface can be investigated in a non-invasive manner. Swept-source optical coherence tomography (SS-OCT) is a specific type of OCT that offers high image resolution as well as improved scan speed [ ].
In terms of clinical application, pretreatment of a prepared cavity and activation mode of SAC could affect interfacial adaptation of nanoceramic resin restoration. The first objective in this study was to investigate if there was a difference in interfacial adaptation among different SACs used for resin nanoceramic inlay cementation. The second objective was to determine if pretreatment of the prepared cavity with polyacrylic acid or universal adhesive can affect interfacial adaptation. The final objective was to compare the activation methods of dentin adhesive and SAC: light activation with a pre-cure method, light activation with a co-cure method, and a chemical activation method.
The null hypotheses tested are as follows:
There is no difference in interfacial adaptation of a resin nanoceramic inlay restoration when it is cemented with different SACs.
When the prepared cavity is treated by polyacrylic acid or universal dentin adhesive before SAC placement, interfacial adaptation is not different from that obtained without a pretreatment.
There is no difference in interfacial adaptation when the dentin adhesive is applied and cured with a pre-cure method versus a co-cure method.
There is no difference in interfacial adaptation and polymerization shrinkage strain when the SAC is polymerized with a dual-cure method versus a self-cure method.
Materials and Methods
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.
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.
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 18.104.22.168). 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).
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.
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.
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.
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.
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 .
Measurement of polymerization shrinkage strain
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 [ ].
First, linear shrinkage strain (Lin%) was calculated by: