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2- Dokuz Eylul University, Department of Electrical and Electronics Engineering, Izmir, Turkey. 3- Ege University, Faculty of Dentistry, Department of Restorative Dentistry, Izmir, Turkey. The purpose of the study was to use the photonic imaging modality of optical coherence tomography (OCT) to compare the marginal adaptation of composite inlays fabricated by direct and indirect techniques.
During the last decade there has been an increasing demand for esthetic restorations in the posterior dentition. The marginal adaptation is one of the important factors that determine the longevity of the restorations25. The OCT was first used in dentistry in 1998, with in vivo imaging of hard and soft oral tissues7.
In the direct group, two coats of glycerin were applied over all preparation walls and margins as a separating medium and left for three minutes for complete drying. In the indirect group, impressions were taken using prefabricated plastic caps with a radius of 10 mm as stock trays to reduce the bulk of the impression materials and a two-step technique (Affinis Precious, Coltane Whaledent, Switzerland) was used. In order to determine marginal discrepancies of the inlay restorations, all 34 teeth were measured by OCT before cementation as a first step. On each margin, OCT measurements were obtained in 200 µm intervals, which resulted in 140 OCT scans on average per tooth. Figure 5 A representative optical coherence tomography (OCT) scan of a tooth from the direct group before cementation. Figure 6 Optical coherence tomography (OCT) scan of a tooth from the direct group after cementation. All preparation walls were cleaned with pumice, and inlay restorations were ultrasonically cleaned in distilled water for 10 min and then air-dried. Marginal discrepancy values were compared by statistical parametric tests using SPSS 13.0 (SPSS Corporation, Chicago, IL, USA).
The OCT scans taken before (Figure 5) and after (Figure 6) cementation enabled to observe and measure marginal discrepancies of the inlay restorations. After assessing all the data statistically, mean marginal discrepancy values were calculated as summarized in Table 1. Optical Coherence Tomography, introduced in 1991, is a powerful tool that produces non-contact, noninvasive tomographic images of biological tissues12. In previous OCT studies, different wavelengths, such as 800 nm22, 830-1280 nm7, 930 nm18,23, 1260-1360 nm11,15, 1310 nm19,30, and 1319 nm21, have been used.
In addition, in the present study the OCT images showed information about the resin cement thickness, its structure, and interaction between the tooth and the restoration. Other imaging methods, such as SEM, micro-CT, or optical microscope, could compare marginal adaptation of inlays. Clinical OCT systems are now becoming an effective, nondestructive, and suitable method for evaluating marginal adaptations of restorations with the development of small, flexible fiber optic OCT probes that can easily access the oral cavity.
Within the limitations of this in vitro study, marginal discrepancies of inlay restorations were quantitatively and noninvasively evaluated by the OCT system. Pistol handpiece with trigger-activated, light-on demand cycle with interrupt and audible tones every 10 seconds for curing time control.
The cavities were randomly divided into two groups according to the inlay fabrication technique. The marginal discrepancy values were increased for all restorations that refer to cement thickness after cementation. Esthetic restorations for Class II preparations include: direct composite restorations, direct composite inlays, indirect composites (inlays and onlays), ceramic inlays, and ceramic onlays27.
Insufficient sealing may lead to leakage of oral fluids along the interface between restorative material and tooth substrate, and can result in postoperative tooth sensitivity, marginal discoloration, and recurrent caries. It has since been used for evaluating marginal or internal adaptation of restorations2,11,16,21-23,30, crack or void evaluation in composites24, and enamel-ceramic interface20. The teeth were approximately the same size and were stored in saline solution for up to 30 days.
The teeth were stored in distilled water at room temperature during the fabrication process. The measurement procedure that reveals the marginal discrepancy was as follows: the tooth was placed in the sample arm and the OCT diode was focused on the tooth, as shown in Figure 3.
The broadband diode light source is a super luminescent diode with central wavelength 930 nm and bandwidth 100 nm.
The table on which the tooth was placed was adjustable with a micrometer screw to achieve a smooth and precise 200 µm incremental shift between successive measurements.
Inlays were luted with self-adhesive dual-cure resin cement (SmartCem2, Dentsply, Konstanz, Germany) (Figure 2). Differences between direct and indirect technique were evaluated by independent-samples t-test. The resin cement thickness, its structure, and interaction between the tooth and the restoration were also observed and measured.
There were statistically significant differences between the marginal discrepancy values of direct and indirect groups.
The OCT images are obtained by measuring the echo time delay and intensity of backscattered light from a specimen.
The studies, which have used 1260 nm and more wavelength source with spatial resolutions of 10 µm, have evaluated the resin-dentin interface of the cavity floor, defects of the composites, and internal adaptation or sealing performance of the resin cements. Self-adhesive resin cement was used for cementation to eliminate the operator factor of etch-and-rinse multi step adhesives. There are studies evaluating restorations in vivo7,15,19 and further studies should be performed.

The following conclusions may be drawn: direct inlays presented smaller marginal gap values than indirect inlays. Stress distributions in adhesively cemented ceramic and resin-composite Class II inlay restorations: a 3D-FEA study. Concurrent evaluation of composite internal adaptation and bond strength in a class-I cavity. Nondestructive assessment of current one-step self-etch dental adhesives using optical coherence tomography. Clinical and semiquantitative marginal analysis of four tooth-coloured inlay systems at 3 years. Non-destructive evaluation of an internal adaptation of resin composite restoration with swept-source optical coherence tomography and micro-CT. Clinical study of indirect composite resin inlays in posterior stress-bearing cavities placed by dental students: results after 4 years.
Swept-source optical coherence tomography as a new tool to evaluate defects of resin-based composite restorations. Evaluation of the marginal fit at implant-abutment interface by optical coherence tomography.
Clinical evaluation of the marginal fit of cast crowns: validation of the silicone replica method. Imaging in vivo secondary caries and ex vivo dental biofilms using cross-polarization optical coherence tomography.
Non-destructive 3D imaging of composite restorations using optical coherence tomography: marginal adaptation of self-etch adhesives. Marginal analysis of resin composite restorative systems using optical coherence tomography.
Non-destructive characterization of voids in six flowable composites using swept-source optical coherence tomography. The clinical challenge of achieving marginal adaptation in direct and indirect restorations.
Influence of post-cure treatments on hardness and marginal adaptation of composite resin inlay restorations: an in vitro study.
An overview of treatment considerations for esthetic restorations: a review of the literature. In vitro evaluation of the marginal and internal discrepancies of different esthetic restorations. Comparison of marginal fit and microleakage of ceramic and composite inlays: an in vitro study.
Sealing performance of resin cements before and after thermal cycling: evaluation by optical coherence tomography. The first group was directly restored on cavities with a composite (Esthet X HD, Dentsply, Germany) after isolating. Composites are limited for direct restoration of the larger stress-bearing posterior Class II cavities due to polymerization shrinkage effects and some limitations in mechanical properties1.
Marginal adaptation of restorations has been evaluated by different methods such as sectioning the luted restorations and then measuring by optical microscope, scanning electron microscope (SEM), micro computed tomography (micro-CT), or transmission electron microscope (TEM) and replica technique.
Most studies have evaluated marginal adaptation of direct composite restorations with OCT2,11,21-23. Teeth roots were embedded in plastic cylinders using a self-curing acrylic resin 3 mm away from the cervical line. Afterwards, two consecutive 2 mm horizontal increments of Esthet X HD (Dentsply Detrey GmbH, Konstanz, Germany) (Figure 2) composite restorative material were applied to cavity walls and approximal contacts obtained by matrix band. All impressions were stored at room temperature (25°C) for 1 hour before pouring to ensure a similar humidity effect on the setting of the impression material. It was necessary to fix the length of the arm by monitoring the signal reflected of the tooth-air interface. The infrared beam was scanned over tooth-marginal discrepancy (air or resin cement)-inlay surfaces.
This allowed for taking OCT B-scans and thus measuring the marginal gap in 200 µm intervals.
After a homogeneity of variance test was performed using Levene test, before and after cementation data were compared by paired samples t-test at a significance level of alpha=0.05. Such tool uses inherent differences in the index of refraction in tissue rather than enhancement with dyes to differentiate tissue types20. It is stated that when measuring subsurface structures, such as micro leakages or demineralized areas, it would be advantageous to have a higher wavelength source of 1310 nm or more because lower wavelength light penetrates less in tissue6. After analyzing the measurements, we found that marginal discrepancy values of direct and indirect inlays were increased 35 µm and 62.75 µm, respectively, after cementation. This may not be appropriate for comparing the before and after cementation marginal discrepancy values.
Different techniques are available to detect marginal adaptation of restorations, but the OCT system can give quantitative information about resin cement thickness and its interaction between tooth and restoration in a nondestructive manner. According to our knowledge there has been few studies evaluating marginal adaptation of indirect restorations using OCT16,30 and no studies have compared the marginal integrity of direct and indirect composite restorations with OCT. The long axis of the tooth was oriented perpendicular to the surface of the acrylic block with a parallelometer (Degussa F1, DeguDent, Hanau, Germany). The composite was anatomically shaped and each increment was light cured for 20 s with a LED-curing unit (Elipar S10, 3M ESPE, Seefeld, Germany) recommended by the manufacturer.
The reference arm length was tuned until a sharp image was seen on the CCD camera, a camera incorporating a charge-coupled device, enabling a coarse adjustment. The interference signal is then separated into its frequency components through an optical grating and sent to a charge-coupled device (CCD). The imaging depth was around 1.7 mm and the measurements had a Signal-to-Noise Ratio (SNR) of 83 dB.
Using the ruler tool on the image processing software, the marginal discrepancy (corresponding to air before cementation and resin thickness after cementation, as shown in Figures 5 and 6) was measured and recorded during each scan. Then, the restoration was positioned into the cavity by using a custom-made tip in a universal machine (Shimadzu, Tokyo, Japan) for standardization of cementation procedures. Because OCT is a non-contact and nondestructive method, it was useful in taking sensitive measurements in different areas such as ophthalmology and cardiology5. In our study, 930 nm wavelength of OCT system (Thorlabs) with spatial resolution of 7 µm and a bandwidth of 100 nm was used to measure the marginal discrepancies between teeth and inlay.
The gold sputtered layer, required for SEM images, can mask details that OCT images do not encounter23.
Marginal adaptations were scanned before cementation with an invisible infrared light beam of OCT (Thorlabs), allowing measurement in 200 µm intervals.
Ceramic materials are resistant to compressive forces, but they are susceptible to tensile stresses and more prone to fracture than composite materials8. On the other hand, in vivo marginal integrity of restorations can be evaluated by SEM with the use of epoxy replicas10.

The aim of this in vitro study was to quantitatively evaluate and compare the marginal adaptation of composite inlay restorations fabricated by direct and indirect techniques under OCT, and also compare the cement thickness of inlays after cementation.
The matrix band was removed and the inlay was cured for additional 20 s through the proximal, lingual, and buccal enamel walls. A) For the mesial and distal surfaces, buccal to inlay, gingival to inlay, and lingual to inlay measurements were performed. Excess cement was removed, then light curing (Elipar) was applied for 20 s through the proximal, lingual, buccal, and occlusal enamel walls recommended by the manufacturer, and kept under 50 N of static load for 6 min (total curing time). Since the imaging depth of OCT system used in this study is 1.7 mm, the internal adaptation of inlays could not be detected due to the 4 mm cavity depth. The increase in the discrepancy value can be related to the resin cement viscosity or problematic discharge of excess cement because of the complex cavity geometry. Compared with the optical microscope, the OCT system penetrates the sample up to a depth of 2-3 mm22, showing its internal structure with high resolution.
Restorations were cemented with a self-adhesive cement resin (SmartCem2, Dentsply), and then marginal adaptations were again measured with OCT.
It is stated that composite materials performed better stress distribution than ceramic materials in Class II cavities1. Although the replica technique is a reliable and valid noninvasive method to determine the adaptation of restorations to tooth structure, deterioration of the silicone replica can occur, and defects in the area of measurement can affect the assessment of the film thickness with a microscope17. The inlay was carefully detached from the cavity with a fine probe and exposed to the post-curing for 2 minutes in a light-curing unit (Triad, Dentsply Trubyte, Canada). After this step, the infrared beam was scanned over the tooth until distance between tooth and inlay restoration (which corresponds to the discontinuity caused by air before cementation or resin cement after cementation) was detected. Finishing was performed with finishing diamonds (Prisma) and polishing disks (SofLex Pop-on, 3M ESPE, St Paul, USA).
Marginal or internal adaptations of composites have been evaluated by OCT either qualitatively23 or quantitatively11,21,22,30.
Similar to the studies that have used 930 nm, the measurement locations were on the surface of the tooth18, not on inner surfaces, and marginal analysis23 was performed. This result was supported by other studies, which found a significant increase in the marginal discrepancy values after cementation28, which was attributed to the increase of hydraulic pressure of the resin luting cement28.
Recently, optical coherence tomography (OCT) was addressed as a noninvasive cross-sectional imaging of the internal biological system at the submicron scale11. After all the inlays (n=17) were fabricated by one operator, they were checked for fit and adjusted with finishing burs under water cooling. The tooth was placed in the reference arm in such a way that the light beam first hit the tooth from the top. In these studies, interfacial gaps were confirmed by using a confocal laser scanning microscope (CLSM) or optical microscope. According to the results of this study, the null hypothesis 1 was rejected because direct inlay technique used in this study seems to have clinically acceptable marginal discrepancy value than indirect one.
The main advantage of OCT over micro-CT system and which makes OCT suitable in the biomedical sector is the absence of toxic effects such as ionizing radiation. Nowadays, many composite systems that can be used by both direct and indirect techniques are available.
It is a promising imaging modality, which does not require cutting and processing specimens and allows the visualization of microstructures of tissue and biomaterials in real time3,11.
Two coats of glycerin were applied to all preparation walls on working cast and left for drying. The light beam was orthogonally scanned to the tooth-inlay interface in such a way that the infrared beam traversed over the tooth surface, air (distance between tooth-inlay), and inlay restoration regions sequentially. In addition, other studies stated that the OCT system can be employed as a quantitative and complementary tool for analyzing the fracture propagation, defects, and gaps15,20. Fabrication stages of indirect composite inlays, including impression and the die production steps, could explain greater marginal discrepancy values in indirect composite inlays. Then, the teeth were randomly divided into two groups according to the inlay fabrication technique: direct and indirect groups (n=17).
Two horizontal increments of composite material (Esthet X HD) were applied and anatomically shaped.
The entire tooth-restoration margins were scanned, which means from one approximal surface to the other including the cavosurface margins. In the present study, the photonic imaging modality of OCT was utilized to quantitatively compare the marginal adaptation of composite inlay restorations fabricated by direct and indirect techniques.
In this technique, the composite is first light cured directly in the inlay cavity and then the inlay is removed from the cavity and post-cured. Some studies used silver penetration into the interfacial gap that behaved as a metallic contrast agent, which enhanced the OCT reflection in their study11,21. Marginal discrepancy values ranging from 48 to 219 µm have been reported for various indirect composite and ceramic inlays9,14,29. Before removing restoration from die, die was placed in Triad Unit for a final 2 minute curing according to the manufacturer’s instructions.
On the other hand, contrast agents were not used in other OCT studies regarding the examination of dental restorations2,16,20,22,23,30.
However, it is recommended that marginal adaptation of inlays should be less than 100 µm10.
In the indirect technique, an impression is taken after cavity preparation; then, it is sent to the laboratory to fabricate inlay restorations. Die stone was scraped away from the inlay margins to prevent accidental chipping of the restoration.
In the present study, the marginal adaptations of direct and indirect inlays were qualitatively, quantitatively, and noninvasively compared by OCT without a contrast agent, and a large cavity scheme was used to represent the clinical situation.
Although inadequate adaptation of inlays can be compensated by resin luting cement at the margins of a restoration, it has been shown that an accurately fitting restoration is prerequisite for long-term success. The indirect technique improves the visual checking of marginal adaptation, proximal contacts, anatomic form, and polymerization shrinkage, compared with direct composite technique26.
For the occlusal surface, buccal to inlay and lingual to inlay measurements were carried out (Figure 4). It is stated that significant resin luting cement wear was also observed around wide marginal adaptation values (>150 µm), and a good marginal adaptation would significantly reduce the wear of resin luting cements in clinical conditions10. Directly fabricated inlays are less expensive, easily built up clinically, and demonstrate better marginal integrity than indirect ones26.
Inlays were then checked in the respective cavities for marginal integrity using a silicone-disclosing medium (Fit Checker, GC-Germany, Munich, Germany). Marginal adaptation, proximal contacts, and polymerization shrinkage can be also controlled with directly fabricated inlays rather than direct composite restorations.

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