Abstract
Objectives
To analyze marginal integrity of resin composites dental restorations using optical coherence tomography (OCT).
Methods
Thirty extracted human premolars had occlusal cavities prepared and were randomly divided according to the restorative systems evaluated: Filtek P90™/P90 Adhesive System™, Filtek Z350™, and Filtek Z250™/Single Bond™ (3 M/ESPE). The teeth were then stored in the dark for 24 h in 0.9% saline solution. Restorations were finished and polished and stored again for 24 h before thermocycling (500 cycles, 5–55 °C). A commercially available OCT system was used (SR-OCT: OCP930SR/Thorlabs) with 930 nm central wavelength. Cross-sectional images were obtained every 250 μm and evaluated using Image J. A-scans were analyzed using the Origin 8.0 program, after a filter treatment using Matlab.
Results
The qualitative analysis of the internal margins did not observe gaps even after A-scan examination, although distinctive patterns were found for each restorative system. Penetration of Single Bond and Filtek P90 self-etch primer into dentin was also observed. A thick adhesive layer was found for Filtek P90 bonding agent.
Significance
Considering the characteristics of the OCT system, the setup used in this study was capable of evaluating the marginal integrity of resin composite restorations and detecting some interaction between dental bonding agents and dental substrates. OCT can be considered a promising method for the evaluation of the internal margins of restorations in vivo .
1
Introduction
Resin composites are now used as a restorative material because of their excellent mechanical and esthetic properties. However, one of the biggest problems associated with their use is inherent shrinkage during the polymerization reaction. The polymerization reaction of dimethacrylate monomers involves approximation for the formation of polymer molecules. Therefore, by the end of the polymerization process, the polymer molecule occupies a smaller space compared with that required by the monomer at the beginning of the reaction. This contraction can cause flaws in the integrity of the tooth–restoration interface, hence, microleakage, marginal discoloration, recurrent caries, postoperative sensitivity and enamel fractures can occur. All of these factors can reduce the longevity of the restoration.
Improvements have been developed over the years always with the goal of enhancing the properties and the longevity of the material in the oral cavity. Most of these changes have occurred in the filler particles; the chemistry behind the organic matrix has undergone little change since the pioneering work of Bowen in 1960. Virtually all commercially available resin composites use dimethacrylates such as TEGDMA, BisGMA or UDMA, all cured by an addition reaction of free radicals in their double bonds . This is because the incorporation of a greater amount of inorganic filler particles promotes a reduction in the organic matrix concentration, responsible for polymerization shrinkage. One of the ways found to increase the concentration of these particles was through a reduction in size, which culminated in the development of nanoparticles for such applications .
The use of nanoscale particles did not necessarily lead to an increase in the concentration of inorganic fillers, but it did lead to improvements in polish retention. Thus, polymerization shrinkage using nanoparticles remains similar to that of their precursors, microhybrid resin composites .
In search of ways to further reduce polymerization shrinkage, innovations in resin monomers emerged as a new challenge. As a result, a new type of resin matrix was developed based on cyclic monomers. These hydrophobic monomers are composed of a siloxane backbone with oxirane rings, and for that reason are called siloranes . According to the manufacturer, the main advantage of this new restorative system is its reduced polymerization shrinkage with mechanical properties similar to resins based on methacrylates. The lower shrinkage is a result of the fact that their reaction is based on the opening of their rings via a cationic reaction. This process allows a gain of space that overlaps the volume contraction that occurs in the subsequent step of the reaction, when the monomers form chemical bonds between them leading to polymer chain formation.
To be used as restorative materials, silorane-based resins need compatible bonding agents, which led to the development of an inherent two-step self-etch adhesive.
Evaluation of the marginal integrity of restorations can be accomplished using various methods. The most common method is the evaluation of marginal leakage by dye penetration, followed by observation of the margins under a stereoscopic microscope and/or scanning electron microscope (SEM). For SEM, sectioning of the tooth/restorations involved is required to assess the presence of internal cracks and irregularities, which does not allow evaluation of the marginal integrity in vivo . However, non-destructive analysis using SEM can be performed by obtaining epoxy replicas of the occlusal surface of the restoration .
In this context, optical coherence tomography (OCT) is a possible technique for analysis of the tooth–restoration interface. Introduced in medicine at the beginning of the 1990s, optical tomography has become a powerful method for image acquisition of internal structures of biological systems and materials. This method obtains sub-surface images in a non-invasive way using light rather than a magnetic field or x-radiation. OCT uses the optical properties (reflection and backscattering) for image generation. Its principle is similar to ultrasound, although the latter uses sound waves and the former uses light. Depending on the source spectral bandwidth, it is possible to obtain images with micrometer (μm) resolution. Furthermore, with the OCT system there is no need for direct contact between the probe or apparatus and the tissue under study or the immersion fluid, as light can easily pass through the air–tissue interface .
The optical setup consists of a Michelson interferometer with a low coherence broadband light source. The light generated in an OCT system is split into two arms: a sample arm, containing the item of interest, and a reference arm that contains a movable mirror.
The reflected light from the sample arm and from the reference arm are then recombined and focused by a spectrometer, where any degree of interference between the beams can be observed, but only if light from both arms have traveled the same optical distance .
Nowadays, superluminescent diodes (SLDs) are the preferred source for light generation in OCT systems. This light source produces a wide range of wavelengths, each one generating its own interference image. The intensity of the interference depends on scattering caused by changes in the structure of the tooth, for example. Areas of the sample that backscatter a lot of light will create greater interference than areas that do not. This reflectivity profile, called an A-scan, contains information about the spatial dimensions and location of structures within the item of interest. By making a series of A-scans, scanning transversely to the light beam, it is possible to obtain a two-dimensional map (B-scan) of the reflected light in each point of the scanned area. In other words, a cross-sectional tomographic image may be achieved by combining a series of A-scans along a line.
OCT has been used previously to evaluate gaps at the tooth–restoration interface. In that study, amalgam and composite restorations had intentional gaps (51–146 μm) made at the interface with an acetate tape. Optical microscope analysis confirmed the results revealed by OCT for the location and size of the gaps . The aim of this laboratory study was to qualitatively evaluate the marginal integrity of the new silorane-based restorations under OCT, and to compare it with conventional methacrylate-based restorations. The hypotheses tested were: (1) OCT can be used as a diagnostic method for evaluating marginal integrity; (2) there are no differences in marginal integrity for the restorative systems tested.
2
Materials and methods
The materials, manufacturers, composition and batch numbers for the materials used in this study are listed in Table 1 .
| Material | Composition: filler class, % weight, % volume | Manufacturer a (batch no.) | Restorative technique |
|---|---|---|---|
| Filtek Z250™ | BisGMA, UDMA and BisEMA BisGMA, zirconia/silica: microhybrid, 82%, 60% | 3M/ESPE (5WK) | 2-mm increments; photoactivation for 20 s |
| Filtek Z350™ | UDMA, TEGDMA and BisEMA, zirconia/silica and silica: nanoparticle, 78.5%, 59.5% | 3M/ESPE (6EB) | 2-mm increments; photoactivation for 20 s |
| Filtek P90™ | Silorane resin, quartz filler and yttria fluoride, stabilizers, pigments: microhybrid, 76%, 55% | 3M/ESPE (9CH) | 2-mm increments; photoactivation for 40 s |
| Adper Single Bond 2™ | BisGMA, HEMA, dimethacrylates, ethanol, water, photoinitiator system, methacrylate functional copolymer of polyacrylic and polyitaconic acids, 5-nm silica particles | 3M/ESPE (9XE) | 1. Etching with phosphoric acid 35%. Applied first to enamel and then to dentin. Wait 15 s and rinse for 30 s. Blot excess water using a humid cotton pellet 2. Immediately after blotting, apply a coat of adhesive with gentle agitation using fully saturated applicator for 20 s. Repeat the application. Gently air thin for 5 s to evaporate solvent. Photoactivation for 20 s |
| P90 System Adhesive™ | Self-etch primer : Phosphorylated methacrylates, vitrebond copolymer, BisGMA, HEMA, water, ethanol, silane-treated silica filler, initiators, stabilizers | 3M/ESPE (9BL) | 1. Apply 1 coat of the self-etch primer using fully saturated applicator for 15 s with gentle agitation. Gently air thin to evaporate solvent and obtain an even film. Photoactivation for 10 s |
| Bond : Hydrophobic methacrylates, phosphorylated methacrylates, TEGMA, silane-treated silica filler, initiators, stabilizers | 3M/ESPE (9BH) | 2. Apply the bond to the entire preparation using fully saturated applicator. Gently air thin until the bond is spread to an even film and does not move any longer. Photoactivation for 10 s |
2.1
Experimental setup
A commercially available OCT system was used (Spectral Radar SR-OCT: OCP930SR/Thorlabs, New Jersey, USA). The superluminescent diode (SLD) light source operates at a central wavelength of 930 nm. This system consists of three main parts: a handheld scanning probe, a base unit and a personal computer (PC) ( Fig. 1 ). The base unit contains the SLD light source. A fiber optic coupler is used to direct the light from a broadband SLD source to the Michelson interferometer, which is located inside the handheld probe. The probe and the reference light travel back through the same fiber to the spectrometer and the imaging sensor located in the base unit. The base unit is connected to the PC, which is equipped with two high-performance data acquisition cards. All required data acquisition and processing is performed via the integrated software package, which includes a complete set of functions for controlling data measurement, collection and processing, and for displaying and managing OCT image files. The maximum image depth is 1.6 mm, maximum lateral scanning is 6.0 mm and the axial resolution is 6.2 μm.
2.2
Marginal integrity evaluation
Thirty caries-free extracted human pre-molars were selected from the tooth bank after approval from the Human Ethics Committee of the University of Pernambuco, Recife, Brazil. Class I occlusal cavities were prepared with diamond burs (standard grain 75–125 μm, no. 3131, Microdont, São Paulo, Brazil) in a high-speed hand piece with a cooled water spray, using one bur per five cavities. Because of the shape of the bur, slightly expulsive cavities were obtained with the following dimensions: 3 mm mesiodistal width, 1.5 mm buccolingual width and 1 mm occlusal depth. All internal angles were rounded. The teeth were then pumiced and randomly divided into three groups ( n = 10) according to the resin composite restorative system evaluated ( Table 1 ).
Resin composites were inserted in two oblique increments. Each increment was photoactivated according to the manufacturer’s recommendations ( Table 1 ) using a halogen light output (Optilight Plus™/Gnatus, São Paulo, Brazil). Just before each specimen preparation, light intensity was measured with an external radiometer (Gnatus, São Paulo, Brazil), always within the range of 550–600 mW/cm 2 .
After storage for 24 h at room temperature in 0.9% saline solution in a dark environment, marginal excesses were removed with an air-cooled, high-speed, fine diamond bur (no. 3118F KG Sorensen, São Paulo, Brazil). Finishing and polishing of the restorations were conducted using low-speed rubber points (Enhance/Dentsply, Rio de Janeiro, Brazil) followed by felt points (Edenta, Hauptstrasse, Switzerland) with extra-fine grain aluminum oxide polishing paste (Diamond R/FGM, Joinville, Brazil). Specimens were then washed with distilled water for 15 min in an ultrasonic bath (Biowash/Bioart Equipamentos Odontológicos, São Carlos, Brazil) to eliminate any debris. After additional 24 h storage in the dark in 0.9% saline solution, the specimens were thermocycled. Alternate baths of 5 ± 3 °C and 55 ± 3 °C were applied at 500 cycles each for 15 s.
Specimens were positioned and individually fixed to the work table with modeling clay. Cross-sectional buccolingual images were obtained every 250 μm from the start of one of the proximal margins to the other. This procedure gave a complete mapping of the internal margins of the restoration.
Images were then qualitatively analyzed using public domain software, Image J ( Imaging Processing and Analysis in Java ) . Whenever doubts on the integrity of internal margins arose, A-scans of specific points were analyzed. Due to the level of noise in OCT images, it was necessary to use a low-pass filter for all images. This kind of filter introduces a distortion in the form of smoothing but is useful to removing isolated lines and pixels while preserving spatial resolution . A 5 × 5 average filter was used for noise reduction using a special program in the Matlab language. This program also provides A-scans from filtered images of specific points as a function of depth, analyzed using Origin 8.0 (Microcal Software Inc, Northampton, USA). For accurate quantitative measurement throughout OCT, it is necessary to know the refractive index of the materials being studied. Thus, the refractive index of all the test materials was calculated. Samples of the materials were obtained using a Teflon mold so that the exact thickness of the material could be determined using a digital caliper (0.01 mm). After obtaining the images by OCT, the refractive index could then be determined by applying the formula:
Refractive index = Optical distance ( obtained by OCT ) Real distance ( mold/caliper ) .
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