Assessment of composite leakage using optical coherence tomography: A systematic review

1. Introduction

Advances in restorative dentistry are now synchronized with modifications of resin composite and adhesive formulations. However, polymerization shrinkage of methacrylate-based resin composites during curing may result in contraction stress and a loss of marginal adaptation [1]. Dental adhesives should provide an equally effective bond to both the tooth tissue and composite resin. An inadequate marginal sealing of the restoration can lead to microleakage [2], postoperative sensitivity [3], and debonding [4], which ultimately reduces the longevity of the restoration.

Polymerization shrinkage, the maximum rate of shrinkage, and polymerization shrinkage stress of dental composites of the resin composite can create marginal microgaps at the outer interface of the restoration and interrupt the integrity of internal adaptation between the resin composite and tooth substrate [1]. Marginal microgaps can be identified by an inspection from outside the tooth structure. To find these microgaps inside a tooth, dye and tracer penetration methods have been previously used [2,3]. Because tracers, such as methylene blue, rhodamine, erythrosin and silver nitrate, can infiltrate into teeth, these tracers were used to assess gaps even though restorations must still be cut out [4]. In addition, once the examined teeth are sectioned for the experimental measurement of gaps, the sample is degraded. Even though sacrificing of the experimental sample is plausible, the degree of staining of the dentin layer defines a straightforward measure that predicts the actual microleakage between the cavity and restoration [5].

Marginal adaptation, bond strength, and interaction with the tooth substrate are common methods for assessing the bonding performance of restorative systems [5]. Conventionally, marginal adaptation tests require multiple sectioning of samples, followed by immersion into a staining solution and surface polishing before examination under a light microscope, a scanning electron microscope, or a transmission electron microscope (TEM) [6–8]. These procedures are time-consuming and limited to experimental studies. Measuring microtensile bond strength and interaction with the tooth substrate does not pose as many clinical implications as studies on marginal adaptation [9-11].

Optical coherence tomography (OCT) is a new nondestructive method for producing high-resolution, cross-sectional images of internal biological structures at the micron scale. Based on low-coherence interferometry, OCT was introduced to backscatter signal intensity within the scattering medium. The signal from serial scans can be transformed into an image using specific software [12]. OCT began being used in dental studies to characterize primary and secondary caries, assess gaps between the composite-tooth interface, and evaluate voids and internal defects in dental restorations.

OCT is a noninvasive cross-sectional imaging method that uses low-coherence interferometry. This technique permits visualization of the microstructure of tissues and biomaterials in real-time without requiring tissue sectioning or specimen preparation. A computer can be used to reconstruct a visual image from the obtained backscatter signal as an input [14].

In the OCT image, an interfacial microgap is observed as a bright spot or line with high signal intensity that changes at the interface appears as a white cluster on the image. When light passes through the interface between two medias with different refractive indices, a portion of light is reflected (Fresnel phenomenon), depending on the incidence angle and refractive index. The refractive index of air is 1.0 (n) and a tooth or resin composite is 1.5–1.6 (n) [15-17]. If incomplete adhesion forms a microgap, air or water may exist at the interface. The OCT system shows a higher signal intensity by reflecting light there. If the microgap is filled with another medium, the reflection is weaker than air. Microfocus X-ray computed tomography (micro-CT) is another useful method to evaluate the internal adaptation of restorations [18-20]. Due to the penetrating ability of X-rays, micro-CT evaluates dental hard tissue irrespective of the depth.

Swept-source OCT (SS-OCT) can construct images using ultra-high-speed scanning of the generated near-infrared laser wavelength and by evaluating the superficial dental layers and restorations quickly and precisely. SS-OCT can improve the penetration of images with a 1050 nanometer wavelength to generate an axial resolution of 5.3 μm and a rate of 100,000 scans per second, which equals a two-fold measure of the highest obtained score; this makes it an excellent candidate for studying dental composites, for the early detection of dental caries, and for extension to other uses [16-18]. Micro-CT and SS-OCT can be useful non-destructive methods for the evaluation of internal adaptation. This systematic review investigates the assessment of leakage in dental restoration using OCT

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