OCT- An Emerging Diagnostic Tool in Sensing Marginal Integrity of Nano Composites

Objectives: The purpose of the present research was to demonstrate images of failure into the structure of nano composites using optical coherence tomography. Methods: Ten human third mandibular molars were taken in the study. A class V cavity was prepared and restored with nano composite universal. 20 OCT images of each tooth were obtained and a total of 200 images were evaluated after restoration. Spectral domain (SD-OCT), at 840 nm and a measured spatial resolution of 10 μm was used for the study, which is based on the Michelson interferometer set-up and is controlled by the software OCT 800 – Complex Square/LabView, that collects data and generates the image. The restored surfaces were scanned in a bucco-lingual direction and tomographic images were obtained. Later, the crown of each tooth was sectioned in a occlusogingival direction (buccolingually) and images of each section were obtained by a Stereo microscope. Images of OCT and stereo microscope were compared and evaluated. Results: Images obtained by OCT helped in identifying failures at surface and in the internal structure of composites. Original Research Article Badami et al.; BJMMR, 15(10): 1-6, 2016; Article no.BJMMR.25699 2 Significance: Images obtained from OCT helps in detecting the failures, gaps, bubbles which adversely effect the longevity of restoration.


INTRODUCTION
Ideal restorative material must have an effective seal between the restoration, maintain aesthetics and add strength to the tooth structure. Even though advancement in the field of restorative materials, the marginal integrity of the restoration remains an uphill task for dentistry. Better the marginal fit and marginal adaptation more will be the longevity of the restorative material. The main drawback of composites is poor marginal adaptation resulting in marginal discoloration, post-operative sensitivity and secondary caries. The factors which govern the failure of composite resin restoration include the quality of bonding to the dental structures and the stress generated on the restoration [1].
The interpreters for the failure and replacement of restoration are the loss of marginal adaptation and the presence of secondary caries [2]. Microleakage is considered as the main culprit in the development of secondary or marginal caries due to loss of marginal fit [3]. Evaluating the integrity of the dental composites by traditional methods such as visual and probing inspection cannot identify gaps or failures into the internal structure of composites, which results in desadaptation, infiltration and loss of material.
The optical coherence tomography (OCT) provides images of biologic microstructure with high spatial resolution (<10_m) [4]. The components of OCT setup include a broadband light source, whose spectral width limits the axial spatial resolution; an interferometer, which generally employs a Michelson design [5]. Spectral domain OCT (SD-OCT) possess several advantages over the time domain OCT with sensitivity, [6] fast acquisition data, [7] etc. hence its use has been widespread [5,8].
Applications of OCT are utilized in branches of dermatology [9] and endoscopy [10]. OCT also has wide range of uses in dentistry like imaging of invitro dental structure by Colston et al. [11] differentiation of healthy teeth and caries lesions in vitro by Warren et al. [12] Recently it's application in diagnosing early caries [13] and evaluation of enamel interface restoration [14] is carried out. OCT images of dental pulps were reported by Kauffman et al. [15] using rat's teeth.
Shemesh et al.did extensive work on OCT in detecting the root canal wall, diagnosis of vertical root fractures [16,17]. On going research of OCT to characterize dental materials [18,19] and pulp dentin complex [20] has been reviewed.
Hence, the aim of the present research was to evaluate the OCT images of nano composites and compare them with stereomicroscopic images.

MATERIALS AND METHODS
Total 10 human mandibular third molars were taken for the study from the department of oral maxillofacial surgery (Mamata Dental College, Khammam). These teeth were cleaned of gross debris, scaled with ultrasonic instruments, washed in distilled water and sterilized in an autoclave. Class 5 cavity was prepared on the buccal surface of the teeth of 3× 2 ×2 mm dimensions. Samples were etched, bonding agent applied and nano composite universal was filled and light cured for 40 sec. Spectral domain optical coherence tomography was used for detecting the gaps and voids. 20 images of OCT are obtained after restoring with nanocomposite and a total of 200 tomographic images are obtained.
The SD -OCT setup uses a broad band light source operating at 840 nm and with a spectral width of 50 nm, a fiber output power 25 mw and a measured spatial resolution of 10 µm. The system is based on the Michelson interferometer set-up: in the reference arm, there is a mirror mounted on a piezoelectric base; in the sample arm, a mirror controlled by a Galvo motor is required for the scanning of light at the sample. Light from the source is first split by a beam splitting mirror and travels down the reference and sample arms, respectively. The reflected and back-scattered light coming from both arms are then recombined at the beam splitter and collected by a spectrometer, consisting of a lens collimator system, 1200l/mm grating and an optical detector (linear CCD cam, ATMEL, 2048 pixels, 12 bits, CA, United States), connected to a computer. The system is controlled by the software OCT 800 -Complex Square, developed from LabView program language, that collects data and generates the image. The image acquisition time is less than 1 s. The images were taken by scanning the buccal surface in a bucco-lingual direction. Images are obtained by scanning the buccal surface in bucco lingual direction. The light penetrated into the restorative material and adjacent tooth structure and tomographic image perpendicular to the long axis is obtained. After the achievement of the images by OCT, the crown of each tooth was sectioned in an occlusogingival direction (buccolingually) with a water-cooled diamond saw. The slices obtained were evaluated by an optical microscope. Stereo microscope performed the microscopic image from each slice. The microscopic images and the correspondent OCT counterpart were compared. The OCT and microscopic images were evaluated for the presence of gaps, voids and bubbles.

RESULTS
Representative OCT images of nano composite universal were obtained from the occlusal margin of class V cavity. Images in Figs. 1, 2 clearly detect the internal structure of composites and the adjacent enamel. OCT images can provide better vision into material about 1.50 mm deep at this wavelength. Failures at surface and interior of the nanocomposite restoration can be envisioned in these images. The superficial limit of the composite material is clearly delineated by a homogenous white line of high intensity.
Figs. 3, 4 shows the microscopic image of the nanocomposite, where gaps and bubbles are present at the surface and internally which are also clearly appreciated from the OCT images obtained.

DISCUSSION
Nanocomposites are composites in which atleast one of the phases shows dimensions in the nanometre range (1 nm =10 -9m ) [21]. Nanocomposite materials have emerged as suitable alternatives to overcome limitations of microcomposites and monolithics, while posing preparation challenges related to the control of elemental composition and stoichiometry in the nanocluster phase. They are reported to be the materials of 21 st century in the view of possessing design uniqueness and property combinations that are not found in conventional composites.

. OCT image showing presence of bubble in the nano composite restorative material
Accurate marginal sealing is crucial for the longevity of composite resin restorations [1,22,23]. Class V cavities were chosen because they remain a challenge for restorative procedures.
Hence,the clinical studies assessing the performance of an adhesive system use class V cavities. The C-factor of these cavities impairs the shrinkage, increasing the stress over the boding interface [22,24]. Mostly, these cavities often present gingival margins in the dentin, which requires additional challenge to obtain a proper marginal seal [23].

. Stereomicroscopic image showing the presence of bubbles in nanocomposite restoration
Earlier Visualization and probing were used in assessing the dental disease but their sensitivity and reliability are uncertain. Dental CT provides three-dimensional images for better diagnosis but the harmful ionizing radiation limit their usage. Innovative techniques, such as a smart ultrasonic devices, [25][26][27][28] LED-based dental optical probes, [29] and laser fluorescence [30,31] help in early detection of caries. OCT is an effective diagnostic tool because it is a noninvasive, nondestructive, non-radiated, and real-time monitoring method.
As OCT method is non invasive and can produce video rate images, it's application in the field of dentistry is being reviewed. Apart from advantages, the disadvantages of OCT include limited penetration depth and scanning range.
Early detection of the failures, gaps helps in increasing the longevity of restoration. Research on various materials should be carried out by OCT for determining the penetration depth, image distortion and minimum detectable gap size which increase the success rate and priority in selection of the restorations.

CONCLUSION
This research analysed the presence of gaps, bubbles which have adverse effect on the marginal integrity and longevity of restorations. Further studies are required to confirm the use of OCT for clinical application.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.