1

Introduction

In recent years, caries pattern has changed and the traditional restorative dentistry model is changing towards less invasive approaches. Resin infiltration has been proposed as one of several minimal invasive caries treatments of non-cavitated carious lesions without sacrificing dental hard tissue . Various experimental and in vivo studies have reported that resin infiltration inhibits the progression of carious lesions and increasingly recommended it for clinical use. Against this background, detailed knowledge about resin infiltration throughout the various process stages is desirable to optimize application strategy and thus infiltration results. In addition to the dehydration of enamel , the etching step and the penetration of the lesion with resin are two key sub-processes.

Optical coherence tomography (OCT) has the potential to display these areas of varying refractive index. The method is based on the principle of low-coherence interferometry and has been previously described in various publications . To our knowledge, there is a no non-destructive imaging tool available to visualize the etching process of natural human carious lesions or the resin infiltration into these lesions. The aim of this investigation was to visualize the etching process and infiltrant penetration into white spot lesions by spectral domain OCT (SD-OCT).

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Materials and methods

Four intact, extracted human molars and premolars with vestibular carious lesion were visually selected (ICDAS code 2) after approval of the study protocol by the Ethics Committee of the University of Leipzig (No. 299-10-04102010). One region of interest (ROI) per tooth was marked by 2 drill holes each in occlusal-cervical direction (diamond bur, micro lance, pointed, 46 μm, 957 AM, Komet/Co. Gebr. Brasseler GmbH) and these marked ROI were imaged by micro computed tomography (, 100 kV, filter Al + Cu, rotation step 0.2°, averaging 6, pixel size 3.5 μm; Skyscan 1172-100-50, Bruker Micro CT, Kontich, Belgium) to verify the spatial extent of the carious lesions to enamel. The ROI were imaged 2- and 3-dimensionally by spectral domain OCT (SD-OCT, Telesto SP5, Thorlabs GmbH, Dachau, Germany). OCT images were acquired under the following conditions: The light was vertically fed into the specimens after the surfaces were carefully dried using a cotton pellet to leave the surface moist without water droplets. Technical specifications of the OCT system were as follows: center wavelength 1310 nm ± 100 nm, sensitivity ≤ 106 dB, axial/lateral resolution < 7.5 (air)/15 μm, field of view 9 mm x 9 mm x 3 mm (pixel size 512 × 512 × 512), A-scan average 1, spacing 6 μm. The carious lesions were treated with Icon (DMG, Hamburg, Germany) according to the manufacturer’s instructions and during each treatment step the ROIs were video-recorded with SD-OCT (CamStudio 2.7.2, Apowersoft Free Screen Recorder 1.2.4). The specimens were cleaned and dried. Hydrochloric acid (Icon-Etch) was applied onto the lesions for 2 min and rinsed with water for 30 s. After drying the lesions with oil- and water-free air, ethanol (Icon-Dry) was applied onto the lesions and allowed to evaporate for 30 s. The infiltrant was applied onto the etched, dried lesions and allowed to penetrate for 3 min. Before light-curing for 40 s. Additionally, deviating from the standard instructions the hydrochloric acid was actively applied onto the lesion surface by moving the application tip. The etching gel was applied onto a glass surface to test for gas development within the gel. The image sequences for etching and infiltration were viewed in time lapse. The specimens were sectioned along the ROI planes using a microtome (Leitz 1600 Sägemikrotom, Ernst Leitz Wetzlar GmbH, Germany), decalcified (2% HCl, 10 s, 20 °C) and deproteinized (10% NaOCl, 30 s, 20 °C). After rinsing (distilled water, 10 s, 20 °C), dehydration (30% – 100% ethanol; 10 min Hexamethyldisilazane, Carl Roth GmbH + Co, Karlsruhe, Germany), air drying and gold coating (10 nm; sputter coater MSC 1, Ing.-B. P. Liebscher, Wetzlar, Germany) the sections were imaged by scanning electron microscopy (SEM, 5 kV; Phenom G2 pro, Phenom World, Eindhoven, Netherlands). The extent of carious lesions and resin infiltration, as visible in the OCT images, were compared with the corresponding information from the μCT images and SEM images, respectively.

2

Materials and methods

Four intact, extracted human molars and premolars with vestibular carious lesion were visually selected (ICDAS code 2) after approval of the study protocol by the Ethics Committee of the University of Leipzig (No. 299-10-04102010). One region of interest (ROI) per tooth was marked by 2 drill holes each in occlusal-cervical direction (diamond bur, micro lance, pointed, 46 μm, 957 AM, Komet/Co. Gebr. Brasseler GmbH) and these marked ROI were imaged by micro computed tomography (, 100 kV, filter Al + Cu, rotation step 0.2°, averaging 6, pixel size 3.5 μm; Skyscan 1172-100-50, Bruker Micro CT, Kontich, Belgium) to verify the spatial extent of the carious lesions to enamel. The ROI were imaged 2- and 3-dimensionally by spectral domain OCT (SD-OCT, Telesto SP5, Thorlabs GmbH, Dachau, Germany). OCT images were acquired under the following conditions: The light was vertically fed into the specimens after the surfaces were carefully dried using a cotton pellet to leave the surface moist without water droplets. Technical specifications of the OCT system were as follows: center wavelength 1310 nm ± 100 nm, sensitivity ≤ 106 dB, axial/lateral resolution < 7.5 (air)/15 μm, field of view 9 mm x 9 mm x 3 mm (pixel size 512 × 512 × 512), A-scan average 1, spacing 6 μm. The carious lesions were treated with Icon (DMG, Hamburg, Germany) according to the manufacturer’s instructions and during each treatment step the ROIs were video-recorded with SD-OCT (CamStudio 2.7.2, Apowersoft Free Screen Recorder 1.2.4). The specimens were cleaned and dried. Hydrochloric acid (Icon-Etch) was applied onto the lesions for 2 min and rinsed with water for 30 s. After drying the lesions with oil- and water-free air, ethanol (Icon-Dry) was applied onto the lesions and allowed to evaporate for 30 s. The infiltrant was applied onto the etched, dried lesions and allowed to penetrate for 3 min. Before light-curing for 40 s. Additionally, deviating from the standard instructions the hydrochloric acid was actively applied onto the lesion surface by moving the application tip. The etching gel was applied onto a glass surface to test for gas development within the gel. The image sequences for etching and infiltration were viewed in time lapse. The specimens were sectioned along the ROI planes using a microtome (Leitz 1600 Sägemikrotom, Ernst Leitz Wetzlar GmbH, Germany), decalcified (2% HCl, 10 s, 20 °C) and deproteinized (10% NaOCl, 30 s, 20 °C). After rinsing (distilled water, 10 s, 20 °C), dehydration (30% – 100% ethanol; 10 min Hexamethyldisilazane, Carl Roth GmbH + Co, Karlsruhe, Germany), air drying and gold coating (10 nm; sputter coater MSC 1, Ing.-B. P. Liebscher, Wetzlar, Germany) the sections were imaged by scanning electron microscopy (SEM, 5 kV; Phenom G2 pro, Phenom World, Eindhoven, Netherlands). The extent of carious lesions and resin infiltration, as visible in the OCT images, were compared with the corresponding information from the μCT images and SEM images, respectively.

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Results