Purpose: Recently, low reactive level laser therapy （LLLT） has been attracting attention in periodontal treatment. However, little is known about the response to irradiation of gingival epithelial cells, which constitute gingival tissue and are directly affected by laser light in the superficial layer. In this study, we determined the ideal laser irradiation conditions and analyzed the temporal expression of relevant genes to investigate the effect of LLL irradiation on wound healing in human gingival epithelial cells.
Methods: Immortalized human gingival epithelial cells （HGK） were seeded at 2.5×10³ cells per well, cultured for 24hours. After incubation for 24hours, the cells were irradiated with lasers of 0.5, 1, 2, 3, 4, 5, and 6W, with no laser irradiation as a control. After laser irradiation, the cell proliferation rate was measured using WST-8, which is a method offering improvement over the MTT assay. Irradiation was performed under the laser irradiation conditions determined from cell proliferation measurement results. Then, qRT-PCR analysis was used to compare the temporal expression of wound healing-related genes （EGF, EGFR, TGF-α, and HB-EGF） at 1, 3, 6, 12, and 24 hours after laser irradiation.
Results: A significant increase in cell proliferation rate was observed in the 3 W laser irradiated group on days 2 and 3 after laser irradiation, compared to the control. Based on the above, the ideal laser irradiation condition for HGK in this study was determined to be 3W （100mJ, 30pps, 10s）. Gene expression analysis showed significant upregulation of EGFR and TGF-α at 6 hours and HB-EGF at 12 hours after laser irradiation compared to the control.
Discussion and Conclusion: The results of this study showed that LLL irradiation of HGK promoted cell proliferation and increased the expression of wound healing-related genes. LLL irradiation might promote wound healing of the gingival epithelium in vitro.

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Introduction: Swept-source optical coherence tomography （SS-OCT） is a new modality for noninvasively capturing cross-sectional images of not only biological structures but also any objects that are transparent to near-infrared light. With no exposure to radiation, SS-OCT can be used safely even for pregnant women and children.
Features: The center wavelength is 1,310nm and the sweep range is 110nm. The wavelength has low extinction to enamel and enables observation of deep internal structures. However, as it does not penetrate the full depth of dentin, only shallow parts can be observed. The shooting time of SS-OCT is much shorter than conventional TD-OCT （time-domain OCT） because the light source produces continuous spectra within the range without moving the reference mirror. Therefore, 2D images can be captured in real time.
SS-OCT images: 〈Sound tooth〉 Enamel and dentin are shown as homogeneous relatively white areas, divided by DEJ as a black line. Because of the short penetration depth of near-infrared light in dentin, deeper parts of dentin cannot be described. 〈Caries〉 Carious parts appear as brighter, whiter areas. Compared with the DEJ depth level, the caries depth can be estimated in some cases. According to past research, SS-OCT’s diagnostic ability exceeds that of conventional diagnostic methods such as visual inspection and X-ray photography, in occlusal and proximal caries. However, X-ray photography is advantageous for the diagnosis of deep caries which penetrates to near the pulp chamber. 〈Other structures〉 SS-OCT is also useful for obtaining images of occlusal tooth wear, enamel cracks, and composite resin. It may also be helpful for identifying the causes of clinical symptoms or finding secondary caries beneath composite resin.
Dental SS-OCT: Octina （Yoshida Dental Mfg. Co., Ltd., Tokyo, Japan） is an SS-OCT product, which was approved as a medical device in 2020 and can be used in dental clinics. The portable body contains all of the equipment needed to take cross-sectional images: probe, light source, computer, and so forth. It is dedicated to clinical use; for example, the air is blown to the mirror in the tip of the probe to avoid misting-over by the patient’s expired air.
Conclusion: SS-OCT is useful for caries diagnosis and observing other dental tissue or structures that can be penetrated by near-infrared light. However, another method may be appropriate in some cases. It is important to choose the best diagnostic technique after understanding the features of each method.

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