In the experiment, the reaction of the rat oral mucosa to fractional mechanical and laser treatment performed by radiation of a diode laser with a wavelength of 980 nm in various combinations of the average power of laser radiation, the pulse duration and energy of the laser radiation, along with the fill factor was investigated. Optical and histological examination of the rat oral mucosa during its regeneration after fractional treatment was performed. It was found that on the 5<sup>th</sup> day after a single-shot fractional laser treatment, the number of mitotically dividing cells exceeds the number of those cells after a single-shot fractional mechanical treatment and in the control area. It has been found that on the 28<sup>th </sup>day after a three-fold fractional treatment, the surface of the mucosa is visually bleached, and after the laser treatment, the connective tissue of the mucosa lamina propria and the submucosal layer contains fibrous structures and cellular elements, mainly myo- and fibroblasts. It has been found that during the regeneration on the 28<sup>th</sup> day after a three-fold fractional laser treatment, the thickness of the epithelium and the thickness of the submucosal layer with lamina propria of mucosa increase in comparison with the control.
Optical and histological methods were used to examination of influence the power and pulse duration of 980-nm diode laser to the dimensions and morphology of tissue around fractional micro injuries created by the radiation of that laser in the oral mucosa of rats <i>in vivo</i>. The power of laser radiation (P) varied in the range of 1÷21 W, and its pulse duration (t<sub>p</sub>) - in the range 50÷500 ms. <p> </p>Histological examination showed that in the mucosa of the oral cavity after the laser fractional irradiation, there following effects are found: a tissue defect, a transudate in the lumen of ablative micro injury, stretching and compacting effect of the nuclei of the basal epithelium, the disappearance of granules of the keratohialin, destroying the structure of the connective tissue, erythrocyte stasis in the vessels, the disappearance of transverse striation in the muscle fibers in muscle layer.<p> </p> It has been found that ablative micro injury begins to form up at P = 5 W, t<sub>p</sub> = 100 ms and affects only the epithelial layer of the mucosa. At P = 7 W, t<sub>p</sub> = 120 ms, the ratio of width to depth of ablative micro injury is 1 : 1, and at P = 10 W, t<sub>p</sub> = 100 ms, an ablative micro column with ratio of 1 : 1.5 is formed in the epithelial and submucosal layers of the mucosa. The laser effect with P = 15 W, t<sub>p </sub>= 200 ms leads to lengthening of the ablation micro-column to 1 : 2, with the bottom of the ablative micro column reaching the muscular layer. With a further growth of laser power or pulse duration, the width of the micro injury increases, and the growth of the micro injury depth is slowed down so that the micro column buildup is ceased.