ORIGINAL ARTICLE Effects of photobiomodulation at different wavelengths on orthodontically induced root resorption in orthodontic retention period: a micro-CT and RT-PCR study Taner Ozturk 1 & Nisa Gul Amuk 2 Received: 17 May 2019 /Accepted: 30 March 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020 Abstract The aim of this study was to evaluate and compare the reparative and inhibitory effects of single wavelength photobiomodulation (SW-PBM) and of cumulative increased wavelength photobiomodulation (CW-PBM) on orthodontically induced inflammatory root resorption (OIIRR). Thirty-three Wistar albino rats were divided into five groups: untreated group (negative control), only relapse group (positive control-1), only retention group (positive control-2), SW-PBM group (650 nm, 100 mW/cm 2 ), and CW- PBM group (532–650–940 nm, 100 mW/cm 2 ). Orthodontic tooth movement was induced experimentally in rats for 10 days with an applied force of 50 cN; retention and therapeutic approaches were performed concurrently. At the end of the experiment, maxillary quadrants were prepared for micro-CT analysis and real-time polymerase chain reaction (RT-PCR) analysis. After the Shapiro-Wilk normality test, Kruskal-Wallis test followed post hoc Bonferroni test and paired samples t test was used for statistical evaluation of the data. Resorption lacunae volume (p < 0.001), number of resorption lacunae (p < 0.05), and percentage of the resorption (PR) lacunae (p < 0.001) decreased with PBM applications when compared with the positive control groups, and the mean PR was similar in the negative control group when compared with SW-PBM group. Receptor activator of nuclear factor kappa B ligand (RANKL) levels of the PBM groups were lower (p < 0.05) than those of the positive control groups. Cyclooxygenase-2 (COX-2) expression levels significantly decreased with PBM administration (p < 0.05). No significant change was found in osteoprotegerin (OPG) expression levels and OPG/RANKL ratios (p > 0.05). PBM applications showed marked inhibitory and reparative effects on OIIRR by modulating the RANKL and COX-2 expression levels. However, the effects of the different wavelengths were similar to each other. Keywords Biostimulation . Cyclooxygenase . Osteoprotegerin . Receptor activator of nuclear factor kappa B ligand . Tooth root repair . Three-dimensional analysis Introduction Orthodontic relapse is a complex, unpredictable, and common problem in orthodontic treatment that occurs due to the ten- dency of the tooth to return in its initial position [1–3]. Despite comprehensive diagnosis and correct treatment planning, orthodontic relapse exists due to crowding or spacing of aligned teeth, return to increased overbite and overjet, and instability of angle class II and III corrections [3]. The main causes of relapse have generally been accepted as remodeling of the periodontal ligament (PDL) fibers and the surrounding alveolar bone [4, 5], which also induces a tooth movement in the reverse direction. Orthodontic tooth movement and ortho- dontic relapse occur through the same process, with an in- crease in osteoclast differentiation in the compression side of PDL [1, 4, 6]. During orthodontic tooth movement, a pressure occurs in the compression side of the PDL that produces ischemic ne- crosis at different levels. The initial removal of this necrotic tissue is achieved by macrophage-like and multinucleated cells activated by biochemical signals derived from the sterile necrotic tissue; the damage that appears at the nearby outer * Taner Ozturk tanertr35@gmail.com 1 Department of Orthodontics, Faculty of Dentistry, Erciyes University, Erciyes Universitesi Dis Hekimligi Fakultesi, Kayseri, Turkey 2 Department of Orthodontics, Faculty of Dentistry, Erciyes University, Erciyes Universitesi Dis Hekimligi Fakultesi, Kayseri, Turkey Lasers in Medical Science https://doi.org/10.1007/s10103-020-03014-1