# Low-Level Light Therapy in Orthodontic Treatment: A Systematic Review
## 1 Introduction  
LLLT in dentistry makes different dental procedures more comfortable for the patient since it is a potential alternative to some standard devices and techniques, such as local anesthesia, scalpels, and drills [1]. LLLT is a \"cold light\" therapy because it maintains a constant temperature throughout the procedure, which is in contrast to others that are utilized for thermal coagulation or tissue cutting and involve increasing the temperature in tissues [2, 3]. Regarding the biomolecular pathway of action, LLLT employs a photochemical mechanism in which energy is transferred to intracellular mitochondrial chromophores, i.e., light-absorbing molecules, such as endogenous porphyrins and respiratory chain components such as cytochrome-C oxidase, which are capable of transferring absorbed laser energy to the mitochondria; at this level, laser energy is converted into metabolic energy via the respiratory chain with the production of adenosine triphosphate (ATP) [4, 5]. The mitochondrial respiratory chains act as the main photoreceptors for LLLT at wavelengths in the visible spectrum, whereas the calcium channels at the cell membrane level serve as the key photoreceptors for LLLT at wavelengths in the infrared spectrum [6, 7]. The short-termactivation of the respiratory chain and oxidation of nicotinamide adenine dinucleotide (NADH) brought on by the respiratory chain's absorption of light results in modifications to the mitochondrial and cytoplasmic redox states. The electron transport chain becomes active and causes the cytoplasm to become more alkaline, the electrical potential of the mitochondrial membrane to increase, the reserve of ATP to increase, and then the synthesis of nucleic acids to begin [8; 9]. LLLT activates cells through two mechanisms:  
1. A direct mechanism through photobiological action (respiratory chain), where intracellular signaling is activated in redox chains (ATP increase);
2. An indirect mechanism of the activation of cells via secondary messengers released by the activated cells directly [10]. Past results have demonstrated great LLLT compliance, no invasiveness, no traumatic side effects, and no collateral impacts [11; 12]. These numerous advantages have made it possible to intervene in a safer and easier way, especially in patients with special needs and in small children [13]. Therefore, over recent decades, the results of LLLT have been studied in many branches of dentistry. For example, it can be used to alleviate the painful sensation caused by the needle insertion of needed anesthesia [14] or to disinfect carious lesions or root canals during ongoing devitalization through photoactivation [15], and it may also be useful in the treatment of sore mouth syndrome, although this has yet to be demonstrated with large-scale clinical trials [16]. Thanks to its effect on osteoblast proliferation and bone formation, its ability to interact selectively with the mouth cavity's tissues, its healing effect, and its biostimulating effect favoring the healing process, the dental laser is particularly suitable for surgery/implantology [9; 17]. In recent years, it has also gained popularity in the orthodontic branch, particularly for tooth movement, root resorption, pain perception during treatment, and the stability of orthodontic miniscrews. Orthodontics has grown considerably in recent decades due to the development of extremely high-performance alloy wires and low-friction brackets. However, for most patients undergoing orthodontic treatment, pain persists as a common problem, particularly in the first 3 to 4 days after fixed orthodontic appliances have been fitted [18; 19]. It is clear that this suggested symptom could reduce patients' compliance [20] or discourage them from treatment [21]. Moreover, the average time for orthodontic treatment varies considerably from case to case, ranging from 12 to 24/36 months.