Selective adsorption of specific salivary biopolymers on the enamel surface leads to the formation of an acquired salivary pellicle.¹ The adsorbed protein layer can be considered a dynamic biofilm that regulates and modulates interactions at the interface between the tooth surface and oral cavity.^{1, 2, 3, 4, 5} The pellicle serves as a lubricant, protecting the dentition against abrasion and attrition.¹ In addition, the salivary pellicle may protect against demineralization of dental hard tissues caused by acid attack.^{1, 2, 3, 4, 5} The adsorbed pellicular phosphoproteins and mucins reduce enamel surface solubility, acting as a perm-selective membrane and regulating not only the diffusion of calcium and phosphate ions, but also the acidic agents.^{1, 2, 3, 4, 5} In addition to its protective functions of lubricating, inhibiting demineralization, and working as a diffusion barrier against chemical attacks of the enamel, the pellicle can act as a reservoir of agents promoting remineralization. The maturation age of the pellicle, when it can effectively inhibit erosive activity, remains unclear.

During orthodontic treatment, the early diagnosis of microscopic changes in soft and hard tissues, including periodontal tissue, is very important to prevent iatrogenic side effects like root resorption and periodontal diseases. Cervical periodontal tissue is the most critical area that reacts first to mal-habits or orthodontic forces, and it is also the place where bacteria deposits in the early stage of periodontal diseases. The early diagnosis of hard tissue changes, such as demineralization, is also very important in maintaining a patient’s health during orthodontic treatment. Many diagnostic devices, including radiographic equipment and intra-oral scanners, are helpful in diagnosing these problems, but have certain limitations in invasiveness and precision. The purpose of this study is to verify the possible utilities of non-invasive diagnostic devices in the orthodontic field that can compensate for these limitations. For this, non-invasive optical diagnostic devices, including optical coherence tomography and optical Doppler tomography, were used in vivo with animal and human examination for hard and soft tissues. These devices can provide real-time three-dimensional images at the histological scale. The results of this study verified these devices can be used in clinical practice during orthodontic treatment and introduced a new diagnostic paradigm differentiating microstructural changes in tissues in orthodontic diagnosis.