Dental Tissue Biology in Orthodontic Tooth Movement

Orthodontic tooth movement (OTM) is based on the principle of applying forces to a tooth for a certain period of time, which evokes a pronounced biological reaction in the dental pulp (DP), periodontal ligament (PDL), alveolar bone and gingiva. The biologic responses of PDL have been systematically investigated at a molecular, cellular, and tissue-level. However, most studies have been performed in animals and the role of the different molecules has been separately tested either in vivo or in vitro. Studies exploring the effects of orthodontic force (OF) on the DP have showed that increased OF involves pulp cell damage and inflammation. Knowledge regarding the details of the conversion from mechanical forces to biochemical signals on human PDL and DP tissue is still deeply lacking, which would have a significant impact on daily dental and orthodontic practice. Clinically, it is known that OTM results from a complex array of interacting biologic factors. Variables such as age and gender strongly influence the rate of OTM. Ageing affects PDL and DP homeostasis: the PDL becomes more fibrotic during maturation and the root apex narrows by deposition of secondary dentin and cementum, which compromises the blood supplies and nerve tissue passing through the apical foramen. Moreover, PDL biology might be altered in patients of different sex, since hormones are considered to be key mediators in the complex interplay guiding OTM. Therefore, understanding age- and sex-dependent responses of dental tissue to OF can optimize our selection of force systems to provide more efficient and safer treatment for all patients.