Biomechanical and biological responses of periodontium in orthodontic tooth movement: up-date in a new decade

Abstract

Nowadays, orthodontic treatment has become increasingly popular. However, the biological mechanisms of orthodontic tooth movement (OTM) have not been fully elucidated. We were aiming to summarize the evidences regarding the mechanisms of OTM. Firstly, we introduced the research models as a basis for further discussion of mechanisms. Secondly, we proposed a new hypothesis regarding the primary roles of periodontal ligament cells (PDLCs) and osteocytes involved in OTM mechanisms and summarized the biomechanical and biological responses of the periodontium in OTM through four steps, basically in OTM temporal sequences, as follows: (1) Extracellular mechanobiology of periodontium: biological, mechanical, and material changes of acellular components in periodontium under orthodontic forces were introduced. (2) Cell strain: the sensing, transduction, and regulation of mechanical stimuli in PDLCs and osteocytes. (3) Cell activation and differentiation: the activation and differentiation mechanisms of osteoblast and osteoclast, the force-induced sterile inflammation, and the communication networks consisting of sensors and effectors. (4) Tissue remodeling: the remodeling of bone and periodontal ligament (PDL) in the compression side and tension side responding to mechanical stimuli and root resorption. Lastly, we talked about the clinical implications of the updated OTM mechanisms, regarding optimal orthodontic force (OOF), acceleration of OTM, and prevention of root resorption.

Fig. 1

When a maxillary incisor is retracted, the bone resorption (−) occurs in the compression side and in the labial side of the alveolar bone, while bone deposition (+) in the tension side and in the palatal side of the alveolar bone. To explain the mechanism, we hypothesis that the PDLCs and osteocytes are the primary sensors responding to mechanical signals and the PDLCs control the soft tissue remodeling, while a PDLC-dependent PDLCs-osteocytes signaling network control the internal hard tissue remodeling and osteocytes control the external hard tissue remodeling

Fig. 2

The process of the transduction from mechanical loadings to biological signals. Step 1: the extracellular mechanobiology of the periodontium (in yellow). Step 2: cell strains (in red). Step 3: cells activation and differentiation (in green). Step 4: tissue remodeling (in blue)

Fig. 3

Hypothetical mechanisms of mechanical signal transduction in PDLCs

Fig. 4

Differentiation and its mechanisms of osteoclast

Fig. 5

Regulation of osteoblast differentiation