Background: The studies on biomechanics of orthodontic tooth movement (OTM) are mainly performed at analytical, tissue and cellular levels. The prime aim of this study was to elucidate the periodontal response to orthodontic force loading by integrating biomechanical and biological approaches.
Methods: We designed and conducted a multilevel study consisting of three parts. (1) At the analytical/theoretical level, 3D finite element (FE) method was used to analyze stress distribution and changing during OTM. (2) At the tissue level, we explored the effects of tensile and compressive forces on the expressions of Type I collagen, matrix metalloproteinases Type I (MMP-1) and tissue inhibitor of metalloproteinase Type I (TIMP-1) in rat's periodontal ligament (PDL) in vivo. (3) At the cellular level, we studied the effects of variant strain patterns and magnitudes on functional expression of rat's osteoblasts in vitro.
Findings: (1) In the 3D FE model, the canine tipping and bodily movements showed different ways in stress distribution and degeneration. However, in both tooth movement modalities, tensile zones and compressive zones had similar stress distribution pattern. (2) Tensile and compressive forces imposed different effects on the expressions of Type I collagen, MMP-1 and TIMP-1 in PDL, with Type I collagen and TIMP-1being increased significantly in the tensile zones and MMP-1 being increased significantly in both zones. (3) Differences in strain pattern (dynamic vs. static) and magnitude (light vs. heavy) resulted in different levels of osteoblast's functional expression indicated by alkaline phosphatase (ALP) and osteocalcin (OC). It was found that dynamic loading was more effective for ALP expression whilst static loading was more effective for OC secretion and 3kPa strain force in vitro was optimal for the both.
Interpretation: It is suggested that there may exist an optimal force system in both magnitude and pattern of loading that could induce efficient OTM.