Survival of periodontal ligament myofibroblasts after short-term mechanical strain in rats and in vitro: Could myofibroblasts contribute to orthodontic relapse?

Objectives

To investigate in vivo whether myofibroblasts formed in the PDL after exposure to short-term high experimental orthodontic forces in rats survive. To study in vitro whether human PDL fibroblasts can differentiate into myofibroblasts and survive when chemical or mechanical stimuli are removed.

Design

Nine 6-week-old male Wistar rats were used in this experiment. Rat molars were exposed to high but rapidly decreasing experimental orthodontic forces by applying a rubber band and analyzed for the presence of myofibroblasts using ASMA staining. In vitro, human periodontal ligament (PDL) fibroblasts were exposed to transforming growth factor β1 (TGFβ1) and/or mechanical stress and monitored for myofibroblast formation and survival after these stimuli were abrogated.

Results

In vivo exposure to orthodontic forces strongly induced myofibroblast formation in the stretched regions of the PDL. Furthermore, many PDL myofibroblasts remained present 6 days after exposure to these short-term high orthodontic forces. Human PDL fibroblasts were shown to differentiate into myofibroblasts after 2 days of TGFβ1 exposure and survive for at least 2 more days after removing chemical stimuli (TGFβ1) or mechanical strain. Under in vitro conditions, both TGFβ1 and mechanical strain for 3 days promoted (myo)fibroblast formation, and these cells persisted for 3 more days after the removal of both stimuli.

Conclusions

PDL myofibroblasts survive after the removal of mechanical strain in vivo and in vitro. This supports the hypothesis that myofibroblasts, which form in response to mechanical strain and chemical cues in the periodontal ligament (PDL), play a role in relapse following orthodontic tooth movement.