Dynamics and observations of long-term orthodontic tooth movement and subsequent relapse in C57BL/6 mice

The risk of relapse is associated with orthodontic treatment and is a major problem. Discussions about the risk of relapse have been made for a long time. However, the detailed mechanism of relapse has not yet been elucidated, although basic research has been conducted to clarify this mechanism. This study aimed to evaluate relapse following orthodontic treatment in mice (C57BL/6) tested via the coil spring method of tooth movement for 21 days and mechanical retention for 7 days after completion of tooth movement. During the experiment, relapse was observed and evaluated over 7 days. At the end of the orthodontic tooth movement, the average distance was 259.6 (± 10.9) μm, and tooth movement was observed in all mice. No significant difference was found in the distance at the end of the experimental tooth movement nor at the end of the 7-day mechanical retention. The distance at the start of relapse observation was 258.6 (± 10.4) μm, whereas that at the end was 155.4 (± 12.4) μm, indicating that the distance decreased significantly. Relative to the total relapse distance over the 7-day period, 45.7 (± 4.3)% of the relapse was observed on Day 0-1. We established a relapse observation mouse model, which allowed us to develop an effective and reproducible method for optimal evaluation of relapse. Our findings clarified that most of the relapse occurs within 7 days at the initial observation stage.