Palatal expansion and relapse in rats: A histologic and immunohistochemical study

•Midpalatal suture expansion and relapse were modeled in rats. •Tooth tilting and midpalatal suture expansion contributed to widening of the maxillary arch. •The relapse of the palatal bone width was the major contributor to the relapse of arch width. •Mechanical force during expansion stimulated remodeling and osteogenic activity in the midpalatal suture area. •After removal of the expansion force, suture tissue remodeling returned to a normal level. Introduction Rapid palatal expansion is an effective intervention for correcting transverse maxillary deficiency in growing patients. However, relapse after treatment is often observed, and the mechanisms of tissue remodeling during expansion and relapse remain unclear. This study aimed to gain insight into such a mechanism. Methods A total of 24 5-week-old male Wistar rats were randomly divided into either the expansion or sham device (control) group. Each rat underwent 7 days of expansion and 7 days of relapse. The width of the dental arch, palatal bone, and suture, as well as the angle of the teeth, were measured. Tissue remodeling in the midpalatal suture was examined using microcomputed tomography and histologic and immunohistochemical analyses. Results The mechanical expansion force caused an increase in arch width, which relapsed after the removal of force. Bilateral tilting of the teeth and midpalatal suture expansion contributed to the widening of the maxillary arch, and only the relapse of the palatal bone width was observed. Histochemical staining showed that suture tissue remodeling was activated by mechanical force in the expansion group and reverted to the level of the control group after relapse. Immunohistochemistry staining revealed that the expression of cathepsin K, osteocalcin, and collagen type I was higher in the expansion group than that in the control group on day 7; however, the difference dissipated by day 14. Conclusions The expansion force stimulated osteogenic activity in the midpalatal suture area. After removal of the expansion force, tissue remodeling went back to the normal level.