With interest, we read the recent study reported by Dudic et al in the May 2013 issue of the AJO-DO (Dudic A, Giannopoulou C, Kiliaridis S. Factors related to the rate of orthodontically induced tooth movement. Am J Orthod Dentofacial Orthop 2013;143:616-21). Above all, we sincerely congratulate the authors for their efforts and contribution. Nevertheless, we have concerns about the amount of tooth crowding of the patients involved in this study.

First, why were the patients required to have severe crowding in both jaws? Other malocclusions—eg, bimaxillary protrusion—might also need premolar extractions, but with no or mild crowding. What’s more, the crowding situation shown in Figure 1 was mild, but the inclusion criterion was for severe crowding.

Second, according to the study, the intra-arch obstacle, which is mainly from the adjacent teeth, significantly decreased the displacement of the teeth. That is, the crowding situations in the experimental and the control teeth influenced the movement velocity and the amount of displacement. However, the authors did not clearly interpret the region of crowding in these patients. Severe crowding in the anterior segment should have less of an obstacle effect on premolar displacement.

In addition, the authors evaluated the tooth displacements when intra-arch or interarch obstacles were present. However, both intra-arch and interarch obstacles could have been present; these would greatly impair the movement velocity and the displacement of teeth. Evaluation of this situation was absent in the study. It might be advisable to extract the premolars early in patients with severe crowding and interarch obstacles to tooth movement if they are scheduled for premolar extractions.