It was fascinating to read the investigation about the factors related to the rate of tooth movement 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). We appreciate the authors for their great efforts in contributing to the theory of tooth movement.

The authors investigated the relationship between buccal tooth tipping and subject-related factors (age and sex) as well as tooth-related factors (location and interference). They reported that younger subjects showed greater tooth movement velocity than did older subjects, and interarch or intra-arch obstacles decreased the amount of tooth displacement. However, some additional considerations should have been mentioned in this study.

First, the authors stated that the subjects met the criterion of severe crowding in both jaws. However, the maxillary dental arches in Figures 1 and 2 do not seem to show severe crowding. Maybe the subjects’ eligibility should have been more carefully verified. In addition, the investigators moved premolars buccally. In such a model, the location of the crowding could affect tooth displacement more than the severity of crowding. Severe crowding on the contralateral control side might not influence tooth displacement in the experimental group, but mild crowding on the experimental side might decrease tooth displacement significantly and influence the result.

Second, the continuity of the force could play a crucial role in orthodontic tooth movement. In similar models, Lundgren et al and Owman-Moll et al reactivated the archwire weekly to prevent force decay. However, in this study, the investigators checked the force after 4 weeks. Because other factors, such as length of the archwire and the patient’s diet, could influence force decay, the variation of force was undoubtedly different among the subjects over such a long period and, accordingly, could have led to different rates of tooth movement in this trial.

Last, it might be disputable to consider that younger subjects showed greater tooth movement velocity than did older subjects simply by comparing the different ages of the participants. The authors failed to exclude other confounding factors such as the patients’ growth patterns, tooth locations, and interferences of teeth in the study design. That is to say, the baseline characteristics of the patients were not comparable in the 2 groups.