Thank you very much for the comments. As mentioned in the letter, it is well known and accepted that the anatomic limitation during incisor retraction is the palatal/lingual cortical plate. We certainly agree with the principle that tooth movement within the alveolar housing is recommended at all times to avoid possible iatrogenic consequences during anterior retraction, such as the loss of marginal bone and root resorption.
The main objectives of our report were to present and to raise the possibility of invasion/approximation of the incisive canal after anterior retraction because of the proximity of the 2 anatomic structures—the tooth roots and the canal. To our knowledge, invasion of the incisive canal after tooth movement and its consequences have not been reported previously. As shown in the article, the incisive canal is located within the alveolar housing in the median plane. It is extended and fused anteriorly from the palatal cortical plate, and the canal border demarcated by thick cortical bone is much closer to the apical third of the incisor roots compared with the palatal cortical plate per se. Thus, anatomically, the maxillary incisors are at risk of touching the palatal plate after a large amount of bodily movement but, in addition, also invading the incisive canal wall in the mesial/palatal aspect of the roots. As shown by patient 1, along with root resorption, the invasion of the incisive canal may induce a 2-walled defect in some locations of the tooth root, since the palatal and also the mesial marginal bone may be damaged, creating a more periodontally compromised condition in the long term.
According to our recent 3-dimensional study, the anteroposterior distance from the apical third of the incisor roots to the incisive canal is about 5 to 6 mm in subjects with normal occlusion. This distance does not necessarily imply the “safety zone” for retraction, because patients with relatively large interroot distances (approximately 40% of subjects) are not at risk of canal invasion or contact even after maximum retraction. However, considering the individual variations of canal dimensions that are frequently reported, clinically we recommend evaluation of the proximity of the incisive canal to the maxillary incisors, along with its dimensional characteristics, when a considerable amount of maxillary retraction is planned.
We sincerely appreciate the feedback and hope our report can be of help in diagnosis and treatment planning.