Authors’ response

Thank you for your interest and for the opportunity to address your concerns. Biomechanics is always a matter of great relevance because if its principles are not taken into account, unfavorable orthodontic results may occur. However, our mechanotherapy produced excellent results due to well-controlled movement of severely impacted molars, weakening the reasons for concern.

  • 1.

    We stated that excessive extrusion was prevented by using a longer cantilever arm. It is necessary to differentiate significant from excessive extrusion. Detectable extrusion of the posterior teeth may occur during usual tooth leveling, deepbite correction, intermaxillary elastics use, rapid maxillary expansion, and molar disimpaction or uprighting, but in general, it is not excessive or contraindicated. Unfortunately, we did not have a lateral head film immediately after torqued cantilever use to evaluate vertical molar displacement with a scientifically valid method. However, supposing that panoramic superposition could produce some reliable information, we could say that the center of resistance (CR) of the molar was vertically unchanged, confirming the reduced extrusion vector and molar rotation around the CR (pure rotation). We have no doubt about the vertical change of the molar crown, but it results mainly from rotation around the CR rather than true molar extrusion (vertical displacement of the CR). Although not always desired, this vertical change can be detected in panoramic radiographs taken during the initial stage of molar uprighting, even when an intrusive force is simultaneously applied. Unfortunately, these follow-up panoramic radiographs are not always available for evaluation. In this case, significant mandibular first molar extrusion and uprighting were achieved using the second molar as distal anchorage; as a consequence, second molar intrusion could be expected. In addition, a reverse curve was used to correct the deepbite, benefiting second molar intrusion and uprighting, since posterior extrusion was minimized by a “low-angle” patient and bite raising built on the posterior teeth. It has been demonstrated that usual preadjusted appliance techniques may be efficient to control unwanted biomechanical side effects during molar uprighting. In fact, the second molar’s periodontal health was significantly improved, discarding occlusal interference sequels; the LAFH increased by 5.2 mm during treatment (11-14 years), whereas the mean increase of LAFH in untreated subjects is about 4.5 mm, discarding the influence of molar uprighting mechanics on LAFH. The transverse occlusal plane was not canted, and the marginal ridges were satisfactorily leveled. Thus, a static scene taken from the initial stage of molar uprighting may not say much about the overall treatment changes and results.

  • 2.

    It has been demonstrated that 22° of molar uprighting accounts for a CR extrusion of 0.6 mm. In this patient, the CR seemed vertically unchanged after 45° of molar uprighting (panoramic superposition). Thus, depending on molar tipping severity, the impact of the CR extrusion on the treatment results may be clinically questionable. Furthermore, after the initial uprighting stage of a mesially impacted mandibular molar, its crown becomes clinically more accessible, and preadjusted appliance mechanics can contribute to vertical control. It is not surprising that the mandibular second molars with severely extruded CR have been successfully treated using cantilever mechanics without any intrusion force. However, in general, mesially impacted molars do not have a significantly extruded CR, exempting vertical control. So, taking into account several scientific studies and our successful treatment results, the conclusion about minimal side effects seems to be quite reasonable. Your Figure 2 predicting final molar positioning based on the initial stage of molar uprighting does not make sense because a torqued cantilever was not the only device used.

  • 3.

    We think that if a pretwisted rectangular wire has its effectiveness compromised by dedeflection, there would be serious problems to obtain third-order tooth movement by twisting a rectangular wire. However, this assumption is not realistic. This torqued cantilever was only activated twice, including the initial activation, producing an extensive uprighting movement. We said that a torqued cantilever could be successfully used, making helical loops or TMA unnecessary, but we did not state that a torqued cantilever is better or worse, because it has not yet been scientifically compared with other devices. Thus, the assumption that stainless steel wire with loops or TMA wire would be better choices is incorrect in this letter. Currently, we are conducting a study to provide more detailed and reliable information regarding the “modus operandi” of this device.

  • 4.

    The arch length shortening was due to second molar and premolar displacements in the first molar area. Space analysis was judged unnecessary because arch length in the first molar area would be normalized after correction of the displaced teeth (space recovery). Although the patient’s forward mandibular rotation pattern might reduce the resorption rate of the anterior border of the ramus, we had no reason to think that retromolar space for KM correction could be a problem because the patient had no asymmetrically reduced mandibular growth, the skeletal and dental midlines were coincident, and the opposite side of the mandibular arch had satisfactory space for second molar eruption. In addition, intermaxillary elastics used for complete Class II correction may have contributed to further increase the retromolar space due to mesial drifting of the buccal segment. In this patient, periodontal surgery during molar uprighting was especially performed to allow repositioning of orthodontic accessories on the buccal surface of the first and second molars. After the periodontal surgery, the retromolar soft tissue again covered the distal surface of the second molar, making it similar to the erupting second molar on the opposite side. Thus, prophylactic periodontal surgery may be a wasted effort, because the presence of second molar operculum is a transitory and not worrying condition if the patient’s age is consistent with the time of molar eruption, the retromolar area is growing, and the patient is asymptomatic. In addition, the posttreatment follow-up showed similar and normal periodontal condition of both mandibular second molars.

Thank you for your comments.

The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.

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Dec 8, 2018 | Posted by in Orthodontics | Comments Off on Authors’ response
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