We read with great interest the Clinician’s Corner article in the April 2018 issue and commend the authors for their creative idea to upright the “kissing molars” (Barros SE, Janson G, Chiqueto K, Ferreira E, Rösing C. Expanding torque possibilities: A skeletally anchored torqued cantilever for uprighting “kissing molars.” Am J Orthod Dentofacial Orthop 2018;153:588-98). Certain statements on the biomechanics and periodontal issues during the molar uprighting concern us.
The authors stated “Mini-implant insertion between the first premolar and the canine allowed use of a longer cantilever arm, which contributed to preventing excessive molar extrusion during uprighting.” Activation of the cantilever (Fig 4) produced not only a pure rotation moment but also an extrusive force. Although the extrusive force was small, significant extrusion of the second molar was detected when we superimposed Figure 6, A and B , after the first 5-month treatment (our Fig 1 ). Special attention should be paid to prevent excessive vertical extrusion during molar uprighting, whether or not the maxillary second molar erupted. Otherwise, overeruption of the mandibular molar may cause occlusal interference or irritation of the maxillary soft tissues.
During inclined molar uprighting, assessing the depth of the center of resistance (CR) of the molar plays a key role in the treatment plan. When the CR of the impacted molar is below the plane of the CR of its normal position, the treatment biomechanics described in this article are perfect. When the CR of the impacted molar is above its ideal position, even a pure rotation of the inclined molar will result in obvious extrusion of the tooth crown (our Fig 2 ). In this situation, an intrusive force should be considered or performed from the beginning of the molar uprighting; this was not mentioned by the authors during the whole process. Thus, the authors might have carefully drawn the conclusion that “This can be achieved with the aid of a skeletally anchored torqued cantilever, which can perform an extensive root uprighting movement with minimal side effects, allowing easier clinical handling of extremely tipped and partially erupted KMs.”.