Avoiding Inferior Alveolar Nerve Injury during Osseous Genioplasty: A Guide for the Safe Zone by Three-Dimensional Virtual Imaging

Sir: We read with great interest the article entitled “Avoiding Inferior Alveolar Nerve Injury during Osseous Genioplasty: A Guide for the Safe Zone by Three-Dimensional Virtual Imaging” by Lin et al.1 The authors grouped and analyzed the distance between the lower edge of the mental foramen and the inferior alveolar nerve tube in the cone-beam computed tomography three-dimensional model of Asian adults who had undergone orthodontic surgery. The results showed that the class III and cleft lip/palate groups presented significantly larger three-dimensional distances than the class I and class II groups. When performing chin osteotomy, if the safety distance of 5 to 6 mm is observed, the risk of intraoperative nerve injury remains as high as 5 to 16 percent. Osseous genioplasty is the most common surgical method for the chin. After osteotomy, the bone can be moved in three directions with high flexibility, and various chin deformities can be corrected. Ritter et al.2 found that there were no cases in which the mental nerve’s course was located more than 5.5 mm below the inferior border of the mental foramen. The authors concluded that the risk of nerve injury was reduced to 2.5 percent at a safe distance of 7.06 mm from the inferior border of the mental foramen. In clinical practice, 1 mm is a large distance, given the narrow surgical area of the chin, which undoubtedly increases the difficulty of the surgery. Furthermore, the lower the osteotomy line, the worse the postoperative stability and effect. Therefore, for patients with malocclusion and cleft lip/palate, osteotomy requires careful consideration. Although the authors provided the number of male and female patients in the study, the measured data were not discussed by sex. Many class III patients presented mandibular deviation and asymmetric jaw development. The authors measured the left and right safety distances of the chin in patients with various deformities and explained that some patients had facial asymmetry, but they were not grouped according to the laterality of their mandibular skew. If the measurement data can be grouped in a more detailed manner, it will be more meaningful for clinical guidance. In the clinical setting, many class III patients still have a protruding or long chin after orthognathic surgery; therefore, many patients choose to undergo genioplasty when removing the titanium plate and titanium nail. McLeod and Bowe3 found that the probability of inferior alveolar nerve injury was higher when bilateral sagittal split osteotomy was combined with genioplasty. The authors’ study may explain this result. The authors showed data of patients treated with orthodontic surgery, but in clinical practice, this procedure is more frequently used for patients with chin deformities, especially microgenia. Although the authors did not evaluate data in this regard, their approach provided us with a warning. For microgenia, especially those with a lower mental foramen, computed tomography reconstruction is required to reconstruct the shape of the inferior alveolar nerve preoperatively, and the surgical procedure should be carefully selected. This requires doctors and patients to reach a consensus about the surgical procedure. ACKNOWLEDGMENTS The study was funded by the 2018 Project of the Beijing Municipal Science and Technology Commission (no. Z181100001718187) and the Initiative for Innovative Medicine from the Chinese Academy of Medical Science (no. 2016-I2M-1-018). DISCLOSURE The authors report no conflicts of interest.