A New Approach to Set the Absolute Midsagittal Plane of the Mandible Using a Similarity Index in Skeletal Class III Patients with Facial Asymmetry

Woo-Jin Han,Yun-Hoa Jung,Jae Joon Hwang,Hyung-Seog Yu,Bong-Hae Cho,Kee-Joon Lee,Sung-Hwan Choi

doi:10.3390/app10238550

Abstract

This study sought to test the feasibility of a newly developed plane called computed modified absolute mandibular midsagittal plane (cmAMP) based on the similarity index (SI) for evaluating the stereoscopical symmetry of the mandible by comparison with other proposed midsagittal planes. This study involved 29 adult patients (15 men, 14 women; average age, 23.1 ± 6.9 years) with skeletal Class III facial asymmetry who underwent bimaxillary orthognathic surgery. Using cone-beam computed tomography images taken before and 1 year after surgery, cmAMP with the highest SI value between the two anterior segments of the hemi-mandible was set by a computer algorithm. Results show that the SI using cmAMP had the highest value (0.83 ± 0.04) before surgery compared to the other midsagittal planes, and was not significantly different from the SI (0.80 ± 0.05) using a facial midsagittal plane (MSP) after surgery. The distance (1.15 ± 0.74 mm) and angle (2.02 ± 0.82°) between MSP and cmAMP after surgery were significantly smaller than those between MSP and other midsagittal planes. In conclusion, the cmAMP plane best matches the two anterior segments of hemi-mandible symmetrically and is the closest to MSP after orthognathic surgery in skeletal Class III patients with facial asymmetry.

Highlights

Facial asymmetry is one of the main reasons for patients to undergo orthognathic surgery [1], especially among Asians, where asymmetry between the right and left hemi-mandibles with mandibular prognathism is common
If a large amount of mandibular setback is required for the improvement of facial asymmetry in Asians, intraoral vertical ramus osteotomy (IVRO) has the advantage of reducing the possibility of injury to the inferior alveolar nerve and less adversely affecting temporomandibular disorders [2,3]
The nonoverlapping surface (3543.14 ± 734.13 mm2 ) and volume (5088.85 ± 2280.33 mm3 ) of the right anterior segment of the hemi-mandible based on computed modified absolute mandibular midsagittal plane (cmAMP) before surgery were significantly smaller than those with midsagittal plane (MSP), absolute mandibular midsagittal plane (AMP), and mandibular midsagittal plane (mAMP) as the reference plane (p < 0.001) (Table 2)

Summary

Introduction

Facial asymmetry is one of the main reasons for patients to undergo orthognathic surgery [1], especially among Asians, where asymmetry between the right and left hemi-mandibles with mandibular prognathism is common. Bimaxillary surgery with mandibular setback procedures, such as the sagittal split ramus osteotomy (SSRO) or intraoral vertical ramus osteotomy (IVRO), is the main approach to correct this asymmetry. SSRO has been preferred over IVRO in asymmetric surgery due to rapid postoperative recovery and reliable stability through rigid fixation. If a large amount of mandibular setback is required for the improvement of facial asymmetry in Asians, IVRO has the advantage of reducing the possibility of injury to the inferior alveolar nerve and less adversely affecting temporomandibular disorders [2,3].

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Discussion

Conclusion