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

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.

Establishment of the mid-sagittal reference plane for three-dimensional assessment of facial asymmetry: a systematic review

“…Previous studies have substantiated that if MSP does not correspond to the external facial structure (facial morphology) and internal structures (skeletal structures), it provides fallacious information, thus affecting treatment planning [15,49]. Therefore, the construction of an accurate MSP is essential not only for quantifying the degree of symmetry between the right and left sides of the face but also for arriving at the correct diagnosis.…”

“…A semi-automatic technique that involves digital landmarking was employed for cephalometric and morphometric MSP construction in nine studies [23,24,28,31,32,40,[47][48][49], whereas a fully automated and landmark-independent technique for morphometric MSP and symmetry plane construction was reported in seven studies [3,16,21,25,9,33,50]. The fully automated MSP construction method was reported to accurately measure the degree of asymmetry and achieve satisfactory skeletal symmetry, as asserted by the authors in their respective studies (Table 3).…”

“…Between 2005 and 2023, a total of 42 studies were identified, focusing on estimating the mid-sagittal plane in patients with facial asymmetry. Out of the selected 42 studies, eight were computational studies [1,3,[21][22][23][24][25][26]; six were observational [14,15,[27][28][29][30]; 21 studies used retrospective analytical design [4,9,9,12,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]; three were experimental studies [17,48,49], and one study was ambispective in nature [50]. The age range of the patients in all the included studies was 7-57 years.…”

“…The reproducibility of the constructed MSP was found to be questionable in 13 of the included studies [16, 17, 21-26, 32, 9, 41, 46, 52] as they failed to report the reliability assessment, which led to a rating of 'high' risk of concern; however, the overall risk was judged to be 'low' in the index test domain. Regarding the reference standard domain, nine studies with ambiguous reference standards were considered to have 'unclear' risk [17,26,31,38,39,41,49,50,52], while the majority of the studies were deemed to have 'low' risk. The Supplementary Figure includes a detailed presentation of the study quality and risk of bias assessment.…”