Three-dimensional assessment of facial asymmetry in class III subjects, part 2: evaluating asymmetry index and asymmetry scores

Clinical Oral Investigations https://doi.org/10.1007/s00784-023-05193-x RESEARCH Three‑dimensional assessment of facial asymmetry in class III subjects, part 2: evaluating asymmetry index and asymmetry scores Deepal Haresh Ajmera1 · Congyi Zhang2 · Janson Hoi Hei Ng3 · Richard Tai‑Chiu Hsung4,5 · Walter Yu Hang Lam6 · Wenping Wang2,7 · Yiu Yan Leung5 · Balvinder S. Khambay1,8 · Min Gu1 Received: 27 May 2022 / Accepted: 28 July 2023 © The Author(s) 2023 Abstract Objectives To evaluate the outcomes of corrective surgical treatment for craniofacial asymmetry using four different methods with the aim of developing the best technique for craniofacial asymmetry assessment. Materials and methods CBCT images of twenty-one class III subjects with surgically corrected craniofacial asymmetry and twenty-one matched controls were analyzed. Twenty-seven hard tissue landmarks were used to quantify asymmetry using the following methodologies: the asymmetry index (AI), asymmetry scores based on the clinically derived midline (CM), Procrustes analysis (PA), and modified Procrustes analysis (MPA). Results Modified Procrustes analysis successfully identified pre-operative asymmetry and revealed severe asymmetry at the mandibular regions compared to controls, which was comparable to the asymmetry index and clinically derived midline methods, while Procrustes analysis masked the asymmetric characteristics. Likewise, when comparing the post-surgical outcomes, modified Procrustes analysis not only efficiently determined the changes evidencing decrease in facial asymmetry but also revealed significant residual asymmetry in the mandible, which was congruent with the asymmetry index and clinically derived midline methods but contradictory to the results shown by Procrustes analysis. Conclusions In terms of quantifying cranio-facial asymmetry, modified Procrustes analysis has evidenced to produce promising results that were comparable to the asymmetry index and the clinically derived midline, making it a more viable option for craniofacial asymmetry assessment. Clinical relevance Modified Procrustes analysis is proficient in evaluating cranio-facial asymmetry with more valid clinical representation and has potential applications in assessing asymmetry in a wide spectrum of patients, including syndromic patients. Keywords Facial asymmetry · 3D · Three dimensional · Asymmetry index · Asymmetry scores * Min Gu 1 Discipline of Orthodontics, Division of Paediatric Dentistry and Orthodontics, Faculty of Dentistry, the University of Hong Kong, Hong Kong SAR, China 2 Department of Computer Science, The University of Hong Kong, Hong Kong SAR, China 3 Faculty of Dentistry, the University of Hong Kong, Hong Kong SAR, China 4 Department of Computer Science, Hong Kong Chu Hai College, Hong Kong SAR, China 5 Division of Oral and Maxillofacial Surgery, Faculty, of Dentistry, the University of Hong Kong, Hong Kong SAR, China 6 Discipline of Prosthodontics, Faculty of Dentistry, the University of Hong Kong, Hong Kong SAR, China 7 Texas A&M University, College Station, TX, USA 8 Institute of Clinical Sciences, College of Medical and Dental Sciences, The School of Dentistry, University of Birmingham, Birmingham, UK 13 Vol.:(0123456789) Clinical Oral Investigations Introduction Aesthetic faces have been proven to influence individuals’ personality development, which can dictate their social, economic, and health status [1, 2]. In tandem with sexual dimorphism, juvenescence, and uniformity, symmetry is also a prerequisite for an attractive face [2, 3]. However, perfect symmetry in humans continues to be a hypothetical concept [4, 5], thus compelling individuals toward reconstructive surgical procedures to restore facial harmony [2, 6]. Nevertheless, the effect of surgical treatment is directly reliant on the precision of the diagnosis, thereby making accurate evaluation an essential and mandatory step before planning the surgical procedure. The advent of 3-dimensional (3D) imaging modalities has provided additional diagnostic tools for clinical use [7]. Accurate and comprehensive knowledge of facial discrepancies can be obtained from 3D images, as it is possible to rotate and view 3D images from different angles [8]. There are numerous methods documented in the literature to quantify facial asymmetry. Several studies have calculated asymmetry 3-dimensionally by measuring linear, angular, and surface distances of several landmarks from the plane of symmetry [9–12]. In addition, others have performed surface area and volumetric measurements [6]. Furthermore, some studies have also used a 3D image-based coordinate assessment approach to compare assorted symmetry factors (region-based asymmetry index) [4, 13, 14], while other studies have calculated asymmetry scores [2, 15]. A midsagittal plane (symmetry plane) is central for the quantification of facial asymmetry and can be constructed by manually selecting the reference landmarks not affected by the asymmetry [6, 16–18] or by deriving it mathematically based on a best-fit superimposition method called “Procrustes analysis.” Several studies have analyzed facial asymmetry using clinical midline and Procrustes methods; however, flaws associated with these methods, such as unreliability and falsified presentation of true asymmetric features, have also been reported [2, 19], which might influence the diagnosis and post-operative treatment outcomes. Hence, a method that yields reliable evaluation of asymmetry is required. Therefore, the present study aimed to compare four different methods and develop the best technique for the assessment of facial asymmetry. Materials and method Sample size calculation By considering the mean difference of 0.66 mm (standard deviation of 0.5) as clinically significant [14], together with a power of 95%, an effect size of 1.32, and alpha level set at 0.05, a minimum sample size of 32 (16 in each group) was 13 calculated with G*Power (version 3.1.9.2, Kiel University, Germany) [20]. Subjects Asymmetry group Twenty-one subjects (7 male and 14 females) aged 18 to 40 years (23.0 ± 3.4 years) from the orthodontic-orthognathic pool who sought surgical treatment at the Prince Philip Dental Hospital, University of Hong Kong, between April 2012 and July 2019 were chosen if they fulfilled the following inclusion criteria: (1) soft tissue chin deviation > 3 mm, (2) bimaxillary surgery with no genioplasty, (3) pre-operative cone-beam computed tomography (CBCT) scan (T0) and post-surgical CBCT scan (T1) taken at least 6 months after surgery, (4) had no history of craniofacial syndromes or craniofacial surgery, and (5) were not diagnosed with hemifacial microsomia or orbital dystopia. Control group Twenty-one age (23.0 ± 3.3 years) and gender (7 males and 14 females) matched subjects from the same hospital who had taken CBCT scans in 2015 for nonsurgical purposes and who satisfied the following inclusion criteria were recruite (...truncated)