THE KOREAN JOURNAL of Original Article ORTHODONTICS pISSN 2234-7518 • eISSN 2005-372X https://doi.org/10.4041/kjod.2019.49.3.170 Relationship between the maxillofacial skeletal pattern and the morphology of the mandibular symphysis: Structural equation modeling Mi So Ahna Objective: The purpose of this study was to investigate the relationship between Sang Min Shinb the facial skeletal patterns and the shape of the mandibular symphysis in adults Tetsutaro Yamaguchic with malocclusion by using a structural equation model (SEM). Methods: Ninety Koutaro Makic adults who had malocclusion and had records of facial skeletal measurements Te-Ju Wud performed using cone-beam computed tomography were selected for this Ching-Chang Koe study. The skeletal measurements were classified into three groups (vertical, Yong-Il Kima,f anteroposterior, and transverse). Cross-sectional images of the mandibular symphysis were analyzed using generalized Procrustes and principal component (PC) analyses. A SEM was constructed after the factors were extracted via factor analysis. Results: Two factors were extracted from the transverse, vertical, and anteroposterior skeletal measurements. Latent variables were extracted for aDepartment of Orthodontics, Dental each factor. PC1, PC2, and PC3 were selected to analyze the variations of the Research Institute, Pusan National mandibular symphyseal shape. The SEM was constructed using the skeletal University Dental Hospital, Yangsan, Korea variables, PCs, and latent variables. The SEM showed that the vertical latent bDepartment of Management variable exerted the most influence on the mandibular symphyseal shape. Information Systems, College of Conclusions: The relationship between the skeletal pattern and the mandibular Business, Dong-A University, Busan, symphysis was analyzed using a SEM, which showed that the vertical facial Korea skeletal pattern had the highest effect on the shape of the mandibular c Department of Orthodontics, School symphysis. of Dentistry, Showa University, Tokyo, Japan [Korean J Orthod 2019;49(3):170-180] dDepartment of Orthodontics, Chang Gung Memorial Hospital, Kaohsiung, Key words: Structural equation model, Morphology of mandibular symphysis, Taiwan Facial skeletal pattern eDepartment of Orthodontics, School of Dentistry, University of North Carolina at Chapel Hill, NC, USA fInstitute of Translational Dental Sciences, School of Dentistry, Pusan National University, Busan, Korea Received October 11, 2018; Revised December 4, 2018; Accepted December 19, 2018. Corresponding author: Yong-Il Kim. Associate Professor, Department of Orthodontics, Dental Research Institute, Pusan National University Dental Hospital, 20 Geumo-ro, Mulgeum-eup, Yangsan 50612, Korea. Tel +82-55-360-5163 e-mail [email protected] How to cite this article: Ahn MS, Shin SM, Yamaguchi T, Maki K, Wu TJ, Ko CC, Kim YI. Relationship between the maxillofacial skeletal pattern and the morphology of the mandibular symphysis: Structural equation modeling. Korean J Orthod 2019;49:170-180. © 2019 The Korean Association of Orthodontists. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 170 Ahn et al • Skeletal patterns and mandibular symphysis shape INTRODUCTION pattern and presents coordinate points in three dimen- sions. Identifying the measurement points on the differ- The mandibular symphysis is located at the midline ent planes could reduce image distortion or errors that of the frontal part of the mandible, and constitutes the occur in cephalometric radiographs. It is also possible to chin and frontal part of the lower face.1 Several factors reconstruct a cross-section of the jaw that is based on affect the growth and shape of the mandibular sym- a reference plane and established on 3D measurements. physis, such as neuroskeletal balance,2 masseter muscle Moreover, CBCT allows for both precise measurement thickness,3 mandibular plane angle,4 overbite,2,5,6 lower and acquisition of reliable data by using the same land- incisor angle,7 occlusal function,8 and inheritance.9 The marks. shape of the mandibular symphysis plays a significant Data analysis of multivariate variables, however, is role in aligning the mandibular incisors during orthog- complicated. Therefore, a statistical method is required nathic surgery and orthodontic treatment. Evaluation of to simplify the relationship among the variables. A the shape of the mandibular symphysis before orthodon- structural equation model (SEM) allows for simultaneous tic treatment is important to prevent possible iatrogenic investigation and estimation of multiple interdependent damage.10,11 relationships. It also allows for analyses that consider Aki et al.12 reported that patients with a large sym- the measurement error related to the observed variables, physis could be treated using increased protrusion of and conceptually expresses the relationship between the the anterior teeth with a higher probability of non- variables by classifying the latent variables. These mod- extraction treatment. However, patients who had a long els demonstrate the relationship between the variables and narrow symphysis were more likely to be treated clearly and express the regression weights of individual using orthodontic extraction treatment to compensate relationships as structural coefficients. for their arch length discrepancy. Clinicians classify the Therefore, this study aimed to visualize the relation- anterior and posterior growth patterns of the mandible ship between the vertical, horizontal, and transverse on the basis of the size and shape of the mandibular facial skeletal patterns, delineate the shape of the man- symphysis.12-14 Compared to the treatment of the pos- dibular symphysis via a SEM, and analyze the relation- terior growth pattern, which is related to mandibular ship between the facial skeletal pattern and the cross- retraction, the treatment of the anterior growth pattern sectional morphology of the mandibular symphysis. tends to require orthodontic treatment and orthognathic Specifically, the goals were to 1) obtain the mandibular surgery. Thus, the shape of the suture affects both the symphysis morphology via geometric morphometrics and overall treatment plan and the classification of the facial perform a principal component (PC) analysis via cross- skeletal morphology. sectional CBCT imaging of patients with skeletal maloc- By using two-dimensional (2D) cephalometric analysis, clusion; 2) extract the facial skeletal factors as latent many studies showed that the shape of the mandibular variables by using factor analysis; and 3) analyze the symphysis correlates with the facial skeletal pattern.1,15 relationship between the facial skeletal pattern and the Retrospective cephalometric analysis showed that indi- mandibular symphysis morphology via a SEM. viduals with brachyfacial patterns had more hard tissue on their chins, and individuals with a larger mandibular MATERIALS AND METHODS plane angle showed higher growth rate of the lower an- terior facial height.1 Samples Many studies have investigated the relationship be- This study used CBCT data of adult patients who tween the mandibular symphysis morphology and the visited the Pusan National University Dental Hospital 2D facial skeletal pattern. However, these studies had between January 2010 and December 2017, and whose some limitations because they did not consider the chief complaints were skeletal malocclusion. In total, three-dimensional (3D) facial skeletal pattern. These 90 patients (39 male and 51 female) were included, studies evaluated the facial skeletal pattern for each and their average age was 22.96 ± 4.5 years. Those cross-sectional dimension. However, a statistical model who had systemic diseases, trauma, surgical history, or that demonstrates the relationship between the form of maxillofacial malformations were excluded. This study the mandibular symphysis and the 3D facial skeletal pat- was reviewed and approved by the institutional review tern is needed for extensive understanding of the maxil- board of the Pusan National University Dental Hospital lofacial complex morphology. (PNUDH-2016-025). Cone-beam computed tomography (CBCT) imaging al- lows for 3D analysis of the facial skeletal pattern, which cannot be performed using lateral or posteroanterior cephalography. CBCT reconstructs the 3D facial skeletal www.e-kjo.org https://doi.org/10.4041/kjod.2019.49.3.170 171 Ahn et al • Skeletal patterns and mandibular symphysis shape Methods planes). 2) Obtaining measurements CBCT imaging The measurements were used as observed variables to CBCT (Pax-Zenith3D; Vatech Co., Ltd., Hwaseong, Ko- analyze the facial skeletal morphology and were clas- rea) was performed for all patients under the same con- sified as three factors: horizontal, vertical, and antero- ditions: 90 kVp, 10 mA, scan time of 24 seconds, voxel posterior factors. The definitions of the factors are pre- size of 0.3 mm, and FOV of 20 × 19 cm. The maxillo- sented in Table 2. facial complex morphology was analyzed and measured The horizontal factor included the distance measure- using a 3D analysis software (InVivo6; Anatomage Inc., ments of the orbit, zygoma, maxilla, and mandible. The San Jose, CA, USA). upper and lower parts of the orbit were measured as the The CBCT images were