The management of non-growing patients with skeletal Class III involves camouflaging or skeletal correction via orthognathic surgery. During orthodontic camouflage, retroclination of the lower incisors are needed to achieve a class I incisor relationship. Conversely, in patients with severe Class III skeletal discrepancies, decompensation of the lower incisors prior to orthognathic surgery is required by uprighting or proclining the lower incisors.

Unfortunately, orthodontic tooth movement in Class III malocclusion cases is always challenging. The anteroposterior thickness of the alveolar bone in the lower incisor region has been reported to be thin in mandibular prognathism and may limit the range of incisor movement [1]. Class III malocclusion patients with long lower face height are frequently present with narrow alveolus, linguoversion of mandibular symphysis (MS) and thin labial bone. Therefore, pronounced tipping movements of the lower incisors can cause labial gingival recession and alveolar bone loss [2]. Presence of thin gingival biotype and dental crowding could further complicate matters.

The understanding of MS morphology is important during orthodontic treatment planning as it provides essential information in determining the limits of orthodontic proclination or retroclination of the lower incisors. It is also an important landmark for surgical planning such as genioplasty, block bone grafting [3], and dental implant insertion. If the lower incisor roots are moved against the cortical plate of the alveolus, severe root resorption and bony dehiscence may occur. Limiting lower incisor movement within the MS during orthodontic treatment is thought to be paramount for achieving better results, stability, and periodontal health [4].

Variation in MS morphology has been associated with individual variation [5], vertical facial proportions [5-10], sagittal skeletal relationship [11], incisor inclination [1, 12, 13], muscular and functional activities [4, 14, 15], and sexual dimorphism [3, 16]. Studies on facial type and morphology of MS have mentioned that those with short face or forward growth of mandible has wider MS while those with long face or backward growth of mandible exhibits elongated and thin MS [8-10, 17]. Moshfeghi et al. [18] further emphasized that mandibular morphology is associated with the direction of mandibular growth. Other studies also suggest that MS in growing children may be predictive for lower face height in adulthood [19]. Nevertheless, it is interesting to know which factors most influence mandibular symphysis.

Majority of studies employ conventional linear and angular measurements as an assessment tool. As MS is curved and round, linear measurements will have limitations in defining and describing the actual shape of MS. Studies also show that conventional cephalometric study based on angular and linear measurements were insufficient to analyse shape changes of complex anatomical forms such as craniofacial complex [20, 21]. Besides, size is usually the confounder in statistical analysis [22]. Craniofacial shape can be affected by various factors such as sexual dimorphism, patient’s weight and built, and genetics. Moreover, visual interpretation of results may be limited, especially when describing shapes.

Geometric morphometric analysis (GMA), in contrast, has the advantages to analyse and compare the overall shape and variation of MS without being affected by its size. This method is able to study the changes as well as the variability of MS shape, with its ability to measure in two-dimensional (2D) and three dimensional (3D) objects. GMA is a tool that incorporates mathematics and biology, commonly used in morphological studies investigating shape variations of objects. It uses Cartesian landmark coordinates that can capture morphologically distinct shape variables, providing a graphical representation of statistical interpretation [23]. Size of the structure would not be a confounder in statistical analysis as GMA analysis is based on landmarks after separating shape coordinates from size, position, and orientation of the landmark configurations.

To best of our knowledge, no other studies have used GMA to study the shape of MS. Therefore, the aim of this study is to investigate the general MS shape variation among Class III skeletal base using geometric morphometric analysis. In addition, we aim to determine which factors most influence the shape variation of MS. The comparison of different vertical facial proportions using GMA would give a clear graphical interpretation of MS shape, that can add value to current literature.

This was a cross-sectional study where lateral cephalometric radiograph (LCR) was taken on patients who seek orthodontic treatment in the Faculty of Dentistry, Universiti Kebangsaan Malaysia (UKM), from 2010–2019. Ethical approval was obtained from the Research and Ethics Committee of UKM (UKM PPI/111/8/JEP-2018-509).

The inclusion criteria for the study were Class III skeletal with ANB <1°, lower incisor angle <99°, aged 11–40 years old, with no significant medical history nor taking bisphosphonates and no history of previous orthodontic treatment. Subjects with missing lower incisors, history of periodontitis, dental anomalies, syndromic and craniofacial deformities as well as poor image quality of the LCR were excluded.

For geometric morphometric studies, the sample size calculation cannot be determined by straightforward application of mathematical formula [24] as there is no clear guideline [25]. A study suggested a minimum sample size calculation based on the number of coordinates and landmarks [26]. Therefore, in this study, 9 landmarks with 2D coordinates (x, y) will yield a minimum sample of 18 (9×2=18) per group to allow for inferential statistics.

A total of 254 LCR were obtained using Planmeca Romexis software (Planmeca ProMax 3D Classic; Planmeca Romexis, Helsinki, Finland) following these exposure parameters: 60–90 kV, 1–14 mA, exposure time between 9–37 seconds and a source-midsagittal plane distance of 1.5 m. The LCR were digitally traced using VistaDent OC (2D) (GAC International, Inc., Bohemia, NY, USA). The subjects were classified based on gonial angle: high (>130°), average (120°–130°) and low (<120°) [12]. The gonial angle were measured from the inclination of the posterior border of ramus and the inferior border of the horizontal body of the mandible [27].

A total of 254 LCR were included in this study. The mean age of subjects was 21.78±5.95 years old. The sample consisted of 94 (37.0%) males and 160 (63.0%) females with mean ANB values of –2.44±2.31. More than half of subjects 132 (52.0%) had retroclined lower incisors whereas 122 (48.0%) had upright lower incisors. Majority (45.7%) had average gonial angle at 124.96°±2.97°, 29.5% had high gonial angle at 133.99°±3.29° and 24.8% had low gonial angle at 114.59°±3.62°.

We aim to investigate the general shape variation of MS among Class III skeletal patterns and determine which factors may have the highest influence on the shape variation of MS. GPA superimposed all samples and removed non-shape information to allow visualization of MS shape variation. The scatter of points appeared more pronounced and elongated in the lingual basal part of MS and alveolar region with lesser variation landmarks observed at the lower border of MS. Therefore, more shape variation was observed around the lingual basal part of MS where the width of MS varies horizontally, from slim to wide. Significant shape variation of alveolar bone was observed surrounding the lower incisors, with vertical variation from short to tall. This is influenced by dentoalveolar compensation of teeth in Class III skeletal pattern.

In this study, PCA identified 14 different shape of MS. The first three PCs represented the majority variation of the sample (74.6%) and were the main features to classify most of the MS in Class III skeletal base. PC1 describes the variation in width and height of the whole MS (narrow and elongated or wide and short). PC2, on the other hand, describes the variation at the level of B-point (inferiorly or superiorly position), inclination of alveolar region (retrocline or procline) and shape changes at the basal of MS (oval or rounded). PC3 describes the variation in height and width of the alveolar part of MS (short and narrow or wide and tall). Bangar et al. [11] found that MS in Class III skeletal bases has reduced concavity on the labial surface and increased vertical dimension compared to Class I and Class II skeletal bases. Maniyar et al. [17] also found that Class III skeletal base with retroclined incisors has taller MS. This is not in agreement with our result where our study indicates that MS variation in Class III skeletal base can be observed at the alveolar part, basal part, or both, where it varies from narrow and elongated or wide and short as mentioned previously. Previous studies indicate that the shape variation of MS could be due to individual variation [5], vertical facial proportions [5-10], sagittal skeletal relationship [11], incisor inclination [1, 12, 13], muscular and functional activities [4, 14, 15], and sexual dimorphism [3, 16]. However, these studies used linear measurements to measure height, width, or inclinations of MS. Linear and angular measurements limit the visual interpretation of result, especially when describing the round and curved MS shapes.

Utilization of GMA not only allowed visualization of MS shape variation, but has also identified that gonial angle (9.2%) has the largest effect on the shape of MS, followed by incisor inclination (1.49%) and sex (1.29%). Gonial angle, also known as angle of the jaw, is an angle measured between the inclination of the posterior border of ramus and inferior border of the horizontal body of the mandible [27, 31]. It was used in this study to define the divergence of mandible and not influenced by other facial structures and landmarks. The gonial angle was also found to be associated with vertical facial proportion where it was significantly increased in hyperdivergent subjects [9].

CVA showed significant shape variation between high, average and low gonial angle groups (P<0.0001). The high gonial angle group had narrow, slim and elongated MS. The average gonial angle group had a relatively shorter alveolar part, wider and more rounded basal part with prominent pogonion compared to high gonial angle group. Low gonial angle, on the other hand, had the shortest and widest the alveolar part. The basal part was also wide, rounder, and bulbous. This finding is similar with previous studies which concludes that those with long face or backward growth of mandible (high gonial angle) exhibits elongated and thin MS [5-9]. Aki et al. [19] also confirmed our findings in that MS morphology is associated with mandibular growth rotation in anterior and posterior directions.

Additionally, different facial types have been shown to be associated with different occlusal forces, where those with long faces have lower bite force magnitude and reduced masticatory muscle size [15, 32]. Skeletal Class III open bite patients with reduced biting forces were reported to have thinner MS compared to patients with normal overbite [4]. Increased neuromuscular activity and training such as masticatory forces, can result in muscle hypertrophy, metabolic and cross-sectional area changes, and may change the fiber-type composition of the muscle towards a larger percentage of slow-type fibers [33]. Low angle group with increased muscle activity [34] have larger muscle cross-sectional area [35] as well as wider trapezoidal-shaped ramus, bigger coronoid, rectangular body, curved basal arch and wider MS.

The findings of this study also show that the ratio of alveolar to basal part was different in the three gonial angle groups. The high gonial angle group has the tallest alveolar part and vice versa for the low gonial angle group. This suggests that craniofacial structures grow differently between low and high angle facial types, where anterior lower facial height growth is greater in high angle group [36]. Additionally, the alveolar part of MS in the retroclined incisor group is longer or taller compared to those in the upright incisors’ group.

As the vertical facial proportion increases, natural dentoalveolar compensation occurs where more teeth extrude and MS elongates to meet with opposing teeth [10, 36]. As a result, the alveolar part of MS in the retroclined incisor group is longer or taller compared to those in the upright incisors’ group, as found in this study. MS elongates to meet with opposing teeth when mandible growth outpaced the upper jaw [37]. Hence, incisor inclination influences the morphology of MS as mentioned by other studies [1, 12, 13].

Our findings also show that alveolar bone at the B-point level was very narrow and constricted in patients with Class III skeletal bases who have high gonial angle. Furthermore, the alveolar bone surrounding lower incisors decreases in thickness as the gonial angle increases. This suggests that orthodontists must be extra careful when moving teeth in patients of the high gonial angle group, as there is higher risk of root movement which can lead to gingival recession, root resorption and bone fenestration [38].

In a study by Swasty et al. [16], males generally have significantly taller MS than females. Our current findings are also in line, where male MS is taller and narrower at the lingual basal part and B point area compared to female (P<0.006). Similar findings were also observed in other studies [3, 6, 11, 16, 39]. The differences in skeletal shape, including dental arches is often reported with significant differences noted [40, 41]. Bite forces, for example, is also reported to influence bone remodeling. Males have almost four times higher bite force than females, at 190 N and 50 N respectively [42].

In conclusion, the shape of MS in Class III skeletal base varied at the alveolar part, basal part or both. The alveolar part ranged from tall and narrow with inferiorly positioned B-point to short, wide with superiorly positioned B-point. The basal part ranged from narrow, elongated, and oval-shaped to wide, short, and bulbous-shaped. The factors identified to highly influence MS shape was gonial angle, followed by incisor inclination and sex.