Factors associated with the morphology of the mandibular symphysis and soft tissue chin

ABSTRACT Objectives: This study aimed to (I) assess the morphology of the symphysis and soft tissue chin associated with sex, age and sagittal/vertical skeletal patterns, and (II) identify the individual and combined contributions of these variables to different portions of the symphysis. Methods: This cross-sectional study included 195 lateral cephalometric radiographs from untreated adults. Alveolar, basal, and soft tissue of the symphysis were measured by an X/Y cranial base coordinate system, and divided in accordance to four predictor variables: sex, age, and sagittal/vertical skeletal patterns. Parametric tests were conducted for comparison and correlation purposes, while multiple regression analysis was performed to explore combined interactions. Results: Alveolar inclination is related to sagittal and vertical patterns, and both explained 71.4% of the variations. Alveolar thickness is weakly predicted and poorly influenced by age. Symphysis height was 10% higher in males, and associated with a vertical skeletal pattern and sex, and both explained 43.6% of variations. Basal symphyseal shows an individual thickness, is larger in males, and vertically short-positioned with age. Soft tissue chin is not necessarily related to the size of the underling skeletal pattern, and enlarges with age, even in adulthood. Conclusions: The symphysis and surrounding tissues are influenced by sex, age, and sagittal and vertical patterns, acting differently on the alveolar, basal and soft tissue portions. Sagittal and vertical skeletal patterns are the strongest association on alveolar symphysis inclination, whereas sex and age acts on the vertical symphysis position and soft tissues thickness.


INTRODUCTION
The mandibular symphysis is the anatomical anterior part of the mandible composed of cortical and alveolar bones. Differently, the chin, or mentum, is the projected part of the mandibular symphysis, constituting a feature unique to modern humans. 1 In turn, the adjacent soft tissue below the lower lip is called soft tissue chin. The symphysis and adjacent tissues make up an interactive and complex anatomical structure, didactically divided into three portions: two hard tissues and one soft tissue. The hard tissues constitute the alveolar and basal portions.
The alveolar ridge accommodates the mandibular incisors, and its inclination usually matches the long axis of the alveolar symphysis. 2 The basal portion constitutes the mandibular symphysis itself, with a more apical location when compared to the alveolar portion. 3 On the other hand, the soft tissue chin represents the integumental mentum, which is supported and designed by the underlying basal symphysis, dentoalveolar projection, and soft tissue thickness. 4 Identifying the factors associated with the morphology of the symphysis and adjacent structures can be useful for basic and applied sciences. Alveolar symphyseal inclination can be affected by anteroposterior orthodontic movement in compensatory or decompensatory treatments for skeletal discrepancies, 5 and also in arch-perimeter changes for tooth-size discrepancies. 6 This aspect has been a concern Dental Press J Orthod. 2021;26(4):e2119347 Evangelista K, Silva MAGS, Normando D, Valladares-Neto J -Factors associated with the morphology of the mandibular symphysis and soft tissue chin 5 when considering a safe anteroposterior movement of the mandibular incisors, preventing periodontal damage such as bone fenestration and dehiscence. 7 The inclination of the long axis of the basal symphysis is one of the characteristics used to predict mandibular rotation and projection during growth. 8 This symphyseal site can also provide autogenous bone for bone grafting prior to dental implant placement. 9 Furthermore, both bone and chin soft tissue play crucial roles in facial aesthetics, and are therefore vital when making surgical case decisions in cases of genioplasty. 10 Previous cross-sectional 3,4,[11][12][13][14][15][16][17] and longitudinal [18][19][20]  The present study hypothesized that different regions of the symphysis and adjacent tissues are influenced individually or in association by different predictor variables. Thus, the aim of this study was to assess the contributions of sex, age, and sagittal/vertical patterns to the morphology variation of the alveolar, basal, and soft tissue portions of the symphysis and surrounding tissues, using a multiple regression model.

MATERIAL AND METHODS
This cross-sectional cephalometric study was approved by the Evangelista K, Silva MAGS, Normando D, Valladares-Neto J -Factors associated with the morphology of the mandibular symphysis and soft tissue chin 7 The following inclusion criteria were followed: adult patients (males > 18y and females > 16y); facial symmetry (detected by facial photographs); presence of all teeth, except third molars; and good quality digital lateral cephalometric radiographs, teeth in maximum intercuspal position, lips at rest and a natural head position. According to the exclusion criteria, radiographic images presenting advanced periodontal disease; signs of facial or dental trauma; syndromes or congenital craniofacial anomalies, such as cleft lip or palate; and previous orthodontic, prosthetic, or surgical procedures, were not considered.
Variables were grouped in accordance with the alveolar, basal and soft tissue portions of the symphysis and surrounding tissues. Digital cephalometric radiographs were obtained with standardized settings (90 kV, 12.6 mA).    Figure 2 shows all cephalometric measurements with CCI values and respective Bland-Altman plots. The results revealed a high intra-examiner agreement for most variables.

RELIABILITY
The G'perp-Pog' (CCI=0.7207) was an exception, probably due to limitations in the geometric arrangement and distance in measuring. The cephalometric method used in this study is a reliable tool to evaluate the morphology of the symphysis and adjacent tissues.    (Table 1).

UNIVARIATE ANALYSIS
The alveolar inclination was similar for both genders, although influenced by the sagittal and vertical patterns. It was anteriorly inclined in Class II and sagittal cases, and posteriorly inclined in Class III and vertical cases. Alveolar symphyseal thickness was not influenced by age or sex, and was thinner in individuals with a Class III skeletal combined with a hyperdivergent pattern.   Tables 4 and 5   Evangelista K, Silva MAGS, Normando D, Valladares-Neto J -Factors associated with the morphology of the mandibular symphysis and soft tissue chin 16  Another interesting data was that alveolar thickness maintained stable with aging. This study used a sample with patients ranging from 16 to 63 years old (most of them young adults).

CORRELATION AND MULTIPLE REGRESSION ANALYSIS
The older group showed a mean reduction of only 0.5 mm in the alveolar symphysis, and it was not considered statistically significant. Pre-surgical treatment in preparation for orthognathic surgery is a major clinical concern, especially in patients found in the soft tissue chin projection, as previously demonstrated in a sample of a balanced face, 11 and could be explained by Enlow's hypothesis of growth equivalence. 24 Symphyseal height is, on average, 10% greater in males than in females, in both balanced and unbalanced faces, as shown by the present sample, and this result is in agreement with Gomez et al. 17 This feature also includes the height of the incisor crown. between the ages of 6 and 18 years old. 25 The present results showed that Gn' thickness increased 1.7mm during adulthood, which was lower than in another study 19 that found a 3.7-mm increase over a 40-year period, considering from late adolescence to late adulthood. Age was also related to shortness of vertical basal symphysis position in the present sample.
One possible explanation for this finding could be physiological tooth wear throughout life, and its consequence for the vertical dimension of the face. 27  This study has limitation regarding the cross-sectional design.
Because the study was not based on a longitudinal data set, the association established needs to be carefully interpreted.
However, the sample groups were allocated with comparable distribution, and also statistical adjustment contributed towards minimizing bias and confounding effects. In addition, the 2-D cephalometric measurements confirmed satisfactory reproducibility. Although 3-D evaluation is the current approach in skeletal researches, it is not recommended for all patients in orthodontics. 28,29 For ethical reasons and ALARA principle, the 3-D tomographic images must balance the risks and benefits, especially in patient with no treatment need (normal occlusion). 28,29 Evangelista K, Silva MAGS, Normando D, Valladares-Neto J -Factors associated with the morphology of the mandibular symphysis and soft tissue chin Genioplasty can be designed to increase or reduce chin size or to straighten an asymmetrical chin. Although the response of soft tissues is similar to bone movement, genioplasty should be performed with discretion and individually (Fig 3). Two of the influence factors, such as age and sex, cannot be manipulated clinically, and together have influence on symphysis height and soft tissues thickness. The aging influence in each gender must be useful on the treatment planning of genioplasty. Additionally, since the morphology and position of the soft tissues can be altered by aging, it is essential to future studies to investigate predictive criteria for changes from hard to soft tissues, and apply this information not only after surgery but also in long term periods. The present study reinforces that the thickness of soft tissue chin is not necessarily related to the size of the underling skeletal pattern and, in addition, the influence of sex and age cannot be disregarded.

CONCLUSIONS
This study concludes that the morphology of symphysis and