A teenager’s mandibular symphysis may look different depending on facial growth pattern—but this study found that age and sex themselves did not significantly change its form between twelve and fifteen. The characteristics and morphology of the mandibular symphysis significantly impact orthodontic diagnosis and treatment plan strategies.

The mandibular symphysis is a significant anatomical reference for facial aesthetic analysis and one of the predictors for the direction of the rotational growth pattern of the mandible. Because its form affects the amount of trabecular bone supporting the lower incisors, it provides valuable information about the anterior lower limit of the dentition during biomechanics for correcting crowding or dento-skeletal malocclusions.

Excessive retraction or proclination of the anterior teeth may generate alveolar bone loss, dehiscence, fenestration, gingival recession, and root resorption, so analyzing the alveolar bone form at the symphysis region can help define therapeutic limits, especially in class II subjects.

Most earlier studies of mandibular symphysis morphology and skeletal pattern used conventional cephalometric or linear measurements, which do not precisely reproduce the morphology of the symphysis curvature. Geometric morphometric analysis has become an important scientific tool in orthodontics for investigating skeletal morphology modifications and explaining complex morphology differences more successfully than traditional morphometric coefficients.

The study therefore used geometric morphometric analysis to evaluate mandibular symphysis form from twelve to fifteen years in a longitudinal retrospective sample of untreated class II subjects. This retrospective study used lateral cephalograms from the American Association of Orthodontists Foundation Craniofacial Growth Legacy Collection.

The archive contains nine known collections of longitudinal craniofacial growth records from the United States and Canada, including cephalograms taken each year in children who never received orthodontic treatment. The researchers searched for male and female subjects with lateral cephalograms at twelve plus or minus six months and fifteen plus or minus six months, labeled T zero and T one.

The preliminary search recruited 227 subjects from the Burlington, Fels, Iowa, and Oregon growth studies databases. The final criteria required skeletal class II malocclusion with an ANB angle greater than four degrees, no fixed appliances, teeth in occlusion, and no extreme craniofacial pattern.

The study sample consisted of 120 subjects, including 64 females and 56 males, and 240 cephalograms, with T zero at twelve years and T one at fifteen years. A Procrustes fit was performed for each dataset, and the points were superimposed by generalized Procrustes alignment before the Procrustes coordinates were projected in Kendall tangent space.

Univariate and multivariate normality was evaluated using the MVN program, version one point six. The normality assessment included the Shapiro–Wilk, Cramer–von Mises, Lilliefors, Mardia, and Royston tests. Principal components analysis used the Procrustes coordinates from T zero and T one to obtain principal components describing the major features of mandibular symphysis change in each group.

The same analysis also included both T zero and T one datasets to increase sensitivity. Discriminant function analysis evaluated the influence of gender and age on mandibular symphysis form change at both time points, with T-square and permutation tests used to obtain a p-value.

Canonical variation analysis evaluated the influence of biotype on morphological change at T zero and T one by analyzing group structure in multivariate data. Procrustes analysis superimposed all specimens and standardized information about rotation, translation, and volume among observations for principal components analysis.

For the twelve-year dataset, seven significant principal components accounted for 98.903 percent of cumulative variation. For the fifteen-year dataset, seven significant principal components accounted for 98.890 percent of overall variation. At both T zero and T one, almost 90 percent of cumulative variance was described by the first two principal components.

Figure two summarizes mandibular symphysis modifications at T-zero, when participants were twelve years old, using Procrustes superimposition followed by principal component analysis. Panel A plots the first two principal components, with male and female scores shown in different colors, while the axes indicate that PC one accounts for fifty-nine point nine percent and PC two for twenty-nine point five percent of the variation.

Panel B visualizes these shape changes, comparing the black consensus curve with the purple mean-of-variation curve. Figure three shows the first two principal components of mandibular symphysis modification at T one, corresponding to fifteen years of age, after Procrustes superimposition and PCA.

In panel A, male and female scores are plotted using different colors, while panel B translates the variation into shape curves: black for the consensus and purple for the mean variation. Together, the figure shows how the main shape axes summarize mandibular change and how individual variation relates to the consensus form.

Discriminant function analysis found no statistically significant difference between males and females in the T zero dataset, with a p-value of zero point 56, or the T one dataset, with a p-value of zero point 11. Age did not influence the form of the chin symphysis at either T zero or T one, with a p-value of zero point 71.

Canonical variation analysis found a statistically significant difference in symphysis form between hyperdivergent and hypodivergent patterns at both time points, with p-values below zero point 001. There were no statistically significant differences between the other skeletal-pattern combinations: normodivergent versus hyperdivergent, or normodivergent versus hypodivergent.

Figure four superimposes the mandibular symphysis curves for male subjects in blue and female subjects in pink. Panel A shows the comparison at T-zero, twelve years, while panel B shows it at T-one, fifteen years. The close overlap of the curves visualizes the authors’ DFA finding of no statistically significant gender differences across observational timing, supporting the two-block partial least squares results.

Figure five superimposes mandibular symphysis curves identified through discriminant function analysis. Panel A shows the comparison at T zero, or twelve years, and panel B at T one, or fifteen years. Blue represents hypodivergent subjects, red represents normodivergent subjects, and green represents hyperdivergent subjects.

This matters because symphysis morphology constrains lower-incisor proclination, retraction, and torque during orthodontic camouflage planning. The two-block partial least squares analysis reported an RV coefficient of zero point 486 with a p-value below zero point 001 for covariation between the initial form and anatomical change associated with residual growth.

PLS one accounted for 87.01 percent of total covariance, while PLS two accounted for 7.57 percent of total covariance. The vector described by PLS one would reflect the mean morphological variation expressed in the principal components analysis and relate to sagittal and vertical intragroup form characteristics attributed to different facial biotypes.

The smaller sample size may have contributed to inflation of the RV value. Table 5 uses two-block partial least squares analysis to assess how the symphysis’s initial position covaries with changes in MS-prime morphology during growth, pooling patients by vertical skeletal pattern.

The reported RV correlation coefficient is zero point four eight six, with a p-value below zero point zero zero one, indicating statistically significant covariation. The first PLS component accounts for 87.01 percent of covariance, and the second accounts for 7.57 percent, bringing cumulative covariance to 94.58 percent.

The largest principal components, PC one and PC two, accounted for almost 90 percent of cumulative variance and expressed form variation along the vertical and sagittal craniofacial axes, generally attributed to different skeletal vertical growth patterns. Subjects in the hyperdivergent subgroup showed a narrower and elongated symphysis compared with the hypodivergent subgroup.

This pattern would suggest that hyperdivergent individuals can be more exposed to iatrogenic injuries such as gingival recession or dehiscence during treatment of mandibular crowding or dentoalveolar compensation of class II. The findings confirm the importance of establishing an appropriate treatment plan based on evaluating mandibular symphysis morphology.

Because the study included only subjects with class II malocclusion, the analysis did not consider the general influence of sagittal skeletal pattern on mandibular symphysis morphology. Figure six superimposes mandibular symphysis curves before and after treatment, with blue showing patients at T zero and red showing patients at T one.

The three panels separate hyperdivergent, normodivergent, and hypodivergent subjects, allowing the authors to examine these shapes within distinct skeletal patterns using discriminant function analysis. This matters because the symphysis is a thin cortical-bone region that can limit lower-incisor proclination, retraction, and torque during dentoalveolar compensation.

The RV value of 48.60 percent showed no significant covariation between the initial form of the symphysis and anatomical change associated with residual growth. Accordingly, clinicians should not expect spontaneous changes in symphysis morphology during orthodontic treatment of adolescents.

This may be important for hyperdivergent subjects because they are generally more exposed to iatrogenic effects during orthodontic treatment, including the risk of dehiscence or fenestration of the lower incisors. The main takeaway is that vertical skeletal pattern, especially the contrast between hyperdivergent and hypodivergent subjects, was linked to symphysis form, while residual growth showed no significant covariation with the initial form.