During facial growth, the maxilla and mandible translate downward and forward. Although the forward displacement of the maxilla is less than that of the mandible, the interarch relationship of the teeth in the sagittal view during growth remains essentially unchanged. Interdigitation is thought to provide a compensatory (tooth movement) mechanism for maintaining the pattern of occlusion during growth: the maxillary teeth move anteriorly relative to the maxilla while the mandibular teeth move posteriorly relative to the basilar mandible. The purpose of this study was to investigate the hypothesis that the human chin develops as a result of this process.
Twenty-five untreated subjects from the Iowa Facial Growth Study with Class I normal occlusion were randomly selected based on availability of cephalograms at T1 (mean = 8.32 yr) and T2 (mean = 19.90 yr). Measurements of growth (T2 minus T1) parallel to the Frankfort horizontal (FH) for the maxilla, maxillary dentition, mandible, mandibular dentition, and pogonion (Pg) were made.
Relative to Pg (a stable bony landmark), B-point moved posteriorly, on average 2.34 mm during growth, and bony chin development (B-point to Pg) increased concomitantly. Similarly, the mandibular and maxillary incisors moved posteriorly relative to Pg 2.53 mm and 2.76 mm, respectively. A-point, relative to Pg, moved posteriorly 4.47 mm during growth.
Bony chin development during facial growth occurs, in part, from differential jaw growth and compensatory dentoalveolar movements.
The chin, or more specifically the protuberance of the bony mandibular landmark, the mentum osseum, is a facial feature unique to modern humans. Humans differ in the forward projection of the mentum osseum compared with higher primates and other species of Homo who lack a similar prominence of this mandibular landmark. Why is this so?
The development of the chin in modern humans has largely been viewed in the literature as an evolutionary change in mandibular architecture brought about by altered function and biomechanical forces as the mandible diminished in size. Recent studies, however, have documented that the formation of the human chin cannot be explained entirely as a function of biomechanics. In contrast to a purely biomechanical explanation, other studies have suggested that modern human chin morphology is the result of a posterior displacement of the mandibular dentition relative to the basal region of the mandible, and the evolution of the human chin is the result of a relative independence of the alveolar and basilar regions of the mandible. This would suggest that the degree of development of the chin is largely a function of the alveolar region of the mandible “drifting back” along the basal region of the mandibular corpus.
It is well established that during facial growth the anterior aspect of the mandibular alveolus at the symphysis is resorptive, while the lower symphyseal border, near Pg, is developmentally stable and exhibits little to no remodeling. As such, formation of the human chin is not the result of bony deposition along its anterior surface. Chen et al, for example, conducted a longitudinal analysis of the mandibular outline, in norma lateralis , using elliptical Fourier analysis. Their data suggest that the prominence of the chin results from the posterior placement of the mandibular incisors relative to the chin rather than from an increase in the relative size of the chin at Pg.
The posterior migration of the mandibular dentition is developmentally linked to the differential growth of the mandible and maxilla. This was originally suggested by Lager. More recently You et al documented that the forward growth of the mandible was, on average, nearly 5 mm more than that of the maxilla in a longitudinal sample. Similarly, while the mandibular dentition moved anteriorly relative to the maxillary basal bone, it migrated posteriorly relative to the mandibular basal bone. The posterior position of the mandibular dentition relative to the basal bone of the mandible is likely due to the interdigitation and function of the mandibular and maxillary dentition in promoting maintenance of the occlusal pattern. As the mandible outgrows the maxilla, the mandibular dentition is, in effect, dragged relatively posterior by the maxilla.
The purpose of this study was to assess the longitudinal development of the chin as a function of differential jaw growth and spatial positioning of the mandible, the maxilla, and the dentition. We did this by testing 2 specific hypotheses in a longitudinal sample of untreated subjects.
Hypothesis 1: The horizontal projection of the chin is a function of differential anterior growth between the mandibular body and the mandibular dentition and its associated alveolar bone; that is, the chin develops as the mandibular dentition exhibits posterior displacement relative to mandibular basilar bone during jaw growth.
Hypothesis 2: Changes in the anterior-posterior position of the mandibular dentition during growth and development follow concomitant changes in the maxillary dentition as a function of maxillary growth relative to the mandible. As such, variation in the position of the mandibular dentition during growth should follow that of the maxilla.
Material and methods
To test our hypotheses, material was obtained from the Iowa Facial Growth Study. This pure longitudinal study began with 183 whites (92 males and 91 females). Included in this study are lateral and anterior-posterior cephalograms, as well as intraoral models, taken every 6 months between the ages of 5 and 12 years and annually thereafter through age 18 years. A final set of records was taken at adulthood. All subjects had a normal angle Class I molar and canine relationship and were free of any facial or skeletal disharmony. Subject participation in the growth study diminished with age, leaving 100 participants at age 12 years and 70 participants in early adulthood.
A subset of 25 subjects (13 males and 12 females) who had never received orthodontic treatment or extractions of permanent teeth (third molars excluded) were randomly selected based on the availability of lateral cephalograms with sufficient image quality in the regions of interest at the ages studied. Time point 1 (T1) was during childhood at around 8 years of age (mean = 8.32 years; SD = 0.24 years), while time point 2 (T2) was during early adulthood, with ages ranging from 16 to 28 years (mean = 19.90 years; SD = 3.88 years). Table I provides descriptive statistics and selected skeletal cephalometric relationships for our sample. Although these subjects had normal (Class I) occlusion during growth, they did exhibit considerable variation in skeletal cephalometric measurements.
For the measurements used in our study, on each of the lateral cephalograms the following landmarks were identified ( Fig 1 ):
A-point (A): the deepest point of the bony concavity of the maxilla between the anterior nasal spine and the prosthion
Maxillary incisor (Mx1): the anteriormost portion of the maxillary incisal edge
Maxillary first molar (Mx6): the most anterior point on the mesial surface of the maxillary first molar crown
B-point (B): the deepest point of the bony concavity of the mandible between the infradentale and Pg
Pg : the most anterior point on the contour of the bony chin
Mandibular incisor (Mn1): the anteriormost portion of the mandibular incisal edge
Mandibular first molar (Mn6): the most anterior point on the mesial surface of the mandibular first molar crown
The lateral cephalograms were traced by hand. For any landmark that did not lie in the midsagittal plane, the midpoint between the right and left points was used. When hand tracing, all cephalometric landmarks were identified using a 0.5-mm mechanical lead pencil. All tracings were verified by a second independent investigator, and discrepancies were resolved by the second investigator. Linear measurements on the tracings were recorded using a stainless steel digital caliper to the nearest tenth of a millimeter and were corrected for enlargement.
The measurements between landmarks are defined in Table II . At T1 and T2, linear measurements to each landmark were made parallel to the FH from a constructed reference line through the sella perpendicular to the FH. The change in the position of each cephalometric landmark, between T1 and T2 (relative to the constructed reference line and parallel to the FH) was calculated as the difference between linear measurements for each landmark taken at T1 and T2. Given the developmental stability of Pg during growth, this T1 to T2 change in each cephalometric landmark was referenced to the amount of T1 to T2 change in the position of Pg. Thus, the measurement “B-Pg” in Table II is the change in linear distance from the reference line to B-point (T2 minus T1) minus the change in linear distance from the reference line to Pg (T2 minus T1). All other measures in Table II were derived similarly by subtracting the T1 to T2 Pg change from the T1 to T2 landmark change.
|Mn1-Pg||Position of central mandibular incisor relative to Pg|
|Mn6-Pg||Position of first mandibular molar relative to Pg|
|A-Pg||Position of the maxilla relative to Pg|
|Mx1-Pg||Position of central maxillary incisor relative to Pg|
|Mx6-Pg||Position of first maxillary molar relative to Pg|
To determine the ability to accurately replicate the cephalometric measurements, interobserver error was tested by remeasuring a subset (40%) of the sample with at least a 24-hour intervening period. Measurements were taken with a high degree of accuracy, as the average error between observations was less that 1% (SD ± 2.7%).
To test our first hypothesis, we examined the Pearson correlation ( r ) and coefficient of determination ( r 2 ) between the developmental change in chin size (B-Pg) and the developmental change in mandibular dentition position (Mn1-Pg and Mn6-Pg). We would predict a significant positive correlation between chin development and the longitudinal change in the position of the dentition such that greater posterior placement of the dentition is associated with greater chin development. We would further predict that a majority of the variance in the developmental change in B-Pg is explained by variation in the developmental change in the position of the mandibular dentition.
Our second hypothesis was similarly tested by examining r and r 2 between the developmental change in mandibular dentition position (Mn1-Pg and Mn6-Pg) and their maxillary counterparts (Mx1-Pg and Mx6-Pg) as well as the anterior projection of the maxilla relative to Pg (A-Pg). Should the change in the position of the mandibular dentition follow the change in the position of the maxillary dentition, presumably due to the effects of interocclusal tooth contacts during function, we would predict that the developmental change in the mandibular dentition should be significantly positively correlated with Mx1-Pg, Mx6-Pg, and A-Pg. We would, in addition, predict that the majority of the variance in the developmental change in the position of the mandibular dentition should be explained by the change in our maxillary variables.
Mandibular rotation has been shown to occur during facial growth in untreated subjects. Given that our measure of chin development is taken relative to the FH, it is potentially sensitive to the variation in mandibular rotation. To assess whether longitudinal change in mandibular plane angle is a confounding factor when measuring longitudinal change in chin size along the FH, we examined the Pearson correlation ( r ) between the developmental change in chin size and the developmental change in the mandibular plane angle (SN-MP). Should the change in horizontal projection of the chin (relative to B-point) follow the change in mandibular plane angle, we would predict that the developmental change in chin size should be significantly negatively correlated with the developmental change in mandibular plane angle.
To further assess chin development and associated mandibular and maxillary growth, we also used generalized Procrustes analysis (GPA) on a series of 15 coordinate landmarks taken on the lateral cephalograms ( Table III , Figure 2 ) at both T1 and T2. GPA is a geometric morphometric method, using 2D or 3D coordinates of definable landmarks that allow comparison of shapes irrespective of size, location, or rotation of the objects. Principal components analysis on Procrustes scaled landmarks (ie, landmarks scaled for size) was used to assess residual shape differences in maxillary and mandibular growth from the 2 time periods.