Chin-throat anatomy: Normal relations and changes following orthognathic surgery and growth modification
To determine if a new facial line (T), tangent to the throat, intersects the mandibular border in anterior (ANT) and posterior (POST) parts in proportions varying with facial configuration, and to evaluate the association between chin projection and throat inclination and the potential for the T-line to reflect this association. Measurements on profile photographs and cephalograms of 135 adults (aged 18–50 years)—45 each of Class I, II, and III (CI, CII, CIII) malocclusions—included ANT and POST, chin-throat (CTA), and mento-cervical (MCA) angles. Pre- and posttreatment measurements were compared in two subgroups (n = 25 each) of CII and CIII orthognathic surgery patients and in CII, division 1 early-treatment patients (n = 63). Statistics included analysis of variance and t-test for group differences, and Pearson correlation for associations among variables. ANT was nearly equal to POST in CI (50.99%) and CIII (51.86%) subjects and shorter in CII (36.01%) subjects. CTA and MCA were greater in CII profiles and smaller in CIII profiles. Significant differences (P < .0001) were observed for ANT, POST, CTA, and MCA between Classes I/II and II/III and for MCA between Classes II/III (P = .016). High correlations were noted between ANT and CTA in Classes I (r = −0.83), II (r = −0.73), and III (r = −0.68). In surgically treated patients, posttreatment measurements approached CI values. In the early-treatment group, ANT increased but remained smaller than POST; CTA decreased by nearly 13%. Chin-throat relations and chin extension are associated and require routine assessment in terms of diagnosis and treatment outcome. A practical tool to assess chin-throat relationship, the T-line bisects the mandibular body nearly equally in Class I faces.ABSTRACT
Objectives:
Materials and Methods:
Results:
Conclusions:
INTRODUCTION
Common cephalometric analyses focus on relations among hard tissues (bone and teeth) and their association with surrounding soft tissues (nose, lips, and chin).1–4 Such assessments disregard the influence of the chin-throat relationship on the profile and, consequently, treatment, possibly because of the recognition that this relationship may not be altered with orthodontics. Chin-throat evaluation is more common in plastic surgery, based on clinical and anthropometric normative data,5 and may be performed directly on patients or on photographs using well-defined soft tissue landmarks. Farkas6 contributed the most recognized anthropometric age-related data on facial proportions.
Different components define the relation of the chin to the rest of the face:
- 1.
Chin position relative to facial structures, most particularly the lips and/or nose (eg, E-line, Holdaway line, Steiner line),1 or cranial references (vertical projections from soft tissue nasion, soft tissue glabella,7 or other8). These assessments represent the vast majority of orthodontic analyses.
- 2.
Form of the chin “button,” evaluated through angular measurements identified differently by various authors: the mento-cervical angle, most notably used by Farkas6 (78.3° ± 7.9° in males; 83.9° ± 9.3° in females); the lower face–throat angle by Legan and Burstone7 (100° ± 7°); and the “lip-chin-throat” angle by Worms et al.9 (110° ± 8°). This measurement is highly dependent on lip position relative to the chin, rather than on chin form.
- 3.
Chin assessment within the submental region (throat length, chin-throat angle7,9,10). Throat length or extension, measured in millimeters, does not necessarily project the relationship between chin and throat, although severely protruded or retruded chins may have an impact on chin/throat perception. The chin-throat angle, also known as the submental-cervical or cervico-mental angle (as opposed to the mento-cervical angle), has been reported at a low of 90°10 but also at 124°.11–13 A recent survey14 of this angle's attractiveness revealed an optimal value of 95°.
The chin-throat relationship requires proper analysis because of its variability. Indeed, a chin may be adequately extended in the anterior region yet be deficient in terms of harmony because of an increased chin-throat angle. The lack of consistent data in the literature, along with our clinical observation of an optimal intersection of the throat line with the mandible to define the chin-throat relationship, led us to formulate this hypothesis: a line tangent to the soft tissue border of the throat (T-line) bisects the body of the mandible in its middle in normal and well-balanced facial proportions.
Our aims were to (1) explore the association between chin projection and throat inclination in various malocclusions and through comparisons of before and after treatment in orthodontically and surgically treated subgroups and (2) determine the potential for the new T-line to assess the chin-throat connection.
MATERIALS AND METHODS
This retrospective study was approved by the institutional review board of the American University of Beirut and included two samples of patients with malocclusion. The first sample comprised 135 pretreatment profile photographs of White adult patients, taken under controlled standardized conditions, and corresponding lateral cephalographs obtained in the same cephalostat (Instrumentarium Dental, Tuusula, Finland) in natural head position (Table 1). The inclusion criteria were the following: age between 18 and 50 years, no prior orthodontic treatment, and absence of craniofacial anomalies. Three groups of 45 patients each were defined as follows: Class I (ANB angle between 0° and 3°); Class II (ANB greater than 4.5°); and Class III (ANB less than 0°). Two subgroups (n = 25 each) were identified among the Class II and Class III patients who were treated with combined orthodontic/orthognathic intervention to compare their measurements before and after surgery.
The second sample consisted of 63 pre- and posttreatment lateral cephalographs of White patients (33 boys, 30 girls) with Class II, division 1 malocclusion who had been treated by growth modification with either a straight-pull headgear or a Frankel function regulator. The subjects were enrolled in a previously published randomized clinical trial in which the inclusion criteria were no prior orthodontic treatment, no craniofacial anomalies, minimum ANB angle of 4.5°, and minimum overjet of 5 mm.15,16 Their ages ranged between 7 and 13.3 years (average: 10.15 ± 1.45 years in boys, 9.32 ± 1.55 years in girls).
In the first sample, the lateral cephalograms and photographs were digitized, superimposed, and analyzed by one investigator using the Dolphin Imaging program, version 9 (La Jolla, Calif). In the second sample, only cephalograms were processed through the imaging program. The following lines and angles were traced (Figure 1): line “T,” tangent to the throat (cervical plane), formed the chin-throat angle (CTA) with the line tangent to the inferior border of the chin (submental plane), and divided the soft tissue border of the mandible, represented by a line joining soft tissue gonion and pogonion, into anterior (ANT) and posterior (POST) mandibular portions. The millimetric measurements of both distances were computed in percentages of the total border for better representation across patients. The mento-cervical angle (MCA) was traced, according to the method of Farkas,6 between the “upper contour of the chin and the surface beneath the mandible,” that is, the intersection between the anterior and inferior slopes of the chin.
Citation: The Angle Orthodontist 87, 5; 10.2319/100916-734.1
Ten records from each sample were digitized and measured twice to determine examiner reliability. The following statistical tests were used: intraclass correlation coefficient for repeated measurements; one-way analysis of variance and post hoc Bonferroni tests, when appropriate, for differences among and between groups; paired t-tests to compare the nonsurgical and surgical Class II and Class III subgroups, as well as pre- and posttreatment measurements in the surgical subgroups; and Pearson product-moment for correlations among various measurements. The level of significance was set at .05. Statistical analyses were conducted using the IBM® SPSS® statistics 21.0 statistical package (Chicago, Ill).
RESULTS
The intraclass correlation coefficients ranged from 0.994 to 0.999 for all parameters, indicating high intraexaminer reliability.
In the Class I group, the T-line bisected the mandibular soft tissue border nearly in its middle (ANT = 50.99%; POST = 49.01%; Table 1). ANT was comparatively reduced (36.01%) in Class II patients. Both MCA and CTA were wider in the Class II than the Class I and Class III groups, in which the findings were similar for angles MCA and CTA, and ANT and POST were almost equal. Comparison among malocclusion groups showed statistically significant differences in all measurements between Class II and both Class I (.054 < P < .0001) and Class III (P < .001) groups, but only for MCA between Classes I and III (P = .016).
The highest correlation was found between ANT and CTA (r = −0.79) in all malocclusions in descending order from Class I to Class III (Table 2), as follows: Class I (r = −0.83); Class II (r = −0.72); and Class III (r = −0.68).
Comparisons between surgical and nonsurgical subgroups within the Class II and Class III groups revealed more severe sagittal discrepancy in the surgery subgroup: higher (Class II; P = < .0001) and lower (Class III; P = .002) ANB (Table 3). None of the other measures were different in the Class III malocclusion, but the anterior sections were shorter and CTA greater among Class II surgical patients (P < .05).
In the surgically treated Class II and Class III patients, the within-group ANT differences before and after surgery were different in the Class II (P < .0001) but not in the Class III subgroup (Table 4). MCA decreased in both classes, the Class III angle moving farther away from Class I. CTA decreased in Class II but remained unchanged in Class III; in both malocclusions, the postsurgical results were within one standard deviation of the Class I values.
In the Class II surgical subgroup, the ANT and POST postsurgical measurements were no longer significantly different from those of the Class I group (P = .21) (Table 4, legend). The correlations between MCA and either ANT or CTA were higher after surgery than before surgery (Table 5). The correlations between ANT and CTA were high before and after surgery. In the Class III surgical subgroup, the nearly equal ANT and POST before and after surgery (Table 4) were not different from those of the Class I group (P > .05). The correlations between ANT and MCA and between MCA and CTA increased from before to after surgery (Table 5). The correlations between ANT and CTA were high pre- and postsurgery. All correlations within the Class II and Class III subgroups were statistically significant (.03 < P < .0001).
In the Class II, division 1 growing patients, ANT was nearly one-quarter (27.48%) of the mandibular distance before treatment, remarkably smaller than POST (72.52%). After treatment, ANT increased to nearly 40% (Table 6). CTA improved from 127° to 115° with treatment, ending with highly correlated ANT and CTA (r = −0.7).
DISCUSSION
This study introduced norms for chin-throat relation in various malocclusions and a simplified method by which to gauge this association. The chin-throat connection is critical in defining chin extension: an acute angle accentuates the perception of anterior projection, and a significantly obtuse angle conveys the impression of reduced extension. The latter is characteristic of aging, along with the development of a double chin, particularly if associated with weight gain.17
T-Line
The proposed tangent to the throat (T) bisected the mandibular border in its middle in acceptable esthetic appearance. When these proportions differed, facial esthetics were compromised and likely compatible with the existence of malocclusion. This premise was more evident in Class II malocclusions, in which the T-line intersected the mandibular border more anteriorly.
The T-line/mandibular intersection was further validated by the results in treated adults and children. The proportions of ANT and POST also improved in the more deviant, surgical Class II dysmorphology, approximating the Class I norms accompanying more esthetically pleasing profiles. In the younger patients, the increase in ANT reflected ameliorated profile esthetics related to favorable differential growth between the jaws and understandably did not reach the outcome achieved with surgery. It is noteworthy that at the time of treatment planning, the concept of T-line was not applied.
Angular Measurements in the Chin-Throat Zone
The average CTA in Class I subjects (116° ± 6.87°) was nearly midway between available norms (90° to 124°).10–14 Differences may be ascribed to the number of subjects in the studies or to gender or population characteristics. Another contribution of this study was determining pretreatment averages for CTA in Class II (132.13° ± 13.13°) and Class III (112.22° ± 13.11°) subjects, presumably defining zones of deviations from the Class I angulation. Class III CTA was closer to that observed in Class I patients. The Class II obtuse angle was less esthetic than either of the others, a conclusion supported in another study by the perception of a CTA angle of 110° as “slightly unattractive,” the “very” and “extremely” unattractive set measured at 125° and 130°.14
This finding supports the concept of more geometric delineation (expansion) of hard and soft tissues as being more esthetic than the constriction of skeletal volume.18 The more obtuse CTA, and the fact that the T-line intersects the mandible more anteriorly in Class II malocclusion, represent a constitutional limitation to ideal correction of the chin-throat zone in this dysmorphology, barring the surgical correction, whereby, on average, the posttreatment CTA (reduced by more than 15°) was closer to the Class I readings. In the younger sample, CTA decreased after treatment (from 127° to 115°), also approximating the Class I values.
These observations indicate the possibility of using a workable soft tissue CTA value of ≤115° until larger samples of “esthetically” defined profiles are evaluated. Recent findings14 indicated preferred CTA attractiveness at 95°, followed by 100° then 90°, angles of 110°, 115°, and 125° representing thresholds for surgical intervention, as assessed by patients, lay people, and clinicians, respectively.14 However, these results were gathered from altered angles on a silhouette profile.14 Research is needed on actual faces and should further indicate whether throat inclination varies with mandibular divergence, a more forward-inclined throat matching a steeper mandibular plane and a more upright throat line accompanying a flatter mandibular outline. Another layer of research might address the contribution of muscle and/or fat in determining throat outlines.
Measurements of the MCA in the Class I subjects (84.15° ± 9.4°) were close to the norms by Farkas6 (78.3° ± 7.9° in males; 83.9° ± 9.3° in females). MCA was greater in Class II (95.06° ± 10.9°) and slightly smaller in Class III (78.26° ± 9.36°) subjects, again demonstrating closer measures between Class I and Class III phenotypes relative to Class II phenotypes.
The correspondence of ANT and POST and CTA in Class I and Class III subjects reflects mandibular proportionality in Class III and supports the finding that most Class III malocclusions are associated with maxillary retrognathism.19 Nevertheless, Classes I and III differed in MCA values (P = .016), this angle being smaller in Class III than in Class I malocclusions, indicating a relatively more prominent pogonion in Class III malocclusions. Further research should investigate differences in T-line and mandibular length between these malocclusions.
Clinical Significance and Implications
The chin-throat relationship, coupled with the position of the chin relative to the lips and nose, allows the practitioner to determine the chin extension in the facial profile, a critical step when the scope of therapeutic influence is only in the lower face. In contrast to the spectrum of linear and angular measurements of optimal chin-throat relationship, the T-line consistently divided the mandibular border into almost-equal parts in individuals with normal skeletal bases and closely approached this proportion in surgically corrected malocclusions. While directly related to the CTA (r = −0.79; Table 2), as both include the throat line, the T-line further depicts the proportionality of the anterior extension of the chin. Indeed, a normal CTA may be found in a patient with deficient chin extension; an obtuse CTA may be noted with normal mandibular length yet with a deficient chin extension delineated through T-line; or an obtuse CTA may coexist with equal ANT and POST components of the mandibular border in the presence of a large mandible. Accordingly, both measurements complement a more accurate judgment; the T-line yields a readily interpretable proportional assessment.
In the most severe malocclusions of the surgical subgroups, ANT was moderately and negatively correlated to the MCA, suggesting that the more obtuse the MCA, the less extended the anterior portion of the chin. Accordingly, the less defined the chin “button” in a Class II malocclusion, the greater the amount of advancement of the mandible (with possible genioplasty) needed.
ANT was also highly and negatively correlated to the CTA, indicating that the more acute the CTA, the more extended the anterior portion of the chin. This finding would imply (and actually confirm the clinical observations) that a greater mandibular setback increases the risk of creating a double-chin profile.20,21 In fact, 19 (76%) of the 25 Class III patients had a maxillary LeFort I osteotomy rather than a mandibular setback or a combination of both because the chin-throat zone was not favorable for the setback, even in patients in whom cephalometric measurements indicated mandibular prognathism in conjunction with maxillary retrognathism.
The anterior projection of the chin correlated better with the MCA after surgery in Class II and Class III malocclusions. The higher correlations between CTA and MCA in the severe Class II and Class III subgroups, but not in the total sample, indicate the greater associations between throat and chin in extreme malocclusions. These findings suggest the practical use of the T-line in facial esthetic diagnosis of these malocclusions, at least as an initial or complementary assessment to other cephalometric measurements.
CONCLUSIONS
- •
The T-line is a practical tool for the evaluation of the relation between throat inclination and mandibular extension, providing a readily interpretable, more individualized assessment that takes into account the proportionality of the face, particularly in orthodontic/orthognathic interventions.
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The hypothesis underlying this research was supported: in Class I occlusions with seemingly well-proportioned faces, the soft tissue throat line bisects the mandible in its middle. In this context, chin extension is affected by the level of inclination of the throat line.
- •
T-line and chin-throat angular measurements indicate more similarity between Class I and Class III subjects in comparison with Class II subjects.
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The incorporation of T-line into orthodontic diagnosis and treatment planning aids individual assessment based on the harmony of adjacent facial structures rather than numeric norms that may vary with age, gender, or ethnicity.
Landmarks and lines used for linear and angular measurements. Go: soft tissue gonion, determined by superimposing hard tissue gonion on the photograph in Dolphin; Pog: soft tissue pogonion; throat line (T-line) tangent to the throat; MCA: mento-cervical angle at the intersection of the tangents to the upper contour and inferior border of the chin; CTA: chin-throat angle between T-line and submental plane; ANT/POST: anterior and posterior portions of the mandibular border determined by the intersection of T-line with Go-Pog line.
Contributor Notes