Depth of Antegonial Notch as a Predictor of Mandibular Growth Potential –A Cephalometric Study

INTRODUCTION

Possibly no other bone can rival the mandible as a target for scientific study. A plethora of work in cephalometrics and other x-ray techniques formed the background for mandibular growth studies. In Orthodontics, knowledge of mandibular growth is beneficial in diagnosis and treatment planning. The ability of an orthodontist to predict future mandibular growth and its rotation could allow therapy to be tailored to the individual. Hence, Orthodontic researchers are continually investigating the possibility of predicting growth of the mandible using single parameter from a profile radiograph.

Consequently, the shape of the mandible is often used by clinicians as an aid in predicting mandibular growth. Factors such as the size of the gonial angle, shape of the lower border of mandible, inclination of the condyle and symphyseal morphology have been associated, by numerous investigators, with the amount and direction of subsequent growth of the mandible^1,2,3,4,5,6 . 

Mandibles which demonstrate an opening or backward and downward rotation during growth experience pronounced apposition beneath the angle with excessive resorption under the symphysis³ . The resultant upward curving of the inferior border of the mandible, anterior to the gonion is known as antegonial notching⁵ . This feature has been constant with both congenital and acquired conditions of the mandible⁷ . If significant dimensional differences are seen in the craniofacial morphologies of subjects with deep and shallow antegonial notches, one can anticipate the mandibular growth and potential by this single parameter on routine cephalograms, the antegonial notch.

Subjects with deep antegonial notching have been reported to have disturbed condylar growth⁷ . Other studies have shown a diminished mandibular growth potential in subjects with pronounced antegonial notching⁶ . Thus this study was undertaken to assess and compare the differences in craniofacial dimensions between deep and shallow antegonial notch groups in an attempt to give the clinician a simple method to assess mandibular growth pattern and potential. 

MATERIALS AND METHOD

The 50 cephalometric films of pretreatment orthodontic patients were obtained. The age group of the subjects varied from 10-14 years. The growth status of the patient was determined by the cervical vertebra maturation. Nongrowing patients and those having a Class III malocclusion were excluded from the study since they often present with extreme growth patterns that could bias the sample selection process. 

Sample selection was based on the depth of the antegonial notch, measured on each subject's pretreatment cephalogram as the distance along a perpendicular line from the deepest point of the notch concavity to a tangent through the two points to greatest convexity on the inferior border of the mandible, on either side of the notch.

Cephalometric landmarks were identified (Fig. 3) and fifteen angular and linear measurements were made on each cephalometric tracing (Fig. 4 and 5). Tracings were done on 0.3 mm acetate paper with a 0.3 lead pencil. Linear measurements were made with a Boley gauge calibrated to 0.1 mm and angular measurements were made with a protractor with an accuracy of 0.5⁰. 

RESULTS

The means, standard deviations and P values for the angular and linear measurements are presented in Table I and II. The data shows that in subjects with deep notching the mandibles develop more vertically than in those with shallow notches. The mean values for SNPg is 2.3⁰ less in the deep notch group and GoGn-SN is 6.55⁰ greater. The BaN-CCGn, FH-GoMe, ANS- XiPm and Occlusal plane inclination angles differ significantly between the two groups, varying from significant to highly significant (Table –I). 

The tendency for vertical growth in deep notch patients is further corroborated by the mean values for total face height (N-Me) and lower face height (ANS-Me) which are respectively 4.3 mm and 2.5 mm longer than the analogous shallow notch values. In addition, the articular and gonial angles are respectively 3.8⁰ and 2.7⁰ more obtuse in deep notch group. The average notch depth of the posterior border of the ramus is 0.7 mm more in the deep notch sample. The mandibular incisor measured to Apo line is placed, on an average 2.3 mm more anteriorly in the deep notch group when compared with its position in the shallow notch group.

The mean value for the antegonial notch depth is 2.6 mm greater in the deep notch group which merely reflects the selection criteria. Some of the variables that record statistically significant differences are the posterior mandibular ramus notch, the mandibular plane angle and facial axis angle.

DISCUSSION

Orthodontics is interested in facial growth and development basically because growth can alter the need for and application of therapy. Successful orthodontic treatment in growing patients may depend heavily on the amount and direction of jaw growth. Lateral cephalometric radiographs are indispensable tools in orthodontic diagnosis, treatment planning and quantifying dentofacial changes over time.

Various parameters have been used to predict mandibular growth with varying success^1,2,3,6. Bjork, with his implant studies³ described structural signs seen in extreme types of mandibular rotators. Among other signs was the contour of the inferior border of the mandible. It was found that in forward rotating mandibles apposition occurs below the symphysis and resorption takes place under the angle. Conversely, in backward mandibular rotation apposition beneath the angle is common and resorption underneath the symphysis is possible³ . The most pronounced area of remodeling is below the angular region and is known as antegonial notching⁵ . Subjects with deep antegonial notching have been reported to have disturbed condylar growth⁷ .

Other studies have shown that the mandibular growth potential is diminished in subjects with pronounced antegonial notching⁶. 

Becker, Coccaro and Converse⁷ explained that the association between pronounced antegonial notching and diminished mandibular growth was due to the failure of growth of the mandibular condyle which prevents the lowering of the jaw. The masseter and medial pterygoid, by their continued growth, cause the bone in the region of the angle to grow downward, producing antegonial notching, thus a relative tension occurs between the angle and the muscle sling in which it is suspended such that bone is deposited under the angle posterior to the notch.

Issacson and associates⁸ suggest that a reduced vertical condylar growth and an excessive vertical growth at the midfacial sutures and the alveolar processes cause a backward rotational growth of the mandible. Thus the anterior part of the corpus is pressed down into the matrix resulting in resorption at the lower surface of the symphysis while the posterior part of the corpus is lifted up from the soft tissue matrix, stretching the periosteum and apposition takes place below the angle.

The purpose of this study was to assess and compare their differences between deep and shallow notch groups in order to determine whether any definite dimensional differences in the craniofacial morphology are associated with different notch depths. Perhaps the most clinically significant feature of this study of differences between deep and shallow mandibular antegonial notch groups is the difference in growth direction of the mandible as indicated by GoGn-SN, BaN-CCGn, FH-GoMe and ANS-Xi-Pm (Table I). This finding is also observed in other studies⁵.

The variation in the vertical growth pattern is also reflected in the difference in facial height (N-Me) between the two groups (Table II). The deep antegonial notch group shows a more vertical mandibular growth pattern resulting in a longer anterior facial height than the shallow notch group. However the lower component of the facial height (ANS-Me) shows only a tendency towards significance.

The deep notch group has reduced SNPg angle (Table I) when compared with the shallow notch group suggesting that the effective horizontal component of mandibular length is less in such patterns. The observed smaller length in deep notch cases is in agreement with the observations of several investigators⁷ who have reported the presence of marked antegonial notching in the mandibles of patients with disturbed or arrested growth of the condyles. Nahoum⁹ have independently reported pronounced notching of the lower mandibular border occurring in conjunction with steep mandibular plane and a large gonial angle. The deep notch cases examined in this study showed a significantly larger mean gonial angle than in subjects with shallow notching (mean Ar-Go-Me is 27⁰ ) greater in deep notch subjects.

A similar association is observed between the size of the gonial angle and direction of mandibular growth by Lundstorm and Woodside¹⁰ and Odegaard¹¹ Enlow¹² stated that as the gonial angle opens, the antegonial notch is accentuated. The mandibular spatial position in deep notch subjects tends to be significantly retrognathic as revealed by more obtuse S-Ar-Go and SN-GoAr angles, the mean differences between the group being 3.7⁰ and 2.9⁰ respectively. This significant difference was also noted by Singer, Mamandras and Hunter.⁸ 

Although the average values of SNA and SNB are respectively 2⁰ and 1.7⁰ smaller in the deep notch group they fail to achieve statistical significance. This finding is in contradiction to those of other studies^5,8. This variation could be due to the lower heritability of sagittal craniofacial dimensions in comparison with vertical ones. Thus the low association between anteroposterior mandibular position and mandibular shape may reflect increased environmental influences that cannot be effectively predicted.

The general length of the mandible (Ar-Pg) is on an average 2.5 mm longer in the shallow notch group and is consistent with observations of other investigators. An additional morphologic feature that shows a very significant difference between the two groups is the posterior notch depth which is found to be greater in the deep notch group (Table II) and is in agreement with the findings of Lambrechts, Harris Rossouw and Stander⁵. 

An explanation for the association between pronounced antegonial notching and a deeper posterior mandibular ramus notch has been advanced by Enlow¹³ . According to the author the angle between ramus and the corpus also can become increased or decreased as a part of the rotation process. It is primarily the ramus, rather than the corpus, that carries out most of the changes involved in the opening and closing of the angle because this is where most of the active remodeling processes occur. In vertical growth patterns as the mandible rocks down and back the ramus-corpus angle closes to compensate for it. Since most of the remodeling occurs in the ramus there is resorption at the posterior border of the ramus that produces a characteristic posterior ramal notch. The process is just the reverse in horizontal growth patterns where the mandible rotates upward and forward. The compensatory remodeling forms a shallow ramal notch. The mean measurements for Li- GoGn and is 3⁰ smaller in the shallow notch group (Table I). It is also seen that the mandibular incisor measured to the Apo line is placed, on average 2.3 mm, more anteriorly in the deep notch group when compared to the shallow notch group and is found to be significant. In the anterior region the teeth attempt to compensate for the rotational pattern which accounts for the varying degree of inclination of the lower incisors seen in each group.

It is already known that deep antegonial notches in adolescents tend to deepen with growth while shallow notches become shallower⁶ . Mandibular morphology thus appears to influence the type of growth pattern of the face. The posterior mandibular ramus notch, the mandibular plane angle and the facial axis were some parameters that recorded statistically significant differences between the means. Hence these variables when observed and measured on cephalogram are good indicators of the type of growth pattern likely to be expressed by the subject.

Treatment considerations:

The results of this study clearly indicate that the treatment plan must vary depending on the notch depths. For instance, the extra oral traction needs of deep notch patients can be fulfilled with high pull and straight pull headgears while shallow notch cases can safely wear low pull headgear. 6 Furthermore, studies have shown that deep notch subjects are more likely to have extractions as compared to their shallow notch counterparts in order to achieve a successful orthodontic result. All mechanics must be aimed at maintaining or improving the facial balance throughout treatment. 

SUMMARY AND CONCLUSION

The shape of the mandibular outline was moderately to highly correlated to variables indicating mandibular growth direction but the correlation with variables indicating anteroposterior jaw relationship was not so strong. Subjects with deep notch had a greater gonial angle than did shallow notch subjects.

The mandibular growth direction in deep notch subjects, as measured by the facial axis, the mandibular plane angle and the occlusal plane inclination, was more vertically directed than for shallow notch patients. Longer facial heights were observed in deep notch subjects with less prominence of the chin. Deeper posterior ramus notch depths were also seen in patients with deep notch. Some of the shortcomings of the study were the relatively small sample size and the fact that only subjects with characteristically deep or shallow antegonial notches were used to determine variables that could assist in predicting future mandibular growth direction. However the purpose of assessing and comparing the differences between deep and shallow notch groups from single profile radiographs was successfully achieved.