INTRODUCTION

The understanding of facial growth and development is necessary for proper orthodontic diagnosis and treatment planning. Cranial base growth is linked to the overall growth of facial bones, especially the maxilla and mandible, either directly or indirectly.^1,2 The cranial base consists of anterior and posterior parts, where the maxilla is directly attached to the anterior part through growth sutures and the mandible is indirectly attached to the posterior part through the temporomandibular joint. Therefore, any change in the amount and/or direction of growth of the cranial base can have direct or indirect effects on the developing maxilla and mandible.³

Previous studies^4–6 found that cranial base angular and linear measurements tended to decrease gradually in skeletal anteroposterior (AP) relationships II, I, and III, respectively. Proff et al.⁵ found that cranial base length and cranial base angle were reduced in subjects with Class III sagittal relationship compared with those with other skeletal relationships. In addition, Chin et al.⁶ suggested that, when the cranial base angle increased, SNB angle decreased. However, other studies^7–9 could not detect such a correlation between sagittal skeletal relationships and cranial base measurements.

It was thought that cranial base angle or flexure angle (N-S-Ar) played an important role in the development of malocclusion. Flattening of this angle can result in more posterior positioning of the glenoid fossa and therefore a greater tendency toward backward positioning of the mandible. In contrast, reduction of the cranial base angle can result in more forward positioning of the glenoid fossa and a tendency toward a Class III skeletal relationship.

Bjork-Jarabak analysis can be a useful tool for understanding the relationship between cranial base growth and the developing maxilla and mandible.² It was proposed that, if the sum of Bjork polygon angles was greater than 400°, the mandible tended to have backward growth rotation, whereas if the sum was less than 392°, the mandible tended to have forward growth rotation.

The influence of cranial base angulation and linear measurements as a factor in the development of AP jaw relationships remains a matter of debate. Therefore, the aim of this study was to assess the linear and angular cranial base measurements in different AP skeletal relationships using Bjork-Jarabak analysis.

MATERIALS AND METHODS

This retrospective cross-sectional study was based on the pretreatment lateral cephalograms of 288 Caucasian adult orthodontic patients (146 women, 142 men; mean ages of 21.24 ± 2.72 and 22.94 ± 3.28 years, respectively) attending orthodontic clinics at the dental teaching clinics/Jordan University of Science and Technology. To access patient files, ethical approval was granted by the Institutional Research Board at Jordan University of Science and Technology. All subjects were Caucasians from the Jordanian population with an average maxillary to mandibular plane angle (MMPA) of 27° ± 5). Patients with a history of previous orthodontic treatment, orthognathic treatment, craniofacial anomalies, facial trauma, or detected asymmetries were excluded.

Lateral cephalograms were in centric occlusion using an Orthoslice 1000 C (Marne La Vallee Cedex 2, France) with a cephalostat at 64 KVp, 16 Ma, and 0.64-second exposure, according to the manufacturer's instructions.

Cephalometric Analysis

Lateral cephalograms were hand traced in a dark room under the same standardized technique. Magnification of radiographs was adjusted using the radiopaque ruler (calibration marker). Points and lines were marked with a 3H pencil. All measurements were performed by the same investigator (E.A.M.) to reduce interexaminer differences.

The measurements derived from Björk-Jarabak analysis (Figure 1) included linear and angular measurements of a Bjork polygon, as presented in Table 1. In addition, the ANB angle (angle formed between point A–Nasion–point B) was measured and used to classify AP skeletal relationships into three groups: Class I, II, or III (Class I: ANB angle between 2–4°, II: ANB angle >4°; and III: ANB angle <2°).

View Figure

• Download as Powerpoint
• Download Figure

Table 1. Cranial Base (Bjork Polygon) Linear and Angular Measurements Used in This Study

View Fullscreen

RESULTS

Sample Characteristics

The distribution of the sample according to AP skeletal relationships is shown in Table 2. The age and gender distribution were similar among groups.

Table 2. Distribution of the Selected Sample According to the AP Skeletal Relationship

View Fullscreen

Angular measurements

The means and standard deviations (SDs) of angular cranial base measurements (Bjork polygon angles) are shown in Table 3. Statistically significant differences among groups were found in the saddle and gonial angles (P < .05), whereas the articular angle and the sum of the Bjork polygon angles (saddle, articular, and gonial) were not significantly different between Class I, II, and III skeletal relationships (P > .05).

Table 3. Means and Standard Deviations (SDs) of Cranial Base (Bjork Polygon) Measurements in Different AP Skeletal Relationships^a

View Fullscreen

The saddle angle was found to be significantly larger in Class II compared with Class III skeletal relationships (P < .05), with a mean difference of 2.17°. However, there was no significant difference in saddle angle between Class I and Class II or between Class I and Class III groups. On the other hand, gonial angle was found to be highly significantly larger in the Class III compared with Class II skeletal relationships (P < .001), with a mean difference of 3.62°. No significant differences were detected in gonial angle between Class I and III or between the Class I and II groups.

Gender Differences

Means and SDs of cranial base measurements in men and women and the differences between them are shown in Table 4. Women were found to have a significantly larger articular angle than men (P < .05), with a mean difference of 1.85°. On the other hand, gender differences were not detected in the saddle and gonial angles.

Table 4. Means and Standard Deviations (SDs) of Cranial Base (Bjork Polygon) Measurements in Men and Women and the Differences Between Them^a

View Fullscreen

Men showed significantly (P < .05) larger anterior (N-S) and posterior (S-Ar) cranial base lengths compared with women, with mean differences of 2.79° and 2.81°, respectively. Ramal height (Ar-Go) and body of the mandible length (G0-Gn) were also significantly (P < .05) larger in men compared with women, with mean differences of 3.47° and 2.49°, respectively.

DISCUSSION

AP growth and positioning of the cranial base are highly associated with the midface and position of the mandible. This can play an important role in jaw growth and directly influence the skeletal pattern.^10,11

All measurements in this study were performed on lateral cephalograms, which have been extensively used to provide guidelines for diagnosis and treatment planning. Bjork-Jarabak analysis was used in this study, because it was shown to be very useful when assessing facial characteristics through only a few measurements.^12,13

Nanda¹⁴ suggested that most of the growth changes of the facial tissues would predominantly take place before the age of 18 years. Accordingly, only adult patients (>18 years) were included in this study to ensure that most of the growth changes had been attained.

Anterior cranial base length (N-S) is the linear distance between points N and S, while the posterior cranial base length is controversial, defined as either S-Ba or S-Ar linear distances. Björk¹⁵ suggested that S-Ar was more easily visualized, whereas Varjanne and Koski¹⁶ considered that S-Ar is too distant and suggested the use of Ba to measure the skull base angle. Many studies supported that the two points (Ba) and (Ar) were highly correlated and the differences between them were negligible.^17,18 Accordingly, S-Ar was used in this study, because the Bjork polygon was used in which (S-Ar-Go) is the articular angle of the polygon, to assess linear and angular cranial base measurements.

In the present study, the saddle angle was found to be smaller in a Class III compared with Class II skeletal relationship. This was in agreement with findings reported by previous studies.^5,6,19–22 On the other hand, Thiesen et al.²³ reported that the saddle angle was not different among different skeletal relationships. However, the sample in the study by Thiesen et al.²³ included growing subjects with an age range between 8 and 17 years old, so the result might not have reflected the adult size after growth completion.

No significant difference in saddle angle was found between Class I and Class II or between the Class I and Class III groups. This was in agreement with previous studies.^23–26 The only difference was between the Class II and Class III skeletal relationships, indicating more forward positioning of the posterior cranial base and, thus indirectly, more forward positioning of the mandible, which is a more common finding in Class III.

In the current study, the articular angle was similar among the different AP skeletal relationships. This was in agreement with Hegde et al.²⁷ Rodriguez-Cardenas et al.,²⁸ on the other hand, found that articular angle was increased in the Class I skeletal relationship compared with other skeletal relationships. The differences between the current results and the previous studies could have been related to the nature of the samples and included age groups. Also, Rodriguez-Cardenas et al.²⁸ used cone-beam computed tomography (CBCT)–synthesized lateral cephalograms, which could be different from the lateral cephalograms used in the current study.

Gonial angle was larger in Class III compared with the Class II skeletal relationship. This was supported by the findings of Gasgoos et al.,²⁹ who found that the gonial angle was larger in the Class III group. Those authors suggested that the increase in the gonial angle resulted from the increase in the effective length of the mandible (Ar-Gn).

The sum of the Bjork polygon angles was similar among the groups in the present study. This was also found by Rodriguez-Cardenas et al.,²⁸ who reported that sum of the Bjork polygon angles showed no significant differences among the three sagittal classes. This finding can also be explained by the fact that the current sample included only those patients with average MMPA. This angle is affected by growth rotation of the mandible, which is linked to the sum of Bjork polygon angles.

The similarity of the anterior and posterior cranial base lengths in the studied groups coincided with the findings of previous studies^5,9,19,22,30 and contradicted that reported by Chang et al.³¹ and Gong et al.,³² who found that anterior and total cranial base lengths were smaller in Class III than in the Class I and II groups. The differences might have been related to the sample included and the age range studied. For example, Chang et al.³¹ studied a sample of Chinese patients whose craniofacial features might be different when compared with a Caucasian population.

In the present study, ramal height (Ar-Go) was smaller in the Class II skeletal relationship compared with the Class I and Class III groups, whereas it was similar between Class I and Class III subjects. This was in agreement with Dong et al.,³³ who found that patients with a Class II skeletal relationship tended to have smaller ramal height (Co-Go) when compared with Class I subjects.

Mandibular body length was larger in the Class III compared with Class II skeletal relationship in the present study. This was supported by the findings of a previous study.⁵ Gasgoos et al.²⁹ also found that subjects with a Class III skeletal relationship had a larger mandibular body length (Go-Pog) and larger effective mandibular length (Ar-Gn) than Class II subjects.

The body length of the mandible was larger in the Class III group. This agreed with the findings of previous studies that reported a longer mandible in Class III cases.^29,34,35

No differences were found in angular measurements between men and women except for articular angle. This was supported by previous findings of Rodriguez-Cardenas et al.,²⁸ who reported that the articular angle was increased in female patients. In the present study, linear measurements were found to be significantly greater in male patients compared with female patients. These findings were supported by previous studies.^36–38 In addition, this was in agreement with Valiathan et al.,³⁹ who found that male patients had longer anterior and posterior cranial base lengths, ramal heights, and mandibular body lengths. They also found that females tended to have larger articular angles than males did.

CONCLUSIONS

• The Class II skeletal relationship has a larger saddle angle compared with Class III.

• The Class III skeletal relationship has a larger gonial angle compared with Class II.

• The Class III skeletal relationship has a larger ramal height and body of the mandible length compared with Class I and Class II.

• Articular angle, anterior (N-S), and posterior (S-Ar) cranial base lengths are not different among different skeletal relationships.

• Females tend to have a larger articular angle than males do.

• Males have a longer cranial base length (anterior and posterior) and longer ramal height and body length of the mandible than females do.