Main

Angle defined Class II Division 1 malocclusion as characterised by a distal relation of the lower teeth to the upper to the extent of more than one-half the width of one cusp and the maxillary incisors being protrusive.1 The craniofacial characteristics of subjects exhibiting Class II malocclusion have been presented by several researchers.2,3,4,5,6,7,8,9 The value of most previous studies is limited9 since they are missing a clear definition of Class II malocclusion, or Class II Division 1 and 2 are combined.6,7 However, when comparing Class II Division 1 and 2 malocclusions, it has been reported there were no significant morphological differences between the two malocclusions except for the maxillary incisor angulation which was used as selection criteria.9 The major differences between Caucasians with Class II Division 1 malocclusion and cephalometric standards10,11 were that the mandible was retrognathic, and a short lower face.9 McNamara indicated that retrusion of the mandible is the most commonly occurring factor contributing to Class II malocclusion and the average position of the maxilla was found to be neutral relative to cranial and cranial-base structures.6

Previous studies indicated that facial proportions show little age and sex variation for patients with a pleasing profile and normal occlusion in various ethnic groups.12,13,14,15,16,17,18

Former studies on population norms showed that there are some ethnic differences in the facial morphology between Chinese and Caucasians.15,19,20,21,22 No data, however, are available for the craniofacial pattern in Chinese with Class II Division 1 malocclusion.

The objectives of this study are to assess the craniofacial pattern of the Chinese children with Class II Division 1 malocclusion and in addition, to compare this sample with Chinese population norms, and Caucasians with Class II Division 1 malocclusion.

Materials and methods

Lateral cephalometric radiographs taken in centric occlusion of 105 Chinese patients, (50 males and 55 females) with Class II Division 1 malocclusion were selected from the files of the Prince Philip Dental Hospital, Faculty of Dentistry, The University of Hong Kong.

The criteria for inclusion in the sample were Chinese of 10–15 years of age, in early permanent dentition, with no previous orthodontic treatment and Class II Division 1 malocclusion as determined by a distal relation of the lower teeth to the upper to the extent of more than one-half the width of one cusp and the maxillary incisors being protrusive from the study models.

A cephalometric software system, Dentofacial Planner 6.3, was used to perform the lateral cephalometric evaluation. The system receives data from a digitiser, maintains a database, and performs angular measurements. The reference points, planes and angles used in their study were defined according to Björk.23

Statistical analysis

Fifty of the lateral cephalometric radiographs were traced twice on different occasions by the same operator and the data were compared using paired 2-tailed Student t-test to assess the systematic error. Later the random error was assessed by tracing ten lateral cephalometric radiographs twice and using the formula ME = (Σd2/2n). Analysis of the data showed that there is no significant difference for any of the two tests.

For the analysis of the means and standard deviations of the angular measurements of the studied sample, an SAS computer program was used. Unpaired Student t-test was performed to test if there were any significant differences among the sexes, occlusal group and ethnic groups.

Cephalometric patterns of this sample were compared with Chinese norms,20 and Caucasians with Class II Division I malocclusion.9 The sample of Chinese norms20 comprised 204 12-year-old boys and girls, and the Caucasian sample of Class II Division 1 malocclusion9 comprised 107 11–13-year-old boys and girls. The comparison between the present sample and the two reference samples,9,20 also included the use of SD-score, ie the differences for the specific variables were expressed in proportions of the standard deviations of the reference samples.24

Results

Since there was no significant sex difference for any of the variables (Table 1), the data for the boys and girls were pooled subsequently.

Table 1 Table 1
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Comparison between Chinese Class II Division 1 and Chinese norms20

The mean and standard deviation values of the angular measurements for the present sample of Chinese children with Class II Division 1 malocclusion were compared with the Chinese norms as shown in Table 2.20

Table 2 Table 2
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There were decreased skull base angle as indicated by lower N-S-Ba angle, prognathic maxilla as indicated by higher S-N-A angle, retrognathic mandible as indicated by reduced S-N-B angle, retruded chin as indicated by small S-N-Pg angle, skeletal class II base relationship as shown by higher A-N-B angle, increased mandibular plane angle as shown by high SN/MnP angle, increased intermaxillary plane angle as indicated by high Mx/MnP angle, proclined upper incisors with respect to the maxillary plane as indicated by increased UI/MxP angle and decreased interincisal angle as shown by low UI/LI angle, all differences being statistically significant.

The maxillary plane angle and angle of the lower incisor relative to the mandibular plane are the only measurements which showed non-significant difference.

Ethnic groups comparison

Table 3 presents the differences between Class II Division 1 malocclusion among Chinese and Caucasians.9 Chinese revealed a significantly more prognathic maxilla and less retrusive mandible than Caucasians9 as indicated by larger S-N-A and S-N-B angles, respectively. The chin was also 1.7° less retrusive in Chinese as shown by significantly higher S-N-Pg.

Table 3 Table 3
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The A-N-B angle was 0.7° larger in the Chinese than in the Caucasian sample, the difference being statistically significant. The maxillary plane angle (SN/MxP) was of the same magnitude in both ethnic groups but the mandibular plane angle (SN/MnP) was significantly larger in the Chinese and the same magnitude of difference was found in the vertical maxillary/mandibular relationship (Mx/MnP). The maxillary incisors (UI/MxP) are significantly more proclined in Chinese but the lower incisor angulation (LI/MNP) is similar in these two ethnic groups.

Discussion

This study was based on a sample in whom subjects were accepted for orthodontic treatment due to relatively high treatment needs, ie the sample comprised Chinese patients with Class II Division 1 cases who have moderate or severe malocclusions.

Previous studies on Chinese with normal occlusion and pleasing profile15,17 have reported that sexual dimorphism in dentoskeletal pattern was unremarkable. Gianelly12 claimed that for children within the age range studied, there appeared to be no need for separate age and sex norms. This study found that there were no statistically significant angular difference between Chinese males and females with Class II Division 1 malocclusion (Table 1). Subsequently as no angular variations were noted, both sexes were pooled together for subsequent evaluations.

The present sample of Class II Division 1 malocclusion, selected on assessment of the dental arch relationship only, has definite class II skeletal problem, reflected by the large ANB angle which is twice as large as the Chinese norm,20 or expressed in SD, score 1.5 times greater (Table 2). There are certain advantages in using the parametric method based on normal distribution theory rather than the percentile method, eg 'it allows any measurements to be expressed as a standard deviation score, and hence located to a particular percentile, which is much more informative than knowing whether they are inside or outside the reference interval'.24 The skeletal Class II pattern in the Chinese sample is more pronounced than in the Caucasian sample, reflected by the significantly larger ANB angle. This may indicate that the Chinese patients with Class II Division 1 malocclusion in Hong Kong referred for/seeking orthodontic treatment have more severe malocclusion compared with the Caucasian sample.9 It has been shown that the overall demand for orthodontic treatment in Hong Kong is still comparatively low.25 Subsequently the ethnic comparison (Table 3) of differences in patients with Class II Division 1 malocclusion has to be interpreted with some caution. It also seems that the inclusion criteria used to collect the samples differed between the two studies, eg in the study by Pancherz et al.,9 cases were included if they were at least half a unit class II, and in the present study cases here included if they were greater than half a unit class II. This may explain why the ANB angle was greater in the present study. A part of the selection criteria is proclination of the upper incisors, assessed on study models, which are more proclined in the Chinese Class II Division 1 sample (6° or +1 SD) compared with the Chinese norm. The maxillary incisors in the Chinese Class II Division 1 sample are 9° (+1.4 SD) larger than the Caucasian sample. However, this difference is similar to the ethnic difference (8°) reported for the population standards.20 For the lower incisors there is no difference between this sample and the Chinese norm,20 or the Caucasian Class II Division 1 sample.9 However compared to the population norms, the position of the lower incisors are unaffected in the Chinese Class II Division 1 sample but proclined in the Caucasian sample, since both the upper and lower incisors are significantly more proclined in Chinese than in Caucasians.18 In the Chinese Class II Division 1 sample the mandible is retrognathic but significantly less than in the Caucasian sample (Table 3).

The skull base angle is more closed in the Chinese class II sample than the Chinese norm.20 This may explain why the Chinese Class II sample has maxillary prognathism and less mandibular retrognathism than the Caucasian Class II sample (Table 3).9 However, the skull base angle for the Caucasian Class II sample is not given, so no direct comparison can be made.

The chin is flatter in the Chinese Class II sample than in the Chinese norm,22 the difference between S-N-B and S-N-Pg being 0.5° and 2.0°, respectively (Table 2). The chin prominence is less affected in the Caucasian Class II sample, the difference between S-N-B and S-N-Pg being 1.1° (Table 3).

The maxillary plane in relation to the anterior skull base (SN/MxP) is the same for all the groups (Tables 2 and 3), and the mandibular plane angles (SN/MnP and SN/MxP) are steeper in the Chinese Class II sample only (Tables 2 and 3). This indicates that the facial vertical proportions of the Chinese Class II sample are long anteriorly and short posteriorly, while in the Caucasian Class II sample the average vertical proportions seem to be close to normal (Table 3). 9,10

Conclusion

The craniofacial morphology of the Chinese Class II Division 1 sample differed significantly from the Chinese population norm and from Caucasians with Class II Division 1 malocclusion.

The authors would like to thank Professor Karlberg and Mr Wilfred Wong for setting up statistical analysis, Miss Sylvia Fong for collection of materials and Miss Sue Cheung for preparation of this manuscript. This research was supported by the Swire Scholarship of the University of Hong Kong 1995–1996.