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Scientific Section |
1 Department of Orthodontics, Eastman Dental Institute for Oral Health Care Sciences, University College London, 256 Gray's Inn Road, London WC1X 8LD, UK
2 Department of Orthodontics, Matsumoto Dental University, 1780 Hirooka-Gohbara, Shiojiri, Nagano 399–0781, Japan
3 Department of Orthodontics, Eastman Dental Institute for Oral Health Care Sciences, University College London, 256 Gray's Inn Road, London WC1X 8LD, UK
Abstract
Objective: To identify the craniofacial features of Japanese girls with Class II division 1 malocclusions.
Method: One hundred and ninety lateral cephalometric radiographs were analysed, and the subjects whose age ranged from 7 years 6 months to 15 years 10 months were divided into three groups by their dentition: middle mixed dentition, late mixed dentition, and early permanent dentition. The mean values of 5 linear and 16 angular cephalometric parameters were compared with established Japanese Class I control values.
Results: Japanese girls with Class II division 1 malocclusion had a significantly small S-N-B angle (p < 0•001), short mandibular ramus (p < 0•05–0•001), and a large mandibular plane angle (p < 0•05–0•001).
Conclusion: Japanese girls with Class II division 1 malocclusion had a high-angle facial pattern associated with the short mandibular ramus.
Key words: Angle Class II Malocclusion, Computer-aided Cephalogram, Japanese Adolescents, Skeletal Class II.
Introduction
Analysis of craniofacial structures using lateral cephalometric radiographs has been used for the prediction of growth, as well as diagnosis and treatment planning in orthodontics for many years. A Class II skeletal pattern with maxillary protrusion and mandibular retrusion, positionally and morphologically, is a frequent dentofacial abnormality in American and European whites (Haynes, 1970
; Proffit et al., 1998), Chinese (Lew et al., 1993), and Japanese (Susami et al., 1971; Kitai et al., 1990). Many studies have attempted to clarify the morphological features of skeletal Class II malocclusion, and most investigators have reported the presence of a retrognathic mandible, proclined upper incisors, and neutral positioned lower incisors in Caucasian (Drelich, 1948; Renfroe, 1948; Henry, 1957; Harris et al., 1972; Hitchcock, 1973; McNamara, 1981), Chinese (Lau and Hagg, 1999), and Japanese Class II patients (Miura et al., 1958; Kuwahara, 1968; Iwasawa et al., 1969,1980). However, investigations of the antero-posterior position of the maxilla and the size of the mandible in Class II subjects have not reported consistent results. Furthermore, the skeletal Class II pattern arises from not only horizontal, but also vertical discrepancies (Adams and Kerr, 1981), aided by the morphology of the cranial base (Bacon et al., 1992). The influence of these morphological features has not been fully evaluated for a Japanese population. As a result, the purpose of the present study was to further define the morphology of Japanese skeletal Class II malocclusion and compare these features to those of normal Japanese Class I data.
Material and Methods
One-hundred-and-ninety lateral cephalometric radiographs of Japanese girls with Class II division 1 malocclusion and who had no history of any orthodontic treatment were examined. All patients had an A–N–B angle > 5 degrees, an Angle's Class II molar relationship, and an increased overjet. The control data represented the cephalometric standard values of Class I Japanese children published by the Japanese Society of Paediatric Dentistry in 1995 (JSPD). The Class II sample were divided into three groups based on dental age:
The mean age of each group is shown in Table 1.
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). In this study, points Po and Or were not used since poor reproducibility has been reported previously (Cooke and Wei, 1991). From these, five linear and 16 angular measurements were compared with those of the Class I standards.
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). The results are presented in Table 2.
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Results
The coefficient of reliability for almost all cephalometric parameters satisfied the level of confidence (>0•90). However, two results, A–B/N–Pog angle and Occ.P/S–N angle, had a low coefficient of reliability (<0•90); these should be viewed with caution (Houston, 1983).
Comparison between the cephalometric measurements of the Class II division 1 Japanese girls and Class I controls are shown in Table 2
.
The results may be summarized as follows:
Cranial Base Relationships
Although the mean anterior cranial base length (S–N) tended to be shorter in subjects with Class II division 1 malocclusions, this was significantly different only at the early permanent dentition stage.
Maxillary Skeletal Relationship
The anteroposterior position of the maxilla evaluated by the S–N–A angle showed a significantly more protrusive maxilla in Class II division 1 female subjects compared with the control. According to the angle between the palatal and S–N plane, the maxilla was positioned approximately at the same vertical position in both the test and control groups.
Mandibular Skeletal Relationship
The anteroposterior position of the mandible was evaluated by the S–N–B and S–N–Pog angles. The mandible in the Class II division 1 group indicated a significant retrusive position. The vertical position of the mandible was evaluated by two linear parameters (N–Me, S–Me) and five angles (S–N/Ar–G, S–N/G–Me, y-axis, N–Ar/S–G, and N–Pog/G–Me). The anterior facial height (N–Me) showed no significant difference in the test and control groups, but S–Me in Class II division 1 group indicated a significant excessive vertical development. It was evident that the following angular measurements showed a significantly excessive vertical development in the Class II sample: S–N/G–Me angle, y-axis, N–Ar/S–Gn angle, and N–Pog/ G–Me angle. In contrast, the test and control subjects had a similar S–N/Ar–G angle. The mean length of the mandibular ramus (Ar–G) was significantly shorter in the Class II division 1 sample, but the mandibular body length (Go–Me) was not significantly different from the control, except for Group 3. Subjects with Class II malocclusion also had a similar gonial angle (Ar–G–Me angle), except for Group 1.
Intermaxillary Relationship
The anteroposterior relationship between the maxilla and mandible was evaluated by the A–N–B angle and the A–B/ N–Pog angle. All were significantly larger in the Class II subjects compared with the controls.
Dentoalveolar Relationship
These were similar in the Class II division 1 and Class I groups except at the early permanent dentition, where the lower incisors were more proclined. Otherwise, the inclination of both upper and lower incisors, and the occlusal plane inclination were similar in both groups.
Discussion
Our study revealed that Class II Division 1 subjects had on average an anteriorly positioned maxilla when compared to JSPD normal controls. When we considered the mandible our results showed a significant retrognathia in the Class II sample according to the S–N–B angle and a shorter mandibular ramus. This agrees with Menezes (1974), who noted that all mandibular dimensions, overall mandibular length, mandibular body length, and vertical ramus were significantly shorter in Class II division 1 subjects. Other investigators have also reported the presence of a short mandibular body length (Nelson and Higley, 1948; Craig, 1951; Henry, 1957). However, in these Caucasian studies, there was no significant difference in the mandibular ramus length between Class II and I. These data indicated that the short mandibular ramus is one of the distinctive features of Japanese female subjects with Class II division 1 malocclusion, and the short posterior facial height (Ar–G) in the present study is the cause of the dolichofacial pattern. Furthermore, the retrusive mandible may be explained by the short mandibular ramus, slightly short mandibular body, and the obtuse gonial angle associated with backward rotation of the mandible.
Conclusions
The characteristic features of Japanese Class II division 1 malocclusion are as follows:
Acknowledgments
The authors sincerely thank Dr Ryuzo Kanomi, private orthodontic practice, Himeji, Japan, for generously providing cephalometric radiographs used in this study.
Notes
E-mail: N.Ishii{at}eastman.ucl.ac.uk
References
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