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Asymmetry in the Condylar Long Axis and First Molar RotationDepartment of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University, 1-8 Yamadaoka, Suita, Osaka, 565-0871, Japan; Correspondence: * corresponding author, hidakao{at}dent.osaka-u.ac.jp
Asymmetric growth occurs frequently in the mandibulofacial region, but little attention has been given to asymmetry in the temporomandibular joint. The purpose of this study was to clarify the feature of asymmetry in the condylar long axis and its relation to upper first molar rotation. Records of 148 pre-orthodontic patients were used. The angle of the condylar long axis and that of the molar rotation were both larger on the left side than on the right side. Positive correlations were found between the corresponding bilateral measurements of condylar long axes and also between those of molar rotations, whereas no correlation was found between the condylar long axis and molar rotation. These findings were found in most subgroups classified by dental age, skeletal pattern, bite force balance, or gender. These results suggest that consistent left-right differences in the condylar long axis and first molar rotation are common.
Key Words: asymmetry • growth • condyle • molar • axis
A mild degree of facial asymmetry, which may be affected by handedness (Keles et al., 1997; Pirilä-Parkkinen et al., 2001), is common in humans (Ferrario et al., 1993, 1995; Haraguchi et al., 2002). However, fewer studies have been done on asymmetries in the temporomandibular joint than on asymmetries in other craniomandibular regions, and the findings of these studies are still controversial (Williamson and Wilson, 1976; Lew and Tay, 1993; Maxwell et al., 1995; Williamson et al., 1998). Furthermore, it seems unfortunate that measuring methods for the condylar long axis (CLA) angle are subjective in these studies. In this study, special attention was paid to reliability of the measurements of the CLA angle. The purposes of this study were as follows: in right-handed subjects, (1) to clarify the feature of asymmetry in upper first molar rotation and the CLA, (2) to investigate if there is any correlation between the two kinds of components, and (3) to examine the potential different aspects observed in subgroups divided by Hellman’s dental age, skeletal pattern, bite force balance, or gender.
Subjects The records of 148 right-handed consecutive patients (63 males and 85 females) who received orthodontic treatment at a clinic were used in this study. The patients’ average age was 11.2 yrs, with a range of 6.5 to 17.2 yrs (Hellman’s dental age: IIIA-IVA). However, those with temporomandibular disorders (TMD) or with uni- or bilateral posterior crossbites were excluded. Informed consent was obtained from all the subjects in accordance with the guidelines of the Osaka University Dental Hospital.
Measurements
Condylar Long Axis Angle (CLA angle)
Molar Rotation Angle (MR angle) We took standardized occlusal photographs of the dental casts to measure the angle of molar rotation, which was the angle between the soft-tissue palatal midline and the line connecting the mesiobuccal and mesiolingual cusps of the molar (Fig. 1C  ). Each landmark was indicated by a marker on the upper dental casts for photography. The landmarks of the cusp tips were placed at or nearest the level tangential to the plane determined by the three most prominent cusp tips. The reference line for the measurement of rotation angles was changed from the soft-tissue palatal midline to the same reference line used in submentovertex measurements. This procedure was performed on the basis that the soft-tissue palatal midline is almost parallel to the median palatine suture (an absolute angular difference of 0.4 ± 0.3° in our preliminary study). We then re-calculated the molar rotation angle to ensure comparability between the measurements. The angle of molar rotation relative to the CLA on the ipsilateral side was defined as the relative MR angle (rMR angle).
Skeletal Pattern
The ANB angle and Frankfort mandibular plane angle (FMA) were measured for evaluation of the skeletal pattern anteroposteriorly and vertically, respectively. Subjects were divided into subgroups according to the ANB angle (Skeletal 3, ANB < 2°; Skeletal 1, 2°  ANB 6°; Skeletal 2, ANB > 6°) and FMA (Low FMA, FMA < 25°; Intermediate FMA, 25° FMA 30°; High FMA, FMA > 30°).
Bite Force Balancing Point
Statistical Analysis The error in measurements was estimated with the use of data obtained from repeated measurements.
No systematic errors were found. Overall, the mean and standard deviation of the random error was 0.6 ± 0.2° for cephalometric measurements and 0.6 ± 0.3° for dental cast measurements.
Left-Right Differences
The rMR angle was also larger on the left side (paired t test, p 0.0076; Table 1), except in the subgroups of Mn L-deviation, BF L-deviation, Skeletal 2, Skeletal 3, and Low FMA.
Correlations
Comparisons Between Subgroups The right condylar angulation tended to increase with dental age (ANOVA, p = 0.0567), and a marginally significant difference was found between IIIA and IVA (Scheffé, p = 0.0724). However, this tendency was not found in the left CLA. This would explain the disappearance of the lateral difference in older dental ages. Gender differences were found. The CLA angle on the right side was larger (paired t test, p = 0.0390) in females. The MR angle on the left side was larger (unpaired t test, p = 0.0219) in males. Furthermore, the rMR angle on both sides was larger (unpaired t test, p = 0.0398) in males. Notable differences were not found among other subgroups.
Most studies on condylar angulation have relied on visualized medial and lateral poles to identify the CLA. However, larger identification errors have been associated with the condylar poles (Lysell and Petersson, 1980; Williamson et al., 1998). The determination of the CLA, and thus the measurement of the CLA angle in the submentovertex projection, is therefore subject to error. Different methods have been used in a few studies (Williamson et al., 1998; Janson et al., 2001) to reduce landmark identification errors to an acceptable level of accuracy. Specifically, the medial condylar point has been defined as the tangential point to the medial condylar border of a line drawn parallel to the mandibular body line, and the lateral condylar point has been determined in the same manner (Janson et al., 2001). This method is similar to the one used in the present study, in that both methods used mandibular lines for determining the CLA. However, the definition of the mandibular body line was not clearly described in that study. In this study, the CLA was defined as the longest line after the preliminary CLA line was rotated. Irregularity in condylar shapes in the axial view could affect the CLA. However, in the present study, condylar shapes seemed to have much less variation than previously reported (Oberg et al., 1971; Krenkel and Grunert, 1989), probably because the subjects were younger in this study. Pre-orthodontic patients with uni- or bilateral posterior crossbites were excluded from this study, because one of the aims of this study was to clarify the features of normally occurring asymmetry in the orofacial region. In unilateral malocclusions, it has been reported that muscular function and craniofacial or temporomandibular structures can have significant degrees of asymmetry (Pirttiniemi et al., 1990; Mimura and Deguchi, 1994; Hesse et al., 1997). Despite the nonexistence of such patients in the study sample, the results of this study indicated a remarkable consistency in the left-right difference of measurements. This does not seem to be extraordinary, because asymmetric growth in the mandibulofacial region occurs quite frequently (Pirttiniemi, 1994). It is of interest to note that mandibular asymmetry is an age- and sex-dependent phenomenon (Melnik, 1992; Huggare and Houghton, 1995). Further, it has also been recently found that such left-right differences are more noticeable in the mandible than in the maxilla (Severt and Proffit, 1997; Haraguchi et al., 2002). In this study, the CLA angle was found to be greater on the left side (3.1°). This was in agreement with previous findings (Williamson et al., 1998), where the mean left angular values were greater by 0.9°. However, earlier findings indicated no significant difference in condylar angulation between left and right sides (Williamson and Wilson, 1976; Lew and Tay, 1993). These conflicting results seem to be a consequence of the different methods (measurement accuracy, sample size) and/or the different population characteristics. For instance, racial differences have been found in body asymmetries (Huggare and Houghton, 1995). The etiology of the consistent left-right difference should be inspected from both genetic and environmental points of view. As for environmental factors, a functional asymmetry in the masticatory system seems most likely to be involved (Vig and Hewitt, 1974). Facial structures have been shown to be strongly dependent on muscular balance (Kiliaridis et al., 1996; Ciochon et al., 1997). However, in this study, bite force balance, which is a functional indicator of the masticatory system, was not associated with left-right differences of CLA and MR angles. Moreover, the remarkable consistency in left-right differences was found, regardless of classification by several factors. These findings suggest the involvement of a genetic pattern of laterality, which may be supported by recent findings in molecular genetics (Collignon et al., 1996; Meno et al., 1996). Lateral differences in the CLA angle were not significant in some subgroups. The disappearance of lateral differences in the subgroups of dental age IIIC and IVA could be ascribed to a lateral difference in changes of CLA angles during that period. Although the result was obtained from cross-sectional data, it is believed that the CLA on the right side tends to rotate distally with dental age, whereas this tendency was not found in the left CLA, indicating lateral growth asymmetry. The condyle is the major growth center of the mandible (Enlow, 1990) where bone formation and resorption have been observed (Thilander et al., 1976), and CLA can vary. Assuming that the change in the CLA is related to the growth of the mandible, the time course of mandibular condylar growth may differ between sides, i.e., a more persistent growth on the right side may explain the CLA angle increasing on the right side with dental age and approaching the CLA angle on the left side. In this case, the amount of residual growth at the condyle would be greater on the right side, leading to chin deviation toward the left side at the end of the growth period. This speculation fits the findings that the mandible tends to deviate toward the left side (Severt and Proffit, 1997; Haraguchi et al., 2002). Positive correlations were found between each bilateral homologous pair of CLA, MR, and rMR angles. The positive correlations imply that consistent lateral differences could not have been caused by a geometric arrangement of the midsagittal reference line or by a functional displacement of the mandible. This is because both would show negative correlations between corresponding bilateral measurements, leading to an increase on one side and a decrease on the other. Hence, it is reasonable to state that the consistent asymmetry observed in this study could have been inherent. The positive correlations for CLA angle agree well with reports from a previous study (Maxwell et al., 1995) in which the correlation coefficients ranged from 0.40 to 0.90, but did not agree with reports from an earlier study (Danforth et al., 1991) in which no statistically significant correlations were found. A plausible explanation for these differences is that each investigator used different methods. In the study showing no correlations, adult dry skulls were used, and CLA was determined visually. In this study, no significant correlation was found between CLA angle and its ipsilateral/contralateral MR angle. This finding was somewhat unexpected and surprising, because there seems to be a functional linkage between these two angles and, moreover, because these two angles showed very similar lateral asymmetries.
The research was partly supported by grants from the Japan Society for the Promotion of Science (No. 12470459). This paper is based on a thesis submitted to the graduate faculty, Osaka University, in partial fulfillment of the requirements for the PhD degree. Received for publication August 5, 2002. Revision received July 18, 2003. Accepted for publication November 4, 2003.
Journal of Dental Research, Vol. 83, No. 2,
109-114 (2004)
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