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Asymmetry in the Condylar Long Axis and First Molar Rotation

Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University, 1-8 Yamadaoka, Suita, Osaka, 565-0871, Japan;

^* corresponding author, hidakao{at}dent.osaka-u.ac.jp

	ABSTRACT

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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

	INTRODUCTION

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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.

	MATERIALS & METHODS

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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
To eliminate inter-operator error and to ensure standardization, one experienced operator took all of the measurements (Lysell and Petersson, 1980). All measuring procedures were performed three times, and three determinations were made for each measurement. Each measurement used in the final analysis was obtained from an average of the three determinations, to reduce measurement error.

Condylar Long Axis Angle (CLA angle)
The submentovertex radiographs were taken by means of an AxialTome60i System (AxialTome Corporation, San Carlos, CA, USA). During exposure, the subjects kept their teeth in centric occlusion under light pressure. The cephalometric tracings were done on acetate paper by a single investigator. The CLA angle was defined as the angle between the CLA and the reference line on the tracing (see Figs. 1A, 1B).

View larger version (58K): [in this window] [in a new window]   Figure 1. Definitions of condylar long axis (CLA) angle and molar rotation (MR) angle. (A) Line (1), which passes through the mediolateral center of the posterior mandible, was constructed by the least-squares method. The preliminary CLA, Line (2), is perpendicular to Line (1) and at a height where the condyle is at its widest. The CLA, Line (3), was obtained by rotating Line (2) within 10° in a clockwise or counterclockwise direction about the point where it intersects Line (1) and by fixing it at the position where the condyle is at its widest. (B) Line (4), the transspinosum axis; Line (5), the interspinosum axis, which was drawn perpendicular to the transspinosum axis from its midpoint as the reference line; Line (6), the median palatine suture. The CLA angle was defined as the angle between Line (3) and Line (5). (C) The angle was measured between the soft-tissue palatal midline and the line connecting the mesiobuccal and mesiolingual cusps of the molar. The reference line for measuring rotation angles was changed from the soft-tissue palatal midline to the same reference line used in the submentovertex measurements, Line (5), and then the molar rotation angle was recalculated.

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 facial midline was defined as a line perpendicular to the line connecting bilateral latero-orbitale points (Lo) through the neck of the crista galli (Nc) (Fig. 2). Menton (Me) deviation more than 2 mm from the facial midline was assumed to be asymmetric and was designated as either mandibular left deviation (Mn L-deviation) or right deviation (Mn R-deviation).

View larger version (25K): [in this window] [in a new window]   Figure 2. Evaluation of skeletal pattern. (A) Lateral. Lo, latero-orbitale points; Nc, the neck of the crista galli; Me, the menton. (B) Anteroposterior/vertical. The ANB angle was an indicator of the anteroposterior skeletal pattern, and the Frankfort mandibular plane angle (FMA), an indicator of the vertical skeletal pattern.

Bite Force Balancing Point
The balancing point was determined by the use of a bite force measuring system (Dental Occlusion Pressuregraph FPD-703, Fuji Film Co., Tokyo, Japan). The system is described elsewhere (Hidaka et al., 1999). A pressure-sensitive sheet (30H Type-R) was placed between the upper and lower dental arches, and the subject bit the sheet firmly. Deviation greater than 1 mm from the median palatine suture was designated as left deviation of the bite force (BF L-deviation) or right deviation of the bite force (BF R-deviation).

Statistical Analysis
Normality of variance was tested with a Chi-square test, the homogeneity of variance with either a Bartlett test (among three groups or more) or an F-test (between two groups), and interaction effects with ANOVA. A Friedman ANOVA was used to examine differences among multiple groups. A value of p < 0.05 was considered as statistically significant.

The error in measurements was estimated with the use of data obtained from repeated measurements.

	RESULTS

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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
As a whole, the MR, CLA, and rMR angles were larger, i.e., there was more distal rotation (p 0.0001), on the left side than on the right side (Table 1). A lateral difference in the MR angles was observed regardless of dental age, skeletal pattern, bite force balance, or gender. Similarly, there was a lateral difference in the CLA angles (paired t test, p 0.035), except in the subgroups of dental age IIIC, IVA, and Skeletal 2.

Correlations
Positive correlations were found between measurements of corresponding bilateral CLA, MR, and rMR angles (Table 2). In all subgroups, positive correlations were still found. However, positive correlations were not significant in Skeletal 2, Skeletal 3, or Low FMA subgroups for any of the measurements, nor in the subgroups of dental age IVA for CLA angles. On the other hand, no correlation was found between the CLA angle and its ipsilateral or contralateral MR angle.

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.

	DISCUSSION

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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.

	ACKNOWLEDGMENTS

 
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 August 5, 2002; Last revision July 18, 2003; Accepted November 4, 2003

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