Malocclusion is an increasing phenomenon in contemporary populations. A large number of malocclusions may result from a combination of dental and skeletal disharmonies. However, they mostly occur due to insufficient supporting bone material (dental arch size) to accommodate the ideal arrangement of teeth (teeth size), creating tooth-size arch-size discrepancies (1, 2). The etiology of malocclusion is very complex. Crown dimensions are considered to contribute to tooth-size arch-size discrepancies and are positively correlated with crowding. The tooth-size arch-size discrepancies and a relative tooth size play an important role among a large number of etiological factors (3-5).
The development of normal occlusion requires a balanced and proportional dental and craniofacial growth. Different environmental and genetic stressors can cause deviations from the typical developmental trajectory and occurrence of malocclusion. In general, malocclusion is a heterogeneous entity caused by multifactorial etiology in which both genetic and environmental factors play an important role. The interaction of genetic and environmental factors is responsible for the variability in expression of malocclusion and a wide spectrum of clinical pictures in affected individuals. Severe forms of malocclusion lead to distorted appearance, impaired masticatory function, and decreased quality of life (1, 6, 7). Better understanding of the underlying etiological mechanism of malocclusion is important for the progress in prevention and treatment of orthodontic anomalies.
Numerous reports have found an association between the dental arch size and malocclusions. There are different aspects of tooth size and malocclusion. In general, secular trends toward increasing tooth size and insufficient supporting bone to accommodate teeth have been observed in recent populations (8).
The asymmetry in tooth size and dental arch asymmetry were recognized as important contributing factors to the etiology of malocclusion (9-11). Different developmental disturbances lead to the emergency of different forms of asymmetry such as directional (DA), fluctuating (FA) or antisymmetry (AS) (12-16). Some asymmetries are subtle and require the use of very precise methods for their detection. It particularly relates to the comparison of paired structures when left-right asymmetries have to be quantified and presented as either directional (DA) or fluctuating asymmetry (FA). Fluctuating asymmetry of bilaterally symmetric structures is always taken as an indicator of developmental instability. Different stressors can diminish the stability of developmental process and interfere with expected developmental path. When adaptive capability of an organism fails to buffer the effects of disturbing stressors, the developmental processes will result in increased deviation from perfect development (15-19).
It is considered that symmetrical development of dental and craniofacial structures means balance and homeostasis with good chances for development of good occlusion. Increased deviations from symmetry are the signs of developmental instability which increases the chance for development of malocclusion (4, 10, 11, 19-22). This observation can have clinical implication since an increased asymmetry could contribute to the development of malocclusion.
FA is considered to be a potential measure of the degree of stress experience by an individual, as well as a reflection of the genotype's ability to compensate for that stress. Developmental instability that results in asymmetry can be caused by decreased genetic control over developmental processes. Asymmetry of dental and dental arch structures means deviation from harmonious development and can also contribute to the development of malocclusion. Some individuals can display higher degrees of genetic susceptibility to different environmental stressors, which will manifest as an elevated level of fluctuating asymmetry in some craniofacial structures. Some variables may display different levels of sensitivity to environmental influences.
The aim of this study was to assess the fluctuating asymmetry of dental arch dimensions in orthodontic patients. The objective was also to evaluate the extent and nature of fluctuating dentoalveolar asymmetry in orthodontic patients with Class I, II, and III malocclusions.
Materials and methods
The samples comprised randomly selected plaster dental casts of 131 patients (62 males and 69 females) from the Department of Orthodontics, School of Dental Medicine, University of Zagreb, Croatia. The distribution of subjects according to sex and malocclusion group is shown in Table 1. The mean age of subject ranged from 14.9±2.1 year for Class I, 14.2±1.4 for Class II, and 17.8±2.9 for Class III. Dental models were scanned and digitized using ATOS II SO ("small objects") scanning technology (GoM mbh, Braunschweig, Germany) according to the method described by Šlaj (23, 24). 3D virtual models were created, scanned and digitized using ATOS ATOS viewer version 6.A.2 software.
(N = 131)
(N = 62)
(N = 69)
|χ2 - test||χ2 = 2.496||df = 2||P = 0.287|
Legend: N – sample size; n – number of subjects with malocclusion
Measurements of dental arch dimensions were taken from virtual three-dimensional dental models (Figure 1). The palatal symmetry axis was obtained by connecting the incisive papilla with most visible posterior landmark over the median palatal raphe. The mandibular midline was obtained as a projection of the maxillary midline to the mandibular model using the anterior and posterior reference points. The models were placed into occlusion (Figure 2) and the midpalatal axis from maxillary arch was transferred onto the mandibular model to determine the mandibular midline. Dental arch widths and depths were measured according to the method described by Cassidy et al. (21) (Figure 3). The median palatal raphe was the reference point for transverse measurements. Dental arch widths were calculated as a distance from the landmarks on each tooth type orthogonal to midpalatal raphe (Figure 3A). Arch depths were measured parallel with the mid palatal raphe. Five depths were defined to quantify various segments of the dental arch and the whole arch (Figure 3B). Total weighted asymmetry (TWA) was calculated using equation suggested by Palmer and Strobeck (14). Total weighted asymmetry (TWA) of dental arch width and dental arch depth was analyzed as a composite measure of total fluctuating asymmetry of dental arches. The asymmetry was calculated for each individual based on the differences between the antimeric teeth according to the following equation:
Therefore, the TWA is the sum of absolute weighted asymmetries for all dental arch measurements in each individual. It was pointed out that such composite measures of asymmetry may be a more effective means of assessing developmental instability than the traditional approach of analysis of single variables (4, 10, 12, 14, 19, 25). The analysis of variance (ANOVA) was used to compare differences between the groups.
The comparison of total weighted asymmetry of the widths of dental arches (TWW) is presented in Table 2. The levels of fluctuating asymmetry were found to be significantly higher in Class III than in Class I and Class II malocclusion. Fluctuating asymmetry in mandible for all types of malocclusion was considerably higher than in maxilla in both sexes (Figure 4).
Legend: N – sample size; M – mean of FA; sd – standard deviation
Table 3 shows differences in asymmetry of dental arch depths between malocclusion groups. It was found that there was no significant difference in maxilla, but males displayed greater asymmetry than females. Male subjects’ mandibles showed higher TWD in Class I, while females displayed significantly higher asymmetry in Class III than in Class I and Class II. Fluctuating asymmetry was higher for Class III in both jaws (Figure 5).
Legend: N – sample size; M – mean of FA; sd – standard deviation
All measurements in the present study were obtained on three dimensional virtual models. Some previous studies showed that measurements obtained on 3D models can be considered reliable and comparable to those obtained with digital calipers in conventional way. Both methods showed a high degree of concordance and reproducibility (24, 26, 27).
In this study, a composite measure of total weighted dental (fluctuating) asymmetry (TWA) was calculated as the sum of asymmetries for particular measurements in each individual. According to Palmer and Strobeck (14) such composite measures of asymmetry are much more effective for assessing developmental instability than measures of fluctuating asymmetry for individual variables. The results of this study did not show significant fluctuating asymmetry for dental arch variables. We have observed significant differences in magnitude of fluctuating asymmetry between malocclusion groups and between the upper and lower jaws.
Some previous studies showed higher asymmetry of maxillary than mandibular teeth (28-31). Harris and Nweeia (28) found significantly higher scores of asymmetry in females than males regarding tooth size. Maxillary teeth were more asymmetric in MD dimensions than mandibular teeth. Harris and Nweeia (28) observed that the pattern of asymmetry corresponds closely with morphogenetic fields of teeth pointing to the importance of genetic and ontogenetic patterns in human dentition. They also observed higher FA in more distal teeth (premolars and molars) (28).
The fluctuating asymmetry of dental arches showed higher asymmetry values in the mandible than in the maxilla. Total weighted asymmetry for dental arch widths (TWW) was much greater in the mandible than in the maxilla in all malocclusion groups. The values of TWW were the highest for subjects with Class III malocclusion. The anteroposterior degree or asymmetry of maxillary arch depths was greater in males than in females. TWA scores for dental arch depths in mandible were the greatest for Class III malocclusion in females. The results imply that the lower jaw is more sensitive to both environmental and genetic stress. The upper jaw appears to be better buffered and displays a lower amount of asymmetry.
Kaur et al. (32) studied the total weighted asymmetry and observed significant correlations with transverse maxillary dimensions. Cases with increased TWDA had increased crowding, arch form asymmetry and transverse deviations in dental arches. Cases with increased TWDA displayed increased crowding and arch form asymmetry due to developmental instability.
Asymmetries in dental occlusion may reflect disturbances in genetic control of development and/or influence of environmental factors (33). The degree of asymmetry can reflect the degree of genetic canalization of dentoalveolar development. Scanavini et al. (34) found higher asymmetry in dental arch dimensions in the mandible than in the maxilla. Similar findings were obtained in some other studies (35, 36). Dental arch measurements are influenced by heredity and environment but it seems that hereditary contribution plays much greater role. The results of some studies show high levels of genetic control for transverse arch measurements (dental arch widths) but considerable postnatal influences of environment was also established (21, 33). Cassidy et al. (21) stated that dental arch size has 50% of genetic component. The highest heritability estimates, about 60% on average, were obtained for dental arch widths. The size of dental arches shows considerable variations within different types of malocclusion. The influence of environmental factors on dental arch variables is also significant. Dental arches change after teeth emergence. The movement due to muscular pressures and oral habits contribute to the variations in size and shape of dental arches (21).
Schaefer et al. (22) observed differences in the magnitude of fluctuating asymmetry for dental arches between the upper and lower jaw. The upper jaw displayed higher FA due to higher sensitivity to developmental disturbances than the lower jaw. Schaefer et al. (22) concluded that fluctuating asymmetry increased in both jaws with environmental stress. However, genetic stress additionally increases FA in the lower jaw.
The observation in this study that dental arch variables in Class III display greater FA suggests an association with greater stress (genetic and/or environmental) during early dentoalveolar development. Increased genetic susceptibility to environmental stress can lead to increased developmental instability and elevated levels of FA in various structures such as dental arch dimensions. Livshits and Kobyliansky (18) stated that some genetic components could make an individual become more susceptible to the pressure of asymmetry in various structures. Besides, they stated that external factors could influence the degree to which each structural asymmetry is manifested.
According to Garn and coworkers (20) asymmetries may be a major contributing factor to malocclusion. Significant asymmetry means imbalance. More balanced and more symmetric patients have a greater likelihood for good occlusion. Patients with an increased fluctuating asymmetry tend to have more dental crowding and more severe malocclusion.
Sprowls et al. (4) obtained positive correlation of the TWDA with the positional dental asymmetries. They believed that increased arch form asymmetry may be associated with dental crowding and increased developmental instability. Individuals with greater asymmetry displayed more severe malocclusion due to developmental instability and higher effect of environmental perturbation. They observed an increase in dental crowding in cases with increased dental fluctuating asymmetry. Sprowls et al. (4) stated that establishing the degree of fluctuating or directional asymmetry in orthodontic patients is equally important as establishing Bolton discrepancies.
Schaefer et al. (22) observed that significant directional asymmetry co-occurred with fluctuating asymmetry in circumstances of increased stress. Therefore, they stated that both FA and DA could be indicators of developmental instability (15, 16, 22). Both types of asymmetries are dynamically inter-related because there is possibility of transition from DA to FA (15, 22). There is evidence to support the findings of associations of DA and FA in facial asymmetry with specific genes (37-39).
Weaver et al. (39) performed the study of gene association with dentoalveolar phenotypes in subjects with malocclusions. They found strong associations of the BMP3, Lats1, and SATB2 genes with fluctuating asymmetry of dental arches. The BMP3 gene was found to be associated with left to right patterning in mammalian development. This gene was found to be important for development of mandibular prognathism. This finding partly helps to explain considerably greater values fluctuating asymmetry in subjects with mandibular prognathism compared to other malocclusion groups. Further research is needed to compare both FA and DA for the same dentoalveolar variables in both jaws.
The TWA values were low but they differed significantly between the groups of malocclusion. Composite measures of fluctuating asymmetry (TWA) for dental and dental arch variables were the highest in Class III, and lowest in Class I malocclusion. Regarding the inter-arch differences, the teeth in the maxilla were more asymmetrical than the teeth in the mandible. Dental arch asymmetry was considerably greater in the mandible than in the maxilla in all malocclusion groups. Males displayed a higher degree of asymmetry than females. The highest fluctuating asymmetry in Class III malocclusion points to the fact that patients with Class III malocclusion experienced the highest level of genetic and environmental stress during early development.