Statistical comparison of the groups

Statistical comparison of the groups is shown in Table 4.

TABLE 4.  t-Test Comparison of the Measurements^a

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Angular measurements

Among the angular measurements, only two of 13 variables (lower incisor to NB and interincisal angles) exhibited statistically significant differences between the groups at P < .05 level. Patients with crowding had smaller values of lower incisor to NB angle but larger values of the interincisal angle. The other dentofacial angular measurements did not show any significant difference.

Linear measurements

Among the linear measurements, six of 12 variables exhibited statistically significant differences between the groups. Overjet, overbite, and WITS appraisal values were significantly greater in the crowded group. In addition, maxillary and mandibular skeletal lengths and mandibular dental measurements were significantly smaller in the crowded group than in the noncrowded group.

Correlations of the measurements

Correlations of the measurements are shown in Table 5. Significant inverse correlations were found between crowding and SNB (r = .246; P < .05), lower incisor to NB angle (r = .339; P < .01), anterior cranial length (r = .233; P < .05), mandibular length (r = .300; P < .01), maxillary length (r = .280; P < .05), and mandibular dental measurements (r = .283; P < .05). The interincisal angle (r = .310; P < .01), overjet (r = .253; P < .05), overbite (r = .222; P < .05), and FMIA (r = .301; P < .01) were directly correlated with crowding.

TABLE 5.  Pearson Correlation Coefficients of the Measurements^a

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Discriminant analysis

Discriminant analysis was performed using eight variables that exhibit significant difference between groups. Table 6 represents Fisher's linear discriminant functions coefficients of the variables and constants. According to the results using the abovementioned eight variables, 81.7% of the original grouped cases were correctly classified (Table 7).

TABLE 6.  Fisher's Linear Discriminant Functions Coefficients^a

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TABLE 7.  Classification Results of Discriminant Analysis^a

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Incisor inclination

Because orthodontics makes it possible to alter the dentoalveolar tooth position, it is very important to determine whether incisor position and inclination contribute to crowding. Retrusion of lower incisors has been found to be significantly correlated with mandibular anterior crowding. Hunter21 reported that the inclination of lower incisors relative to the mandibular plane was significantly greater in the spaced cases at nine years. Sanin and Savara1 also found in their longitudinal study that children who presented no crowding in the permanent dentition had more labially inclined mandibular incisors in the mixed dentition. On the other hand, Bishara et al11 stated that the uprighting of the incisors could not, by itself, be a determinant of the severity of the anterior or total change in arch length discrepancy.

Our results revealed that inclination of the lower incisor is associated with mandibular anterior crowding in the mixed dentition stage. All measurements related to lower incisors were smaller in the crowded group. Among these, statistically significant differences were noted in lower incisor to NB angle, interincisal angle, and mandibular dental measurements.

It is well known that retrusion of teeth results in available arch loss. Therefore, oral habits like thumb or lip sucking must be avoided in the mixed dentition stage. These habits often cause retrusion of lower and protrusion of upper incisors and result in mandibular anterior crowding.

The interincisal angle is very important in controlling continuous alveolar eruption of incisors. An increased interincisal angle is often associated with increased overbite. Lingual inclination of the upper incisors forces lower incisors backward and causes crowding. Axial inclinations of the incisors must be proper so as to function as an occlusal stop for continuous alveolar eruption.

Overjet and overbite

According to our results, both overjet and overbite measurements were significantly greater in the crowded group. The lower incisor inclination was strongly related with both overjet and overbite. Retrusion of the lower incisors may be associated with an increase in overjet and overbite. Therefore, it was an expected result to find a difference in overjet and overbite between the crowded and noncrowded group.

Cranial base dimensions

Melo et al14 compared cephalometric measurements of patients with and without crowding and could not find any difference between groups except in the S-SE (anterior cranial base) variable. The normal group showed a tendency for larger values in the measurements of the anterior cranial base when compared with the crowded group. They also assumed that the subjects in the normal group had longer anterior cranial base lengths, whereas subjects in the crowded group had longer posterior cranial base lengths. We also found a significant inverse correlation between crowding and anterior cranial base length, but the relationship was not strong enough to differentiate the crowded group from the normal group. According to these results, we could assume that anterior cranial base dimensions have little effect on crowding.

Maxillary and mandibular relationship

Among the measurements of maxillary and mandibular relationship, the only significant difference was determined in the WITS appraisal. WITS values were significantly greater in the crowded group, indicating a tendency toward a Class II jaw dysplasia. Although the difference was not significant, the mean ANB angle was also greater in the crowded group. Thus, we can state that mandibular anterior crowding is more likely to occur in retrognathic cases.

Maxillary dimensions

Melo et al14 evaluated indicators of crowding found in the primary dentition, which may lead to future mandibular anterior crowding in the mixed dentition stage. They found maxillary and mandibular dental arch lengths were possible indicators for crowding in the early mixed dentition. In our study, maxillary skeletal lengths (Co-A) of the crowded group were significantly smaller. The SNA angle was also smaller in the crowded group, but the difference was statistically nonsignificant. Thus, we concluded that bimaxillary and bialveolar retrusion cases are more likely to experience mandibular anterior crowding than bimaxillary and bialveolar protrusion cases.

Mandible dimensions

Berg10 compared cephalometric variables of patients with and without crowding and found that variables related to lower jaw dimensions (Ar-Po, SNB) had significantly smaller values in the crowded group. Several other reports have also indicate that crowding occurred more frequently in less prognathic cases.4822

In our study, we used mandibular skeletal length (Co-Gn) and SNB to compare the sagittal dimensions of the mandible in cases with and without crowding. The SNB angle was greater in the crowded group, but this difference was statistically nonsignificant. Mandibular length (Co-Gn) was significantly smaller in the crowded group. Our results support the results of Leighton and Hunter8 and Sakuda et al17 who found shorter mandibular body lengths in cases with crowding.

Growth direction of the mandible

Leighton and Hunter8 reported that the angles between the S-N line and the mandibular and occlusal planes were significantly larger for the cases with crowding at both mixed and permanent dentition stages. They also associated downward and deficient growth of the mandible with both upright or retroclined mandibular incisors and with crowding. On the other hand, Lundström22 found no correlation between arch dimension changes, changes in the incisor position, and direction of mandibular growth. Our results differed from that of Leighton and Hunter8 and did not exhibit any significant difference in measurements regarding the growth direction of the mandible (Table 4). Mean values of the occlusal plane and gonion-gnathion to SN line, Y axis, and FMA angles were all similar in crowded and noncrowded groups. Therefore, we can conclude that although both forward and backward rotating underlying patterns may have a considerable effect on the environment for the alignment of the teeth, this study failed to show an association between the growth direction of the mandible and the degree of mandibular anterior crowding at the mixed dentition stage.

Anterior and posterior facial heights

Leighton and Hunter8 reported shorter posterior facial heights for cases with crowding. In our sample we used posterior/anterior face height ratio and found no significant difference between the groups. Therefore, our results did not confirm Leighton and Hunter's8 findings.

Correlation with crowding

Conflicting results existed in the literature evaluating the correlation between crowding and dentofacial dimensions. Berg10 reported significant correlation between crowding and overjet, Ar-ii, Ar-b, and Go-Ar measurements. However, Miethke and Behm-Menthel15 reported that no correlation existed between crowding and vertical skeletal dimensions and lower incisor position.

In our sample, we found significant inverse correlation between crowding and SNB, lower incisor to NB angle, anterior cranial length, mandibular length, maxillary length, and mandibular dental measurements. We found direct correlations between crowding and interincisal angle, overjet, overbite, and FMIA. Although, correlation coefficients are not high enough to have clinical importance, any of the abovementioned dentofacial characteristics, whether alone or combination with other factors can be associated with the development of incisor crowding.