Editorial Type:
Article Category: Research Article
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Online Publication Date: 03 Feb 2016

Morphological relationship analysis of impacted maxillary canines and the adjacent teeth on 3-dimensional reconstructed CT images

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DOI: 10.2319/071516-554.1
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ABSTRACT

Objective:

To examine whether there is a relationship between maxillary canine impaction and the morphologic characteristics of the maxillary dentition, especially the root of the lateral incisor.

Materials and Methods:

In this study, we selected only patients with unilateral maxillary canine impaction to compare the morphologic characteristics of the dentition on the impaction side and the clinically normal eruption side. The sample size was decided to be 40 based on the pilot study. To minimize bias depending on sex and location of the maxillary canine impaction, we selected equal numbers (20) of boys and girls, and equal cases (20) of buccal impaction and palatal impaction. Under the aforementioned conditions, the mean age was 13.5 ± 2.3 years. The multislice spiral computed tomography images of these 40 subjects were converted into three-dimensional (3D) reconstructed images using the OnDemand 3D program (Cybermed Co, Seoul, Korea). Then we measured the morphologic characteristics of the individual teeth on the obtained 3D teeth images.

Results:

Length and volume of the maxillary lateral incisor's roots were significantly smaller on the impaction side compared with the normal eruption side (P = 0.001 and P = 0.006, respectively). The width and volume of the canine's crown were significantly greater on the impaction side compared with the normal eruption side (P = 0.020 and P < .0001, respectively).

Conclusion:

These results might help to prove the hypothesis that the smaller-sized lateral incisor roots and greater-sized canine crowns are the influential etiologic factors in maxillary canine impaction.

Keywords: Maxillary canine impaction; Lateral incisor morphology; Guidance theory; 3D reconstruction

INTRODUCTION

Impairments in tooth eruption, such as tooth impaction and ectopic eruption can occur in all 32 permanent teeth during the period of mixed dentition. The maxillary canines, with the exception of the third molars, are known to be particularly vulnerable to such problems.1–3 Thus, the prevalence of maxillary canine impaction reaches 1% to 5%.4–7

To date, multiple studies have investigated the factors involved in maxillary canine impaction. In general, maxillary canine impaction shows a female predilection; it occurs 2–3 times more often in females than in males.8–11 Moreover, it has been reported that maxillary canine impaction occurs at an incidence 3–6 times higher on the palatal side compared with the buccal side.12–14 However, contrary reports show that maxillary canine impaction occurs 2–3 times higher on the buccal side compared with the palatal side in East Asians. These reports suggest that the side of impaction varies depending on the ethnic population.10,15,16

Studies on maxillary canine impaction have been conducted to examine various causative factors, such as excessive or inadequate space for the eruption of permanent teeth. In 1983, Jacoby13 reported that 85% of palatal impactions have sufficient space for eruption and 83% of buccal impactions have insufficient space for eruption, suggesting that excessive or inadequate space for maxillary canine eruption is involved in the position of maxillary canine impaction. Follow-up studies have examined the relationship between maxillary canine impaction and space for eruption.17–19 Moreover, it is now well-established that morphologic anomalies of the maxillary lateral incisors are closely associated with maxillary canine impaction.9,20–23 Similarly, it has also been reported that there was a significant decrease in the mesiodistal width of the maxillary teeth in patients with maxillary canine impaction.24,25

Regrettably, no studies have clarified the underlying mechanisms involved in the impaction and eruption of the maxillary canines.2,26–28 Of the several hypotheses proposed, the most plausible is the guidance theory, which suggests that impairments occur in the normal eruption of the maxillary canine in patients where the guiding function of the maxillary lateral incisor's root is lost. This leads to speculation that the incidence of maxillary canine impaction would be relatively higher in patients with peg lateralis or missing maxillary lateral incisors. Supporting this theory, studies have been conducted demonstrating that maxillary canine impaction is associated with the shape of the lateral incisor and inadequate space for maxillary canine eruption. To date, however, no studies have been conducted to examine the relationship between the root of the maxillary lateral incisor and the eruption of the maxillary canine.

Recently, active studies have been conducted using three-dimensional (3D) reconstructed computed tomography (CT) images with a cone beam CT and a multislice spiral CT (MSCT) in the field of maxillary canine impaction. Most of these studies have focused on the characteristics of the impacted canine or the resorption of the adjacent teeth.29–32

The aim of this study was to analyze the morphologic characteristics of the impacted canines and the adjacent teeth by using the 3D reconstructed images. In addition, we tried to verify the relationship between the features of the teeth and maxillary canine impaction.

MATERIALS AND METHODS

Subjects

This study was approved by the Institutional Review Board at the Seoul National University School of Dentistry (No. S-D20120015). We conducted the current study in patients between 10 and 18 years old who underwent orthodontic examinations and were diagnosed with maxillary canine impaction at the Seoul National University Dental Hospital during a 5-year period ranging from July 2007 to June 2012. Of these, the patients who underwent CT examination for diagnostic purposes were enrolled in the current study. The exclusion criteria for this study were as follows: (1) patients with the presence of congenitally missing or malformed lateral incisors; (2) patients in whom the canine is distally impacted toward the first premolar; (3) patients with definitive obstructions (eg, odontoma or supernumerary teeth); (4) patients with systemic disease; (5) patients with craniofacial anomalies (eg, cleft lip or palate); (6) patients with several impacted teeth or congenitally missing teeth.

In this study, we performed a subgroup analysis to examine whether the morphologic characteristics of the adjacent teeth had a relationship with maxillary canine impaction; we selected only patients with unilateral canine impaction on the left or right. Furthermore, to compare the exact shape of the crown and root in every dentition, we excluded patients where the root of the incisor was resorbed by the impacted canine. Thus, we attempted to examine whether there were differences in the morphologic characteristics of the dentition between the impaction side and the clinically normal eruption side in the same patient.

We were able to identify 89 patients who satisfied the aforementioned criteria in our database.

3D Assessment

The MSCT images of these subjects were obtained using the SOMATOM Sensation 10 (Siemens AG, Erlangen, Germany) at a slice thickness of 0.75 mm. Then, we converted the CT DICOM (digital imaging and communication in medicine) images into 3D reconstructed images using the OnDemand 3D program (Cybermed Co, Seoul, Korea). Radiodensity was set between +1000 and +3000 on the Hounsfield unit (HU) scale so that all the hard tissue of a tooth could be expressed.

With these rare 3D reconstructed images, we acquired the 3D tooth images by eliminating the adjacent anatomic structures (Figure 1a). After removing the premolars and molars, we identified the maxillary central incisors, the maxillary lateral incisors, and the maxillary canines for this study (Figure 1b through g).

Figure 1. Reconstructed 3D MSCT DICOM images with the surrounding structures removed using the OnDemand 3D program (a). Maxillary right canine separated from the reconstructed 3D image (b). Maxillary right lateral incisor (c). Maxillary right central incisor (d). Maxillary left central incisor (e). Maxillary left lateral incisor (f). Maxillary left canine (g).Figure 1. Reconstructed 3D MSCT DICOM images with the surrounding structures removed using the OnDemand 3D program (a). Maxillary right canine separated from the reconstructed 3D image (b). Maxillary right lateral incisor (c). Maxillary right central incisor (d). Maxillary left central incisor (e). Maxillary left lateral incisor (f). Maxillary left canine (g).Figure 1. Reconstructed 3D MSCT DICOM images with the surrounding structures removed using the OnDemand 3D program (a). Maxillary right canine separated from the reconstructed 3D image (b). Maxillary right lateral incisor (c). Maxillary right central incisor (d). Maxillary left central incisor (e). Maxillary left lateral incisor (f). Maxillary left canine (g).
Figure 1. Reconstructed 3D MSCT DICOM images with the surrounding structures removed using the OnDemand 3D program (a). Maxillary right canine separated from the reconstructed 3D image (b). Maxillary right lateral incisor (c). Maxillary right central incisor (d). Maxillary left central incisor (e). Maxillary left lateral incisor (f). Maxillary left canine (g).

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

As shown in Figures 2 and 3, we measured the following five characteristics in each tooth: (1) mesiodistal width of the crown (mm); (2) anatomic height of the crown (mm); (3) volume of the crown (mm3); (4) length of the root (mm); and (5) volume of the root (mm3). Then we divided these data into two groups (the impaction-side group and the eruption-side group) in order to compare the data between the two groups.

Figure 2. Mesiodistal width of the crown (mm) measured from the widest point of the crown (a). Anatomic height of the crown (mm) measured from the lowest buccal CEJ point to the incisal tip in the lateral view image. Length of the root (mm) measured from the lowest buccal CEJ to the root apex in the lateral view image (b).Figure 2. Mesiodistal width of the crown (mm) measured from the widest point of the crown (a). Anatomic height of the crown (mm) measured from the lowest buccal CEJ point to the incisal tip in the lateral view image. Length of the root (mm) measured from the lowest buccal CEJ to the root apex in the lateral view image (b).Figure 2. Mesiodistal width of the crown (mm) measured from the widest point of the crown (a). Anatomic height of the crown (mm) measured from the lowest buccal CEJ point to the incisal tip in the lateral view image. Length of the root (mm) measured from the lowest buccal CEJ to the root apex in the lateral view image (b).
Figure 2. Mesiodistal width of the crown (mm) measured from the widest point of the crown (a). Anatomic height of the crown (mm) measured from the lowest buccal CEJ point to the incisal tip in the lateral view image. Length of the root (mm) measured from the lowest buccal CEJ to the root apex in the lateral view image (b).

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

Figure 3. The tooth was separated into the crown part and the root part using a line made by connecting the lowest buccal CEJ and lowest palatal CEJ (a). Then, the crown and root volume were measured using the OnDemand 3D program. This cross-sectional image of the root was obtained by separating the crown and root part (b).Figure 3. The tooth was separated into the crown part and the root part using a line made by connecting the lowest buccal CEJ and lowest palatal CEJ (a). Then, the crown and root volume were measured using the OnDemand 3D program. This cross-sectional image of the root was obtained by separating the crown and root part (b).Figure 3. The tooth was separated into the crown part and the root part using a line made by connecting the lowest buccal CEJ and lowest palatal CEJ (a). Then, the crown and root volume were measured using the OnDemand 3D program. This cross-sectional image of the root was obtained by separating the crown and root part (b).
Figure 3. The tooth was separated into the crown part and the root part using a line made by connecting the lowest buccal CEJ and lowest palatal CEJ (a). Then, the crown and root volume were measured using the OnDemand 3D program. This cross-sectional image of the root was obtained by separating the crown and root part (b).

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

Sample-Size Determination

The pilot study was carried out with 10 patients (5 with buccal impaction and 5 with palatal impaction) according to the same terms and methods. Three categories with expected differences in average value between the two groups were chosen: lateral incisor's length of the root, lateral incisor's volume of the root, and canine's volume of the crown.

The following equation is a method to calculate the sample size when comparing the mean value of two groups with equal sample numbers.

.

Three categories were applied to this equation, and the minimal sample size was 35.5, 38.6 and 29.6, respectively, for each category. Therefore, we decided that 40 should be the sample size in our study (n = 40).

In each group, we controlled the factors to minimize bias depending on the sex and location of the maxillary canine impaction. Accordingly, we had an equal number of boys and girls (n = 20), and an equal number of cases of buccal impaction and palatal impaction. Under all conditions mentioned earlier, we randomly selected 40 out of 89 patients. As a result, the mean age of the 40 subjects was 13.5 ± 2.3 years, and there were 17 impacted canines on the right and 23 impacted canines on the left.

Statistical Analysis

Since the variable distribution of this study does not satisfy normality, a parametric approach method could not be carried out. Therefore, we used the Wilcoxon signed rank test, which is a nonparametric approach method, in this study. Statistical analysis was performed using SPSS Windows version 12.0 software (SPSS Inc, Chicago, Ill). The level of statistical significance was set at 5%.

RESULTS

In this study, intraexaminer agreement was carried out in order to verify the reproducibility of the measurement of all sample values carried out by an examiner. Since the sample size was 40, total number of teeth measured was 240 (80 central incisors, 80 lateral incisors, and 80 canines). Out of 240 teeth, 30 randomly selected teeth (10 central incisors, 10 lateral incisors, and 10 canines) were remeasured. Then these values carried out two times were verified using intraclass correlation coefficients (ICCs) (Table 1). The ICC value shows that the intraexaminer reliability is satisfying in all categories.

Table 1. Intraexaminer Agreement Analysis Using Intraclass Correlation Coefficient (ICC)
Table 1.

As shown in Figure 4, there were no significant differences in all parameters of the morphologic characteristics of the maxillary central incisor between the two groups. There were significant differences in the length (9.77 ± 1.43 mm, 10.65 ± 1.12 mm) and volume (278.26 ± 51.39 mm3, 331.32 ± 96.04 mm3) of the root of the maxillary lateral incisor between the two groups (P = 0.001 and P = 0.006, respectively) (Figure 5). Furthermore, there were no significant differences in any parameters of the crown of the maxillary lateral incisor. In the morphologic characteristics of the maxillary canine, the width (8.03 ± 0.80 mm, 7.78 ± 0.61 mm) and volume (626.97 ± 149.24 mm3, 515.87 ± 136.05 mm3) of its crown showed a significant difference between the two groups (P = 0.020 and P < .0001, respectively) (Figure 6).

Figure 4. Statistical comparison of central incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test.Figure 4. Statistical comparison of central incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test.Figure 4. Statistical comparison of central incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test.
Figure 4. Statistical comparison of central incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test.

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

Figure 5. Statistical comparison of lateral incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).Figure 5. Statistical comparison of lateral incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).Figure 5. Statistical comparison of lateral incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).
Figure 5. Statistical comparison of lateral incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

Figure 6. Statistical comparison of canines' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).Figure 6. Statistical comparison of canines' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).Figure 6. Statistical comparison of canines' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).
Figure 6. Statistical comparison of canines' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).

Citation: The Angle Orthodontist 87, 4; 10.2319/071516-554.1

DISCUSSION

In this study, we examined the relationships between maxillary canine impaction and the morphologic characteristics of the maxillary central incisor, maxillary lateral incisor, and maxillary canine. There have been previous studies on this topic, but their results are of limited significance in that they analyzed the gross features of the crown based on the diagnostic casts from the impressions. However, in this study, we could examine the shape of the incisors' roots and the morphologic characteristics of the impacted canine using 3D reconstructed CT images.

First, there were no statistically significant differences in the measurements associated with the morphologic characteristics of the maxillary central incisor between the two groups (Figure 4). As we expected, the shape of the crown and root of the central incisor had no significant correlation with the eruption of the maxillary canine.

Previous studies have shown that anomalies of the maxillary lateral incisors had a significant correlation with maxillary canine impaction.9,20–23 Moreover, a significant decrease in the mesiodistal dimension of the maxillary teeth, including the maxillary incisors, has also been reported in patients with maxillary canine impaction.24,25 However, all of these studies were conducted to examine the crown of the teeth erupting into the oral cavity. Little is known about the precise mechanisms involved in the impaction and displacement of the maxillary canine.2,26–28 Of the mechanisms proposed, the guidance theory. which suggests that the root of the maxillary lateral incisor plays an important role in inducing the normal eruption of the maxillary canine, has been accepted as the most plausible explanation. According to this theory, the eruption of the maxillary canine would be deviated if the function of the root of the maxillary lateral incisor was impaired, indicating that the root of the maxillary lateral incisor plays a key role in the eruption of the maxillary canine.

In the current study, we mainly analyzed the differences in the shape of the crown and root of the maxillary lateral incisors between the two groups. Although previously unavailable, it is now possible to examine the shape of the root using 3D-reconstructed CT images. As shown in Figure 5, there were no significant differences in the parameters of the crown of the maxillary lateral incisor between the two groups. However, there were statistically significant differences in the length and volume of the root between the two groups. In other words, although there was no significant difference in the size of the crown of the maxillary lateral incisor between both sides in the same individuals, the root of the lateral incisor had a shorter length and smaller size on the impaction side compared with the normal eruption side.

Previous studies have presumed that the shape of the root would be smaller than that of the crown of the maxillary lateral incisor. However, we found differences in the length and size of the root between both the right and left sides in the same individuals. Although this study cannot prove the guidance theory, nor can it prove that smaller roots of the maxillary lateral incisor result in deviations in the eruption of the maxillary canine, our results are sufficient to suggest that individuals with smaller roots of the maxillary lateral incisor are vulnerable to impaired eruption of the canine.

As shown in Figure 6, our results showed that there was a significant difference in the width and volume of the crown of the maxillary canine between the two groups. In other words, our results showed that the size of the maxillary canine was greater on the impaction side compared with the normal eruption side. Although the result shows that there is a statistically significant difference in the canine width, it is difficult to assign great significance since the actual difference (0.25 mm) is smaller than the voxel size (0.75 mm). Nevertheless, it is possible to interpret such meaning since the canine crown volume, which is actually more important, showed significant difference. These results suggest that there is a possibility that normal eruption might be impaired due to insufficient space in patients with greater crowns of the maxillary canine, which is consistent with previous reports suggesting that maxillary canine impaction might arise from inadequate space for its eruption.2,13,19 In actual clinical cases, adequate space needs to be acquired before the orthodontic traction of an impacted maxillary canine is carried out. Using the aforementioned results, the practitioner needs to think about acquiring more space than the normally erupted canine's mesiodistal size on the opposite arch.

CONCLUSIONS

  • The length and volume of the maxillary lateral incisor's root were significantly smaller on the impaction side compared with the normal eruption side (P = 0.001 and P = 0.006, respectively). This indicates that there is a high correlation between maxillary canine impaction and smaller lateral incisor root sizes.

  • The width and volume of the crown were significantly greater on the impaction side compared with the normal eruption side (P = 0.020 and P < .0001, respectively). This indicates that there is a high correlation between maxillary canine impaction and greater crown sizes.

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Copyright: © 2017 by The EH Angle Education and Research Foundation, Inc.
<bold>Figure 1.</bold>
Figure 1.

Reconstructed 3D MSCT DICOM images with the surrounding structures removed using the OnDemand 3D program (a). Maxillary right canine separated from the reconstructed 3D image (b). Maxillary right lateral incisor (c). Maxillary right central incisor (d). Maxillary left central incisor (e). Maxillary left lateral incisor (f). Maxillary left canine (g).

<bold>Figure 2.</bold>
Figure 2.

Mesiodistal width of the crown (mm) measured from the widest point of the crown (a). Anatomic height of the crown (mm) measured from the lowest buccal CEJ point to the incisal tip in the lateral view image. Length of the root (mm) measured from the lowest buccal CEJ to the root apex in the lateral view image (b).

<bold>Figure 3.</bold>
Figure 3.

The tooth was separated into the crown part and the root part using a line made by connecting the lowest buccal CEJ and lowest palatal CEJ (a). Then, the crown and root volume were measured using the OnDemand 3D program. This cross-sectional image of the root was obtained by separating the crown and root part (b).

<bold>Figure 4.</bold>
Figure 4.

Statistical comparison of central incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test.

<bold>Figure 5.</bold>
Figure 5.

Statistical comparison of lateral incisors' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).

<bold>Figure 6.</bold>
Figure 6.

Statistical comparison of canines' morphologic features between the impaction side group and the eruption side group using a Wilcoxon signed rank test (*P < .05).

Contributor Notes

Corresponding author: Dr Ki-Taeg Jang, Department of Pediatric Dentistry & Dental Research Institute, School of Dentistry, Seoul National University, 275-1 Yongon-dong, Chongno-gu, Seoul 110 768, Korea (e-mail: jangkt@snu.ac.kr)
Received: 01 Jul 2016
Accepted: 01 Dec 2016
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