INTRODUCTION

In general, canines play an essential role in establishing a functional occlusion as well as serving as the foundation of an esthetic smile.¹ Aside from the third molars, maxillary canines are the most commonly impacted teeth.^2,3 Their reported prevalence in the literature ranges from 1% to 3% of the population.^4–6

The etiology of impacted canines is unclear and appears to be multifactorial. There are two main theories why canine impaction occurs: guidance and genetics. There are some possible guidance factors such as overretained primary canine, peg or missing lateral incisor, spaced dentition, abnormal tooth bud eruption, or abnormal eruption rate.⁷ Genetic factors are also present in many forms. Peck et al.⁵ reported that maxillary impacted canines are more common in females than in males. Baccetti⁸ noted a relationship between canine impaction and other dental anomalies. He also stated that transcription factors such as MSX1 and PAX9 may have a role in palatal canine displacement.⁹

Impacted canines may result in several complications such as arch length shortening, migration of adjacent teeth, esthetic implications, cystic formation, or canine ankylosis.¹⁰ Moreover, one of the most severe complications associated is root resorption of adjacent teeth. As a result, this affects the longevity of adjacent teeth. Therefore, early detection and prevention would decrease the need for canine exposure and simplify orthodontic treatment.

Two possible predictors of successful outcome are the mesiodistal location of the crown and angulation of the tooth.¹¹ If the canine angulation to the mid-sagittal plane was greater than 25° on a panoramic radiograph, the root resorption increased by 50%.¹² In addition, if the angle was more than 31°, there was less chance for the permanent canine to erupt after primary canine extraction.¹³ However, Warford et al.¹⁴ reported that angulation added little supplementary predictive value. Sector location of impacted canines on a panoramic radiograph could be helpful in predicting the treatment time.¹⁵ Furthermore, the more mesially located the canine crown, the less likelihood of eruption after primary canine extraction. In addition, it was found that if the tip of an unerupted canine did not pass more than half the root of the lateral incisor, the success rate was about 90%. However, if it did, then the success rate decreased to 64%.¹¹

Conventional two-dimensional radiographs are commonly used to assess impacted canines. Among them, panoramic radiographs are usually taken for an overview, initial diagnosis, canine localization, prediction of tooth eruption, treatment plan, and treatment outcome assessment. However, the diagnostic information obtained from the panoramic radiograph is limited because of many drawbacks, such as distortion, magnification, artifacts, blurred images, or structural superimposition.¹⁶ Thus, panoramic radiographs were judged not to be a reliable method in detecting root resorption, especially on the palatal or buccal aspect or in subjects with early or mild resorption.¹⁷ Even though the panoramic radiograph is a standard diagnostic tool in orthodontics, the risk of misinterpretation when using the panoramic radiograph is high.

In comparison to panoramic radiographs, cone-beam computed tomography (CBCT) has improved localization of impacted teeth, identifies relevant pathology, and has a high ability and reliability in detecting root resorption by eliminating overlap of dental structures. Furthermore, CBCT has lowered the cost and radiation dose compared with CT by reducing the cone shaped X-ray beam, size of the scanner, and scanning time.¹⁸ Moreover, Kau et al. stated that, in complicated orthodontic cases such as in canine impaction or cleft lip and palate, three-dimensional radiograph is mandatory and CBCT is the imaging of choice.¹⁹ On the other hand, ALARA principles and Sedentex CT guidelines stated that CBCT examination should not be used indiscriminately and should be applied in selected orthodontic cases in which conventional radiographs cannot provide sufficient diagnostic information.²⁰ Furthermore, some studies found that the treatment planning for impacted canines did not differ irrespective of whether a 2D or 3D radiograph was used.²¹

The intention of the present study was not to suggest that panoramic radiographs could be used to detect root resorption and replace CBCT. However, the aim was to provide information that could increase the detection probability of root resorption by clinicians through the application of a simpler tool and, in addition, help to judge the role of CBCT application. In general, the objective was to assess the mesiodistal position of maxillary impacted canines on panoramic radiographs and compare these with the labio-palatal position of maxillary canines and the possibility of root resorption of adjacent permanent teeth on CBCT.

MATERIALS AND METHODS

This was an institution review board–approved, retrospective radiographic/chart review study (RSRB00065125). The data were collected from July 1, 2015, to November 1, 2016, at the Orthodontic Clinic, University of Rochester Eastman Institute for Oral Health.

Regardless of race, gender, or ethnic origin, this study sample of 64 subjects was collected from a pool of 215,100 patients who had panoramic and cephalometric radiographs as well as clinical examinations performed at the Orthodontic Clinic. Eighty-eight maxillary canines were diagnosed as being impacted according to the patient's clinical examination and panoramic radiograph evaluation. These patients were then referred for CBCT scans after they demonstrated delayed eruption of maxillary canines or impacted canines. Normally, the root of the maxillary canine is completely formed at 15 years of age.⁸ Hence, it was decided to divide the subjects into two age groups: one group less than 15 years old and the other group more than 15 years old. The age range of subjects was approximately between 10 and 68 years old.

The inclusion criteria in selecting subjects were unilateral or bilateral maxillary impacted canines identified on panoramic radiographs and CBCT scans available within a maximum interval of 6 months between the procedures provided that no orthodontic treatment had been performed. Other patients were eliminated when they had insufficient panoramic radiographs or CBCT scan quality, syndromes, history of dental trauma, odontogenic tumors or cysts around the impacted canines, ectopic canines, previous orthodontic treatment, or panoramic radiograph and CBCT scan taken more than 6 months apart.

All digital panoramic and CBCT radiographs were obtained from an iCAT FLX machine (Imaging Sciences International, LLC, Hatfield, Penn). The scanning parameters of iCAT FLX CBCT scans were 120 kVp, 17.8 s, 5 mA, voxel size 0.30 mm, and field of view 23 × 17 cm. The exposure parameters of the iCAT FLX machine for panoramic radiographs were 94 kVp, 20 s, 5 mA.

Panoramic radiographs and CBCT images from 64 patients were analyzed. The sector location of the maxillary canine cusp tip in relation to the adjacent incisors was divided into five sectors on panoramic radiographs by using the Alessandri method.¹¹

Sector Locations of Maxillary Canine Cusp Tips (Figure 1) on Panoramic Radiographs

• Sector 1: area distal to the line tangent to the distal heights of contour of the lateral incisor crown and root.

• Sector 2: area bounded by sector 1 and the long axis of the lateral incisor.

• Sector 3: area bounded by sector 2 and a line tangent to the mesial heights of contour of the lateral incisor crown and root.

• Sector 4: area bounded by sector 3 and the long axis of the central incisor.

• Sector 5: area bounded by sector 4 and the midline between the upper central incisors.

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The labio-palatal position of the maxillary impacted canine and resorption of maxillary incisors were assessed in static cross-sectional reformatted CBCT images (Figure 2a,b). Root resorption of permanent incisors was classified as resorption or no resorption. The labio-palatal position of the maxillary impacted canine was classified as labial, mid-alveolus, or palatal.

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Sector location on the panoramic radiograph was compared with the labio-palatal position of the maxillary impacted canine as well as the possibility of maxillary incisor root resorption on CBCT. All panoramic radiographs and CBCT images were masked to hide the patient's identity and were displayed in a random order for evaluation. In addition, CBCT scans were reconstructed using Dolphin software version 11.7 (Dolphin Imaging & Management Solutions, Chatsworth, Calif). Chi-square and Fisher exact tests were used for correlation assessment between panoramic and CBCT findings. The level of significance was set at P < .05.

All panoramic and CBCT volumes were reviewed twice by the first operator (Dr Ngo) at an interval of 1 month. The assessed variable on panoramic radiographs was tooth sector location. In addition, reviewed variables on CBCT comprised type of impacted canine, position of the canine crown in relation to the dental arch, and occurrence of root resorption of adjacent incisors. The repetition of the above variables was assessed by using the kappa statistic at a level of P < .05 to determine intraobserver and interobserver agreement. Ten randomly selected panoramic radiographs and CBCT volumes were reviewed twice by the first operator (Dr Ngo) at an interval of 1 month to determine intraobserver agreement. A second operator (Dr Said) rated 10 randomly selected panoramic and CBCT images independently to evaluate interobserver agreement with the first operator (Dr Ngo).

RESULTS

Intraobserver agreement was high for the panoramic image and CBCT results, showing kappa values of 0.9054 for sector location, 1 for labio-palatal location, and 1 for root resorption (P < .001). Interobserver reliability was also high: kappa values of 0.9054 for sector location, 0.8526 for labio-lingual location, and 0.8833 for root resorption (P < .001).

The study included 64 patients: 27 males (10–68 years old) and 37 females (11–31 years old). The mean age was 17.5 (±11.6) years for males and 15.6 years (±3.9) for females (Table 1).

Table 1 Distribution of Maxillary Impacted Canines According to Gender and Age

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In the <15 year group, labially impacted canines were more frequent in sector 1 and palatally impacted canines were more frequent in sectors 2, 3, and 5; all cases in sector 5 showed palatal impaction. In the >15 year group, palatally impacted canines were most frequent in sectors 3, 4, and 5; all cases in sector 5 showed palatal impaction. Of the total 88 impacted maxillary canines, 23 (26.1%) were impacted labially, 7 (8%) in the mid-alveolus and 58 (65.9%) palatally. Labially impacted canines were more frequent in sector 1, and palatally impacted canines were most frequent in sectors 3, 4, and 5. There was a statistically significant association between the sectors of the canines and the labio-palatal position of the canines (P = .001; Table 2).

Table 2 Relationship Between Sector Location on Panoramic Radiographs and Labio-palatal Position of Maxillary Impacted Canines on CBCT Images

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In the <15 year group, all sectors showed root resorption of permanent incisors. In the >15 year group, sectors 2, 3, 4, and 5 showed root resorption. The >15 year group showed more frequent root resorption of permanent incisors (62.5%) than the <15 year group (29.2%), but there was no statistically significant difference between the two groups (Table 3).

Table 3 Relationship Between Sector Location of Maxillary Impacted Canines on Panoramic Radiographs and Resorption of Adjacent Permanent Incisors on CBCT Images^a

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For the total group, nonresorption of permanent incisors was present in 49 (55.7%) patients and mainly at sectors 1, 2, and 3, showing significant differences according to sector location. Resorption of permanent incisors was present in 39 (44.3%) patients and was observed mainly in sectors 4 and 5 with significant differences according to sector location (P = .001; Table 3).

In both age groups, root resorption was more frequently observed in the palatally impacted canines, but the difference was not significant (Table 4).

Table 4 Relationship Between Labio-palatal Position and Resorption on CBCT Images^a

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DISCUSSION

Maxillary impacted canines are more common in females than males.²² According to Ericson and Kurol,¹² the sex ratio between male and female is 2:3. In the current study, there were more female subjects (58%) than male subjects (42%), resulting in a ratio of almost 1.37:1. Walker et al.²² speculated that the difference in growth and development between the sexes, as well as genetics, may be reasons for the above finding. Another possible reason could be that females seek orthodontic treatment more frequently than males.

Lindauer et al.²³ reported that most canines destined to become palatally impacted had cusp tips overlapping or mesial to the lateral incisor root. In the <15 year group of the current study, sector 1 showed more frequent labially impacted canines. Sectors 2, 3, 4, and 5 showed more frequent palatally impacted canines. In the >15 year group, the sample size in each category of sector 1 was too small to see a pattern. Sectors 3, 4, and 5 showed more frequent palatally impacted canines, and sector 2 showed the most frequent labially impacted canines. In addition, among five sectors, mid-alveolus positioned canines were found most commonly in sector 2. In total, most labially impacted canines were located in sector 1, mid-alveolus impacted canines were more frequent in sector 2, and palatally impacted canines were positioned mainly in sectors 3, 4, and 5. This suggests that sector location on panoramic radiographs could be used to predict the labio-palatal position of impacted canines.

Sector location was suggested as a better predictor for canine impaction possibility than angulation. Warford et al.¹⁴ reported that 48.6% of impacted canines were found in sectors 3, 4, and 5. Lindauer et al.²³ found that 41.5% of impacted teeth occurred in sectors 3, 4, and 5. The present study, in comparison, showed a higher percentage of 63.6% of impacted canine positioning in sectors 3, 4, and 5. However, most of the impacted canines were located in sectors 2, 3, and 5, with an even higher percentage of 73.9%.

Treatment for root resorption of adjacent teeth caused by canine impaction remains challenging. The presence or absence of root resorption will affect the treatment plan. Early diagnosis and intervention of root resorption caused by impacted canines may save time, expense, and avoid more complicated treatment.¹⁰ In the present study, root resorption was associated with 39 of 88 impacted canines (44.3%). The percentage of root resorption was higher than the 40.5% reported by Liu et al.,²⁴ 38% found by Ericson and Kurol,²⁵ and lower than the 66.7% detected by Walker et al.²² Differences in sampling, structure material, and patient age may be reasons for these different results. Ericson and Kurol¹⁸ reported that canines in sectors 3, 4, and 5 comprised 65% of the cases of root resorption. They also found that when the cusp of the canine was positioned mesially to the lateral incisor (in sectors 4 and 5), the risk of complications increased threefold. In the current study, 41 of 49 impacted canines in sectors 1, 2, and 3 did not show root resorption of adjacent permanent teeth, while 21 of 39 impacted canines in sectors 4 and 5 showed root resorption of adjacent permanent teeth. The root resorption of adjacent permanent teeth cannot be accurately judged from a conventional radiograph alone.¹⁰ CBCT imaging was proven to be significantly better than the panoramic radiograph in determining root resorption. The current study suggested that, when impacted canines are found in sectors 4 and 5 on the panoramic radiograph, CBCT should be considered for those with suspected incisor root resorption.

CONCLUSIONS

• Labially impacted canines as seen on CBCT were more frequent in sector 1, mid-alveolus impacted canines were more frequent in sector 2, and palatally impacted canines were more frequent in sectors 3, 4, and 5 on panoramic radiographs.

• Resorption of permanent incisors was mainly observed in sectors 4 and 5.

• Sector analysis of maxillary impacted canines on a panoramic radiograph can guide practitioners in their judgement to apply CBCT.