INTRODUCTION

A displaced canine is one diverging from its normal path of eruption either to erupt in an unusual position or to become impacted buccally or palatally.¹ The reported prevalence of maxillary canine displacement or impaction varies among different populations. It is reported to be about 2% to 3% in a white population.² Numerous sources in the literature suggested that the majority of ectopic canines in whites were palatal.^3,4 The specific etiology behind palatally displaced maxillary canines (PDC) is unknown. Two broad theories exist: guidance and genetic.⁵ However, the weight of evidence favors a primarily genetic multifactorial inheritance model.^6,7

Dental abnormalities were often found during the diagnosis of orthodontic patients.⁸ The maxillary canines were the second most common teeth to experience impaction after third molars,^9,10 and patients with PDC often exhibited other associated dental anomalies,^11–13 such as agenesis, microdontia, and dental transpositions, suggesting that these events may have the same genetic origin.¹² Several studies have been carried out regarding the association of PDCs with congenitally missing teeth, in particular, associations with missing maxillary lateral incisors.^14,15 Baccetti¹² reported that associations between different tooth anomalies was clinically relevant. The prevalence of PDC was greater in patients with microdontia. Maxillary first molar agenesis was associated with a higher prevalence of other permanent tooth agenesis.^16,17 Also, third molar agenesis was significantly associated with agenesis of permanent lateral incisors and second premolars.¹⁷ Peck et al.¹³ suggested an increased prevalence of third molar and mandibular second premolar agenesis associated with PDCs.

It was reported that 50% of third molars present some form of anomaly, either remaining unerupted or partially erupted or they are absent from the oral cavity.¹⁸ Third molar agenesis constitutes the most common developmental anomaly of the human permanent dentition. A wide range of prevalence of third molar agenesis in different populations was reported; it varied from 9% to 41%.^19,20

Impaction of permanent teeth occurs when teeth lack the ability to erupt or there is a barrier in the path of eruption. The incidence of impacted third molars has been documented in developed countries of the world. Carter and Worthington²¹ reported an average worldwide rate of third molar impaction of 24.40%.

No study has previously investigated the association between PDCs and third molar anomalies (agenesis and impaction). The aim of the present study was to investigate the association between third molar agenesis and pattern of impaction, missing teeth, and peg-shaped lateral incisors with palatal canine displacement.

MATERIALS AND METHODS

Ethical approval was obtained from the institutional review board, Jordan University of Science and Technology (Irbid, Jordan).

The pretreatment orthopantomograms for 438 white patients with maxillary canine displacement taken as part of their comprehensive orthodontic treatment were used in this study (Table 1). Ages ranged from 18 to 31 years, with an average age of 19.84 ± 5.54 years.

Table 1 Distribution of Patients According to Gender

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A total of 338 patients with normally erupted canines who had a preexisting radiograph (age averaged 20.59± 3.54 years) as part of their orthodontic treatment were included in the study. Their records were analyzed and served as the controls. Exclusion criteria were age younger than 18 years, poor quality records, syndromic or cleft lip/palate patients, and a history of previous orthodontic treatment.

All orthopantomograms were evaluated to record the following:

• Missing third molars.

• Third molar eruption status. Third molar was recorded as erupted if the highest portion of the tooth was on a level with the occlusal plane.

• Type of impaction: based on the inclination of third molar longitudinal axis relative to the second molar (Winter classification), teeth were categorized as vertical (10° to −10°), mesioangular (1°1 to 80°), horizontal (81° to 100°), and distoangular (−11° to −80°).

• Dental anomalies: missing teeth and peg-shaped lateral incisors were recorded.

• Upper third molar space: the distance between the distal contact point of the upper second molar and the outer cortex of the maxillary tuberosity (Figure 1).

• Lower third molar space: the distance between the distal contact point of the lower second molar and a point made from the intersection of a tangent of the anterior border of the ramus of the mandible and a tangent of the upper border of the body of the mandible (Figure 1).

• Angulation between the second and third molars: the angle between the long axis of both second and third molars.

• Third molar width: the distance between mesial and distal contact points of the third molar.

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RESULTS

Distributions of third molar status in the unilateral PDC, bilateral PDC, and control groups are shown in Table 2. At least one upper third molar was missing in 48% of PDC patients (51% in unilateral and 40% in bilateral PDC patients, respectively; P < .01). In the control group, only 5% of patients had at least one upper third molar missing. The difference between the PDC and control group was significant (P < .001). Gender differences were not detected.

Table 2 Distribution of Third Molar Status in Unilateral PDC, Bilateral PDC, and Control Groups^a

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At least one lower third molar was missing in 19% of PDC patients (21% in unilateral and 15% in bilateral PDC patients, respectively). In the control group, only 3% of patients had at least one lower third molar missing. The difference between the PDC and control group was significant (P < .001). Gender differences were not detected.

Of the PDC patients, 38% had upper third molar impaction (34% in unilateral and 50% in bilateral PDC patients, respectively; P < .001). In the control group, 26% of patients had at least one impacted third molar. The difference between the PDC and control groups was significant (P < .001). Gender differences were not detected.

Of the PDC patients, 67% had at least one lower third molar impacted (66% in unilateral and 70% in bilateral PDC patients, respectively; P < .001). In the control group, 28% of patients had lower impacted third molars. The difference between the PDC and control groups was significant (P < .001). Gender differences were not detected.

Distributions of third molar types of impaction in unilateral PDCs, bilateral PDCs, and control groups are shown in Table 3. The most common type of impaction in PDC patients was vertical in the upper arch and mesioangular in the lower. In the control group, the most common type of impaction was distoangular and mesioangular in the upper and lower arches, respectively. The difference between the PDC and control groups was significant (P < .001). Gender differences were not detected.

Table 3 Distribution of Third Molar Types of Impaction in the Unilateral PDC, Bilateral PDC, and Control Groups^a

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Means, standard deviations, mean differences and P values for third molar variables in the PDC and control groups are shown in Table 4. Upper and lower third molar width and spaces were reduced in PDC patients when compared with the control group (P < .001).

Table 4 Means, SD, Mean Differences, and P Values for Third Molar–Related Variables in PDC and Control Groups^a

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The distribution of dental anomalies associated with the unilateral PDC and bilateral PDC groups when compared with the control group are shown in Table 5. In PDC patients, missing teeth were recorded in 5.6% (4.5% were in unilateral and 1.1% were in bilateral PDC patients, respectively). In the control group, missing teeth were recorded in 14% of the patients. The difference between the PDC and control groups was significant (P < .001). Gender differences were not detected.

Table 5 Distribution of Dental Anomalies Associated With the Unilateral PDC and Bilateral PDC Groups When Compared With the Control Group^a

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In the PDC patients, a peg-shaped lateral incisor was recorded in 9.1% (2.3% were in unilateral and 6.8% were in bilateral PDC patients, respectively). In the control group, a peg-shaped lateral incisor was recorded in 1% of patients. The difference between the PDC and control groups was significant (P < .001). Gender differences were not detected.

DISCUSSION

The present study investigated the association between PDCs and third molar agenesis and pattern of impaction in a large sample of orthodontic patients.

The female:male ratio was 2.6:1, which was consistent with previous studies that reported a higher prevalence of displaced canines in females.²² Also, the patients in this study were collected from the archive of the orthodontic clinic, which may explain the higher female ratio. The demand for orthodontic treatment was shown to be higher in females than in males as they are more concerned about their esthetics and alignment of their teeth.²³

The patients included in this study were at a minimum age of 18 years to confirm the absence/presence of their third molars. Richardson²⁴ reported that third molar development could occur as late as 14 or 15 years of age.

The ratio between unilateral and bilateral displacement in the current study was 2.7:1, consistent with previous reports. It was reported that unilateral canine displacements were more common than bilateral canine displacements by a factor of 5:1.²⁵ Likewise, Sambataro et al.²⁶ reported that only 8% of canine impactions were bilateral.

The findings of the present study revealed a significant increase in third molar agenesis in PDC patients. At least one upper third molar was missing in 48% of PDC patients, in agreement with Peck et al.,¹³ who stated that an increased association of developmentally missing third molars was seen in association with PDCs. Also, this was consistent with Scerri et al.,²⁷ who reported third molar agenesis in 39% of patients in a PDC group compared to 29% in the control group. Furthermore, Camilleri²⁸ found that third molar agenesis was evident in 27.5% of PDC patients. Third molar agenesis was increased in PDC patients in the current study, a 2-fold increase in occurrence when compared with patients without PDCs for the same population.¹⁹

Third molar agenesis was greater in the maxilla than in the mandible, which was consistent with other studies.^19,29 However, the prevalence of maxillary third molars agenesis was similar for the right and left sides in unilateral PDC patients.

In the current study, unilateral PDC patients showed more third molar agenesis than the bilateral PDC patients in both the maxillary and mandibular arches. This was unexpected, as it might be assumed that bilateral PDC patients would have more complex dental anomalies. This was not consistent with Camilleri,²⁸ who reported third molar agenesis in 39% and 25% of bilateral and unilateral PDC patients, respectively.

In the present study, the rate of third molar impaction in PDC patients was 38% and 67% in the maxillary and mandibular arches, respectively, with no gender predilection. This was significantly higher than that reported by Hattab et al.¹⁹ for a similar population. Hattab et al.¹⁹ found third molar impaction in one third of the patients with a higher predilection (by 5.2%) for the maxilla. The difference can be explained by the sample type. PDC patients in the current study were more likely to have a malocclusion, dental anomalies, and potential crowding and so were likely to have a higher incidence of third molar impaction than a random population sample.

In the present study, the most common angulation pattern was vertical in maxillary arch and mesioangular in mandibular arch. This was consistent with previous studies that reported that vertical impactions in the upper arch and mesioangular impactions in the lower arch were common.^30,31

Upper and lower third molar widths were reduced in the PDC patients when compared with the control group. The presence of generalized tooth-size reduction previously reported in PDC patients may explain this finding.³²

Third molar space width was reduced in PDC patients, which may explain the high third molar impaction rate reported in patients with PDC in the current study. This confirms previous reports that a shortage of retromolar space was a major factor in the etiology of third molar impaction.^33,34 The combined effect of mesial angulation of lower third molars to the mandibular plane and retromolar space deficiency may explain the increased impaction rate in the mandibular arch.

In the present study, PDCs were significantly associated with peg-shaped lateral incisors. This was consistent with other reports in orthodontic samples.¹⁴ The occurrence of peg-shaped lateral incisors in the PDC group was approximately 9.1% (2.3% plus 6.8% in unilateral and bilateral PDC groups, respectively). Although this percentage was less than that reported by others,²⁷ the frequency of peg-lateral incisors in PDC patients represented a 5-fold increase in occurrence when compared with patients without PDCs. However, the rate reported by Scerri et al.²⁷ was for both agenesis/peg or diminutive maxillary laterals and not for peg-shaped lateral incisors alone, which may explain the higher previously reported rate.

The incidence of tooth agenesis other than third molars was also evaluated. Approximately 5.6% of PDC patients showed at least 1 or more absent teeth. This was consistent with the rate of congenitally missing teeth in PDC patients reported by Camilleri.²⁸ The incidence found in the current study was lower than the control group of orthodontic patients. This was consistent with Fraga et al.,³⁵ who reported no significant association between tooth agenesis and the PDC anomaly. Also, Herrera-Atoche et al.³⁶ concluded that there was a lack of association between agenesis and maxillary canine impaction in Mexicans. However, although the incidence of agenesis in PDC patients was lower than that in the control group, it still represented an increase of 2 times that of the normal population. The high incidence of hypodontia found in the control group may be explained as the patients included in this group were orthodontic patients who would be expected to have a greater tendency for dental anomalies such as impaction, delayed eruption, agenesis, and peg-shaped lateral incisors.³⁷

There was no difference in the number of missing teeth between the bilateral and unilateral PDC patients. This may suggest that the genetic coding for canine anomalies are linked with, but not identical to, those coding for tooth agenesis.⁷

Limitations of this study included a high female-to-male ratio. Also, the control group was based on an orthodontic population rather than the general population, which meant there was a higher presence of malocclusion or some other dental anomalies such as hypodontia, microdontia, or impaction.

CONCLUSIONS

• PDC patients showed a higher prevalence for third molar agenesis and impaction.

• The most common type of third molar impaction in the upper arch was vertical and distoangular in the PDC and control groups, respectively.

• Upper and lower third molars width and space were reduced in PDC patients.

• There were no significant associations between tooth agenesis (other than third molars) and the PDC anomaly.