Introduction
The aims of this research were to study the prevalence of palatally displaced maxillary canines (PDCs) in subjects with congenitally missing maxillary lateral incisor (MxI2) teeth and to assess the dental and occlusal features associated with this displacement in these subjects.
Methods
The material for this study consisted of the pretreatment orthodontic casts of 246 subjects (78 male, 168 female) with missing MxI2 teeth. These were divided into 2 groups: subjects with PDC (displacement group) and subjects with no PDC (nondisplacement group). A nondisplacement comparison group was selected by matching the displacement group according to age, sex, type of malocclusion, and unilateral or bilateral absence of the MxI2 teeth.
Results
Of 246 subjects, 31 (12.6%) were found to have PDC. Sex distribution and type of congenital absence of MxI2 teeth were not different in the displacement and the nondisplacement groups. There was no significant difference between the dental and occlusal features in the displacement and the comparison groups.
Conclusions
There was a strong association between PDC and missing MxI2; this supports the guidance theory. In subjects with MxI2 agenesis, no other dental or occlusal features were found to play a role in PDC.
The main physical component of the smile is a complete dentition comprising 4 types of teeth. The maxillary permanent canines are important for an attractive smile as well as for a functional occlusion. Congenital absence of a maxillary lateral incisor (MxI2) or maxillary canine displacement is therefore expected to have a detrimental effect on facial esthetics.
The maxillary canine is the second most frequently impacted tooth in the dental arch after the third molars. The prevalence rate of palatally displaced maxillary canines (PDCs) varies between 0.8% and 2.8%. In 70% to 85% of canine displacements, the canine is located palatally to the dental arch.
The etiology of a PDC is obscure and probably multifactorial. There are many theories as to why canine displacement occurs, but they can be separated into 2 major categories: the guidance theory and the genetic theory. The guidance theory refers to an excess of space in the apical region of the maxillary bone during the eruption of the permanent canine, because of either hypoplasia or aplasia of the MxI2. This theory regards the distal aspect of the MxI2 root as the guide that allows the canine to erupt safely into position. Previous studies have established a strong association between small, peg-shaped, missing MxI2 teeth, and PDC, indicating that the presence of a MxI2 root with the correct length, formed at the right time, is an important variable needed to guide the erupting canine in a favorable direction. In agreement, Becker et al found congenitally missing MxI2 teeth in 5.5% of a large sample of patients with PDC (2.4% times more than in the general population) and peg-shaped MxI2 teeth in 17.2% of the sample.
Sacerdoti and Baccetti reported a significant association of unilateral PDC with MxI2 agenesis in a large sample of orthodontic patients. Furthermore, PDC showed reciprocally significant associations with bilateral small MxI2 teeth. On the other hand, PDC was reported to be more associated with absent third molars and second premolars, and with peg-shaped MxI2 teeth than with absent MxI2 teeth.
The genetic theory suggests that PDC has a complex of genetically determined tooth anomalies with reported familial recurrence of canine displacement, and reported associations between canine displacements and other dental anomalies, with 33% of patients with displaced canines reported to have other congenitally missing teeth. Inheritance on an autosomal dominant basis had been proposed for PDCs. The genetic theory is supported by the fact that there seem to be more than twice as many PDCs in females compared with males (sex bias with genetic links involving the sex chromosomes). Moreover, and based on analysis of available data gathered from many sources, PDCs were cited to be approximately 5 times more common in Europeans than in Asians and were found to occur bilaterally.
On a different track, McConnell et al implicated deficiency in maxillary width as a local mechanical cause for PDCs. In contrast, Al-Nimri and Gharaibeh recognized excessive palatal width as an etiologic factor for PDCs. On the other hand, Langberg and Peck observed no statistically significant difference in the anterior and posterior maxillary arch widths between subjects with PDCs and a comparison sample.
Although it is agreed that the absence of MxI2 teeth is frequently associated with PCDs, the prevalence rate of this displacement in subjects with absent MxI2 teeth has not been previously reported. The aims of this research were to study the prevalence rate of PDCs in subjects with congenitally missing MxI2 teeth and to investigate the dental and occlusal features that could contribute to this displacement. The early recognition of MxI2 agenesis might aid in the identification of patients with a high risk of developing a PDC, thus facilitating earlier interception.
Material and methods
The material for this study comprised the pretreatment orthodontic casts of 246 nonsyndromic patients (78 male, 168 female) with congenitally missing MxI2 teeth. These casts were selected from 6060 orthodontic casts collected over 8 years from the orthodontic department at the Dental Teaching Center, Jordan University of Science and Technology in Irbid, orthodontic clinics at the Ministry of Health and Royal Medical Services in Amman, Jordan, and private orthodontic practices in Amman, Jordan. The patients’ ages ranged between 15 and 35 years.
Subjects with congenital absence of 1 or 2 MxI2 teeth confirmed by radiographic examination were selected (congenitally missing MxI2 group). For these subjects, the eruption status of both maxillary canines was determined. If 1 or both maxillary canines were not present in the orthodontic cast, the patient’s radiographs were examined to confirm canine displacement.
Subjects with PDCs were diagnosed based on the following criteria: the displaced canines should have a radiographically fully formed root apex, and the position of the displaced canine relative to the dental arch was determined by the parallax technique.
The 246 subjects with congenitally missing MxI2 teeth were divided into 2 groups. The first group comprised subjects with PDCs (displacement group). The second group included subjects with no canine displacement (nondisplacement group). A comparison group was formed from the nondisplacement group by matching each subject in the displacement group with a subject from the nondisplacement group according to age, sex, type of malocclusion, and type of congenital absence of MxI2 teeth (unilateral or bilateral).
The following parameters were obtained from the orthodontic casts for the displacement and comparison groups: type of malocclusion, determined directly from the orthodontic cast based on Angle’s classifications ; unilateral or bilateral congenitally missing MxI2 teeth; presence of a peg-shaped MxI2; other missing or anomalous maxillary teeth, recorded by direct observation from the orthodontic cast and confirmed by radiographic examination; the mesiodistal width of each maxillary tooth, measured from the mesial anatomic contact point to the distal anatomic contact point of each tooth; the maxillary arch space condition, calculated by subtracting the total sizes of the teeth from the arch perimeter; the mesiodistal width of the congenitally missing maxillary teeth was not included in space condition calculation; the mesiodistal width of a displaced canine was judged to be equal to that of the contralateral permanent tooth (if present); in subjects with bilateral displacement, the mesiodistal width of the displaced canine was measured from the posttreatment casts, and the arch perimeter was measured from the mesial aspect of the maxillary first molar to the mesial aspect of its contralateral tooth; and the interpremolar and intermolar widths of the maxillary arch; interpremolar width was measured by the caliper tips of the dental Vernier placed into the deepest portion of the central fossae of the maxillary first premolars at their junctions with the most lingual aspect of the buccal cusp, and intermolar width was recorded with the caliper tips placed into the deepest portion of the central fossae at their junction with the most lingual aspect of the mesiobuccal cusp.
All orthodontic cast measurements were made at least twice by the same examiner (E.B.). If a difference between the 2 measurements was apparent, a third reading was made, and the aberrant one was discarded. The mean of the 2 closest measurements was used in the calculations.
The measurement error was calculated according to Dahlberg’s double determination method. The results of the measurement error were 0.35 mm for arch perimeter, 0.29 mm for mesiodistal tooth width, and 0.19 and 0.22 mm for interpremolar and intermolar arch widths, respectively.
Statistical analysis
Means and standard deviations for each group were calculated for all variables by using SPSS software (version 11, SPSS, Chicago, Ill). The differences in the sex and malocclusion distributions and the type of congenitally missing MxI2 teeth between the displacement and the nondisplacement groups were determined by using the chi-square test, and the Student t test was used to determine the differences in the dental and occlusal features between the displacement and the comparison groups. P values less than 0.05 were considered significant.
Results
Of 6060 pretreatment orthodontic casts, 246 subjects (4.1%) with nonsyndromic congenitally missing MxI2 teeth were found and studied. The ages of these subjects varied between 15 and 35 years, with an average of 19.2 ± 4.2 years.
In the congenitally missing MxI2 group, females were more frequently affected than males by a ratio of nearly 2:1 (168 [68%] females, 78 [32%] males). Bilateral congenital absence of MxI2 teeth was more frequent (58.1%) than unilateral absence (41.9%).
Of the congenitally missing MxI2 sample, 28 subjects (11.4%) had an associated contralateral peg-shaped MxI2.
MxI2 agenesis occurred most frequently in subjects with Class I malocclusions (42.3%), followed by Class III and Class II Division 2 malocclusions (25.2% and 20.3%, respectively), and least frequently in Class II Division 1 malocclusion (12.2%, Table I ).
| Type of malocclusion | Displacement group (n = 31) | Nondisplacement group (n = 215) | Total sample (n = 246) |
|---|---|---|---|
| Class I | 16 (51.6%) | 88 (40.9%) | 104 (42.3%) |
| Class II Div 1 | 3 (9.7%) | 27 (12.6%) | 30 (12.2%) |
| Class II Div 2 | 9 (29.0%) | 41 (19.1%) | 50 (20.3%) |
| Class III | 3 (9.7%) | 59 (27.4%) | 62 (25.2%) |
In the congenitally missing MxI2 group, 18 subjects (8.4%) had missing teeth other than the MxI2 in the maxillary arch, and 47 subjects (21.9%) had missing teeth in the mandibular arch. Agenesis of MxI2 teeth was mostly associated with missing mandibular incisors (12.6%) closely followed by mandibular second premolars (12.2%) and then maxillary second premolars (7.3%). Bilateral absence of MxI2 teeth was more associated with missing other teeth than unilateral absence (32% and 14%, respectively). Class II Division 2 was the most frequent type of malocclusion to be associated with other missing teeth (38%) followed by Class II Division 1 (27%) and then Class III and Class I malocclusions (21% and 19%, respectively).
The congenitally missing MxI2 sample was subdivided into displacement and nondisplacement groups according to whether MxI2 absence was associated with PDCs.
Of the sample of 246 subjects with missing MxI2 teeth, 31 (12.6%) were found to have PDCs, 5 of whom were bilaterally affected, with females more affected than males (71% and 29%, respectively). In subjects with PDC, bilateral congenital absence of MxI2 teeth was more evident (64.5%) than unilateral absence (35.5%). All subjects with bilateral PDCs had associated bilateral MxI2 agenesis. Fifteen (58%) of the 26 subjects with unilateral PDC, had bilaterally missing MxI2 teeth, 7 subjects (27%) had homolateral unilateral missing MxI2 teeth, and only 4 subjects (15%) had contralateral unilateral absence of MxI2 teeth. Of 31 subjects with PDCs, only 1 (3.2%) had associated peg-shaped MxI2 teeth.
PDCs occurred most frequently in subjects with Class I malocclusion (51.6%) followed by Class II Division 2 malocclusions (29.0%) and least frequently in Class II Division 1 and Class III malocclusions (9.7% for each, Table I ).
Only 1 subject (3.2%) in the displacement group had other missing maxillary teeth, and 5 subjects (16.1%) were missing at least 1 mandibular tooth.
There was no significant difference between the congenitally missing MxI2 displacement and nondisplacement groups with regard to sex distribution ( P = 0.838), type of MxI2 agenesis ( P = 0.457), distribution of type of malocclusion ( P = 0.131, Table I ), number of subjects with missing teeth other than the MxI2 ( P = 0.546), and number of subjects with peg-shaped MxI2 teeth ( P = 0.270).
Comparing the mesiodistal width of each tooth in the displacement group with its counterpart in the comparison group ( Table II ) showed no statistically significant difference between the 2 groups.
| Tooth | Side | Displacement group (n = 31) | Comparison group (n = 31) | Mean difference | Significance |
|---|---|---|---|---|---|
| Central incisor | Right | 8.60 (0.6) | 8.52 (0.6) | 0.08 | 0.587 |
| Left | 8.58 (0.7) | 8.46 (0.6) | 0.12 | 0.473 | |
| Canine | Right | 7.52 (0.5) | 7.49 (0.5) | 0.03 | 0.847 |
| Left | 7.51 (0.5) | 7.45 (0.5) | 0.06 | 0.683 | |
| First premolar | Right | 6.71 (0.5) | 6.77 (0.5) | −0.06 | 0.645 |
| Left | 6.68 (0.4) | 6.73 (0.5) | −0.05 | 0.670 | |
| Second premolar | Right | 6.14 (1.1) | 6.34 (0.5) | −0.20 | 0.401 |
| Left | 6.33 (0.5) | 6.34 (0.5) | −0.01 | 0.919 |
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