Introduction
The aim of this research was to identify the etiologic factors associated with palatally impacted canines and buccally impacted canines in a Chinese population by using the cone-beam computed tomography technique.
Methods
Pretreatment cone-beam computed tomography scans of 170 Chinese subjects with impacted maxillary canines and 170 age- and sex-matched subjects without impaction were used. Impacted canine subjects were divided into 2 groups: those with palatally impacted canines and those with buccally impacted canines. One rater analyzed the cone-beam computed tomography data for qualitative and quantitative variables of the teeth, dental arch, and skeletal components. The measurements were compared by using analytical statistical methods.
Results
The mesiodistal dimension of the lateral incisor was significantly smaller in the palatally impacted canine group than in the other group (by an average of 0.4-0.5 mm; analysis of variance [ANOVA], P <0.001). Both anterior maxillary dental (interpremolar) width and skeletal width (interjugal points) in the buccally impacted canine group were significantly smaller than in the palatally impacted canine and control groups (ANOVA, P <0.001), whereas the intermolar widths and posterior mandibular widths were similar among the groups. The groups with palatally impacted or buccally impacted canines had significantly increased prevalence values of peg-shaped lateral incisors and incisor impaction, respectively (chi-square or Fisher exact tests, P <0.001). After excluding subjects who also had lateral incisor anomalies, the prevalence values of supernumerary teeth, missing premolars, or third molars combined were not different among the impaction and control groups. The average mesiodistal location of the canine cusp tip was significantly different between the buccally impacted canines and the palatally impacted canines groups; it was distal and mesial to the lateral incisor long axis, respectively.
Conclusions
In Chinese subjects, buccal canine impaction is mostly associated with anterior transverse (dental and skeletal) deficiency and incisor impaction, whereas palatal impaction is mostly associated with small or missing lateral incisors, consistent with the guidance theory. Likely, preimpaction migrations of the canines are mainly buccal for buccal impactions and excessively mesiopalatal for palatal impactions.
Maxillary canine impaction is a common clinical problem in dentistry. With a 1% to 3% prevalence in the general population, most impacted canines are palatally displaced in white patients but buccally displaced in Asians. To date, different etiologic factors and theories have been proposed for either type of impaction, as detailed in several recent reviews. Briefly, it is commonly thought that palatally impacted canines are associated with hypoplastic or missing lateral incisors (guidance theory) or with aplasia of premolars and third molars and supernumerary teeth (genetic theory). Indirectly supporting these theories, palatally impacted canines are often present with adequate arch space, except in 1 study, and patients with palatally impacted canines even tend to have smaller dimensions of maxillary anterior teeth than do the control subjects, suggesting that space deficiency is not a cause for palatally impacted canines. In contrast, buccally impacted canines are commonly thought to be associated with dental arch and skeletal (premaxilla) deficiencies, although recently it was reported that buccally impacted canines were also present in a small but significant number of patients without crowded anterior teeth.
These theories were mostly based on studies of white patients. Studies on the etiology of maxillary canine impaction in Asians are scarce. With different predilections for palatally impacted canines and buccally impacted canines between white and Asian people, it is necessary to ascertain whether the theories and factors established for white patients are also applicable to Asians. Additionally, previous studies have mostly relied on 2-dimensional (2D) radiographs (panoramic, posteroanterior cephalometric), which have limitations such as image enlargement, distortion, structure overlap, and poor positioning. The use of dental casts together with x-rays in some studies for dental measurements might also increase the chance of analysis errors because of the magnification associated with x-rays but not with dental casts. With the advent of cone-beam computed tomography (CBCT) in dentistry, these limitations can now be minimized. More specifically, by rendering 3-dimensional (3D) views of teeth and bone at a reasonably high resolution, 1 CBCT scan is sufficient for detailed characterization of canine impaction and systematic analysis of potential etiologic factors from local teeth and bones.
The purpose of this study was to identify etiologic factors associated with palatally and buccally impacted canines in a Chinese population with the CBCT technique. Qualitative and quantitative variables of tooth, dental arch, and skeletal components were assessed and compared between canine impaction and control patients. We hypothesized that buccally and palatally impacted canines in Chinese patients have different etiologic factors in a fashion similar to white patients.
Material and methods
Pretreatment CBCT data for 170 subjects with maxillary canine impaction (experimental group, 101 buccal and 69 palatal impactions) and 170 age- and sex-matched subjects (control group) were obtained from a clinic at Nanjing Medical University in China. The inclusion criteria for the experimental group were patients (1) 12 to 30 years of age at the time of the CBCT scans and (2) with clinically diagnosed unilateral or bilateral maxillary canine impaction. The exclusion criteria for the experimental group were (1) previous orthodontic treatment, (2) dental trauma or anterior maxillary dental surgery history, and (3) dental age younger than the late mixed dentition. At this university clinic, all orthodontic patients were questioned about trauma histories and examined for signs of dental trauma, and the findings were documented in the pretreatment examination form. Because dental trauma is a confirmed risk factor for maxillary canine impaction, patients with a positive trauma history or signs were excluded from this study to prevent confounding.
The same inclusion and exclusion criteria except for the maxillary canine impaction were applied to the control groups. As in many other countries including the United States, a national policy or guideline for prescribing CBCT scans for orthodontic patients has yet to be established in China. At this university clinic, for patients with relatively complicated dental and skeletal orthodontic problems, CBCT scans were prescribed in place of panoramic and cephalometric imaging as part of their pretreatment records. CBCT scans were rarely prescribed for patients with only minor orthodontic problems or for progressive and final records. As a result, all control subjects in this study had relatively complicated orthodontic problems except for maxillary canine impaction.
Access to and analyses of all subjects’ pretreatment CBCT data were approved by a domestic institutional review board.
All CBCT records included were obtained with the patients in an upright position by using the same CBCT machine (NewTom VG; QR, Verona, Italy) and the same parameters: 16-cm diameter field of view, 110 kV, 1-20 mA (pulsed mode), and 0.3-mm voxel size.
All CBCT data (DICOM files) were assigned new names by using randomly generated codes that removed patient identification information. These data were subsequently analyzed by 1 rater (B.Y.) using standardized methods and protocols (detailed below) established after a pilot session.
DICOM files were first imported into Mimics software (version 13.0; Materialise, Leuven, Belgium) and segmented to display the teeth and skull for 3D surface rendering at different Hounsfield unit ranges (1553-2850 for teeth; 226-3071 for skeletal). The segmentation ranges were chosen based on default threshold values of dental enamel and bone in the program. Then, the image was reoriented according to the protocol of Swennen et al to adjust the skull to a standard position. Canine impaction and qualitative variables ( Table I ) were assessed and confirmed in the volumetric and 3 orthogonal views. The landmarks for measuring quantitative variables ( Table I ) were identified in the volumetric view by using the digitizing landmarks tool of the program. After verification in the sagittal, coronal, and axial views, these landmarks were used to measure quantitative image variables. The linear measurements to the nearest 0.01 mm were calculated by the program. Intrarater reliability of all categorical and quantitative variables was assessed by repeating the analysis of 40 patients after a 1-month interval.
| Quantitative variables | Values (mm or °) and definition |
|---|---|
| MD (tooth mesiodistal width) | Maximum mesiodistal crown diameter of maxillary anterior teeth (U1, U2, U3) |
| BL (tooth buccolingual width) | Maximum buccolingual crown diameter of maxillary anterior teeth (U1, U2, U3) |
| IP1 (anterior dental arch width) | Intermaxillary arch width between the deepest points of the central fossae of the maxillary first premolars (U4) |
| IM1 (posterior dental arch width) | Intermaxillary arch width between the deepest points of the central fossae of the first molars |
| J-J (maxillary skeletal width) | Linear distance between the right and left jugal points (intersection of the outline of the maxillary tuberosity and the zygomatic buttress) |
| AG-AG (mandibular skeletal width) | Linear distance between the right and left antegonion points (lateral inferior margin of the antegonial protuberance of the mandible) |
| NC-NC (nasal cavity width) | Linear distance between the right and left lateral piriform rims |
| U3-U2/U4 (canine interdental distance) | Perpendicular distance from U3 cusp tip to the long axial plane of adjacent teeth |
| Angle: U3-U2/U4 (canine interdental angles) | The acute angle between U3 and the long axes of adjacent teeth |
| U3-MSP (canine-midsagittal plane distance) | Perpendicular distance from U3 cusp tip to the midsagittal plane |
| U3-OP (canine-occlusal plane distance) | Perpendicular distance from U3 cusp tip to the occlusal plane |
| Categorical variables | Values (positive finding) and definition |
|---|---|
| Supernumerary teeth | Presence of 1 or more supernumerary teeth |
| Maxillary incisor impaction | Presence of 1 or more impacted maxillary incisors |
| Peg-shaped lateral incisors (U2) | The greatest U2 MD width at the cervical margin was less than 66% of U1 width |
| Maxillary lateral incisor (U2) agenesis | Congenital absence of 1 or both maxillary lateral incisors |
| Premolar agenesis | Congenital absence of 1 or more premolars |
| Third molar agenesis | Congenital absence of 1 or more third molars |
Statistical analysis
Cohen’s kappa (for categorical variables) and intraclass correlation tests (for quantitative variables) were used to assess intrarater reliability. Tooth dimension measurements were compared between groups by Student t tests. One-way analysis of variance (ANOVA) was used to compare tooth, dental arch, and skeletal widths among the buccal impaction, palatal impaction, and their matched control groups. If significance was found, Tukey-Kramer tests were used for post hoc pair-wise comparisons. Chi-square or Fisher exact tests (if incidence was <5) were used to compare the prevalence of dental anomalies in each group. Nonparametric Wilcoxon tests were used for impacted canine-related linear and angular measurements because of the nonnormality of the data.
Results
Overall, intrarater reliability for image assessment was excellent for both categorical variables (kappa tests, κ >0.9) and quantitative variables (intraclass correlations, r >0.9).
In the unilateral canine impaction subjects ( Table II ), measurement of the mesiodistal and buccolingual widths of the central and lateral incisors yielded similar results on the impaction and normal sides. However, the mesiodistal widths of the canines on the impaction side was significantly greater (on average, by 0.2 mm) than those on the normal side, whereas the buccolingual widths were similar between the impaction and normal sides ( Table II ).
| Group | Side | Central incisor | Lateral incisor | Canine | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Impacted | Normal | P ∗ | Impacted | Normal | P ∗ | Impacted | Normal | P ∗ | ||
| Unilateral buccal impaction (n = 93) | MD | 8.82 ± 0.61 | 8.77 ± 0.57 | NS | 7.33 ± 0.73 | 7.27 ± 0.69 | NS | 8.35 ± 0.55 | 8.13 ± 0.55 | <0.01 |
| BL | 7.47 ± 0.58 | 7.49 ± 0.50 | NS | 6.74 ± 0.70 | 6.85 ± 0.59 | NS | 8.51 ± 0.63 | 8.54 ± 0.66 | NS | |
| Unilateral palatal impaction (n = 63) | MD | 8.69 ± 0.46 | 8.62 ± 0.47 | NS | 6.77 ± 0.73 | 6.79 ± 0.69 | NS | 8.28 ± 0.50 | 8.01 ± 0.45 | <0.01 |
| BL | 7.38 ± 0.58 | 7.44 ± 0.55 | NS | 6.53 ± 0.80 | 6.63 ± 0.71 | NS | 8.43 ± 0.71 | 8.44 ± 0.76 | NS | |
∗ P values based on paired t tests between impacted and normal sides.
In all bilateral impaction subjects and their controls, the dimensions of the anterior teeth were similar between the right and left sides (not shown), so the averages of both sides were used and combined for all unilateral impaction subjects and controls, respectively, to compare the differences among buccally impacted canines, palatally impacted canines, and their control groups. As shown in Table III , for the mesiodistal dimension, the lateral incisors in the palatally impacted canine group were significantly smaller than in the buccally impacted canine and control groups, whereas no difference was shown for the central incisor or the canine. Even after excluding subjects with peg-shaped lateral incisors in each group, the average mesiodistal dimension of the lateral incisor in the palatally impacted canine group was still significantly smaller than in the buccally impacted canine group. For the buccolingual dimension, all maxillary anterior teeth in the canine impaction groups were significantly smaller than those in their respective control groups, but there was no difference between the palatally impacted canines and the buccally impacted canines groups.
| Measurement | BIC (n = 101) | BIC-C (n = 101) | PIC (n = 69) | PIC-C (n = 69) | P value ∗ | P value ∗ | P value ∗ |
|---|---|---|---|---|---|---|---|
| Mean ± SD | Mean ± SD | Mean ± SD | Mean ± SD | BIC/BIC-C | PIC/PIC-C | BIC/PIC | |
| Central incisor MD | 8.79 ± 0.54 | 8.80 ± 0.47 | 8.65 ± 0.46 | 8.66 ± 0.50 | NS | NS | NS |
| Lateral incisor MD | 7.29 ± 0.67 | 7.32 ± 0.58 | 6.77 ± 0.65 | 7.23 ± 0.55 | NS | <0.001 | <0.001 |
| Lateral incisor MD (peg-shaped lateral subjects excluded) | 7.41 ± 0.56 (n = 91) | 7.38 ± 0.50 (n = 97) | 7.08 ± 0.47 (n = 42) | 7.27 ± 0.53 (n = 66) | NS | NS | 0.006 |
| Canine MD | 8.26 ± 0.52 | 8.31 ± 0.48 | 8.18 ± 0.44 | 8.21 ± 0.50 | NS | NS | NS |
| Central incisor BL | 7.45 ± 0.50 | 7.75 ± 0.49 | 7.43 ± 0.55 | 7.67 ± 0.47 | <0.001 | 0.034 | NS |
| Lateral incisor BL | 6.77 ± 0.59 | 7.35 ± 0.55 | 6.58 ± 0.69 | 7.22 ± 0.53 | <0.001 | <0.001 | NS |
| Canine BL | 8.50 ± 0.62 | 8.98 ± 0.50 | 8.44 ± 0.69 | 8.82 ± 0.54 | <0.001 | 0.001 | NS |
| IP1 | 35.70 ± 2.06 | 37.29 ± 2.29 | 36.87 ± 2.43 | 37.10 ± 2.20 | <0.001 | NS | 0.005 |
| IM1 | 48.10 ± 2.82 | 48.73 ± 2.98 | 48.55 ± 2.81 | 48.90 ± 2.84 | NS | NS | NS |
| J-J | 73.86 ± 4.37 | 76.99 ± 2.87 | 77.42 ± 5.09 | 77.83 ± 3.37 | <0.001 | NS | <0.001 |
| AG-AG | 86.08 ± 4.35 | 86.46 ± 3.84 | 87.53 ± 4.42 | 87.07 ± 4.40 | NS | NS | NS |
| NC-NC | 23.63 ± 1.50 | 23.49 ± 1.62 | 24.03 ± 2.01 | 23.76 ± 1.65 | NS | NS | NS |
Maxillary interpremolar width in the buccal impaction group was significantly smaller than in its control group and the palatal impaction group ( Table III ), whereas the intermolar widths were similar among all groups. For the skeletal width measurements, maxillary width (J-J) was significantly smaller in the buccally impacted canines group than in the other groups, whereas mandibular width (AG-AG) and nasal cavity width (NC-NC) were similar among groups.
The prevalence of dental anomalies possibly related to canine impaction is shown in Table IV . For buccal canine impaction, only the prevalence of incisor impaction was significantly higher than in its control group. For palatal canine impaction, the prevalence of peg-shaped lateral incisors was significantly higher ( P <0.001) than in its control group and the buccally impacted canines group. There was a missing lateral incisor in only 3 subjects, and the prevalence was not significantly different among groups, but all 3 subjects were from the palatally impacted canine group. The prevalence of each anomaly of supernumerary teeth or missing premolars or molars was not significantly higher in the palatally impacted canines group than in its control group or the buccally impacted canines group. When all 3 anomalies were combined, their prevalence in the palatally impacted canines group was significantly higher than that in the control group, but only when subjects who also had lateral incisor anomalies with any of these 3 anomalies were not excluded ( Table IV ).
| Type of anomaly | BIC | PIC | BIC-C | PIC-C | P value | P value | P value |
|---|---|---|---|---|---|---|---|
| n = 101 | n = 69 | n = 101 | n = 69 | BIC/BIC-C | PIC/PIC-C | BIC/PIC | |
| Impacted incisors | 20, 19.8% | 6, 8.7% | 0, 0% | 0, 0% | <0.001 ∗ | NS ∗ | NS |
| Peg-shaped lateral incisors | 10, 9.9% | 27, 40.3% | 4, 4.0% | 3, 4.3% | NS | <0.001 ∗ | <0.001 |
| Missing lateral incisors | 0, 0% | 3, 4.3% | 0, 0% | 0, 0% | NS ∗ | NS ∗ | NS ∗ |
| Supernumerary teeth | 6, 5.9% | 10, 15.5% | 3, 14.5% | 2, 2.9% | NS | NS | NS |
| Missing premolars | 5, 5.0% | 4, 5.8% | 1, 1.0% | 0, 0% | NS | NS | NS |
| Missing third molars | 31, 30.7% | 25, 36.2% | 24, 23.8% | 16, 23.2% | NS | NS | NS |
| Anomalies of supernumerary teeth, missing premolars, or third molars combined (subjects also had lateral incisor anomalies included) | 37, 36.6% | 35, 50.7% | 28, 27,7% | 18, 26.1% | NS | 0.003 | NS |
| Anomalies of supernumerary teeth, missing premolars, or third molars combined (subjects also had lateral incisor anomalies excluded) | 32, 31.7% | 19, 27.5% | 27, 26.7% | 16, 23.1% | NS | NS | NS |
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