The early diagnosis and interception of potential maxillary canine impaction is the most desirable approach for correcting their path of eruption. However, there is still a lack of evidence regarding the effect of rapid maxillary expansion (RME) on labially impacted canines. This study aimed to investigate the age-related effect of RME on labially impacted maxillary canines in order to reduce the risk of their impaction in the mixed dentition and to examine the proper timing of interceptive treatment.
All patients aged 7-10 years were treated with an RME appliance using the same protocol. The distance to the occlusal plane, axis to the midline, and distribution in different sectors—depending on the patients’ age—were evaluated for maxillary canines before and after treatment on panoramic radiographs in order to detect changes in the position of the impacted canines. These geometric measurements in the impacted canines were also validated by observing the nontreated canines at each age.
Significant differences existed between the impacted canines and the erupted canines in all 3 categories in all age groups. RME treatment modulated the position of the impacted canines in all age groups. Interestingly, a statistically significant difference before and after RME in all categories was detected in patients aged <8 years. A discriminant analysis also showed a positive association of RME treatment with the risk of labially impacted canines. The standardized regression coefficients showed that the angulation of the maxillary canine was the most important predictor for impaction.
Our findings indicate that RME treatment in the early mixed dentition was effective for managing labially impacted maxillary canines. An age of 7-8 years with early mixed dentition might be the most appropriate timing for therapeutic intervention on the basis of RME treatment for buccal canine impaction.
Labially impacted maxillary canines were diagnosed in a 7-year-old patient.
Three differential assessments all used panoramic radiographs.
Rapid maxillary expansion in a young patient can produce the most desirable outcome.
The maxillary canines play pivotal roles in smile esthetic and the occlusal function. The impaction of these teeth has been identified as a risk factor for root resorption of adjacent teeth, ankylosis of the canine, and cyst formation. The incidence of impacted maxillary canines, defined as the antimere completely erupting into the bone, is 2% of the population; the impaction of these teeth is the most frequent after that of third molars. The reason for such a high incidence of impactions has been hypothesized as follows: the tooth germ of the maxillary canine forms high in the anterior wall of the antrum but below the floor of the orbit. The eruption path leading to their correct position in the maxillary arch is long and tortuous. The total arch length for the permanent teeth is primarily established at the time of the eruption of the first molars, with the canines being late in erupting. Specifically, an arch length deficiency has long been thought to be a primary etiologic factor for maxillary canine impaction.
The clinical management of impacted maxillary canines is an important task for most orthodontists. Early diagnosis and careful observation are necessary to ensure the successful treatment of these teeth because the management can involve multiple disciplines, substantial time, financial costs, and risks of gingival recession. , The impaction of maxillary canines can be judged in advance on the basis of clinical perception, palpations, panoramic radiographs, , lateral cephalography, posteroanterior cephalography, and cone-beam computed tomography (CBCT). , Although CBCT imaging is more accurate and reliable than conventional 2-dimensional (2D) radiography, , it is still controversial as a first-line imaging method because of the relatively high radiation dose required for the early diagnosis of tooth impaction, especially in a pediatric population. , An interceptive approach to managing impacted canines is recognized as a useful procedure for preventing the risk of impaction. However, precise information on the most appropriate timing for applying an interceptive approach, which is considered valuable for clinicians, is still lacking.
Maxillary expansion has been proposed as a major interceptive treatment for palatally impacted canines that can eliminate or reduce the severity of developing malocclusion. Palatally impacted canines are usually associated with genetic determinants, such as the congenital absence of the lateral incisors or the second premolars and peg-shaped lateral incisors. In contrast, a previous study found that 83% of labially impacted canines were associated with a lack of available arch length, suggesting that the appropriate correction of this discrepancy may effect desirable changes in the eruption of maxillary canines. However, there are few reports on the effects of interceptive treatment for labially impacted canines.
Our primary objectives in the present study were (1) to determine whether the use of rapid maxillary expansion (RME) affects the position of labially impacted maxillary canines by assessing panoramic radiographs in the mixed dentition, and (2) to identify the appropriate timing of the interceptive approaches for impacted maxillary canines. Our results support the establishment of clinical guidelines for the diagnosis and management of labially impacted maxillary canines with a relatively lower radiation dose and cost.
Material and methods
A diagram of this retrospective case-control study is shown in Figure 1 . Eligible subjects were selected retrospectively from the archives of our office among those who participated in a regular dental checkup between October 2001 and September 2016, ranging in age from 7 to 10 years (n = 5107). The exclusion criteria were as follows: (1) systemic disease, (2) supernumerary teeth, (3) fused teeth, (4) congenitally missing teeth, (5) craniofacial syndrome, and (6) cleft lip and palate ( Fig 1 ).
First, we compared the panoramic radiographs of 82 patients with labially impacted canines (26 males [mean age, 9.0 years] and 56 females [mean age, 8.8 years]) to those of 90 randomly selected patients who did not undergo orthodontic treatment (40 males [mean age, 8.9 years] and 50 females [mean age, 8.7 years]). We defined a canine as being impacted if it met all 3 diagnostic criteria for canine displacement or if it caused root resorption of adjacent teeth on CBCT, as described previously. The assessment and confirmation of the position of impacted maxillary canines were performed according to the method described in a previous study. As shown in Figure 2 , all cases were evaluated for the distance to the occlusal plane (d, mm), the angle made by the long axis of the impacted canine with the midline (α, °), and the position of the canine crown in different sectors, depending on the location of the tip of the tooth (S) ( Fig 2 ).
Next, the effect of RME was assessed in 64 subjects in the impacted maxillary canine group (17 males [mean age, 9.0 years] and 47 females [mean age, 8.8 years]) ( Fig 1 ). All subjects were treated with bonded RME appliances (5 mm of active expansion; at the end of the expansion, all patients retained the acrylic splint expander for 5 months) ( Fig 3 , A ). The screws were expanded at a rate of 2 turns/d (0.2 mm of expansion per turn), and new bone formation in the midpalatal suture was confirmed by occlusal x-ray before the removal of the RME appliance ( Fig 3 , B ). Panoramic radiographs were taken before and after RME and then traced to evaluate the position of the maxillary canines. Each measurement was performed by 1 investigator (M.H-K.). The study was approved by the institutional ethics committee of the Japanese Orthodontic Society (2018-5).
All of the data were presented as the mean and standard deviation. A 2-tailed Student t test, Welch t test, or Mann-Whitney U test was performed for the statistical analyses between the erupted group and the impacted group using the GraphPad Prism 6.0 software program (GraphPad Software, San Diego, Calif). Statistical analyses for the changes in the maxillary canines after treatment with RME were performed using a paired t test or Wilcoxon signed rank test. Discriminant analyses were performed on the groups in order to assess the treatment outcomes of RME using the XLSTAT version 2018.1 (AddinSoft, New York, NY). Discriminant analyses were performed to validate the positional diagnosis of labially impacted maxillary canines and to examine the effect of RME. Standardized coefficients of the discriminant function were used to determine the correlation of variables’ contribution to impacted maxillary canines. P values of <0.05 were considered to indicate statistical significance.
We compared the position of the labially impacted canine with the control group in order to validate the previously reported geometric measurements. All age groups showed a highly statistically significant difference between the mean location of the impacted canine and the unaffected control in all 3 categories ( P <0.0001), suggesting the high sensitivity of our geometric measurements for identifying impacted canines ( Table I ). Our analysis failed to find any significant differences in the laterality of the canine position in either the control or impacted groups (data not shown). According to the confusion matrix, 92.9% of untreated canines were classified correctly ( Table II ). Sensitivity and specificity comparisons were also plotted graphically using a receiver-operating characteristic curve analysis and the area under the receiver-operating characteristic curve, indicating a high diagnostic accuracy (area under the curve, 0.969) ( Fig 4 ). On the basis of these results, the combination of the variables selected in the analysis was proven highly effective for distinguishing impacted canines.
|Age, y||n||Distance (d, mm)||Angulation (α, °)||Sector location (S)|
|7||17||34||21.5 ± 2.3 ∗||18.4 ± 2.6||20.8 ± 8.9 ∗||7.5 ± 7.4||0.8 ± 0.8 ∗||0.1 ± 0.4|
|8||35||47||20.4 ± 3.6 ∗||14.3 ± 4.7||25.7 ± 12.6 ∗||7.0 ± 6.7||0.9 ± 0.8 ∗||0.06 ± 0.2|
|9||29||29||19.4 ± 3.2 ∗||10.5 ± 5.8||25.3 ± 8.9 ∗||5.4 ± 7.3||1.1 ± 0.3 ∗||0 ± 0|
|10||18||28||19.3 ± 4.0 ∗||7.3 ± 7.3||27.8 ± 19.6 ∗||2.0 ± 7.6||1.5 ± 0.8 ∗||0 ± 0|