Maxillary canine impaction is a common challenge encountered by orthodontists in clinical practice. It is estimated that the prevalence of maxillary canine impaction ranges between 1 - 3%, with a buccal: palatal ratio in Caucasians identified as 1 : 6.¹ Contrastingly, a study conducted on a Korean population reported a higher prevalence of buccal impaction, with a ratio of 3 : 1.² The etiopathogenesis of maxillary canine impaction differs between buccal and palatal impactions; buccal impaction is primarily due to an arch-length discrepancy, whereas palatal impaction may be attributed to genetic factors or a lack of eruption guidance.³

Studies aiming to predict the impaction of maxillary canines have focused on canine angulation as a predictive factor. Power and Short suggested that if the canine’s angulation to the midline exceeds 31 degrees, the likelihood of improvement diminishes.⁴ Katsnelson et al. identified an association between buccal canine impaction and a canine angulation to the occlusal plane greater than 65 degrees.⁵ A predictive model for canine impaction employing CBCT, which includes variables such as canine rotation and its angle to the midline and occlusal plane, was reported by Alqerban et al.⁶

The depth of canine impaction can be influenced by canine angulation.⁷ Previous assessments of canine angulation, using the midline as a reference, were contingent on anterior dental relationships. Warford et al. used skeletal landmarks, such as the condyle’s superior point, as a measurement reference.⁸ Despite utilizing panoramic imaging, canine inclination assessments are more accurately conducted via computed tomography.⁹ For children and adolescents, the primary demographic for impacted tooth treatment, CT’s small field of view is preferred to reduce radiation exposure.¹⁰ Hence, the palatal plane established in small field CT imaging could serve as a skeletal reference plane for gauging canine angulation. Zeno and Ghafari suggested that the severity of palatal canine impaction could be discerned through inclination measurements like the canine angulation to the palatal plane.¹¹ Nevertheless, research on buccally impacted canines remains sparse.

Moreover, impacted maxillary canines experience restricted root development either within the floor of the nose or against the maxillary sinus cortical bone,¹ potentially hindering growth due to nearby anatomical structures. Thus, early intervention plays a crucial role in the development of the maxillary canine root.¹² The canine root length in cases of buccal impaction was found by Dekel et al. to be significantly shorter than its contralateral counterpart, by an average of 1.3 mm.¹ Conversely, Silva et al. found no substantial difference in root length between impacted canines and their contralateral sides.¹³ Hettiarachchi et al. reported that the average root length of palatally impacted canines was shorter by 2.6 mm than the contralateral side.¹⁴ However, Leonardi et al. observed no significant disparities in the length and volume of palatally impacted canines when compared to the contralateral side.¹⁵

In light of these considerations, it becomes apparent that three-dimensional assessments of buccally impacted maxillary canines using a skeletal reference line are limited. Furthermore, available studies on the root development of impacted canines yield conflicting outcomes. Consequently, this study aims to assess the three-dimensional position and root morphology of unilateral buccally impacted canines, comparing them with the contralateral side by utilizing cone-beam computed tomography.

This study involved patients diagnosed with unilateral buccal impaction of maxillary canines at the Dankook University Jukjeon Dental Hospital. Pretreatment CBCTs for patients presenting with buccally localized unilateral maxillary impacted canines were retrospectively acquired. The contralateral normally localized maxillary canines served as controls. The institutional review board of Dankook University Jukjeon Dental Hospital approved the study under the number 2402001002. The sample comprised 31 individuals (5 male, 26 female), with ages ranging from 8 to 30 years (mean 13.52 ± 4.84 years) (Table 1). The inclusion criteria included: (1) unilateral buccally localized impaction of a maxillary canine (patients younger than the typical age for canine eruption are included if there is no change in the angle and direction of the canine even after six months of observation), and (2) availability of high-quality CBCTs capturing both the impacted and the contralateral normally localized canines. The exclusion criteria encompassed: (1) diagnosed craniofacial congenital anomalies or syndromes, (2) dental traumatic injuries, (3) the presence of adjacent anomalous or missing teeth, and (4) a history of orthodontic treatment.

Table 1

Gender ratio and mean age of the study population

Gender (n)	Male	5 (16%)
	Female	26 (84%)
Age (Mean ± SD)		13.52 ± 4.84

SD, standard deviation.

Computed tomographic analysis

The CBCT images in this study were analyzed using Invivo 6 plus, version 6.5 software (Anatomage, San Jose, USA) by a single investigator (J.E.J). Variables such as rotation, angulation, torque, root volumes, root lengths, and the presence of hooked apices were measured. The palatal plane, defined by the horizontal line connecting both orbitales and passing through the ANS and PNS, served as the horizontal reference. The midpalatal plane, passing through the ANS and PNS and perpendicular to the palatal plane, was used as the vertical reference. A vertical plane through the ANS and perpendicular to both the palatal and midpalatal planes functioned as the transversal reference (Fig. 1).

Fig. 1. Orientation of reference planes. (A) Axial view, (B) Sagittal view, (C) Coronal view.

Rotation was quantified in the axial view as the angle between a tangent to the buccal contour of the tooth and the midpalatal plane. Inclination was determined in the coronal view as the angle between the tooth’s long axis and the midpalatal plane. Torque was evaluated from the sagittal view as the angle between the tooth’s long axis and the palatal plane (Fig. 2). Further, the canine’s root length, root volume, and the presence or absence of an apical hook were assessed. Root length was measured along the long axis from the canine tip to the root apex using a three-dimensional model of canine segmentation. Root volume was calculated from the volume of the entire tooth (including the crown) using a three-dimensional model of canine segmentation. An apical hook was considered present if the angulation in the apical third of the canine root exceeded 50 degrees relative to the root’s long axis (Fig. 3).

Fig. 2. Canine three-dimensional position measurements. (A) Rotation, (B) Angulation, (C) Torque.

Fig. 3. Measurement of canine root characteristics. (A) Root length, (B) Root volume, (C) Apical hook.

Statistical analysis

All statistical analyses employed SPSS Statistics for Windows, version 25.0 (IBM Corp., Armonk, USA). Descriptive statistics calculated the mean and standard deviation for each variable. The Shapiro-Wilk test checked for normality, while the Levene test assessed homogeneity. Fisher’s exact test and the Mann-Whitney U test were used to compare the gender ratio and age between groups with canine impaction and the control group, respectively. The paired-sample t-test compared dependent variables between the impacted canines and the contralateral controls.

Pearson correlation analysis was used to examine the relationship between age and differences in each variable. The intraclass correlation coefficient, evaluating the reliability of the measurements, was analyzed by re-assessing 8 randomly selected CBCT images two weeks after the initial measurements by the same investigator. A significance level of P < 0.05 was established for all tests.

The Intraclass Correlation Coefficient evidenced high reliability across all assessed variables, ranging from 0.957 (impacted canine angulation) to 0.999 (impacted and contralateral canine apical hook).

When comparing the impacted canine (IC) with the contralateral canine (CC), all variables exhibited significant differences at except for the apical hook. The impacted canine (IC) demonstrated, on average, 17.89 degrees more mesiopalatal rotation and 16.73 degrees more mesiodistal angulation compared to the contralateral canine (CC). Regarding torque, root length, and volume, the contralateral canine (CC) presented higher values than the impacted canine (IC). The increased torque in the contralateral canine (CC) implies a more upright position. The root length of the impacted canine (IC) was notably shorter than that of the contralateral canine (CC) by an average of 1.68 mm, and the root volume was significantly lesser by an average of 48 mm³ (Table 2).

Table 2

Comparison of variables according to canine impaction

	IC	CC	Mean difference	IC/CC ratio (%)	P value
Rotation (°)	61.68 ± 3.42	43.79 ± 2.73	17.89 ± 3.45	140.85	< 0.001***
Angulation (°)	28.30 ± 4.07	11.57 ± 0.87	16.73 ± 4.05	244.60	< 0.001***
Torque (°)	50.81 ± 4.34	72.64 ± 1.49	-21.83 ± 4.30	69.95	< 0.001***
Root length (mm)	20.14 ± 0.37	21.81 ± 0.49	-1.68 ± 0.27	92.34	< 0.001***
Root volume (mm³)	639.97 ± 24.47	688.67 ± 21.93	-48.70 ± 11.76	92.93	< 0.001***
Apical hook (n)	1 (0.03%)	0 (0%)	-	-	0.326

IC, Impacted canine; CC, Contralateral canine.

* P < .05, ** P < .01, *** P < .001.

The correlation analysis between the absolute value of the difference in variables ([IC-CC]) and age is detailed in Table 3. A significant positive correlation (P < 0.001) was evidenced between the differences in angulation and torque across canines, and a significant positive relationship was also observed between the differences in root length and volume (P < 0.016). A significant positive correlation was found between age and the difference in root length (P < 0.004), indicating that the disparity in root length between the impacted canine and the contralateral normal canine escalates with age. Nonetheless, the correlation between age and the difference in root volume did not achieve statistical significance (Table 3).

Table 3

Correlation between age and the absolute value of the difference in variables between the impacted canine and contralateral canine

		Age	Rot [IC-CC]	Ang [IC-CC]	Tor [IC-CC]	RL [IC-CC]	RV [IC-CC]
Age	Pearson correlation	1
	P value
Rot [IC-CC]	Pearson correlation	0.074	1
	P value	0.698
Ang [IC-CC]	Pearson correlation	0.073	-0.092	1
	P value	0.701	0.630
Tor [IC-CC]	Pearson correlation	0.240	-0.044	0.684**	1
	P value	0.202	0.816	0.000
RL [IC-CC]	Pearson correlation	0.507**	0.078	0.234	0.242	1
	P value	0.004	0.683	0.214	0.198
RV [IC-CC]	Pearson correlation	-0.131	-0.156	0.260	0.326	0.435*	1
	P value	0.489	0.409	0.165	0.079	0.016

IC, Impacted canine; CC, Contralateral canine; Rot, Rotation; Ang, Angulation; Tor, Torque; RL, Root length; RV, Root volume.

* P < .05, ** P < .01, *** P < .001.

Apart from the third molars, the maxillary canine is the tooth most frequently encountered as impacted.^3,16 Specifically, buccal impaction of the maxillary canine predominantly occurs among Asians.¹⁷ Sajnani and King have demonstrated that impaction of the maxillary canine can be identified post the age of 8,¹⁸ and clinically, the likelihood of canine impaction may be inferred from the presence of a labial bulge in patients aged between 9 to 10 years.³ Impaction of the canine is known to lead to complications such as ankylosis, cystic formations, and displacement and root resorption of neighboring teeth if not timely addressed.^8,19,20 Furthermore, the maxillary canine undergoes eruption via mesial, palatal, and occlusal migration following the formation of the tooth germ.²¹ If this migration process is impaired, it could hinder root development, resulting in a shortened root or root dilaceration. Consequently, the need for a predictive model to facilitate timely intervention in cases of canine impaction is evident.

This study was designed to undertake a three-dimensional comparison of impacted canines with their contralateral counterparts in patients exhibiting unilateral buccal impaction of the maxillary canine. Utilizing CBCT images, this exploration examined differences in rotation, angulation, torque, root length, root volume, and apical curvature between the left and right maxillary canines in cases of impaction. There is a scarcity of research analyzing unilateral maxillary buccal impacted canines using CBCT images and skeletal reference lines. In this work, the palatal plane was utilized as a three-dimensional skeletal reference, offering a reliable benchmark while minimizing radiation exposure for adolescent patients. In the current study, the CBCT images of maxillary impacted canines, including the palatal plane, had a field of view of 100 × 50 mm, which provides a lower radiation dose compared to the field of view required for full skull scanning that includes reference planes such as the occlusal plane. Conflicting outcomes have been reported in previous studies concerning root development in impacted canines, with research on buccally impacted canines also being limited.^1,13-15

Earlier panoramic studies predominantly relied on canine angulation alone as an indicator of impaction risk. Yet, the findings of this study reveal that not only canine angulation but also rotation and torque should be considered as indicators of impaction risk when using three-dimensional computer tomographic imagery. These findings align with those from prior CBCT studies.^6,11 In the impaction group (IG), it was observed that the impacted canine (IC) exhibited an average increase of 17.89 degrees in mesiopalatal rotation and an average increase of 16.73 degrees in mesiodistal angulation compared to the contralateral canine (CC). The rotation of impacted canines should be considered not only as a predictive factor for impaction but also in surgical approaches and force mechanics during forced eruption of the impacted tooth. Studies focused on the rotation of impacted canines are rare. Drawing parallels from observations on the lateral incisor, the tooth adjacent to the impacted canine, Barros et al. described a frequent mesiolabial rotation of lateral incisors in individuals at high risk of maxillary canine impaction.²² Similarly, Chanshu et al. noted mesiolabial rotation in a patient with an impacted maxillary canine.²³ Dekel et al. reported a mean mesiobuccal rotation of 18 degrees in the lateral incisors of patients with buccally impacted maxillary canines.¹ While these studies align with the current findings, further research focusing on the canine is warranted.

Regarding root length and volume, the contralateral canine (CC) demonstrated superior values compared to the impacted canine (IC), suggesting that canine impaction influences root development. The finding that there is a difference in root length and volume between impacted canines and normal canines suggests that treatment of the impacted tooth at an appropriate time is necessary to promote root growth. This should be considered in the force mechanics during treatment and in the prognosis after treatment of the impacted tooth. Cao et al. postulated that buccally impacted maxillary canines encounter restricted space for root development owing to limited arch space.¹² Dekel et al. found a significant reduction in the root length of buccally impacted maxillary canines compared to their contralateral equivalents, by an average of 1.3 mm; however, no significant disparity in root volume was observed.¹ Leonardi et al. reported no variance in root volume between impacted canines and their contralateral teeth, yet their study was focused on palatally impacted maxillary canines.¹⁵ No significant difference was detected in the presence of an apical hook between the impacted canine (IC) and the contralateral canine (CC), potentially due to the mean chronological age of the sample being 13.52 years, which precedes the completion of canine apex development. According to Dekel et al., albeit not statistically significant, impacted canines displayed a fourfold increase in apical hooks compared to their contralateral normal counterparts.¹ Cao et al. observed a statistically significant higher occurrence of apical hooks in impacted canines than in normal ones, attributing this to the proximity of impacted canines’ roots to the maxillary sinus or nasal floor.¹² Nonetheless, the mean chronological age in these studies was greater than that in the current investigation.

A pronounced positive correlation was established between the disparity in root length between the impacted canine and the contralateral normal canine and age. This signifies that with advancing age, the discrepancy in root length between the impacted and contralateral normal canine intensifies, underscoring the impact of canine impaction on root development as shown by this study’s results.

One limitation of this study is the limited number of samples included, necessitating further research with a larger sample size to develop a predictive model for buccal impaction of the maxillary canines.

The palatal plane can serve as a skeletal reference plane for predicting canine impaction with the advantage of a low radiation dose.

Predictors of impaction are not limited to canine angulation but also include rotation and torque.

Compared to the contralateral normally positioned canine, the unilateral buccally impacted canine exhibited an increased mesiopalatal rotation by an average of 17.89 degrees and a mesiodistal angulation by an average of 16.73 degrees.

The unilateral buccally impacted canine demonstrated a reduction in root length by an average of 1.68 mm and a decrease in root volume by an average of 48 mm³ compared to the root of the contralateral normally localized canine.