Material and methods

This retrospective study involved the analysis of clinical and radiographic records collected from the practice of a specialist orthodontist. Ethical approval to conduct this research was received from the University of Queensland Human Research Ethics Committee B on February 19, 2018 (approval no. 2017002083).

All patients had at least 1 impacted maxillary canine (either palatal or labial/midalveolar), which was surgically exposed at the commencement of treatment and retrieved with fixed preadjusted edgewise appliances. No generic protocol was used, as mechanics for each patient were individualized according to their specific clinical requirements. Broadly, however, transpalatal arches were used in the case of any palatal impaction, and fixed orthodontic appliances were in position before surgical exposure. All patients received open exposures—either conventionally if the canine was palatal, or via apically repositioned flaps if the canine was labial or midalveolar in site. Traction of the canine involved movement of the canine away from adjacent teeth, extrusion of the tooth and assessment regarding its viability, alignment of the canine, and correction of any torque discrepancies. All patients were treated by a single orthodontist (N.P.). Included patients were consecutively debanded cases that met the selection criteria; however, this was a limited subset of all patients with ectopic canines treated within the practice (ie, only those completed patients for whom a pretreatment CBCT scan was indicated).

The inclusion criteria were as follows: (1) the presence of at least 1 impacted maxillary canine, (2) aged <21 years at the commencement of treatment, (3) treatment successfully completed with fixed preadjusted edgewise appliances, and (4) available CBCT radiographic dataset (taken before treatment commencement).

Patients were then excluded if: (1) the ectopic canine was woven or transposed or if it was extracted as part of orthodontic treatment (n = 3), and (2) the radiographic dataset was taken after the commencement of treatment (n = 2) (ie, without an available pretreatment radiographic dataset).

The final sample consisted of 37 patients (21 females and 16 males) with a mean age of 15.1 years. A sample of age- and sex-matched control patients successfully treated with fixed preadjusted edgewise appliances (but without impacted maxillary canines) were also collected from the same clinician. This control sample consisted of 39 patients with a mean age of 15.1 years, with 23 females and 16 males. Table I outlines the characteristics of both samples.

For both treatment and control groups, patients’ age at the start of treatment, sex, and total treatment duration were recorded. Treatment was deemed to have been successfully completed on the removal of fixed appliances. This involved the attainment of well-aligned arches with correction of the impacted canine; the orthodontist and patient were both satisfied that the goals of treatment had been met, and there were no reported treatments that had been prematurely completed. The assessment of successful treatment completion for patients with impacted canines was no different from any other orthodontic case, and this included (but was not limited to) the successful alignment of the previously impacted canine.

CBCT datasets obtained immediately before treatment were imported into InVivo 3D Imaging software (Anatomage, San Jose, Calif) for interpretation and landmark identification.

If both maxillary canines were impacted, the bilateral status was recorded, but only the more severely impacted canine (on the basis of sector, height above the occlusal plane, and α-angle) was chosen for radiographic analysis. For bilaterally impacted canines, the recorded side of impaction reflects the canine chosen for analysis. The buccopalatal site of impaction was determined on the CBCT by the position of the canine cusp tip in relation to a line tangential to the palatal surfaces of the roots of the adjacent lateral incisor and first premolar at the level of the canine tip. This enabled clear radiographic demarcation into a palatal site, or a combined grouping of canines located labially or in a midalveolar position.

CBCTs were assessed using a mathematical approach. The 3D coordinates (x, y, and z values) of relevant anatomic landmarks were recorded within the InVivo software. From these coordinates, using methods of 3D Euclidean geometry, we were able to calculate the vertical and horizontal displacement, angulation, apical curvature, buccolingual inclination, and crown rotation of the canine. We list all these characteristics and define them rigorously in the legend for Figure 1 .

Direct CBCT measurements. Horizontal (in millimeters), the absolute horizontal distance of the canine cusp tip ( T ) to the midsagittal plane ( lh ); Vertical (in millimeters), the absolute vertical distance of T to the occlusal plane ( lv ); α angle (°) , the angle between a line perpendicular to the occlusal plane and the long axis of the canine ( Cm – Cp − vector κ ); β angle (°) , the angle between the long axis of the lateral incisor ( I to J ) and vector κ ; BL (°) , the buccolingual inclination measured as the angle between a line perpendicular to the coronal plane (vector λ ), and vector κ ; Hook (°) , the apical curvature ( Θ ) measured as the angle between the apical hook ( H – Ap ) and vector κ ; Hook (in millimeters), the length of the apical hook ( lk ) as measured by the distance between the line passing through RSc – RSa and Ap (measuring the projection of the apical hook from the external contour of the canine root). If H was undefined (ie, no apical hook was present), this length was manually adjusted to 0 mm; Rotation (°) , describing canine crown rotation ( Φ ); Vector μ, defined by projecting the vector D – M onto the occlusal plane; Vector λ, defined perpendicular to the coronal plane, pointing anteriorly; Φ , defined as the angle between λ and μ (if μ points toward the midsagittal reference plane [MSP]); alternatively, it is defined as 360° minus the angle between λ and μ (if μ points away from the MSP); Φ is defined as 0° when vector μ points anteriorly and increases as vector μ rotates mesiopalatally.

We first identified the points that determined the 3 orientation planes. The occlusal plane was defined as the plane passing through the mesiobuccal cusp tip of the maxillary right and left first permanent molars and the midpoint between the incisal edges of the central incisors. The midsagittal was defined as the plane perpendicular to the occlusal plane and passing through the anterior and posterior nasal spines—B ₁ and B ₂ , respectively. Finally, the coronal was defined as the plane perpendicular to the occlusal and midsagittal planes at B ₂ .

Next, we identified the endpoints of the long axis of the lateral incisor (denoted as I and J in Fig 1 ). To do so, we generated a cross-sectional slice bisecting apicocoronally along the long axis of the lateral incisor on the side of the impacted canine.

Afterward, we analyzed the impacted canine: after recording the coordinates of the cusp tip ( T ), we generated a radiographic slice bisecting the canine apicocoronally along the long axis, with the labial surface of the canine facing out of the computer screen (ie, the canine was oriented directly without mesiodistal rotation). In this view, we were able to record the coordinates of the mesial and distal crown convexity (denoted as M and D in Fig 1 , respectively), the center of the canine root in cross-section at the level of the cementoenamel junction on the mesial and distal surfaces (denoted as Cm ), and a point 5 mm from Cm along the long axis of the canine root (denoted as Cp ).

Finally, we achieved a view of the apical anatomy of the impacted canine. To do so, we rotated the canine around the long axis until we obtained a profile view through the long axis of the root and the apical tip ( Ap ). In that view, we recorded the coordinates of the apical structures: the point of deviation of the apical hook away from the long axis of the root ( H ), the coronal point on the canine root surface from which the apical hook protrudes, located on the midcross-sectional slice through the length of the root and hook ( RSc ), and the apical point on the canine root surface from which the apical hook protrudes, located on the midcross-sectional slice through the length of the root and hook ( RSa ).

The collected data were sufficient for us to calculate the characteristics listed in the legend of Figure 1 using recognized geometric formulas. Table II outlines the pretreatment characteristics of the sample of patients with impacted canines.

Statistical analysis

The intraexaminer reliability for radiographically derived variables was assessed from repeated measurements taken at least 2 weeks apart for 30 patients. We constructed 95% confidence intervals for the mean differences in the repeated measurements ( Table III ).

Table III

Intraexaminer reliability

Variables	Estimate	Lower	Upper
Horizontal (mm)	−0.011	−0.160	0.138
Vertical (mm)	0.063	−0.032	0.158
α angle (°)	−0.203	−0.699	0.293
β angle (°)	1.027 ∗	0.093	1.960
Buccolingual inclination (°)	0.362	−0.054	0.778
Rotation (°)	0.923	−0.181	2.027
Hook (°)	2.539	−1.512	6.590
Hook (mm)	−0.009	−0.114	0.096

 

∗ P <0.05.

Linear regression was employed to assess whether patients involving the management of an impacted canine take longer to treat than age- and sex-matched control patients without an impacted canine ( Table IV ). Here, we note the use of Godambe information matrices throughout the manuscript when computing standard errors to mitigate against bias effects from model misspecification because of missing covariates and mild deviations from linearity.

Table IV

Effect of site of canine impaction on treatment time (controlling for age and sex), compared with controls without an impacted canine

Variables	Estimate	SE	t value	P value
(Intercept)	143.000	40.622	3.520	0.001 ∗∗∗
Age	−0.239	2.613	−0.091	0.928
Sex †	−22.546	9.221	−2.445	0.017 ∗
Palatal site	46.656	10.866	4.294	<0.001 ∗∗∗
Labial/midalveolar site	41.484	12.722	3.261	0.002 ∗∗

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