Treatment duration for impacted maxillary canines: A comparison between surgical exposure and conservative non-surgical orthodontic space opening

Impacted maxillary canines may be defined as those teeth that may have completed root development, but unaided eruption is not expected to occur.¹ In European populations, the prevalence of maxillary canine impaction is approximately 2 per cent,^2,3 with a 2:1 female bias.⁴ Approximately 85 per cent of ectopic canines lie palatally, and this is thought to result from a genetic pre-disposition,^5,6 as well as environmental influences in the form of unfavourable guidance provided by the maxillary lateral incisor.^7,8 In Asian populations however, labial canine impaction is more common than palatal impaction, and is associated with a tooth-size archlength discrepancy.⁹ While palatal sites of impaction have been discussed extensively in the orthodontic literature, the impaction of canines in a labial site has been rarely investigated.

The management options for palatally-impacted canines involve interception by the extraction of the deciduous canine,^10–12 space opening using fixed appliances or rapid maxillary expansion,^13,14 leeway space maintenance using headgear or a transpalatal arch,^15,16 or a combination of techniques.^16–18 For those canines that fail to favourably respond, surgical exposure or removal is likely required.¹⁹ While surgical exposure is the definitive management for impacted maxillary canines, this is associated with additional financial expense, and patients report significant pain and morbidity following the surgical procedure.^20,21 Techniques that successfully avoid surgical exposure are therefore preferred.²²

Olive¹⁴ first presented a technique involving the opening of extra space to assist the eruption of a palatally-displaced canine. It was found that 75 per cent of cases were successfully managed by space opening alone, while the remaining 25 per cent requiring surgical exposure. Even if space-opening was unsuccessful, the severity of canine impaction and the extent of required surgical exposure was reduced. The success and time for canine eruption by applying space opening was predominantly related to the sector of tooth displacement.²³ However, only the time to canine eruption was evaluated, and the sample size was limited to include only palatally-impacted canines.

While patients may desire non-surgical intervention, shorter treatments are also preferred as long treatment times are associated with an increased risk of tooth decalcification and root resorption.²⁴ A total treatment time is also a proxy indicator for case complexity, and may influence the decision to proceed immediately with surgical exposure, or indeed, canine extraction.

Given the importance of treatment time in the management of impacted canines, the aim of the present study was threefold.

• Firstly, to identify whether patients who presented with impacted maxillary canines take longer to treat than patients without impacted canines.

• If overall treatment duration was found to be increased, it was subsequently sought to identify patient and radiographic factors that might be contributory.

• It was finally sought to describe pre-treatment characteristics of patients predictably treated with space-opening, as an alternative non-surgical management strategy for impacted maxillary canines.

These aims were investigated in a sample involving both labially- and palatally-impacted canines.

The present retrospective study involved an analysis of clinical and radiographic records collected from the practice of a specialist orthodontist (RO). Ethical approval to conduct this research was received from the University of Queensland Human Research Ethics Committee B (approval number 2017002083) in February 2018.

All patients were diagnosed as having at least one impacted maxillary canine, which was retrieved either non-surgically or surgically, using fixed edgewise appliances to conduct treatment.

The inclusion criteria included:

All patients identified with impacted canines and available records were considered for inclusion.

Patients were then excluded if:

After the application of the criteria, the selected cases comprised 73 patients (55 female and 18 male) whose mean age was 13.5 years. In addition, a sample of age- and gender-matched control patients without impacted maxillary canines was collected. These patients were also successfully treated by the same clinician using fixed edgewise appliances. The control sample consisted of 45 patients whose mean age was 13.3 years and comprised 33 females and 12 males. For all patients, the age at the start of treatment, gender, and total treatment duration were recorded. Table I outlines the pre-treatment characteristics of the impacted canine cases, and the matched control group.

Patients with an impacted canine were managed either surgically or non-surgically. The treatment modality was classified as either conventional orthodontic treatment only (OT; n=49), surgical exposure at the commencement of treatment (S; n=11), or initial conventional orthodontics with subsequent surgical exposure (OT + S; n=13). Orthodontic treatment in the OT and OT + S groups was previously reported and involved the extraction of the deciduous canine, followed by fixed appliances and an open-coil spring to create significant space in the canine region for all cases.^{14,23,25–27} In addition, 12 patients were treated using rapid maxillary expansion, one patient was treated with a high-pull headgear, one patient was treated using a Clark Twin Block, and two cases were managed by premolar extractions. If space opening successfully resulted in canine disimpaction, treatment was deemed to have commenced at the beginning of this initial phase. If the space opening was not successful (but immediately followed by further treatment involving surgical canine exposure), the starting date of treatment was still taken from the commencement of the initial attempt at space opening. The decision that non-surgical space opening had been unsuccessful (with the need for subsequent surgical exposure) was a patient-centred decision consistent with the guidelines presented by Olive in 2002.¹⁴ Surgically managed canines underwent open exposure with power thread applied to traction the canine towards a rigid base archwire. Active treatment was deemed to have been successfully completed when the patient’s and orthodontist’s clinical goals of well aligned arches and the correction of the impacted canine were met on removal of the fixed appliances.

Twenty-five per cent of impacted canine patients demonstrated bilateral impaction. If both maxillary canines were impacted, the bilateral status was recorded, but only the more severely impacted canine (based on sector, height above the occlusal plane, and α-angle) was chosen for radiographic analysis.

The site of impaction was noted from the orthodontist’s clinical examination, and via radiographic localisation determined by the principal researcher. As labial and mid-alveolar position was likely to have a similar clinical appearance, there was no separation into labial or mid-alveolar sites. Of the impacted canine sample, 71 per cent of impactions were palatal, and 29 per cent were in a mid-alveolar/labial position.

Table II describes the pre-treatment characteristics of the impacted canine patients within the sample.

Radiographs obtained immediately prior to treatment for the impacted canine patients were digitally analysed, after analogue radiographs were first converted into a digital format. Two-dimensional radiographs were imported into Dolphin Imaging software (Patterson Dental; Chatsworth, CA, USA) for the determination of angular and linear measurements. If radiographs were only available in digital format without an attached scale, angular variables and categorical classifications were conducted, but no linear measurements were undertaken. Figure 1 (and associated footnote) outlines the variables assessed on the OPGs using conventional measurement techniques. Figure 2 (and associated footnote) outlines the classification system employed to describe horizontal and vertical canine displacement, with the horizontal sector classified according to Lindauer’s modification,^14,28 and the vertical displacement classified according to the KPG index initially proposed by Kau et al.²⁹

Figure 1.

OPG and reconstructed OPG measurements – linear and angular measures (1) α-angle (°): Angulation of the canine according to the angle formed by the long axis of the impacted maxillary canine, and a line bisecting the central incisor axes (2) β-angle (°): Angulation of the canine according to the angle formed by the long axis of the impacted maxillary canine, and the long axis of the adjacent lateral incisor. (3) Vertical (mm): Absolute distance from the occlusal plane, measured perpendicularly from the canine cusp tip to the occlusal plane (defined as the plane connecting the mesiobuccal cusp of the upper first permanent molar, and the disto-incisal edge of the upper central incisor on the same side). (4) Horizontal (mm): Absolute distance from the mid-sagittal plane, measured perpendicularly from the line bisecting the central incisor axes and the canine cusp tip.

Figure 2.

OPG and reconstructed OPG measurements – Sector and Vertical (KPG). Vertical (KPG): The vertical component of the KPG index categorises the perpendicular distance of the canine cusp tip to the occlusal plane, relative to the central incisor (after Kau, Pan, Gallerano et al. 2009).²⁹ Grade 0: Canine cusp tip is in the proper vertical position. Grade 1: Cusp tip is in the coronal region. Grade 2: Cusp tip lies in a horizontal plane adjacent to the cervical third of the incisor root. Grade 3: Cusp tip lies in a horizontal plane adjacent to the middle third of the incisor root. Grade 4: Cusp tip lies in a horizontal plane adjacent to the apical third of the incisor root. Grade 5: Cusp tip is supra-apical to the incisor root. Sector: The sector classification categorises the horizontal displacement of the canine cusp tip, relative to the lateral incisor (after Lindauer et al. 1992’s modification²⁸ to Ericson and Kurol’s original classification). Sector I: canine cusp tip is distal to the distal outline of the lateral incisor root. Sector II: canine cusp tip is mesial to the distal outline of the lateral incisor root, but distal to the long axis of the lateral incisor. Sector III: canine cusp tip is mesial to the long axis of the lateral incisor, but distal to the mesial outline of the lateral incisor root. Sector IV: canine cusp tip is mesial to the mesial outline of the lateral incisor root.

It was suspected that there might be a correlation between the various radiographic factors which collectively described canine impaction severity.³⁴ For example, the α-angle and β-angle measurements both assess the same characteristic of canine angulation, and an angle change around a given centre of rotation will geometrically alter both the vertical and horizontal position of the canine cusp tip. Multicollinearity describes a situation in which two or more supposedly ‘independent’ variables are highly (but not perfectly) correlated, making statistical discernment of their individual effects impossible.³⁵ The present analysis of multicollinearity confirmed previous findings,^34,36–38 and the current theory that the various canine radiographic measurements were highly correlated with one another. Clinically, this would support the assessment of a canine’s overall severity of impaction to predict the treatment duration, as opposed to the evaluation of individual variables (as is performed in research contexts). Nevertheless, the R² indicated that the group of variables describing radiographic position of the impacted canine accounted for just 24.03 per cent of the variability in treatment time. In total, all available variables were able to explain only 32.83 per cent of the total variability in treatment duration. This might indicate that there were important (i.e. biological) variables unaccounted for in the model; alternatively or in conjunction, treatment time is naturally highly variable and unpredictable, even in the presence of available covariates.

Many of the proposed interceptive techniques for ectopic canines operate through the creation or maintenance of arch circumference, yet the appropriateness of each may vary depending on the individual patient’s inter-arch transverse coordination, anteroposterior molar and skeletal relationship, and available leeway space. Given that labial sites of impaction are associated with arch-length discrepancies, the success of space opening (either with or without premolar extractions) in the management of canine impactions would seem intuitive. While palatal sites of impaction are aetiologically distinct, it is hypothesised that the removal of any obstruction or deciduous tooth creates a favourable environment and pathway for permanent canine eruption; the creation of space generates tension within the transseptal fibres connecting the ectopic canine and adjacent first premolar and lateral incisor, and thereby applying a force to encourage the ectopic canine to improve its position.^39,40 Space opening via fixed appliances works similarly to other interceptive techniques, although the magnitude of space creation is larger, and space is created in the precise location required. While an excess of space is initially created, the increase in overjet and midline deviation is temporary,²⁶ with resolution occurring following alignment of the canine, closure of residual spaces, and conventional correction of the malocclusion. The present findings indicated that space opening was not a panacea for all patients presenting with impacted canines, but that appropriate case selection and treatment implementation are crucial. The present study did not seek to highlight the superiority of one treatment modality for every case of canine impaction. Instead, it was considered beneficial to identify those patients who would benefit from conservative orthodontic space opening, and conversely, confirm the severity of canine displacement warranting appropriate surgical exposure.

Space opening is likely to be successful in the management of most labially-impacted canines, and for palatally-impacted canines found in sectors I and II or vertical (KPG) classification 1. All such patients in the present study cohort were able to be managed non-surgically. The average age of patients who were treated by space-opening was slightly reduced, which potentially indicated a greater responsiveness to non-surgical intervention. In a clinical assessment of a subset of the same patients, Ling et al. compared the pulpal, periodontal, and aesthetic outcomes between space opening and accompanying spontaneous eruption, with surgical exposure and orthodontic traction.^26,27 Both treatment modalities had comparable periodontal and pulpal outcomes; however, surgically exposed and extruded canines had poorer alignment (related to greater intrusion, and significantly greater rotation) than canines that spontaneously erupted after space opening. These aesthetic differences resulted in assessors being more likely to correctly identify a previously impacted canine in the surgically-treated group, than the space opening group. Importantly, subjects were generally satisfied or very satisfied with the colour and position of the treated canines. The reduced morbidity, as well as the aesthetic and stability benefits, strongly suggest conservative orthodontic space opening as the initial management option of choice for appropriately identified patients.

A surgical exposure was more likely to be recommended for patients who were older, and who presented with a vertical (KPG) classification of 4, or a high sector classification. Notwithstanding that 20 per cent of patients with a sector IV classification were successfully treated by orthodontic space opening alone, surgical exposure had greater predictability in the management of such severe impactions.

The linear regression analysis presented in Table V indicated a statistically insignificant tendency for space opening to take longer compared to surgical exposure, all other factors (including severity of impaction) being equal. However, the cases that are recommended for conservative space opening and surgical exposure are not equivalent, as all but the most severely impacted cases were successfully managed conservatively. The results from Table VIII indicate that there were no significant differences in treatment time between the surgical and non-surgical management of palatally-impacted canines, given appropriate case selection. Even though there was a tendency for faster treatment in surgical cases, this was likely offset by the severity of the impaction. Treatment time is therefore similar for the surgical and conservative management of the “clear-cut” surgical (S) and non-surgical (OT) cases, respectively. However, a clinical dilemma arises in those patients with “borderline” impactions which may be reasonably treated either surgically or non-surgically. In the present study, 67 per cent of sector III canines responded favourably to space opening and erupted without surgical intervention. For these cases, clinician and patient related preferences might sway the decision between surgical exposure from the outset, or conservative space opening and reassessment during treatment. Even so, the treatment modality (and related factors) explained only 8.34 per cent of the variation in treatment time. Given these findings, conservative space opening is recommended as the initial treatment of choice for all but the most severely impacted maxillary canines. Even if surgical exposure was required after initial space opening, it is suggested that canines often improve in their radiographic position, which subsequently facilitates a reduction in the extent of required exposure.

The present study contributes significantly to the existing knowledge base regarding impacted canines. Overall treatment duration was investigated, rather than the time for canine eruption or alignment. The alignment of an impacted canine is only the first stage of therapy, and patients perceive that treatment is ongoing until fixed appliances have been removed, and hence a report on overall treatment time is important. Secondly, the presented method of space opening carries significantly less morbidity than surgical exposure and was found to be effective and comparably efficient for a significant number of impacted canines. Finally, the current work includes, and significantly expands, upon the original samples of patients from 2002 and 2005.^14,23 This expanded sample includes both non-surgical and surgically managed patients, as well as palatal and labial/mid-alveolar sites of impaction (the latter of which is underreported in the broader literature). Considering the promising initial findings, the data from this larger cohort also represents progressiveness in the application of the originally-described technique with more severe cases recommended and both patient and orthodontist prepared to persevere with space opening.

However, it is acknowledged that a significant limitation of the present research was its retrospective nature, and the likely associated bias in favour of the intervention being reported.⁴¹ The overall malocclusion and extraction/non-extraction treatment plan may have influenced treatment duration, for which there was no account. Only treatment time was assessed, but not the appointment interval nor the number of patient appointments. While it is important to discuss overall treatment duration, the effect of the treatment modality or the pre-treatment level of impaction on the duration of treatment might be diluted to insignificance over the entire course of treatment, as these variables would only delay the time required for initial alignment of the canine. While clinically useful conclusions have be drawn, a larger sample size would have afforded even greater statistical power for analyses involving surgical subgroups with limited numbers of patients. Care was taken in the analysis to provide 95% confidence intervals so that readers might independently evaluate the potential effect sizes for clinical significance, likelihood of statistical significance, and the degree of certainty. Nonetheless, the sample was as large as practical, comprising the complete sample of relevant patients collected over a single orthodontist’s practicing career. Finally, only treatment duration was assessed to evaluate the success and efficiency of space opening mechanics. Other patient-centred factors including acceptance of surgical morbidity, aesthetic perception of spacing while awaiting canine eruption, or emphasis given to treatment efficiency were not evaluated, and this warrants further investigation and comparison with surgical exposure.

Ultimately, the findings of the present study reinforce the responsibility of all clinicians to screen for canine impaction in all of their patients, as early diagnosis (at a younger age, and before worsening ectopia) maximises the potential for effective, non-surgical management. The promising study findings indicate the need for prospective randomised controlled trials to properly clarify the utility of orthodontic space opening for the management of impacted maxillary canines.²²

• Patients presenting with impacted maxillary canines, both labial/mid-alveolar and palatal, take significantly longer to treat than conventional orthodontic cases without impacted canines.

• Radiographic variables describing canine position are highly correlated, and this collection of variables has a significant influence on treatment duration.

• Overall, treatment duration is highly variable, as only a third of this variability is explained by the examined factors.

• There was a statistically insignificant tendency for surgically-exposed canines to be treated faster than non-surgically managed canines. However, case selection differed substantially.

• Orthodontic space opening is likely to be successful for labially-impacted canines, and all but the most severely palatally-impacted canines. For appropriately treatment planned cases, successful orthodontic space opening has a similar treatment time compared to cases that require surgical exposure.