13: Aligners and impacted canines
Edoardo Mantovani, David Couchat, Tommaso Castroflorio
Introduction
Except for the third molars, the impaction of the upper canine is the most common in the permanent dentition, and its recovery is nearly always recommended. The importance of canines, both from a functional and an aesthetic point of view, is crucial to set a proper occlusion. Furthermore, possible adverse sequelae of canine impaction1 can be as follows:
The prevalence of upper canine impaction is ranging between 0.3% and 2.4%, depending on the population, age, sex, and ethnicity.2–5
The impacted maxillary canines are more common in white populations6 and in female patients, with a male to female ratio of approximately 1:3.7
Impactions are unilateral in the majority of cases, and the occurrence on the palatal side is three times higher than on the labial side.8,9
Some systemic endocrine or infectious diseases are related with failed eruption of one or more teeth (Fig. 13.1).10 They act as predisposing factors but always in conjunction with a local pathologic condition, such as11:
These factors can be associated with impactions of every tooth and are usually related to incisors or premolars. Therefore, other causes can be identified regarding impacted canines. Since impacted upper canines have been diverted or are angulated aberrantly during development, it has been assumed that eruption of the canine is strongly influenced by environmental factors.12,13
The maxillary canine has the longest path of eruption, and a long time period is needed. This could explain the higher percentage of inclusion compared to other teeth.
The upper canine begins its development from the superior part of the maxilla. At age 2 years, the crown is located in correspondence of the apex of deciduous canine, mesially inclined.14 When the permanent incisors are erupted, the close relationship between the crown of the canine and the distal aspect of the root of lateral incisor is particularly important.15 Since the upper cuspid is one of the last teeth to reach its position, the lack of space in the arch can have a great influence on the prevalence of impactions, especially regarding the labial ones.16
The studies that have investigated palatal impactions pointed out the increased incidence of missing or peg-shaped laterals.11,17 This leads to the formation of two theories: the genetic theory and the guidance theory.3,6,18 Both theories share the belief that certain genetic features occur in association with the palatal displacement of maxillary canines. The right side of any patient is genetically identical to the left side. Since many studies indicated 60% to 75% preponderance of unilateral canine impaction, it is reasonable to state that local factors are the prevailing elements.13
Zilberman demonstrated that anomalies of the lateral incisors in patients with palatally displaced maxillary canine (PDC) teeth were found to be four times that of the general population.19 The canine impaction has been related with abnormalities regarding the shape and length of the root of the lateral incisor rather than its agenesis (Fig. 13.2).20
However, missing, small, and peg-shaped lateral incisors are three varieties of expression of a single genetic factor. A peg-shaped or small lateral incisor on one side of the mouth and a missing on the other can be frequently seen (Fig. 13.3).
According to the guidance theory of canine impaction, these factors create a genetically determined environment in which the developing canine is deprived of its guidance, thus influencing it to adopt an abnormal eruption path.
Early diagnosis and treatment
A tooth is impacted when it fails to erupt into the dental arch within the expected developmental window. Therefore, an early diagnosis is crucial to reduce the consequent issues. Palpation of the labial fornix to assess the crown of the erupting canine is the first clinical attempt needed to identify a possible impaction. In case of a well-marked prominence absence in the late mixed dentition, orthopantomography (OPG) is mandatory (Fig. 13.4).21
The early identification signs on radiographs of an abnormal pathway of eruption is needed to prevent canine retention and maxillary incisor root resorption.22
The deciduous canine extraction is recommended when limited or absent resorption of its root can be detected, in class I uncrowded malocclusions.21,23
Ericson and Kurol,21 to evaluate the need of primary canine extraction and its corrective effect, determined a method for detection of the permanent canines, based on the following (Fig 13.5):
The success rate of early extractions will vary depending on the position of the permanent canine on OPG. If the crown of the permanent canine is distal to lateral incisor root axis, the primary canine extraction normalized the erupting position of the permanent canine in 91% of the cases. In contrast, the success rate decreased to 64% if the permanent canine crown were mesial to the midline of the lateral incisor root (Fig. 13.6).24
Bonetti et al. demonstrated that deciduous canine and first molar extractions are more effective as a preventive approach to promote eruption of retained maxillary permanent canines positioned palatally or centrally.25
On the lateral cephalometric radiograph the normal inclination of the canine compared to the perpendicular to the Frankfurt plane should be about 10 degrees (Fig. 13.7). Higher values are related with increased need for orthodontic treatment.26
Hong et al., using cone-beam computed tomography (CBCT) data, stated that the maxillary transverse dimension had no effect on the occurrence of PDC.27 Baccetti demonstrated that, in PDC cases not requiring maxillary expansion, the use of a transpalatal arch (TPA) in combination with deciduous canine extraction can be effective for the permanent canine eruption.28
On the contrary, there is a strict relationship between the lack of space and the labially impacted canines, in particular a transverse maxillary deficiency located in the anterior portion of the dental arch.29
Research using the CBCT approach stated that buccal canine impaction is mostly associated with anterior transverse (dental and skeletal) deficiency.30
Subjects with unilateral or bilateral impacted maxillary canines have smaller maxillary transverse dimensions than subjects without impaction.31
The effect of rapid palatal expansion as a predictor of automatic eruption has been previously demonstrated.32,33
Early treatment of impacted canines is mandatory in case of severely resorbed incisors. When resorption process is halted, the incisors do not suffer from increased mobility or discoloration in the long term.34
Late diagnosis
Diagnosis of upper canine impaction after the expected age of eruption is primarily clinical, with or without the presence of the corresponding deciduous canine. Ectopic or absent canine prominence is usually detected during the examination. The information provided by OPG gives an overall picture but cannot determine the proper position of the canine. However, when it is possible to identify the cause of failed eruption (e.g., a mechanical obstacle such as odontoma), its removal can allow the tooth to erupt spontaneously.
Lindauer,35 in his study using panoramic x-ray, found that 22% of PDC had their cusp tip distal to the lateral incisor and remained undetected.
CBCT systems provide three-dimensional (3D) images and useful data for a more accurate locating of impacted teeth.36
CT investigations have proven to be superior in detecting root resorption compared with conventional radiographic methods (intraoral and panoramic radiographs). The amount of resorption detected by CT scanning was approximately 50% higher.37 Root resorption of the maxillary permanent incisors caused by ectopic eruption of the permanent canine has an overall prevalence of 12%, with a prevalence that is four times as high in girls as in boys.38
Dental follicles of the ectopically erupting canines are on average wider than those of the normally erupting canines.39 During eruption, the follicle of the erupting maxillary canine frequently resorbs the periodontal contours of adjacent permanent teeth but not the hard tissues of the roots.
Resorption of neighboring permanent teeth during maxillary canine eruption is most likely an effect of the physical contacts with active pressure during eruption and cellular activities. The resorptive mechanism seems to be confined to the dental follicle and related to metabolic activation.40 Yan found no significant difference of resorption prevalence between subjects with buccal and palatal impactions. The dominant predictor for resorption was contact relationship less than 1 mm.41
Another recent CBCT study found no significant correlation between follicle width and the variables of gender, impaction side, and localization of maxillary impacted canines.42 Other factors influencing diagnosis and treatment planning, such as ankylosis and root dilaceration, can be identified mostly on CBCT images. Furthermore, CBCT data can provide useful information about shape and size of the impacted canine, especially if further intraarch space is required (Table 13.1).43
According to Becker,44 the major reasons for failure are inadequate anchorage (48.6%), mistaken location and directional traction (40.5%), and ankylosis (32.4%). There is no age limit for orthodontic recovering of impacted canines, but the chance of success decreases with age. A study undertaken in adult patients found 69.5% success rate of impacted maxillary canine treatment among the adults compared with 100% among the younger controls, even though the overall length of orthodontic treatment was similar. All the failed canines were found in the older adult subgroup (>30 years of age).45
Treatment planning and orthodontic management
The main goal of every orthodontic treatment is not only the correction of malocclusion but also a good alignment and healthy periodontal tissues. Regarding impacted canines, the eruption should be in the center of the alveolar ridge.46
During physiologic eruption there is a fusion between keratinized gingiva and reduced enamel epithelium with the formation of the junctional epithelium.47 When this occurs, a proper arrangement of periodontium with an adequate band of keratinized tissue, correct sulcular depth, and connective fibers inserted on cementoenamel junction (CEJ) can be found.48 If a canine erupts in the alveolar mucosa, lack of junctional epithelium may occur, leading to further mucogingival issues (Fig. 13.8).49,50
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