Biomechanics-Based Management of Impacted Canines
Sumit Yadav, Ravindra Nanda
Maxillary canines are important both esthetically and functionally and are the second most commonly impacted teeth after the third molars.1–6 The frequency of unerupted maxillary canines is approximately 0.8% to 2% in the general population, with ethnic variations.4 The incidence of canine impaction is twice as common in females as in males and the teeth are palatally impacted in 85% of these patients and buccally impacted in 15%.6 Patients with impacted maxillary canines are perceived to be more difficult and time consuming to treat than those with a routine malocclusion.
Position of the impacted canines and treatment mechanics are the two major factors affecting total treatment time and the final position of the canines in the oral cavity.7,8 With new technologies such as cone beam computed tomography (CBCT), the impacted canine can be precisely located in three dimensions.7 An optimal force system is then needed for the treatment and this system is defined as that which results in a greater biological response with minimal tissue damage, resulting in rapid tooth movement with few or no deleterious effects.9
Traditional force systems are attenuated as a result of deactivation during tooth movement or because of the physical properties of the force delivery system.10 Appliance design has been focused on improving force delivery. Although an ideal spring that can deliver a continuous force day after day is only a theoretical possibility, various inter-arch and intra-arch mechanics have been devised for the successful management of impacted canines. Oppenhuizen11 and Jacoby12 devised an extrusion spring for palatally impacted canines using prefabricated 0.018-inch stainless steel archwires. Bowman and Carano13 devised new directional force springs called Kilroy I and Kilroy II, used for palatally and buccally impacted canines, respectively. Haydar et al.14 used microscrews with either an elastic traction or a ligature wire for management of the impacted tooth. Vardimon et al. recommended the use of magnets to treat impacted canines on the basis of a less invasive surgical procedure, effective forces at short distances, and controlled spatial guidance.14a
However, none of these authors have discussed the mechanics involved with the use of their appliances. The use of improper mechanics (direction and magnitude of force applied, direction and magnitude of reactionary force) when treating impacted and ectopically erupted canines increases the chance of root resorption of the adjacent teeth. A knowledge of mechanics is essential for effective treatment and an important step toward evidence-based treatment.10,15
Diagnosis of Canine Impaction
The diagnosis of canine impaction is based on both clinical and radiographic examinations.
The following clinical signs along with radiographic diagnosis might be indicative of impacted canines:
Numerous radiographic methods have been proposed for the accurate localization of the impacted canines:
1. Periapical films: A single periapical film provides the orthodontist with a two-dimensional representation of the impacted canines and surrounding dentition.16 To evaluate the position of the canine buccolingually, a second periapical film should be obtained by one of the following methods:
• Clark’s rule (or same lingual opposite buccal [SLOB] rule): Two periapical films are taken of the same area, with the horizontal angulation of the cone changed when the second film is taken. If the impacted canine moves in the same direction as the cone, it is lingually positioned. If the impacted canine moves in the opposite direction, it is situated closer to the source of radiation and is therefore buccally located.
• Buccal object rule: The basic premise of this technique deals with the foreshortening and elongation of the images on the films. If the vertical angulation of the cone is changed by approximately 20 degrees in two successive periapical films, the buccal object will move in the direction opposite to the source of radiation. On the other hand, the lingual object will move in the same direction as the source of radiation.
3. CBCT: CBCT can exactly locate the position of impacted canines in three dimensions of space. This three-dimensional imaging technique can also assess any damage to the roots of adjacent teeth and the quantity and quality of bone surrounding each tooth.17,18
Some research and much debate have surrounded the merits of open versus closed eruption techniques. There appear to be subtle differences between the two eruption techniques in relation to treatment duration and deleterious effects on the periodontal tissue. Two schools of thought exist regarding the eruption techniques; however, the choice between using an open or a closed eruption technique is typically based on individual preference. A simple palatal impaction (cusp tip of the canine at the same level of the cement-enamel junction of lateral incisor or central incisor) usually requires open surgical exposure whereas closed surgical exposure is usually favored when the tooth is more deeply embedded in the bone since open surgical exposure may necessitate excessive removal of the surrounding bone. Individual palatally impacted canines may lead to autonomous eruption after open surgical exposure; however, the orthodontic literature lacks sufficient clinical evidence as to which palatally impacted canine will respond favorably to which eruption technique.19–21 Closed eruption technique has been much criticized regarding the direction of tooth eruption (direction of force) and its deleterious effects (root resorption of lateral incisor). However, with the advent of three-dimensional radiography (CBCT), the exact location of the impacted tooth can be identified and an optimal force system can be applied, which may result in rapid tooth movement with minimal deleterious effect on the impacted and the adjacent tooth.20,21
The etiology of impacted canines is obscure and multifactorial. However, primary etiologic causes of maxillary canine impactions include (1) arch length deficiency (buccal canine impactions), (2) disturbances in tooth eruption sequence (hormonal or disease induced), (3) trauma to the maxilla or maxillary dentition, (4) prolonged retention of deciduous canine, (5) rotation of tooth buds, and (6) pathological lesions such as cysts and odontomas localized in the area of permanent canines and lateral incisors. It is hypothesized that palatally impacted canines are associated with a hypoplastic or missing lateral incisor (guidance theory) or with aplasia of the premolars or third molars (genetic theory). Palatally impacted canines are often present with adequate arch space whereas buccally impacted canines are thought to be associated with dental arch deficiencies.
The permanent maxillary canine lacks the guiding force during the process of eruption into the oral cavity because of extra space in the apical part of the maxilla resulting from a hypoplastic or missing lateral incisor. This theory purports that impacted canines are frequently found in persons with peg-shaped or missing maxillary lateral incisors. Even if these impactions are genetically determined, guidance theory states that the palatal canine impaction usually occurs as a result of local environmental disturbances.