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Biomechanics-Based Management of Impacted Canines
CHAPTER 7 Biomechanics-Based Management of Impacted Canines Sumit Yadav and Ravindra Nanda
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axillary 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 intraarch 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.
Clinical Examination The following clinical signs along with radiographic diagnosis might be indicative of impacted canines: 1. Delayed eruption or migration of the permanent maxillary lateral incisors 2. Delayed eruption of the permanent canine (beyond 14 to 15 years of age) 3. Prolonged retention of the deciduous canine (beyond 14 to 15 years of age) 4. Absence of normal canine bulge 5. Presence of palatal bulge Radiographic Diagnosis 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 121
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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. 2. Occlusal films: These serve as an adjunct along with the periapicals in determining the buccolingual position of the impacted canine.16 3. CBCT: CBCT can exactly locate the position of impacted canines in three dimensions of space. This threedimensional 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
SURGICAL EXPOSURE 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 threedimensional 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
ETIOLOGY 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.
Guidance Theory 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. Genetic Theory According to genetic theory, the eruption anomaly of the maxillary permanent canine is the result of a developmental disturbance of the dental lamina. Evidence for this theory is found in familial and bilateral occurrences, sex differences, and the increased incidence of other dental issues such as ectopic eruption of the first molars and infraocclusion of the primary molars.22–25 Research has even shown that palatally impacted canines usually coexist with either hypodontia or agenesis of the third molars.26 Transcription factors such as MSX1 and PAX9, which have been correlated with agenesis of molars, might be involved in palatal canine impactions.27 Baccetti showed that unilateral palatally impacted canines correlated significantly (p <0.05) with aplasia of maxillary lateral incisors whereas bilateral palatally impacted canines have significant correlation (p <0.05) with agenesis of third molars, supporting the genetic etiology of palatal canine displacement.28
BIOMECHANICAL CONSIDERATIONS Cantilever springs with a single force direction and point of force application are commonly used for the management of impacted and ectopically erupted canines (Fig. 7-1). Cantilever springs produce force on the impacted and ectopically placed canines in all three spatial planes, depending on the position of the canines (see Fig. 7-1; Fig. 7-2). The major force directed on the impacted and ectopically placed canine is vertical (extrusive force) and labial/lingual. In horizontal canine impactions, a pure couple force system with high moment-to-force ratio is advantageous to upright the impacted canine before pulling it out in the oral cavity. Cantilever-based appliances or springs (one-couple force systems) are designed to have a low load deflection rate. The
CHAPTER 7 Biomechanics-Based Management of Impacted Canines
D
D
123
MFa
MF
MC
Mc MFp
F
F f
F F
f d Figure 7-1 Anteroposterior view: A cantilever spring design for extrusion of a canine (a one-couple force system). The dotted line indicates the passive state of the spring while the solid design shows that it is in the activated state; in other words, from this point on the spring will gradually undergo deactivation. The force (F) exerted on the canine and molar per Newton’s Third Law of Motion is equal and opposite. Once activated, the spring generates a couple in the auxiliary tube (MC), where MC = F × D. D is the distance between the center of resistance (CRES) of the molar and the point of application of the force on the canine. MC can also be calculated by the product of the force of the couple (f) and the length of the auxiliary tube (d); that is, MC = f × d. Because the force does not pass through the CRES of the canine, it generates a moment due to force (MF). On the molar, as the force is assumed to pass through the CRES, no moment due to force is generated. Note: Always differentiate between a moment due to a force and a moment due to a couple.
force systems delivered by these appliances (springs) tend to stay optimal and consistent in their magnitude, which prevents them from having any deleterious effects on the surrounding periodontium through the entire range of tooth movement. Additionally, an acceptable magnitude of force is maintained in the appliance during treatment to avoid
Figure 7-2 Transverse view: A cantilever spring design for extrusion of a canine (a one-couple force system). MFp: Since the force is buccal to the center of resistance (CRES) of the molar, a moment due to the applied force is generated. Likewise, a moment is generated on the canine (MFa). Here, the net moment acting on the molar (M) can be obtained by subtracting the moment due to the force from the moment due to the couple (MC). Therefore, M = MC – MFp.
frequent reactivations. A single force is exerted on the canine for eruption and alignment. The reactionary force and the moment are dissipated on the molar (see Figs. 7-1 and 7-2), which can be controlled by using a palatal arch and/or ligating the molar to the rest of the arch. Any spring that allows a modification of the force system during the period of getting an impacted canine out in the oral cavity and aligned in the arch is advantageous as long as clinicians are aware of structural mechanics and spring geometry. Text continued on p. 132
CASE REPORT 1 A 13-year-old postpubertal girl presented with a chief complaint of crooked anterior teeth. Extraoral examination revealed a dolichofacial growth pattern with a straight soft tissue profile and an obtuse nasolabial angle. Intraoral examination showed that she had a bilateral Class I molar relationship and palatally impacted maxillary canines (Fig. 7-3). After leveling and aligning, a 0.032-inch CNA (Connecticut new arch form, Ortho Organizers, Carlsbad, CA.) transpalatal arch and a 0.019-inch × 0.025-inch CNA archwire were placed in the maxillary arch. Cantilever springs (0.017-inch × 0.025-inch CNA
wire) applying 80 g of force (occlusal/extrusive) were placed bilaterally from the auxiliary tube of the molar bracket (Fig. 7-4). Springs were activated at each visit and after 5 months maxillary canines erupted in the oral cavity. At this point, the direction of the force was changed from occlusal to buccoocclusal and force was reduced to 30 g. After 12 months of cantilever mechanics, the crowns of both maxillary canines were in the arch (see Fig. 7-4). Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA archwire. Total treatment time was 18 months (Fig. 7-5). Continued
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CASE REPORT 1—cont’d
A
B
D
C
E
G
F
H Figure 7-3 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views.
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CASE REPORT 1—cont’d
A
B
C
D
E
F
G
H
I
Figure 7-4 A-F, Mid-treatment intraoral (occlusal) views. G–I, Mid-treatment intraoral views. Continued
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CASE REPORT 1—cont’d
A
B
D
C
E
G
F
H Figure 7-5 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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CASE REPORT 2 arch. A cantilever spring (0.017-inch × 0.025-inch CNA) was placed from the right auxiliary tube of the molar bracket (Fig. 7-7). Seventy-five grams of bucco-occlusal force were applied and after 6 months the canine erupted into the arch. Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA and a 0.017-inch × 0.025-inch braided stainless steel archwire. Total treatment time was 16 months (Fig. 7-8).
An 11.7-year-old postpubertal girl was referred by her general dentist regarding a missing right maxillary canine. Extraoral examination revealed a dolichofacial growth pattern with a straight soft tissue profile (Fig. 7-6, A–C). Intraoral examination showed that she had a Class I molar relationship and a palatally impacted right maxillary canine (Fig. 7-6, D–H). After initial leveling and aligning, a 0.032-inch CNA transpalatal arch and a 0.019-inch × 0.025-inch CNA archwire were placed in the upper
A
B
D
C
E
G
F
H Figure 7-6 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views. Continued
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CASE REPORT 2—cont’d
A
B
C
E
F
D Figure 7-7 A–F, Mid-treatment intraoral views.
A
B
D
C
E
G
F
H Figure 7-8 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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CASE REPORT 3 A 12-year-old postpubertal girl presented with a chief complaint of irregularly placed upper front teeth. Extraoral examination revealed a brachyfacial growth pattern with a convex soft tissue profile and obtuse nasolabial angle (Fig. 7-9, A–C). Intraoral examination showed that she had a bilateral Class I molar relationship and highly placed maxillary canines in the buccal vestibule (Fig. 7-9, D–H). The deciduous maxillary left canine was overretained and the patient had a crossbite tendency due to a narrow transverse maxillary dimension (see Fig. 7-9, D–H). The deciduous left maxillary canine was extracted and a hyrax expander was placed to
correct the transverse discrepancy (Fig. 7-10, A–E). Cantilevers (0.017-inch × 0.025-inch CNA) were placed bilaterally from the auxiliary tube of the molar bracket to bring the highly placed canines into the arch (see Fig. 7-10, A–E). Further alignment of the canines was done using a 0.016-inch nickeltitanium (Ni-Ti) wire piggybacked over a stiffer 0.017-inch × 0.025-inch CNA base archwire (Fig. 7-10, F–K). Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA and a 0.016-inch × 0.022-inch braided stainless steel archwire. Total treatment time was 15 months (Fig. 7-11).
A
B
C
D
E
F
G
H Figure 7-9 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views. Continued
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CASE REPORT 3—cont’d
A
B
D
C
E
F
G
H
I
J
K
Figure 7-10 A–K, Mid-treatment intraoral views.
CHAPTER 7 Biomechanics-Based Management of Impacted Canines
CASE REPORT 3—cont’d
A
B
C
D
E
F
G
H Figure 7-11 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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The therapeutic approach to impacted and ectopically placed canines is interdisciplinary. Although many factors account for the final outcome, the three-dimensional force system acting on the canines is perhaps the single most important factor. Correction of impacted and ectopically placed canines usually requires eruption of the canines into the oral cavity and a buccolingual movement to get the canines into the arch. The single force system (cantilever mechanics) has been used effectively to bring about controlled three-dimensional movement in both impacted and ectopically placed canines in the direction predetermined by the treatment goals. The force required to get the canines (impacted or displaced) into the arch is relatively low. However, to prevent the undesirable tipping of the maxillary first molar, a transpalatal bar (0.032-inch round CNA wire) was used in all three case reports (in the third case an inactivated hyrax was used as the transpalatal bar). Furthermore, the cantilever springs were made from a 0.017-inch × 0.025inch archwire, which has a low load deflection rate, thus reducing the frequency of reactivation. In all of the case reports, patients were treated with a closed-eruption surgical exposure technique and the canines were brought into the arch through attached mucosa, which resulted in an adequate zone of attached keratinized gingiva, good bone support, and no excessive periodontal probing depths.
SUMMARY Orthodontic and esthetic outcomes are frequently compromised in patients with impacted or ectopically placed maxillary canines. In the three case reports presented in this chapter, careful treatment planning and meticulous mechanics at each step helped us to achieve the desired results without increasing the treatment time significantly.
REFERENCES 1. Bishara SE. Clinical management of impacted maxillary canines. Semin Orthod. 1998;4:87–98. 2. Peck S, Peck L, Kataja M. The palatally displaced canine as a dental anomaly of genetic origin. Angle Orthod. 1994;64:249–256. 3. Thilander B, Myrberg N. The prevalence of malocclusion in Swedish schoolchildren. Scand J Dent Res. 1973;81:12–21. 4. Ericson S, Kurol J. Radiographic assessment of maxillary canine eruption in children with clinical signs of eruption disturbance. Eur J Orthod. 1986;8:133–140. 5. Bedoya MM, Park JH. A review of the diagnosis and management of impacted maxillary canines. J Am Dent Assoc. 2009;140:1485–1493. 6. Cooke J, Wang HL. Canine impactions: incidence and management. Int J Periodontics Restorative Dent. 2006;26:483–491. 7. van Vlijmen OJ, Kuijpers MA, Berge SJ, et al. Evidence supporting the use of cone-beam computed tomography in orthodontics. J Am Dent Assoc. 2012;143:241–252.
8. Holberg C, Steinhauser S, Geis P, Rudzki-Janson I. Cone-beam computed tomography in orthodontics: benefits and limitations. J Orofac Orthop. 2005;66:434–444. 9. Burstone CJ, Tanne K. Biomechanical basis of tooth movement [in Japanese]. Nihon Kyosei Shika Gakkai Zasshi. 1986;45:541–551. 10. Yadav S, Chen J, Upadhyay M, Jiang F, Roberts WE. Comparison of the force systems of 3 appliances on palatally impacted canines. Am J Orthod Dentofacial Orthop. 2011;139:206–213. 11. Oppenhuizen G. An extrusion spring for palatally impacted cuspids. J Clin Orthod. 2003;37:434–436. 12. Jacoby H. The “ballista spring” system for impacted teeth. Am J Orthod. 1979;75:143–151. 13. Bowman SJ, Carano A. The Kilroy spring for impacted teeth. J Clin Orthod. 2003;37:683–688. 14. Haydar SG, Uckan S, Sesen C. A method for eruption of impacted teeth. J Clin Orthod. 2003;37:430–433. 14a. Vardimon AD, Graber TM, Drescher D, Bourauel C. Rare earth magnets and impaction. Am J Orthod Dentofacial Orthop. 1991;100(6):494–512. 15. Yadav S, Chen J, Upadhyay M, Roberts E, Nanda R. Three-dimensional quantification of the force system involved in a palatally impacted canine using a cantilever spring design. Orthodontics (Chic.). 2012;13: 22–33. 16. Sajnani AK, King NM. Diagnosis and localization of impacted maxillary canines: comparison of methods. J Investig Clin Dent. 2012;4:252– 256. 17. Alqerban A, Jacobs R, Fieuws S, Willems G. Comparison of two cone beam computed tomographic systems versus panoramic imaging for localization of impacted maxillary canines and detection of root resorption. Eur J Orthod. 2011;33:93–102. 18. Botticelli S, Verna C, Cattaneo PM, Heidmann J, Melsen B. Two- versus three-dimensional imaging in subjects with unerupted maxillary canines. Eur J Orthod. 2011;33:344–349. 19. Parkin NA, Deery C, Smith AM, Tinsley D, Sandler J, Benson PE. No difference in surgical outcomes between open and closed exposure of palatally displaced maxillary canines. J Oral Maxillofac Surg. 2012;70: 2026–2034. 20. Parkin N, Benson PE, Thind B, Shah A. Open versus closed surgical exposure of canine teeth that are displaced in the roof of the mouth. Cochrane Database Syst Rev. 2008;(4):CD006966. 21. Mathews DP, Kokich VG. Palatally impacted canines: the case for preorthodontic uncovering and autonomous eruption. Am J Orthod Dentofacial Orthop. 2013;143:450–458. 22. Peck S, Peck L, Kataja M. Site-specificity of tooth agenesis in subjects with maxillary canine malpositions. Angle Orthod. 1996;66:473–476. 23. Peck S, Peck L, Kataja M. Concomitant occurrence of canine malposition and tooth agenesis: evidence of orofacial genetic fields. Am J Orthod Dentofacial Orthop. 2002;122:657–660. 24. Peck S, Peck L. Palatal displacement of canine is genetic and related to congenital absence of teeth. J Dent Res. 1997;76:728–729. 25. Pirinen S, Arte S, Apajalahti S. Palatal displacement of canine is genetic and related to congenital absence of teeth. J Dent Res. 1996;75:1742– 1746. 26. Becker A. Palatal displacement of canine is genetic and related to congenital absence of teeth. J Dent Res. 1997;76:1526. 27. Litsas G, Acar A. A review of early displaced maxillary canines: etiology, diagnosis and interceptive treatment. Open Dent J. 2011;5:39–47. 28. Baccetti T. A controlled study of associated dental anomalies. Angle Orthod. 1998;68:267–274.
M
axillary 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 intraarch 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.
Clinical Examination The following clinical signs along with radiographic diagnosis might be indicative of impacted canines: 1. Delayed eruption or migration of the permanent maxillary lateral incisors 2. Delayed eruption of the permanent canine (beyond 14 to 15 years of age) 3. Prolonged retention of the deciduous canine (beyond 14 to 15 years of age) 4. Absence of normal canine bulge 5. Presence of palatal bulge Radiographic Diagnosis 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 121
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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. 2. Occlusal films: These serve as an adjunct along with the periapicals in determining the buccolingual position of the impacted canine.16 3. CBCT: CBCT can exactly locate the position of impacted canines in three dimensions of space. This threedimensional 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
SURGICAL EXPOSURE 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 threedimensional 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
ETIOLOGY 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.
Guidance Theory 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. Genetic Theory According to genetic theory, the eruption anomaly of the maxillary permanent canine is the result of a developmental disturbance of the dental lamina. Evidence for this theory is found in familial and bilateral occurrences, sex differences, and the increased incidence of other dental issues such as ectopic eruption of the first molars and infraocclusion of the primary molars.22–25 Research has even shown that palatally impacted canines usually coexist with either hypodontia or agenesis of the third molars.26 Transcription factors such as MSX1 and PAX9, which have been correlated with agenesis of molars, might be involved in palatal canine impactions.27 Baccetti showed that unilateral palatally impacted canines correlated significantly (p <0.05) with aplasia of maxillary lateral incisors whereas bilateral palatally impacted canines have significant correlation (p <0.05) with agenesis of third molars, supporting the genetic etiology of palatal canine displacement.28
BIOMECHANICAL CONSIDERATIONS Cantilever springs with a single force direction and point of force application are commonly used for the management of impacted and ectopically erupted canines (Fig. 7-1). Cantilever springs produce force on the impacted and ectopically placed canines in all three spatial planes, depending on the position of the canines (see Fig. 7-1; Fig. 7-2). The major force directed on the impacted and ectopically placed canine is vertical (extrusive force) and labial/lingual. In horizontal canine impactions, a pure couple force system with high moment-to-force ratio is advantageous to upright the impacted canine before pulling it out in the oral cavity. Cantilever-based appliances or springs (one-couple force systems) are designed to have a low load deflection rate. The
CHAPTER 7 Biomechanics-Based Management of Impacted Canines
D
D
123
MFa
MF
MC
Mc MFp
F
F f
F F
f d Figure 7-1 Anteroposterior view: A cantilever spring design for extrusion of a canine (a one-couple force system). The dotted line indicates the passive state of the spring while the solid design shows that it is in the activated state; in other words, from this point on the spring will gradually undergo deactivation. The force (F) exerted on the canine and molar per Newton’s Third Law of Motion is equal and opposite. Once activated, the spring generates a couple in the auxiliary tube (MC), where MC = F × D. D is the distance between the center of resistance (CRES) of the molar and the point of application of the force on the canine. MC can also be calculated by the product of the force of the couple (f) and the length of the auxiliary tube (d); that is, MC = f × d. Because the force does not pass through the CRES of the canine, it generates a moment due to force (MF). On the molar, as the force is assumed to pass through the CRES, no moment due to force is generated. Note: Always differentiate between a moment due to a force and a moment due to a couple.
force systems delivered by these appliances (springs) tend to stay optimal and consistent in their magnitude, which prevents them from having any deleterious effects on the surrounding periodontium through the entire range of tooth movement. Additionally, an acceptable magnitude of force is maintained in the appliance during treatment to avoid
Figure 7-2 Transverse view: A cantilever spring design for extrusion of a canine (a one-couple force system). MFp: Since the force is buccal to the center of resistance (CRES) of the molar, a moment due to the applied force is generated. Likewise, a moment is generated on the canine (MFa). Here, the net moment acting on the molar (M) can be obtained by subtracting the moment due to the force from the moment due to the couple (MC). Therefore, M = MC – MFp.
frequent reactivations. A single force is exerted on the canine for eruption and alignment. The reactionary force and the moment are dissipated on the molar (see Figs. 7-1 and 7-2), which can be controlled by using a palatal arch and/or ligating the molar to the rest of the arch. Any spring that allows a modification of the force system during the period of getting an impacted canine out in the oral cavity and aligned in the arch is advantageous as long as clinicians are aware of structural mechanics and spring geometry. Text continued on p. 132
CASE REPORT 1 A 13-year-old postpubertal girl presented with a chief complaint of crooked anterior teeth. Extraoral examination revealed a dolichofacial growth pattern with a straight soft tissue profile and an obtuse nasolabial angle. Intraoral examination showed that she had a bilateral Class I molar relationship and palatally impacted maxillary canines (Fig. 7-3). After leveling and aligning, a 0.032-inch CNA (Connecticut new arch form, Ortho Organizers, Carlsbad, CA.) transpalatal arch and a 0.019-inch × 0.025-inch CNA archwire were placed in the maxillary arch. Cantilever springs (0.017-inch × 0.025-inch CNA
wire) applying 80 g of force (occlusal/extrusive) were placed bilaterally from the auxiliary tube of the molar bracket (Fig. 7-4). Springs were activated at each visit and after 5 months maxillary canines erupted in the oral cavity. At this point, the direction of the force was changed from occlusal to buccoocclusal and force was reduced to 30 g. After 12 months of cantilever mechanics, the crowns of both maxillary canines were in the arch (see Fig. 7-4). Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA archwire. Total treatment time was 18 months (Fig. 7-5). Continued
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CASE REPORT 1—cont’d
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H Figure 7-3 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views.
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CASE REPORT 1—cont’d
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Figure 7-4 A-F, Mid-treatment intraoral (occlusal) views. G–I, Mid-treatment intraoral views. Continued
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CASE REPORT 1—cont’d
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H Figure 7-5 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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CASE REPORT 2 arch. A cantilever spring (0.017-inch × 0.025-inch CNA) was placed from the right auxiliary tube of the molar bracket (Fig. 7-7). Seventy-five grams of bucco-occlusal force were applied and after 6 months the canine erupted into the arch. Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA and a 0.017-inch × 0.025-inch braided stainless steel archwire. Total treatment time was 16 months (Fig. 7-8).
An 11.7-year-old postpubertal girl was referred by her general dentist regarding a missing right maxillary canine. Extraoral examination revealed a dolichofacial growth pattern with a straight soft tissue profile (Fig. 7-6, A–C). Intraoral examination showed that she had a Class I molar relationship and a palatally impacted right maxillary canine (Fig. 7-6, D–H). After initial leveling and aligning, a 0.032-inch CNA transpalatal arch and a 0.019-inch × 0.025-inch CNA archwire were placed in the upper
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H Figure 7-6 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views. Continued
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CASE REPORT 2—cont’d
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D Figure 7-7 A–F, Mid-treatment intraoral views.
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H Figure 7-8 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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CASE REPORT 3 A 12-year-old postpubertal girl presented with a chief complaint of irregularly placed upper front teeth. Extraoral examination revealed a brachyfacial growth pattern with a convex soft tissue profile and obtuse nasolabial angle (Fig. 7-9, A–C). Intraoral examination showed that she had a bilateral Class I molar relationship and highly placed maxillary canines in the buccal vestibule (Fig. 7-9, D–H). The deciduous maxillary left canine was overretained and the patient had a crossbite tendency due to a narrow transverse maxillary dimension (see Fig. 7-9, D–H). The deciduous left maxillary canine was extracted and a hyrax expander was placed to
correct the transverse discrepancy (Fig. 7-10, A–E). Cantilevers (0.017-inch × 0.025-inch CNA) were placed bilaterally from the auxiliary tube of the molar bracket to bring the highly placed canines into the arch (see Fig. 7-10, A–E). Further alignment of the canines was done using a 0.016-inch nickeltitanium (Ni-Ti) wire piggybacked over a stiffer 0.017-inch × 0.025-inch CNA base archwire (Fig. 7-10, F–K). Finishing and detailing were accomplished with a 0.017-inch × 0.025-inch CNA and a 0.016-inch × 0.022-inch braided stainless steel archwire. Total treatment time was 15 months (Fig. 7-11).
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H Figure 7-9 A–C, Pre-treatment facial views. D–H, Pre-treatment intraoral views. Continued
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CASE REPORT 3—cont’d
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Figure 7-10 A–K, Mid-treatment intraoral views.
CHAPTER 7 Biomechanics-Based Management of Impacted Canines
CASE REPORT 3—cont’d
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H Figure 7-11 A–C, Post-treatment facial views. D–H, Post-treatment intraoral views.
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The therapeutic approach to impacted and ectopically placed canines is interdisciplinary. Although many factors account for the final outcome, the three-dimensional force system acting on the canines is perhaps the single most important factor. Correction of impacted and ectopically placed canines usually requires eruption of the canines into the oral cavity and a buccolingual movement to get the canines into the arch. The single force system (cantilever mechanics) has been used effectively to bring about controlled three-dimensional movement in both impacted and ectopically placed canines in the direction predetermined by the treatment goals. The force required to get the canines (impacted or displaced) into the arch is relatively low. However, to prevent the undesirable tipping of the maxillary first molar, a transpalatal bar (0.032-inch round CNA wire) was used in all three case reports (in the third case an inactivated hyrax was used as the transpalatal bar). Furthermore, the cantilever springs were made from a 0.017-inch × 0.025inch archwire, which has a low load deflection rate, thus reducing the frequency of reactivation. In all of the case reports, patients were treated with a closed-eruption surgical exposure technique and the canines were brought into the arch through attached mucosa, which resulted in an adequate zone of attached keratinized gingiva, good bone support, and no excessive periodontal probing depths.
SUMMARY Orthodontic and esthetic outcomes are frequently compromised in patients with impacted or ectopically placed maxillary canines. In the three case reports presented in this chapter, careful treatment planning and meticulous mechanics at each step helped us to achieve the desired results without increasing the treatment time significantly.
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