A current trend in the orthodontic literature is to identify new methods for increasing the
efficiency of orthodontic treatment, from the de-velopment of brackets and wires to the use of de-vices or surgical methods to accelerate tooth move-ment. It is still possible, however, to improve treatment efficiency by using basic biomechanical principles to create predictable force systems while controlling anticipated side effects.
This two-part overview will focus on the
TARANPREET K. CHANDHOKE, DMD, PHDRAVINDRA NANDA, BDS, MS, PHDFLAVIO A. URIBE, DDS, MDS
Clinical Applications of Predictable Force SystemsPart 1 One-Couple and Two-Couple Systems
OVERVIEW
(Authors’ Note: Dr. Charles Burstone, one of the great minds in orthodontic history, dedicated his life to advancing our specialty through academics and research. His life’s work, and most notably his contributions to the study of biomechanics, have had a considerable impact on the way we practice orthodontics today. With these articles, we pay tribute to his memory and to all that he did for us as a teacher, icon, innovator, visionary, and clinician.)
Dr. UribeDr. NandaDr. Chandhoke
Dr. Chandhoke is an Assistant Professor and Dr. Uribe is an Associate Professor, Postgraduate Program Director, and Dr. Charles J. Burstone Endowed Professor, Division of Orthodontics, Department of Craniofacial Sciences, University of Connecticut School of Dental Medicine, 263 Farmington Ave., Farmington, CT 06030. Dr. Nanda is UConn Orthodontic Alumni Endowed Chair, Division of Orthodontics, and Professor and Head, Department of Craniofacial Sciences, University of Connecticut School of Dental Medicine. Drs. Uribe and Nanda are Contributing and Associate Editors of the Journal of Clinical Orthodontics, respectively. E-mail Dr. Uribe at [email protected].
Clinical Applications of Predictable Force Systems
most fundamental of biomechanical principles: one-couple force systems. In Part 1, the advan-tages of the one-couple system are reviewed, ac-companied by cases illustrating simple methods for incorporating it into daily practice. In Part 2, we will present new applications of these princi-ples using skeletal anchorage.
Intrusion and Utility Arches: One-Couple vs. Two-Couple Systems
Deep overbite, a problem frequently encoun-tered in patients seeking orthodontic treatment, can have implications in terms of both normal function and smile esthetics. A key to the diagno-sis of deep-bite malocclusions is to determine the etiology: overextrusion of the anterior teeth, undereruption of the posterior teeth, or a combina-tion of the two. The incisor display should be ex-amined on smiling and at rest to determine the best approach to orthodontic management.1 If excessive incisor and gingival display are evident, treatment should be planned to intrude the maxil-lary incisors.
While an intrusion arch and a utility arch appear to be similar, they exert significantly dif-ferent effects, since the intrusion arch is a one-couple system and the utility arch is a two-couple system. The intrusion arch, as described by Bur-stone, is a rectangular TMA* overlay wire insert-ed into the auxiliary tube of the first molar and ligated to the base archwire at a single point be-tween the central incisors or between the lateral and central incisors.2 The rigid stainless steel base archwire consists of two posterior segments and an anterior segment that bypasses the canine, al-lowing the anterior and posterior dental segments to function as distinct units with a collective center of resistance (CRes) that can be approximated. From a sagittal perspective, this setup produces a one-couple system with a distal tipback (clock-wise) moment, an extrusive force on the molar, and an intrusive force on the incisors (Fig. 1A). It should be noted that because the point of attach-
ment of the intrusion arch to the base archwire is anterior to the CRes of the incisors, it will generate a small counterclockwise moment. This can be controlled by using a tight cinch-back of the intru-sion wire to fix the length of the wire3 or by using a three-piece intrusion arch (described later).
The utility arch, as developed by Ricketts, differs from the intrusion arch in that it extends from the molars and is inserted directly into the incisor brackets.4 This setup results in a two-couple system, with a clockwise moment on the molars and an intrabracket couple at the incisors resulting in a counterclockwise moment and incisor flaring (Fig. 1B). The force system can change dramati-cally if, for example, the incisors are inclined pal-atally or labially. The inclination of the incisors can cause the utility arch to twist upon insertion into the bracket slot, adding lingual or labial root
*Registered trademark of Ormco Corporation, Orange, CA; www.ormco.com.
Fig. 1 A. Force diagram of intrusion arch (expect-ed dental movements shown in gray). B. Force diagram of utility arch.
B
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V-bend placed at the midpoint between the molar and incisor, two equal and opposite moments are generated at the incisor and the molar when both brackets are engaged. When the V-bend is moved distally to one-third the distance from the molar, a moment and vertical force are generated at the molar, with a single vertical force (and no moment) in the opposite direction at the incisor. This “one-couple system” will occur only when the two brackets are passive and the bend is placed at one-third the distance from the molar. The cantilever is an example of a one-couple system, since it is typically inserted into the molar tube with a single point of attachment to the anterior tooth. It is im-portant to reemphasize that to generate a force without a moment, the wire must not be inserted into the anterior brackets, but rather ligated on top of the base wire segment, below or above the bracket wings. In this case, the location of the bend becomes secondary in regard to the moments and forces generated.6,7 Because additional forces and moments are present in the transverse dimension, however, we need further studies to examine these force systems in three planes of space.
torque in addition to the expected forces and mo-ments. The torque within the bracket prescription and the material properties of the wire can also affect the force system. If lingual root torque is incorporated due to the bracket or tooth inclina-tion, the resulting extrusive force will impede the objective of intruding the anterior segment.2 Since the force system of the utility arch is considerably more complex and difficult to predict than that of the intrusion arch, it clearly needs to be used with caution when correcting vertical problems.4
The placement and size of the V-bend, as well as the material properties of the archwires, can significantly impact the forces and moments generated by either the intrusion arch or the utility arch. Classically, the V-bend position has been described using a “two-tooth” model of the poste-rior molar and the anterior incisor. In clinical sce-narios, however, segments of teeth are more com-monly ligated together, so that they function as posterior and anterior units. This adds complexity in predicting where each unit’s CRes is located and how that unit will respond. From the traditional two-dimensional, sagittal perspective,5 with the
Fig. 2 A. Initial placement of intrusion arch for maxillary incisor intrusion in 12-year-old female patient. B. Improved incisor positions after five months of treatment.
B
A
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The objective of any system for correcting deep overbite is to apply forces low enough to achieve intrusion while avoiding deleterious side effects such as root resorption. Although the intru-sion and utility arches were initially designed with stainless steel wires, new materials like beta tita-nium (including TMA) and nickel titanium provide the elasticity and force-deflection ratios needed to reduce the force exerted by the V-bend and allow easier engagement into the molar and incisor at-tachments.8-10 Compared to Elgiloy** and TMA utility arches of the same size in a three-dimen-sional model, TMA intrusion arches have been shown to generate less force at the incisors as well as lower forces and moments at the molar.11,12
A typical indication for using a continuous intrusion arch is when overextruded upper incisors are accompanied by excessive gingival display. Figure 2A illustrates the treatment of a 12-year-old female patient presenting with this scenario. An intrusion arch fabricated from Connecticut New Arch (CNA***) wire was overlaid and tied at the lateral incisors. Figure 2B indicates the amount of incisor intrusion achieved in five months; note the step between the upper lateral incisors and ca-nines, indicating effective intrusion of the incisors.
One-Couple Systems for Controlling Vertical Incisor Positions
Three-Piece Intrusion Arch
For a case where even minimal proclination of the incisors is undesirable, a three-piece intru-sion arch can be used (Fig. 3). In this modification, cantilevers are placed bilaterally and attached in line with the CRes of the incisors. This allows the line of force to be directed vertically through the CRes, thus minimizing the anterior moment pro-duced by the one-couple system of a continuous intrusion arch.13,14 A light distal force can also be applied with an elastomeric chain between the molar and the anterior segment, redirecting the
path of intrusion through the CRes along the long axis of the incisor instead of a true vertical.13
Figure 4A shows a 67-year-old male patient who presented with overerupted lower incisors and a deep bite. Because the lower incisors were pro-clined and further flaring had to be avoided, a three-piece intrusion arch was used. Anterior and posterior stainless steel segments were placed, with the anterior segment extending distally to the level of the canines. CNA cantilevers were at-tached to the anterior segment distal to the incisors to approximate the CRes of the lower incisors. Note the difference in the incisor positions com-pared to the posterior dentition after four months of intrusion (Fig. 4B).
Asymmetrical Intrusion Arch
A common technique for correcting a cant of the anterior occlusal plane is to use unilateral seating elastics to extrude the undererupted denti-tion. Because the elastics are anchored between the arches, however, the opposing dentition may be extruded, which is not always a desirable ef-fect.15 Moreover, the cant may require intrusion of one side rather than extrusion of the contralateral side, which is not possible to achieve with inter-maxillary elastics. A more predictable method is to tie a continuous intrusion arch to the anterior segment on the side requiring intrusion. Again, the base archwire is segmented into posterior and an-terior components to allow the anterior segment to
**Registered trademark of Rocky Mountain Orthodontics, Denver, CO; www.rmortho.com.***Trademark of Henry Schein Orthodontics, Carlsbad, CA; www.henryscheinortho.com.
Fig. 3 Force diagram of three-piece intrusion arch.
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terior dentition. To create a more predictable force system, anterior and posterior wire segments were placed, and a cantilever extending from the molar tube was attached distal to the left lateral incisor. Figure 5B shows the patient seven months later, with the anterior segment leveled and ready for placement of a continuous wire.
Extrusion Arch
As its name implies, the extrusion arch is similar to the intrusion arch, but with the wire
move freely. One drawback is that the free, untied side of the intrusion arch can extend or bow toward the vestibule, potentially causing irritation for the patient. A modification of this one-couple system is to extend a unilateral cantilever anteri-orly from the molar auxiliary tube and hook it onto the anterior base-wire segment.
Figure 5A illustrates a case in which a uni-lateral cantilever was used to correct an anterior cant with overerupted upper left incisors. Initial placement of a continuous wire would have ex-tended the cant distally by extruding the left pos-
Fig. 4 A. Initial placement of three-piece intrusion arch for mandibular incisor intrusion in 67-year-old male patient. B. Significant incisor intrusion achieved after four months of treatment.
B
A
Fig. 5 A. Initial placement of unilateral cantilever for asymmetrical incisor intrusion in 18-year-old female patient with maxillary anterior cant. B. Cant correction completed after seven months of treatment.
A B
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inverted for incisor extrusion. Although anterior box elastics are often used for the correction of anterior open bites, they can also cause undesirable extrusion of the opposing incisors. An extrusion arch is a more reliable method for extruding the incisors in a single arch.16 To prevent any posterior extension of the open bite, the stainless steel base archwire is again segmented. The overlaid extru-sion arch extends from the molar and is tied at a single point to the anterior base arch. The V-bend is placed adjacent to the molar attachment, produc-ing a counterclockwise moment and intrusive force at the molar and an extrusive force at the incisor Fig. 6 Force diagram of extrusion arch.
Fig. 7 A. Initial placement of extrusion arch with intermaxillary elastics to control posterior segment in 14½-year-old male patient. B. Bite closure noted two months later. C. Placement of continuous wire after four months of extrusion-arch treatment.
A
B
C
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Class II elastics were used to control the counter-clockwise moment on the posterior segment and thus prevent development of a lateral open bite. Two months later, some bite closure could be seen (Fig. 7B); after another two months, a continuous wire was placed (Fig. 7C).
One- and Two-Couple Torquing Arches
The classic torquing arch, extending from the molar tube and engaged directly into the incisor brackets, is another example of a two-couple sys-tem with intrabracket couples at both the molar
segment (Fig. 6). Because the extrusive force at the incisor is anterior to the CRes of the anterior segment, a small clockwise moment can result in retroclination of the incisors. If proclination of the incisors is needed, an anterior force will be re-quired. For this purpose, a “stop-advance” can be incorporated into the extrusion arch by placing crimpable stops distal to the V-bend or by placing the V-bend precisely at the molar tube.
Figure 7A shows a 14½-year-old male patient with an anterior open bite and no evident parafunc-tional habits. Stainless steel base segments were placed along with a CNA extrusion arch. Short
Fig. 8 A. Force diagram of two-couple torquing arch. B. Force diagram of one-couple torquing arch.
BA
Fig. 9 A. Initial placement of one-couple torquing arch for lingual root torque of maxillary central incisors in 14-year-old female patient. B. Improvement in central incisor torque noted after five months of treat-ment.
B
A
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the V-bend is placed closer to the incisor segment, a counterclockwise moment and extrusive force are generated at the incisors, and an intrusive force is exerted on the posterior segment (Fig. 8B).
Figure 9A shows a case in which the maxil-lary central incisors needed lingual root torque. The central incisors were co-ligated, and a rigid stain-less steel base archwire was engaged through the posterior dentition, bypassing the incisors. The base archwire was designed to lie flush against the la-bial surfaces of the incisor crowns and directed beneath the bracket wings to control flaring and extrusion of the crowns. Note the improvement in central incisor torque after five months (Fig. 9B).
and incisor segments.17 The V-bend is placed ad-jacent to the incisor segment to allow for a large anterior moment. As discussed previously, how-ever, it is challenging to predict the sizes of both moments in a two-couple system. If the torquing arch can move freely, the system will result in a counterclockwise moment and extrusion at the incisor segment and intrusion at the molar (Fig. 8A). If incisor crown tipping needs to be avoided, a tight cinch-back of the torquing arch can be placed distal to the molar to generate a distal force against the incisors.17
The torquing arch can also be designed as a one-couple system: a point contact is created on the posterior base archwire by means of a hook, and only the incisor brackets are engaged. When
Fig. 10 A. Force diagram of cantilever for extru-sion of maxillary canine (red cantilever indicates occlusal activation), with remaining teeth brack-eted and base archwire used for anchorage. B. Force diagram of cantilever with posterior an-chorage from fiber-reinforced composite (FRC).
B
A
Fig. 11 A. Limited treatment of 47-year-old female patient requiring extrusion of upper left canine, showing initial placement of cantilever and FRC bonding of posterior segment. B. Canine extru-sion after three months of treatment.
A
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One-Couple Systems for Controlling Movement of Single Teeth
Cantilevers for Extrusion and Intrusion
The intrusion and extrusion arches described above are used to control segments of teeth, whose centers of resistance can vary in all three dimen-sions. The CRes of a single tooth may be less vari-able and easier to predict, making a cantilever the simplest one-couple system. Lighter forces can also be applied to move single teeth, thus reducing unwanted side effects on the anchorage segment.
Figure 10A illustrates one of the most com-mon applications of a cantilever, for extrusion of a maxillary canine. The cantilever is fabricated with a V-bend mesial to the molar, then inserted into the molar auxiliary tube, activated with an occlusal bend, and engaged with a single-point contact at the canine. This setup produces a verti-cal extrusive force on the canine and a vertical force and moment on the molar. A rigid stainless steel base archwire on the remaining dentition counteracts side effects at the molar. Alternatively, the posterior segment can be reinforced as a unit by bonding brackets only to the posterior teeth with a segmented base archwire or by bonding the entire unit with fiber-reinforced composite (FRC, Fig. 10B).
Figure 11A shows a patient who required limited treatment to force eruption of the upper left canine and increase its biological width for crown preparation. After endodontic treatment of the canine, the premolar-to-first-molar segment was secured with FRC. A CNA cantilever was extend-ed from the molar bracket to a point of contact on the superior surface of the canine bracket. Figure 11B demonstrates the stability of the posterior seg-ment after three months. The patient then had a new crown restoration placed without further orthodontic treatment.
In a case where a single tooth has over-extruded, an initial phase of cantilever mechanics can be utilized to level the tooth with the adjacent dentition before engaging it with a continuous archwire (Fig. 12). This will help limit extrusive side effects on the adjacent teeth.
Fig. 13 A. Initial placement of cantilever in 16-year-old male patient requiring intrusion of lower right canine. B. Canine intrusion after two months. C. Placement of continuous archwire after five months of treatment.
B
A
C
Fig. 12 Force diagram of cantilever for mandib-ular canine intrusion.
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Figure 13A shows a patient in whom the low-er right canine was overextruded. The remaining dentition was leveled with a rigid stainless steel archwire, leaving the canine free. An intrusive cantilever was extended from the canine to the molar tube. With the rest of the mandibular arch serving as a rigid anchorage unit, minimal side effects would be expected from the clockwise mo-ment at the molar. Figure 13B shows the difference in the canine bracket level two months later. At five months, the canine was nearly level with the adjacent dentition, and a continuous nickel titani-um archwire was placed (Fig. 13C).
Cantilevers for Transverse Correction
Another application of the cantilever is for transverse corrections such as buccal movement of
a single tooth. The same force system described from a sagittal perspective can be converted to an occlusal view (Fig. 14). The cantilever is inserted into the molar tube, with a V-bend extending buc-cally in the transverse plane. This will produce a clockwise (mesial-in) moment or 1st-order rotation and a lingual force at the molar and a labial force on the tooth to be brought into the arch. A rigid, continuous stainless steel base archwire can be used in conjunction with a transpalatal arch to pre-vent rotation and lingual displacement of the molar.
Figure 15A illustrates a case in which the upper right canine had erupted palatal to the re-maining dentition. Engagement of a continuous wire would have been difficult and would have caused significant palatal tipping of the adjacent teeth. Instead, stainless steel wire segments were placed, and an .032" TMA transpalatal arch was attached to the first molars. A CNA cantilever was extended buccally from the molar and tied at a single point to the canine bracket. Figure 15B shows the patient seven months later, when a con-tinuous nickel titanium archwire was placed to incorporate the canine.
Uprighting Spring
While it is often advantageous to apply a single force and no rotational moment to a single tooth, there are cases in which such a moment is actually desirable. In another application of the cantilever—commonly referred to as an uprighting spring—a force and a couple are applied to a single
Fig. 14 Force diagram of cantilever for transverse correction of canine position.
Fig. 15 A. Placement of cantilever for buccal movement of upper right canine in 18-year-old female patient. B. Improvement in canine position and placement of continuous archwire after seven months of treatment.
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tooth for uprighting. A bracket is bonded to the tooth, and the cantilever is hooked to the base archwire and engaged in the bracket, resulting in 2nd-order movement of both the crown and the root (Fig. 16). Crown movement can be controlled by applying a light force in the opposite direction
Fig. 17 A. Presurgical 46-year-old female patient requiring considerable uprighting of lower left canine. B. Placement of uprighting spring two months into treatment, after initial derotation of canine. C. Improved inclination of canine after two months of treatment with uprighting spring (four months of total treatment). D. Patient following orthognathic surgery, 12 months after initiation of orthodontic treatment.
with an elastomeric chain or by co-ligating the tooth to the adjacent teeth.
Figure 17A shows a patient with a severely tipped lower left canine prior to orthognathic sur-gery. A predictable force system was needed to upright the canine and minimize the duration of presurgical orthodontic treatment. First, a light elastomeric chain was applied to derotate the ca-nine by applying a distal force with anchorage from the posterior segment. Two months later, an uprighting spring was placed to produce distal root tip and mesial crown rotation (Fig. 17B). At four months, the crown position had improved but fur-ther root correction was required. An elastomeric chain was then extended from the posterior seg-ment to apply a distal force to the canine and thus restrict further mesial crown tipping (Fig. 17C). Once sufficient uprighting was seen, the surgical archwires were placed. Note the canine correction after mandibular advancement surgery, 12 months into treatment (Fig. 17D).
Fig. 16 Force diagram of uprighting spring for mandibular canine correction.
BA
C D
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Conclusion
This article has described conventional meth-ods by which one-couple systems can be utilized to achieve predictable and efficient effects. It should be noted that the biomechanical principles described here have been a review of the classic literature regarding these force systems. A major limitation of the classic models is that they have all been described from a two-dimensional per-spective, whereas many changes are often noted in other dimensions. For this reason, studies using both in vivo measurements and well-designed three-dimensional models need to be conducted to better understand how archwires function in all three planes of space, so that clinicians will be able to use these techniques with optimal efficiency and predictability.
(TO BE CONTINUED)
ACKNOWLEDGMENT: We would like to give a special acknowl-edgment to Drs. Nandakumar Janakiraman and Leyla Davoody, who provided cases for this article.
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