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Improving the predictability of clear aligners
Author’s Accepted Manuscript Improving the predictability of clear aligners S. Jay Bowman
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S1073-8746(16)30061-5 http://dx.doi.org/10.1053/j.sodo.2016.10.005 YSODO482
To appear in: Seminars in Orthodontics Cite this article as: S. Jay Bowman, Improving the predictability of clear aligners, Seminars in Orthodontics, http://dx.doi.org/10.1053/j.sodo.2016.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improving the Predictability of Clear Aligners S. Jay Bowman DMD, MSD S. Jay Bowman DMD, MSD is a Diplomate of the American Board of Orthodontics and a member of the Edward H. Angle Society of Orthodontists. He developed and teaches the Straightwire course at The University of Michigan, is an Adjunct Associate Professor at Saint Louis University, an Assistant Clinical Professor at Case Western Reserve University, Visiting Clinical Lecturer at Seton Hill University, and Milton Sims Visiting Professor at the University of Adelaide. He maintains a private specialty practice of orthodontics in Portage, MI.
I like predictability because I know what I'm getting into.
-Actress Katherine Heigl
Addressing Limitations of Clear Aligner Treatment The advent of using a series of clear aligners came at the turn of the 21st century, despite the basis for this concept originating over 55 years earlier.1 Although, the original fanfare of plastic aligners equitably replacing traditional metal braces quickly tarnished as early adopters became painfully aware of limitations of the concept.2-9 Some practitioners vacated, some vacillated, and others simply dug-in to doggedly invent ways to improve this innovation. Patients desire “braceless” correction of their smiles10 and thus far, the efforts to give them what they want has led to continual advances and increasing number of applications and adjuncts for aligners in orthodontics.11-23
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Compliance, Consequences, and Quality of Life with Clear Aligners If treatment with aligners was to be seen as completely successful, it was not enough to occasionally straighten some teeth just in the esthetic zone. Predictable, consistent, and completed results without adverse effects were requirements. First and foremost was a justified concern for patient compliance in wearing removable devices during the course of possibly years of treatment. As Buzz Behrents cautioned, compliance is the single most important factor in treatment success.
Cooperation in wearing aligners differs from that of fixed braces in that wires and brackets cannot be taken-off and disregarded by the patient, so the treatment might proceed towards at least “straighter” teeth. In contrast, if aligners are not worn as prescribed, the patient receives no result. Consequently, an alternative track for the non-compliant patient must be a part of informed consent.
The introduction of so-called Compliance Indicators for the Invisalign® product was evaluated during testing of their Teen product (targeted for the audience with the highest concerns for adherence to wear). The
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soluble, color-fading dots were found to reasonably demonstrate if patients had been consistently wearing their aligners.24 If used as a “policing” device, however, much like embedded timers in headgears or functional appliances, their effectiveness in actually eliciting cooperation is still in question.24 More importantly, compliance by orthodontic patients with either braces or plastic is, on-average, favorable and comparable.25 Non-compliant patients likely suffer consequences of prolonged or compromised results no matter the treatment method chosen.
In terms of adverse effects, although there are still concerns for decalcification, gingivitis, root resorption, etc. with either fixed braces or removable aligners, the oral hygiene advantage for removable appliances is significant—as long as they are, in fact, re-inserted after cleaning and eating. All in all, it was not surprising that the quality-of-life satisfaction of teen aligner patients was found to be quite high.25
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Aligner Tracking A common concern running consistently throughout the history of aligner treatments has been that of teeth not following the predicted or desired movement. This is despite the fact that the forces and moments produced with plastic are comparable to those generated with fixed braces.26 For instance, Chisari et al.13 found that just 57% of the tooth movement programmed for a single incisor was actually realized in a period of an eight-week investigation. In addition, researchers from the University of Illinois11 reported only a 41% mean accuracy of predicted tooth movement. The reasons for these frustrations are multi-factorial and require a variety of responses to resolve. For example, specific teeth oft times do not remain seated within the confines of the plastic aligner trays as treatment progresses. This may produce an air gap between tooth and plastic termed “lag” or “tracking error.” Strategies to reduce “tracking errors” will be described below.
Rohit Sachdeva has described the process of orthodontic finishing as “reducing the errors that have accumulated during treatment.” These errors can crop-up quickly during the sequence of aligners, resulting in a loss of tracking and treatment “running off the rails.” These errors
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include those of diagnosis and treatment planning, but with aligners they accrue at the outset. They include issues with impression taking, laboratory scanning of those impressions, tolerances in the creation of laboratory models (poured or printed), tolerances in the process of the molding of plastic over said models, limitations of the plastic materials themselves, and, of course, inconsistent compliance.
Much like issues in manufacturing of metal wires and braces, there are tolerances permitted in fabrication that may limit the accuracy of tooth movement. The same situation occurs in molding plastic trays, especially when “simulated/predicted tooth movements” are involved. In addition, thin plastic may mold around the undercuts and interproximal spaces between teeth but might provide insufficient force to achieve the desired tooth movement. The flexibility of the material and its conformation from the incisal edge to the gingival margin of individual teeth, combined with the necessary duration of forces applied to specific teeth, can be problematic. It seems there must be a compromise between heavier forces from thicker or less pliable plastic versus thinner or more resilient materials.27,28 In any event, some tooth movements (extrusion, torque, rotations) especially for certain teeth
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(maxillary laterals and cuspids) have been clearly demonstrated to be less predictable and require extra attention.1-3,11-13,29-32 These types of concerns led directly to the creation of bonded composite attachments with the intent to increase the surface area for plastic aligners to “grip” onto teeth.
More importantly, the design of the sequence and velocity of the movement of teeth during treatment requires the greatest consideration. Sequential or simultaneous tooth movement plans are desirable in different situations. Simply expecting teeth to move from start to a finished occlusion without a specific plan in-between for how these teeth will overcome obstacles (i.e., collisions, contact points, occlusal and muscular forces, insufficient force or contacts from the plastic) is unrealistic at best.9
One of the earliest adjuncts to help seat aligner trays, attempt to maintain “tracking,” and increase occlusal forces to elicit tooth movement was the introduction of aligner “chewies” (Chewies Aligner Tray Seaters, Dentsply Raintree Essix, York, PA).15,33 These plastic “cotton rolls” are prescribed for patients to use at least the first few days
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after changing to each new pair of aligners. Like the concept of a tooth positioner, the patient places the device in a site where teeth are not tracking (i.e., a visible “air” gap is seen between the occlusal or incisal of a tooth and the aligner). The patient then bites and holds onto the chewies repeatedly, over a few minutes of time daily, with their aligners in place.15,33 The intent is to help seat the aligner on the teeth that are not tracking, slightly intrude the teeth adjacent to a tooth in question, and the added perturbations may help to accelerate the remodeling in that site to stimulate the tooth to move as prescribed.
Lack of Space Crowded dentitions offer only two options for resolution: create more space (expand) or reduce tooth mass (IPR or extraction). Expansion implies either development/growth of new bone in the buccal alveolus. This may potentially position teeth into less stable locations due to change in equilibrium in the facial musculature and an associated potential loss of bone as teeth are pushed beyond the envelope of the alveolus. Certainly, some modicum of expansion appears to be tolerable or at least “retainable.” Unfortunately, aligners tend to expand a most unstable site: the mandibular intercanine dimension.34 IPR to reduce
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the dimensions of teeth to provide for their accommodation within the arch perimeter is useful, but should be limited in scope. Extractions are a time-tested, evidence-based approach to resolving significant arch length discrepancies; however, the results with aligners have been unpredictable. These more complex cases often benefit from adjuncts such as elastics, mini-screw anchorage, or even the use of sectional fixed braces.16,35-44
In the simplest terms, if there is insufficient space for a tooth to move, it won’t. To prescribe the unraveling of anterior crowding with aligners seems such a simple procedure. Without the creation of space, the “binding” collisions or contacts between irregular incisors will prevent movement. As noted earlier, the options to solve this are obvious and they can be combined (mild expansion + mild IPR; extraction + IPR).
Selecting IPR from the outset makes little sense, unless the original discrepancy is quite minor. That leaves the application of some degree of expansion (labially, laterally). Expansion to resolve minor crowding may be as simple as prescribing the opening of “visible” space (>0.2 mm) between specified teeth. Then reasonable amounts of IPR could be
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performed more safely. Some perform this structural reduction chairside with “lightning strips or disks.” Others provide abrasive strips to patients with prescribed directions to “floss” the strips between contact points to create small amounts of space. Once space is achieved, then more predictable correction of rotations and labiolingual discrepancies can be accomplished. Any small residual space that remains can be easily closed with the aligners as treatment progresses.
If extractions are chosen, then an entirely different group of concerns must be addressed from control of anchorage, torque (root rotation), tipping (bodily movement), and ensuring interproximal contacts are created at the end.36-39,45-46 The application of intramaxillary or intramandibular elastics to miniscrews,42-44 intermaxillary elastics, and the increase in size and type of composite attachments may be needed to increase predictability.16
Predictable sequencing of space closure or pushing individual teeth into alignment for any of the previous scenarios may be best handled in small segments of tooth movement.16,20,23 In other words, to control anchorage in small increments, teeth adjacent to a specific tooth that is out of
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position are prescribed to be held in place (no movement) until the irregular one is moved to place and so on. This detailed sequencing of small parts of a treatment plan are intended to reducing tracking errors that are sometimes experienced during simultaneous movements of large numbers of teeth.
Rotations Although it seems quite obvious that attempting to rotate some teeth with plastic is unlikely to occur due to their anatomy.7 For example, expecting to reliably rotate a “round” premolar seems futile unless a “handle” is placed on it to increase its profile asymmetrically. Molded composite attachments47-48 have increased the predictability of rotational control to some degree.49-50 But, the simple application of a prominent handle, of even a specific engineered design, cannot be depended-upon without accompanying proper treatment planning.
The most unpredictable teeth to achieve rotation for in the esthetic zone are the maxillary laterals and cuspids;11 despite the use of seemingly enormous attachments. Rotating these teeth appears to require the most attention to detailed treatment sequencing.15 Consider the typical
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position of a maxillary lateral incisor often accompanying a Class II Division 2 malocclusion. Simply prescribing that this tooth be “straight” at the end of aligner treatment is absurd. Space must first be created or this tooth will simply not move. This may be accomplished by “expansion” laterally, but also arch perimeter may be increased as lingual root torque is applied to often “rabbited” central incisors. Once visible space is open, then any desired IPR can be provided, but no rotation of the incisor can be started until there is room.
The shape, size, and the position of the lateral incisor between two larger neighboring teeth preclude much contact with plastic. This is further compromised if there is a desire to extrude the lateral and/or intrude the central incisors. There is simply insufficient retention of plastic stretched over the blade-shaped lateral. Consequently, a variety of attachments have been designed and modified for use on laterals with varying degrees of success.49 Unless combined with the prerequisite creation of space, the dimensions or design of an attachment are pointless. In fact, the iatrogenic effect of applying these forces without space can actually produce more intrusion, squeezing the tooth apically; thereby, increasing tracking error. More predictable has been the introduction of adjunctive
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forces from orthodontic elastics as described below.
Expecting a tooth to be rotated into an ideal prescribed position, using flexible plastic (with inherent errors or limits in tolerances), without asking for “over-rotation” also seems dubious. For instance, it is probably necessary to prescribe 2-3 degrees of “over-rotation” to ever be able to simply reach “ideal.” Besides, over-correction has been a timetested tenet in orthodontics.
Another option is to use detailing pliers (The Vertical, Clear Collection, Hu-Friedy, Chicago, IL) 15,51 to produce indentations into the plastic to create accented rotational couples (FIG 1.) Finally, just because the tooth was moved to the desired spot does not guarantee it will remain. It is specifically advisable to consider an Edward’s supercrestal fiberotomy, for significantly rotated teeth that you do not want to return to their original position (i.e., lateral incisors in Class II Division 2).
Insert Fig. 1
Torque and Root Angulation
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Control of root movement with aligners has been an unfailingly constant concern.7,52-53 Rotation of roots (torque), bodily tooth movement, and root paralleling have been deemed to be limitations of applying relatively “loose” forces with plastic. Fixed braces have the advantage of a more intimate junction between the forces generated by the deformation of wires bound in brackets bonded directly to the teeth.
Plastic aligners, despite often covering the majority of surface area of teeth, still do not have the same connection; therefore, the crown of the tooth may slip away from the plastic. Accordingly, to alter the angle of roots (moving them some distance through bone) is much more difficult than simple crown tipping. Complicate this with the flexibility of plastic, including the differential in resilience and force levels from the gingival margin to the occlusal of the aligners, and to create rotational couples for roots become difficult. If simultaneous movements of crown rotation, intrusion, extrusion, and unresolved crowding are added to the mix, predictability can be problematic.
The application of torque specifically for maxillary anterior teeth was identified early in aligner history as unpredictable.12,32,54 As a result,
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most change noted in the angle of incisors occurred due to “flaring,” “rounding-out the arch,” or facial tipping as crowding was resolved using expansive pushing forces. Unintended excess overjet or bimaxillary protrusion may result. For example, the Class II division 2 malocclusion seems an easy fix if you simply tip the upper central incisors labially. Unfortunately, the resulting overjet accompanying a straighter “social six” is not exactly what most patients have in mind.
The addition of so-called “torque ridges” to enhance forces for lingual root torque (limiting labial crown tipping) was first evaluated during the Invisalign Teen Research project.25 These ridges were added to the plastic at the gingival margin on the labial and at the incisal edge of the lingual to generate a rotational couple to tip roots palatally (and viceversa if labial root torque was desired). These ridges can also be added individually anywhere along the aligner tray using The Horizontal pliers (Clear Collection, Hu-Friedy, Chicago, IL )(FIG. 2).15,51
Insert Fig. 2
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Results from the Teen Study25 demonstrated that the predicted actual degrees of torque produced were reasonably achieved (FIG. 3). But, due to the flexibility of plastic and the inherent difficulty in moving roots through bone, prescribing over-correction of torque is suggested in order to just achieve the intended angulation. In addition, caution is warranted as these additional forces may tend to push the plastic incisally and increase tracking error.53 Keeping the trays seated onto the incisor crowns using some programmed intrusion may improve the response. Finally, do not ignore the unintended posterior effects from anterior torque. Loss of posterior anchorage seen with mesial tipping and intrusion of the mesial marginal ridge of molars may be attendant to anterior torque expression.16
Insert Fig. 3
What is so important about anterior torque? More attention than ever is being paid to proper coupling of upper and lower anterior teeth as the population of adult orthodontic patients has increased. The development of crowding, coincident with deep overbites, is often associated with “upright incisors” (obtuse interincisal angle) and the
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constricted envelope of anterior function. The long-term consequence may be substantial loss of tooth structure from a combination of functional and dysfunctional wear. Mix-in any unidentified Bolton tooth-size discrepancies (e.g., small maxillary lateral incisors), acidic diet, or gastrointestinal reflex disease, and the damage to teeth can be dramatic. Consequently, it may not be simply enough to line-up the front teeth in short order to only satisfy patient’s esthetic requests. Informed consent begs proper diagnosis and treatment planning to address possible functional concerns.
Addressing root angulation or torque is not limited to only anterior teeth. The American Board of Orthodontics described that the most common error found in case reports that failed their examination was, in fact, insufficient posterior buccolingual inclination (posterior torque), 55,56 a concern shared in other reports.57-60 Unless the angle of posterior teeth is specifically addressed in the virtual set-up, an excessive curve of Wilson, prominent palatal cusps of uppers, improper posterior intercuspation with inappropriate posterior overjet may result (FIG 4)58 Marshall et al57 advised, “For proper occlusion, there should be no significant difference between the heights of the buccal and lingual cusps of molars and
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premolars.”
Insert Fig. 4
Attention to detail in set-ups by evaluating the posterior teeth from the lingual view can assist in correcting these errors. In addition, increasing posterior collisions during the completion of treatments can significantly reduce the iatrogenic creation of posterior openbites from “passive posterior intrusion” that can occur by simply wearing plastic trays longterm.61 Recent changes in some proprietary software have permitted orthodontists to specifically address these issues with their own individual manipulation of tooth movement in virtual set-ups.
Extrusion Asking for extrusion where occlusal surfaces of teeth are “capped” with plastic (without substantial undercuts for retention) is unrealistic. At 29.6% effectiveness, extrusion was reported to be the least accurate tooth movement with aligners.11 Without some type of retention, the plastic will simply slip occlusally or incisally, leaving the tooth behind; resulting in a loss of aligner contact often termed “aligner lag.” These teeth are
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not lagging behind; they had no chance to be moved. Once again, the knee-jerk reaction was to increase the size and shape of composite attachments to attempt to urge teeth occlusally with bigger handles for the plastic to grip onto.47,48,62 In situations where extrusion was still not seen, the concept of aligners itself was blamed for the “lag” without thinking through the limitations and finding solutions.
If the intent is to extrude an incisor, then certainly adequate retention of the plastic is required to produce the extrusive force. We cannot expect a blade-shaped tooth to track occlusally as if magically attracted to plastic above it. But, is just a more prominent attachment simply enough?
Tight interproximal contacts or significant collisions (even without overlap or crowding) can stop extrusion; therefore, adequate space must be produced prior to attempting extrusion. This is especially important when considering the shape of teeth adjacent to one that is to be “pulledup.” Incisors or canines are often narrower gingivally and taper to a wider dimension incisally. In those instances, extrusion is an obvious non-starter without space creation. Finally, the application of root tip or
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torque along with labiolingual positioning also play intimate roles in extrusion. Sequential tooth movement to achieve the desired vertical change is much more complex than simply communicating “an order” with a lab tech to just pull the tooth occlusally.
Adjunctive Forces The addition of adjunctive forces to enhance clear aligners is not a new idea.6 For example, the application of orthodontic elastics to enhance or facilitate various biomechanics were some of the earliest and most obvious additions. But, attempting to improve aligner “tracking” and reduce the so-called “lag” for certain teeth has been the most perpetual dilemma affecting predictable results. Much like using intermaxillary elastics with braces to increase intercuspation when producing a solid final occlusion, a combination of bonded buttons and elastics can also be used with clear aligners. In these scenarios, buttons bonded on the buccal surfaces of posterior teeth can be used to apply “triangle, box, or up-and-down” elastics to seat teeth into the aligners and into the final occlusion prescribed in the set-up.15,63 Therefore, the application of orthodontic elastics that are connected to the aligner plastic, bonded buttons on teeth, or to miniscrew anchors have increased the variety of
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achievable movements possible.
Boot-strapping for Predictability The predictability of single tooth extrusion and/or rotation can be enhanced using orthodontic elastics in a so-called “boot-strap” arrangement.64 Bootstrap mechanics involve placing bonded buttons at the gingival margin on the facial, lingual, or both surfaces of a tooth that is lagging.15,63 An orthodontic elastic is then stretched across the occlusal surface of the seated clear aligner to attach to buttons and/or notches cut into the tray on opposite sides. The gingival margin of the aligner plastic must be relieved to avoid the buttons so as to permit the tray to seat fully (FIG. 5,6).
Insert Fig. 5
Insert Fig. 6
Intrusion of a segment of teeth can also be supported using elastics supported by miniscrew anchorage. The possibilities include applying the orthodontic elastics or chain from buccal and/or palatal elastics to
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buttons bonded to teeth (while relieving plastic from the trays to avoid those attachments).15,63 As an example, bonded buttons can be placed at the gingival margin of all incisors (for a patient with a deep overbite) or of molars (for hypererupted posteriors). Orthodontic elastics from those buttons are stretched to miniscrews inserted interradicularly apical to the teeth to be intruded.15
Posterior intrusion to enhance the closure of anterior openbites is another option with elastics and miniscrews. Sequential intrusion of posterior teeth combined with some extrusion of anterior teeth to increase the curve of Spee22,23 can be reinforced with bootstrap elastics. In this scenario, elastics are stretched across the occlusal of seated aligners from miniscrews to buttons, notches in trays, or to another miniscrew in bone on the other side of the alveolus.15
Skeletal Discrepancies Addressing skeletal discrepancies were originally seen to be an anathema to be avoided with aligners.3,4 Only limited changes in over or underjet were attempted with aligners, while expecting little if any anteroposterior correction.65,66 Later, maxillary posterior distalization was attempted,
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but without consideration of anchorage concerns. This led to the application of intermaxillary Class II elastics to make-shift notches cut into the plastic trays, then to buttons bonded to the trays, and finally to buttons directly bonded to the teeth (or some combination of those options).20 Once elastics were found to improve the chance of orthodontic movement for Class IIs, then employing sequential molar distalization supported by elastics20 was a logical strategy that has proven successful with aligners.67 Currently, elastics on aligners have also found their way into correcting Class IIs, IIIs, and openbites; especially for growing individuals.
Other options for improving the predictability of skeletal correction have included “pre-cursor” adjuncts used prior to beginning clear aligners. These have included the use of the Carrière arm (Carrière Distalizer, Ortho Organizers, Henry Schein, Melville, NY) to hold Class II elastics to distalize posterior segments14,68,69 or typical, stand-alone distalizers, such as the Pendulum or Distal Jet. Any of these devices may be supplemented with mini-screw anchorage to avoid reciprocal adverse effects.
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For example, the Horseshoe Jet (Specialty Labs, Atlanta, GA), is a laboratory modification of the Distal Jet (American Orthodontics, Sheboygan, WI), supported by miniscrews. These appliances are used to produce molar distalization14,70-73 prior to initiating clear aligner treatment. After posterior teeth are moved into a Class I occlusion, clear aligners are prescribed to hold the molars in place while the remaining anterior teeth are retracted; just as they would be with fixed braces. This retraction is often supported with Class II elastics or even Class I intramaxillary elastics from mini-screws to reduce the possibility of anchorage loss.14,51,72
Finally, the introduction of a variety of methods intended to accelerate the rate of tooth movement have arisen in recent years. For example, the use of a device to produce microvibrations or perturbations (AcceleDent, OrthoAccel, Bellaire, TX) with the intent of stimulating tooth movement has been anecdotally described to reduce treatment time with clear aligners, warranting further research. 16,44,74
Predicting the Future of Clear Aligners Technology has continued to improve clear aligner treatments since their
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inception. Replacing impressions with increasingly more accurate intraoral scanners, 3D printing instead of plaster models, individual tooth movement and staging in virtual set-ups, and improvements in aligner materials and attachments are all examples of recent advancements. Simultaneously, general practitioners with limited training are providing more “social six” treatments (disregarding the fit and function of teeth) and nearly every orthodontic lab (in or out of the orthodontist’s practice) is offering some type of clear plastic product. Can all options (specialist or general dentist; scanner with computer setup versus bench model set-up) provide equal or similar results? Clearly, aligner treatments are here to stay, but are we all equally trained to serve the best interests of our patients using these devices or are some depending upon lab techs to provide the diagnosis and prescribe the treatment plan? All practitioners simply need to be as honest with ourselves as we are with those that choose to have us treat them. Products noted in the manuscript: (Clear Collection pliers, Hu-Friedy, Chicago, IL) (Invisalign®, Align Technology, San Jose, CA) (Chewies Aligner Tray Seaters , Dentsply Raintree Essix, York, PA) (Carrière Distalizer, Ortho Organizers, Henry Schein, Melville, NY) (Horseshoe Jet, Specialty Labs, Atlanta, GA)
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(Distal Jet, American Orthodontics, Sheboygan, WI) (AcceleDent, OrthoAccel, Houston, TX)
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29. Kamatovic M. A retrospective evaluation of the effectiveness of the Invisalign appliance using the PAR and irregularity indices [dissertation]. Toronto (Ont.): University of Toronto, 2004.
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30. Gomez JP, Peña FM, Martínez V, Giraldo DC, Cardona CI. Initial force systems during bodily tooth movement with plastic aligners and composite attachments: A three-dimensional finite element analysis. Angle Orthod. 2015 May;85(3):454-60. doi: 10.2319/050714-330.1. Epub 2014 Sep 2.
31. Dasy H, Dasy A, Asatrian G, Rózsa N, Lee H, Kwak JH. Effects of variable attachment shapes and aligner material on aligner retention. Angle Orthod 2015 Nov;85(6):934-40. doi: 10.2319/091014-637.1.
32. Simon M, Keilig L, Schwarze J, Jung BA, Bourauel C. Treatment outcome and efficacy of an aligner technique—regarding incisor torque, premolar derotation and molar distalization. BMC Oral Health. 2014 Jun 11;14;68. doi: 10.1186/1472-6831-14-68.
33. Tuncay, O. Clinical Reports & Techniques. 2005;Fall 6(2):1.
34. Grünheid T, Gaalaas S, Hamdan H, Larson BE. Effect of clear aligner therapy on the buccolingual inclination of mandibular canines and the intercanine distance. Angle Orthod 2016 Jan;86(1):10-6. doi: 10.2319/012615-59.1. Epub 2015 May 22. 35. Boyd, R.L.: Esthetic orthodontic treatment using the Invisalign appliance for moderate to complex malocclusions, J. Dent. Ed. 72:948-967, 2008. 36. Hönn M, Göz G. A premolar extraction case using the Invisalign system, J. Orofac. Orthop. 67:385-394, 2006. 37. Miller RJ, Doung TT, Derakhshan M. Lower incisor extraction treatment with the invisalign system, J. Clin. Orthod. 36:95-102, 2002 38. Womack WR. Four-premolar extraction treatment with Invisalign, J. Clin. Orthod. 40:493500, 2006 39. Giancotti A, Greco M, Mampieri G. Extraction treatment using Invisalign technique, Prog. Orthod. 7:32-43, 2006.
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40. Uribe F, CutreraA, Nanda R.:A segmented appliance for space closure followed by Invisalign and fixed appliances. Orthod. (Chic) 12:386-395, 2011.
41. Samoto H, VlaskalicV. A customized staging procedure to improve the predictability of space closure with sequential aligners. J Clin Orthod 2014 48(6):359-367.
42. Nak-Chun Choi, Young-Chel Park, Yong-Min Jo, Kee-Joon Lee. Combined use of miniscrews and clear appliances for the treatment of bialveolar protrusion without conventional brackets. Am J Orthod Dentofacial Orthop. 2009 May;135(5):671-81. doi: 10.1016/j.ajodo.2006.12.025 43. Park YC, Chu JH, Choi YJ, Choi NC. Extraction space closure with vacuum-formed splints and miniscrew anchorage. J Clin Orthod. 2005; 39: 76–79.
44. Ojima K, Dan C, Nishiyama R, Ohtsuka S. Schupp W. Accelerated extraction treatment with Invisalign. J Clin Orthod 2014; 48(8):487-499.
45. Li W, Wang S, Zhang Y. The effectiveness of the Invisalign appliance in extraction cases using the the ABO model grading system: a multicenter randomized controlled trial. Int J Clin Exp Med. 2015 May 15;8(5):8276-82. eCollection 2015.
46. Baldwin DK, King G, Ramsay DS, Huang G, Bollen AM. Activation time and material stiffness of sequential removable orthodontic appliances. Part 3: premolar extraction patients. Am J Orthod Dentofacial Orthop. 2008 Jun;133(6):837-45. doi: 10.1016/j.ajodo.2006.06.025.
47. Kuo E and Duong T. Invisalign attachments: Materials. In: The Invisalign System. Tuncay, O. editor. Quintessence Publishing, London 2007 ISBN:91-98.
48. Boyd RL and Vlaskalic V. Three-dimensional diagnosis and orthodontic treatment of complex malocclusion with the Invisalign appliance. Semin Orthod 2001;7:274-293.
49. Nicozisis JL. Tripping the plastic fantastic. Orthod Products 2013;Nov. 28-34.
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50. Humber, P. Rotating canines using the Invisalign system. Aesthetic Dentistry Today 2013 Feb;7(1):30-34.
51. Bowman SJ. Clear collection instruments for clear aligner treatments. Orthod Practice US. 2015 May/June;6(3);74-78.
52. Zhang XJ, He L, Guo HM, Tian J, Bai, Y, Li S. Integrated three-dimensional digital assessment of accuracy of anterior movement using clear aligners. Korean J Orthod 2015 Nov;45(6):275-81. doi: 10.4041/kjod.2015.45.6.275. Epub 2015 Nov 20.
53. Hahn W, Zapf A, Dathe H, Fialka-Fricke J, Fricke-Zech S, Gruber R, Kubein-Meesenburg D, Sadat-Khonsari R. Torquing an upper central incisor with aligners--acting forces and biomechanical principles. Eur J Orthod. 2010 Dec;32(6):607-13. doi: 10.1093/ejo/cjq007. Epub 2010 May 12.
54. Castroflorio T, Garino F, Lazzaro A, Debernardi C. Upper-incisor root control with Invisalign appliances. J Clin Orthod 47:346-351, 2013.
55. American Board of Orthodontics. Why case reports do not pass the ABO Phase III clinical examination. Am J Orthod 110:559-560, 1996.
56. Bowman SJ. Addressing concerns for finished cases: the development of the Butterfly Bracket System. Part I. J Ind Orthod Soc 36:73-75, 2003.
57. Marshall S, Dawson D, Southard KA, Lee AN, Casko JS, Southard TE. Transverse molar movements during growth. Am J Orthod Dentofacial Orthop 124:615-624, 2003.
58. McLaughlin RP, Bennett JC, Trevisi H. Systemized Orthodontic Treatment Mechanics. Mosby, St. Louis, 2001.
59. Yang-Powers LC, Sadowsky C, Rosenstein S, BeGole EA. Treatment outcome in a graduate orthodontic clinic using the American Board of Orthodontics grading system. Am J Orthod Dentofacial Orthop: 122:451-455, 2002.
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60. Sondhi A. An analysis of orthodontic prescriptions: their strengths and weaknesses. Lecture, AAO Annual Session, Honolulu, May 6, 2003. 61. Schupp,W.; Haubrich, J. and Neumann, I.: Treatment of anterior open bite with the Invisalign system, J. Clin. Orthod. 44:501-7, 2010. 62. Boyd, R.L.: Complex orthodontic treatment using a new protocol for the Invisalign appliance, J. Clin. Orthod. 42:525-547,2007. 63. Bowman SJ. Clear collection instruments for clear aligner treatment: part 2. Orthod Practice US 2015 July/August;6(4):48-52. 64. Giancotti, A. and Ronchin, M.: Pre-restorative treatment with the Invisalign system, J. Clin. Orthod. 40:679-82, 2006 65. Fischer K. Invisalign treatment of dental Class II malocclusions without auxiliaries, J. Clin. Orthod. 44:665-672,2010 66. Schupp W, Haubrich J, Neumann I. Class II correction with the Invisalign system, J. Clin. Orthod. 44:28-35,2010. 67. Klein BM. A cephalometric study of adult mild Class II nonextraction treatment with the Invisalign System. Master’s Thesis. 2013. St. Louis: Saint Louis University – Center for Advanced Dental Education.
68. Sandifer CL, English JD, Colville CD, Gallerno RL, Akyalcin S. Treatment effects of the Carrière distalizer using lingual arch and full fixed appliances. J World Fed Orthod 2014; 3:e49e54.
69. Colville CD. Carrière distalizer and Invisalign combo for Class II treatment. Invisalign Insights 2012 http://www.aligntechinstitute.com/RealStories/Documents/newsletter_OR_201209.pdf
70. Bowman, S.J.: Thinking outside the box with miniscrews. In: Microimplants as Temporary Anchorage in Orthodontics, McNamara JA Jr, Ribbens KA, (Eds). Craniofacial Growth Series, The University of Michigan, Ann Arbor, 45:327-390, 2008.
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71. Ludwig B, Baumgaertel S, Bowman SJ. Mini-Implants in Orthodontics: Innovative Anchorage Concepts. Chicago, Quintessence, 2008.
72. Bowman SJ. Settling the Score with Class IIs Using Miniscrews In: Temporary Skeletal Anchorage Devices—A Guide to Design and Evidence-Based Solutions. Kim, K.B., Editor, Springer, 2014.
73. Bowman, SJ. The evolution of the Horseshoe Jet. In: Skeletal Anchorage in Orthodontic Treatment of Class II Malocclusion: Contemporary Applications of Orthodontic Implants, Miniscrew Implants and Miniplates, Papadopoulos, M.S., Editor, St. Louis: Mosby Elsevier, 2015.
74. Schupp W, Haubrich J, Ojima K, Boisserele W, Morton J. Aligner Orthodontics: Diagnosis, Treatment Planning, Orthodontic and Orthopedic Treatment. Quintessence publishing, London, 2015.
Corresponding Author:
Dr. S. Jay Bowman Kalamazoo Orthodontics 1314 West Milham Avenue Portage MI 49024 269-344-2466 [email protected]
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FIGURE LEGENDS FIG 1. Ineffectual movement of a crowded and rotated central incisor due to insufficient space. Additional space must be created by either IPR and/or expanding to open visible space on either side of the affected tooth; otherwise, no movement is possible. Once space is created, a rotational couple is necessary to produce the desired change. Holding or preventing movement of the teeth adjacent to the rotated tooth in question (by staging in the set-up) and using accent pliers to place indents on opposite sides of the plastic (The Vertical pliers, Clear Collection, Hu-Friedy, Chicago, 15,51
IL) may enhance the intended biomechanical effect.
FIG 2. Torque ridge indentations to increase the force applied to the crown to enhance a rotational couple with intent to change the angulation of a tooth. These ridges can be prescribed in some commercial appliances such as Invisalign® or added using The Horizontal pliers (Clear 15,51
Collection, Hu-Friedy, Chicago, IL).
FIG 3. During the Invisalign Teen Study
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torque ridge indentations were added to aligner trays
with the intent to affect a rotation couple to improve the predictability of producing desired angulation of upper incisors. Although the “torque” amounts reasonably matched the planned “set-ups,” prescribing over-correction of this root movement may be required in order to achieve intended goals.
FIG 4. The American Board of Orthodontics determined that the most common error noted in case reports failing the ABO examination were inappropriate buccolingual inclination or torque of posterior teeth.
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Attention to detail in preparing virtual set-ups (e.g., adding buccal root torque)
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15,63
FIG 5. Preparing bootstrap biomechanics.
Tear Drop pliers are used to cut notches in mesial
and distal embrasures of the aligner plastic. Hole Punch pliers clear the aligner plastic to allow the addition of a bonded button. Orthodontic elastics are stretched from the button to the teardrop notches or, alternatively, to another button bonded on the opposite side of the tooth. (Clear Collection pliers, Hu-Friedy, Chicago, IL).
FIG 6. Bootstrap mechanics
15,63
to forcibly erupt “lagging” teeth using orthodontic elastics.
Adult female with Class II Division 2 malocclusion, deep overbite, crowding, rotations, and uneven maxillary anterior teeth. Initially, Invisalign ® treatment featured angulated, horizontal beveled attachments bonded to the facial of the lateral incisors.
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After limited improvement
noted in the overbite, torque of central incisors, and extrusion/rotation of the lateral incisors, a bootstrap set-up was created at “refinement.” Clear buttons were bonded at gingival margin on the facial and metal buttons on the lingual of both upper laterals. Plastic was relieved from the aligners to accommodate those buttons using The Hole Punch pliers.
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Orthodontic elastics were
stretched from the lingual to the palatal buttons across the seated plastic aligners. The virtual setup featured arch expansion with visible space created adjacent to the laterals, forced extrusion of the laterals, intrusion and torque for central incisors, intrusion of lower anterior teeth, and overrotation for the lateral incisors.
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www.elsevier.com/locate/ysodo
PII: DOI: Reference:
S1073-8746(16)30061-5 http://dx.doi.org/10.1053/j.sodo.2016.10.005 YSODO482
To appear in: Seminars in Orthodontics Cite this article as: S. Jay Bowman, Improving the predictability of clear aligners, Seminars in Orthodontics, http://dx.doi.org/10.1053/j.sodo.2016.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improving the Predictability of Clear Aligners S. Jay Bowman DMD, MSD S. Jay Bowman DMD, MSD is a Diplomate of the American Board of Orthodontics and a member of the Edward H. Angle Society of Orthodontists. He developed and teaches the Straightwire course at The University of Michigan, is an Adjunct Associate Professor at Saint Louis University, an Assistant Clinical Professor at Case Western Reserve University, Visiting Clinical Lecturer at Seton Hill University, and Milton Sims Visiting Professor at the University of Adelaide. He maintains a private specialty practice of orthodontics in Portage, MI.
I like predictability because I know what I'm getting into.
-Actress Katherine Heigl
Addressing Limitations of Clear Aligner Treatment The advent of using a series of clear aligners came at the turn of the 21st century, despite the basis for this concept originating over 55 years earlier.1 Although, the original fanfare of plastic aligners equitably replacing traditional metal braces quickly tarnished as early adopters became painfully aware of limitations of the concept.2-9 Some practitioners vacated, some vacillated, and others simply dug-in to doggedly invent ways to improve this innovation. Patients desire “braceless” correction of their smiles10 and thus far, the efforts to give them what they want has led to continual advances and increasing number of applications and adjuncts for aligners in orthodontics.11-23
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Compliance, Consequences, and Quality of Life with Clear Aligners If treatment with aligners was to be seen as completely successful, it was not enough to occasionally straighten some teeth just in the esthetic zone. Predictable, consistent, and completed results without adverse effects were requirements. First and foremost was a justified concern for patient compliance in wearing removable devices during the course of possibly years of treatment. As Buzz Behrents cautioned, compliance is the single most important factor in treatment success.
Cooperation in wearing aligners differs from that of fixed braces in that wires and brackets cannot be taken-off and disregarded by the patient, so the treatment might proceed towards at least “straighter” teeth. In contrast, if aligners are not worn as prescribed, the patient receives no result. Consequently, an alternative track for the non-compliant patient must be a part of informed consent.
The introduction of so-called Compliance Indicators for the Invisalign® product was evaluated during testing of their Teen product (targeted for the audience with the highest concerns for adherence to wear). The
2
soluble, color-fading dots were found to reasonably demonstrate if patients had been consistently wearing their aligners.24 If used as a “policing” device, however, much like embedded timers in headgears or functional appliances, their effectiveness in actually eliciting cooperation is still in question.24 More importantly, compliance by orthodontic patients with either braces or plastic is, on-average, favorable and comparable.25 Non-compliant patients likely suffer consequences of prolonged or compromised results no matter the treatment method chosen.
In terms of adverse effects, although there are still concerns for decalcification, gingivitis, root resorption, etc. with either fixed braces or removable aligners, the oral hygiene advantage for removable appliances is significant—as long as they are, in fact, re-inserted after cleaning and eating. All in all, it was not surprising that the quality-of-life satisfaction of teen aligner patients was found to be quite high.25
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Aligner Tracking A common concern running consistently throughout the history of aligner treatments has been that of teeth not following the predicted or desired movement. This is despite the fact that the forces and moments produced with plastic are comparable to those generated with fixed braces.26 For instance, Chisari et al.13 found that just 57% of the tooth movement programmed for a single incisor was actually realized in a period of an eight-week investigation. In addition, researchers from the University of Illinois11 reported only a 41% mean accuracy of predicted tooth movement. The reasons for these frustrations are multi-factorial and require a variety of responses to resolve. For example, specific teeth oft times do not remain seated within the confines of the plastic aligner trays as treatment progresses. This may produce an air gap between tooth and plastic termed “lag” or “tracking error.” Strategies to reduce “tracking errors” will be described below.
Rohit Sachdeva has described the process of orthodontic finishing as “reducing the errors that have accumulated during treatment.” These errors can crop-up quickly during the sequence of aligners, resulting in a loss of tracking and treatment “running off the rails.” These errors
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include those of diagnosis and treatment planning, but with aligners they accrue at the outset. They include issues with impression taking, laboratory scanning of those impressions, tolerances in the creation of laboratory models (poured or printed), tolerances in the process of the molding of plastic over said models, limitations of the plastic materials themselves, and, of course, inconsistent compliance.
Much like issues in manufacturing of metal wires and braces, there are tolerances permitted in fabrication that may limit the accuracy of tooth movement. The same situation occurs in molding plastic trays, especially when “simulated/predicted tooth movements” are involved. In addition, thin plastic may mold around the undercuts and interproximal spaces between teeth but might provide insufficient force to achieve the desired tooth movement. The flexibility of the material and its conformation from the incisal edge to the gingival margin of individual teeth, combined with the necessary duration of forces applied to specific teeth, can be problematic. It seems there must be a compromise between heavier forces from thicker or less pliable plastic versus thinner or more resilient materials.27,28 In any event, some tooth movements (extrusion, torque, rotations) especially for certain teeth
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(maxillary laterals and cuspids) have been clearly demonstrated to be less predictable and require extra attention.1-3,11-13,29-32 These types of concerns led directly to the creation of bonded composite attachments with the intent to increase the surface area for plastic aligners to “grip” onto teeth.
More importantly, the design of the sequence and velocity of the movement of teeth during treatment requires the greatest consideration. Sequential or simultaneous tooth movement plans are desirable in different situations. Simply expecting teeth to move from start to a finished occlusion without a specific plan in-between for how these teeth will overcome obstacles (i.e., collisions, contact points, occlusal and muscular forces, insufficient force or contacts from the plastic) is unrealistic at best.9
One of the earliest adjuncts to help seat aligner trays, attempt to maintain “tracking,” and increase occlusal forces to elicit tooth movement was the introduction of aligner “chewies” (Chewies Aligner Tray Seaters, Dentsply Raintree Essix, York, PA).15,33 These plastic “cotton rolls” are prescribed for patients to use at least the first few days
6
after changing to each new pair of aligners. Like the concept of a tooth positioner, the patient places the device in a site where teeth are not tracking (i.e., a visible “air” gap is seen between the occlusal or incisal of a tooth and the aligner). The patient then bites and holds onto the chewies repeatedly, over a few minutes of time daily, with their aligners in place.15,33 The intent is to help seat the aligner on the teeth that are not tracking, slightly intrude the teeth adjacent to a tooth in question, and the added perturbations may help to accelerate the remodeling in that site to stimulate the tooth to move as prescribed.
Lack of Space Crowded dentitions offer only two options for resolution: create more space (expand) or reduce tooth mass (IPR or extraction). Expansion implies either development/growth of new bone in the buccal alveolus. This may potentially position teeth into less stable locations due to change in equilibrium in the facial musculature and an associated potential loss of bone as teeth are pushed beyond the envelope of the alveolus. Certainly, some modicum of expansion appears to be tolerable or at least “retainable.” Unfortunately, aligners tend to expand a most unstable site: the mandibular intercanine dimension.34 IPR to reduce
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the dimensions of teeth to provide for their accommodation within the arch perimeter is useful, but should be limited in scope. Extractions are a time-tested, evidence-based approach to resolving significant arch length discrepancies; however, the results with aligners have been unpredictable. These more complex cases often benefit from adjuncts such as elastics, mini-screw anchorage, or even the use of sectional fixed braces.16,35-44
In the simplest terms, if there is insufficient space for a tooth to move, it won’t. To prescribe the unraveling of anterior crowding with aligners seems such a simple procedure. Without the creation of space, the “binding” collisions or contacts between irregular incisors will prevent movement. As noted earlier, the options to solve this are obvious and they can be combined (mild expansion + mild IPR; extraction + IPR).
Selecting IPR from the outset makes little sense, unless the original discrepancy is quite minor. That leaves the application of some degree of expansion (labially, laterally). Expansion to resolve minor crowding may be as simple as prescribing the opening of “visible” space (>0.2 mm) between specified teeth. Then reasonable amounts of IPR could be
8
performed more safely. Some perform this structural reduction chairside with “lightning strips or disks.” Others provide abrasive strips to patients with prescribed directions to “floss” the strips between contact points to create small amounts of space. Once space is achieved, then more predictable correction of rotations and labiolingual discrepancies can be accomplished. Any small residual space that remains can be easily closed with the aligners as treatment progresses.
If extractions are chosen, then an entirely different group of concerns must be addressed from control of anchorage, torque (root rotation), tipping (bodily movement), and ensuring interproximal contacts are created at the end.36-39,45-46 The application of intramaxillary or intramandibular elastics to miniscrews,42-44 intermaxillary elastics, and the increase in size and type of composite attachments may be needed to increase predictability.16
Predictable sequencing of space closure or pushing individual teeth into alignment for any of the previous scenarios may be best handled in small segments of tooth movement.16,20,23 In other words, to control anchorage in small increments, teeth adjacent to a specific tooth that is out of
9
position are prescribed to be held in place (no movement) until the irregular one is moved to place and so on. This detailed sequencing of small parts of a treatment plan are intended to reducing tracking errors that are sometimes experienced during simultaneous movements of large numbers of teeth.
Rotations Although it seems quite obvious that attempting to rotate some teeth with plastic is unlikely to occur due to their anatomy.7 For example, expecting to reliably rotate a “round” premolar seems futile unless a “handle” is placed on it to increase its profile asymmetrically. Molded composite attachments47-48 have increased the predictability of rotational control to some degree.49-50 But, the simple application of a prominent handle, of even a specific engineered design, cannot be depended-upon without accompanying proper treatment planning.
The most unpredictable teeth to achieve rotation for in the esthetic zone are the maxillary laterals and cuspids;11 despite the use of seemingly enormous attachments. Rotating these teeth appears to require the most attention to detailed treatment sequencing.15 Consider the typical
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position of a maxillary lateral incisor often accompanying a Class II Division 2 malocclusion. Simply prescribing that this tooth be “straight” at the end of aligner treatment is absurd. Space must first be created or this tooth will simply not move. This may be accomplished by “expansion” laterally, but also arch perimeter may be increased as lingual root torque is applied to often “rabbited” central incisors. Once visible space is open, then any desired IPR can be provided, but no rotation of the incisor can be started until there is room.
The shape, size, and the position of the lateral incisor between two larger neighboring teeth preclude much contact with plastic. This is further compromised if there is a desire to extrude the lateral and/or intrude the central incisors. There is simply insufficient retention of plastic stretched over the blade-shaped lateral. Consequently, a variety of attachments have been designed and modified for use on laterals with varying degrees of success.49 Unless combined with the prerequisite creation of space, the dimensions or design of an attachment are pointless. In fact, the iatrogenic effect of applying these forces without space can actually produce more intrusion, squeezing the tooth apically; thereby, increasing tracking error. More predictable has been the introduction of adjunctive
11
forces from orthodontic elastics as described below.
Expecting a tooth to be rotated into an ideal prescribed position, using flexible plastic (with inherent errors or limits in tolerances), without asking for “over-rotation” also seems dubious. For instance, it is probably necessary to prescribe 2-3 degrees of “over-rotation” to ever be able to simply reach “ideal.” Besides, over-correction has been a timetested tenet in orthodontics.
Another option is to use detailing pliers (The Vertical, Clear Collection, Hu-Friedy, Chicago, IL) 15,51 to produce indentations into the plastic to create accented rotational couples (FIG 1.) Finally, just because the tooth was moved to the desired spot does not guarantee it will remain. It is specifically advisable to consider an Edward’s supercrestal fiberotomy, for significantly rotated teeth that you do not want to return to their original position (i.e., lateral incisors in Class II Division 2).
Insert Fig. 1
Torque and Root Angulation
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Control of root movement with aligners has been an unfailingly constant concern.7,52-53 Rotation of roots (torque), bodily tooth movement, and root paralleling have been deemed to be limitations of applying relatively “loose” forces with plastic. Fixed braces have the advantage of a more intimate junction between the forces generated by the deformation of wires bound in brackets bonded directly to the teeth.
Plastic aligners, despite often covering the majority of surface area of teeth, still do not have the same connection; therefore, the crown of the tooth may slip away from the plastic. Accordingly, to alter the angle of roots (moving them some distance through bone) is much more difficult than simple crown tipping. Complicate this with the flexibility of plastic, including the differential in resilience and force levels from the gingival margin to the occlusal of the aligners, and to create rotational couples for roots become difficult. If simultaneous movements of crown rotation, intrusion, extrusion, and unresolved crowding are added to the mix, predictability can be problematic.
The application of torque specifically for maxillary anterior teeth was identified early in aligner history as unpredictable.12,32,54 As a result,
13
most change noted in the angle of incisors occurred due to “flaring,” “rounding-out the arch,” or facial tipping as crowding was resolved using expansive pushing forces. Unintended excess overjet or bimaxillary protrusion may result. For example, the Class II division 2 malocclusion seems an easy fix if you simply tip the upper central incisors labially. Unfortunately, the resulting overjet accompanying a straighter “social six” is not exactly what most patients have in mind.
The addition of so-called “torque ridges” to enhance forces for lingual root torque (limiting labial crown tipping) was first evaluated during the Invisalign Teen Research project.25 These ridges were added to the plastic at the gingival margin on the labial and at the incisal edge of the lingual to generate a rotational couple to tip roots palatally (and viceversa if labial root torque was desired). These ridges can also be added individually anywhere along the aligner tray using The Horizontal pliers (Clear Collection, Hu-Friedy, Chicago, IL )(FIG. 2).15,51
Insert Fig. 2
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Results from the Teen Study25 demonstrated that the predicted actual degrees of torque produced were reasonably achieved (FIG. 3). But, due to the flexibility of plastic and the inherent difficulty in moving roots through bone, prescribing over-correction of torque is suggested in order to just achieve the intended angulation. In addition, caution is warranted as these additional forces may tend to push the plastic incisally and increase tracking error.53 Keeping the trays seated onto the incisor crowns using some programmed intrusion may improve the response. Finally, do not ignore the unintended posterior effects from anterior torque. Loss of posterior anchorage seen with mesial tipping and intrusion of the mesial marginal ridge of molars may be attendant to anterior torque expression.16
Insert Fig. 3
What is so important about anterior torque? More attention than ever is being paid to proper coupling of upper and lower anterior teeth as the population of adult orthodontic patients has increased. The development of crowding, coincident with deep overbites, is often associated with “upright incisors” (obtuse interincisal angle) and the
15
constricted envelope of anterior function. The long-term consequence may be substantial loss of tooth structure from a combination of functional and dysfunctional wear. Mix-in any unidentified Bolton tooth-size discrepancies (e.g., small maxillary lateral incisors), acidic diet, or gastrointestinal reflex disease, and the damage to teeth can be dramatic. Consequently, it may not be simply enough to line-up the front teeth in short order to only satisfy patient’s esthetic requests. Informed consent begs proper diagnosis and treatment planning to address possible functional concerns.
Addressing root angulation or torque is not limited to only anterior teeth. The American Board of Orthodontics described that the most common error found in case reports that failed their examination was, in fact, insufficient posterior buccolingual inclination (posterior torque), 55,56 a concern shared in other reports.57-60 Unless the angle of posterior teeth is specifically addressed in the virtual set-up, an excessive curve of Wilson, prominent palatal cusps of uppers, improper posterior intercuspation with inappropriate posterior overjet may result (FIG 4)58 Marshall et al57 advised, “For proper occlusion, there should be no significant difference between the heights of the buccal and lingual cusps of molars and
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premolars.”
Insert Fig. 4
Attention to detail in set-ups by evaluating the posterior teeth from the lingual view can assist in correcting these errors. In addition, increasing posterior collisions during the completion of treatments can significantly reduce the iatrogenic creation of posterior openbites from “passive posterior intrusion” that can occur by simply wearing plastic trays longterm.61 Recent changes in some proprietary software have permitted orthodontists to specifically address these issues with their own individual manipulation of tooth movement in virtual set-ups.
Extrusion Asking for extrusion where occlusal surfaces of teeth are “capped” with plastic (without substantial undercuts for retention) is unrealistic. At 29.6% effectiveness, extrusion was reported to be the least accurate tooth movement with aligners.11 Without some type of retention, the plastic will simply slip occlusally or incisally, leaving the tooth behind; resulting in a loss of aligner contact often termed “aligner lag.” These teeth are
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not lagging behind; they had no chance to be moved. Once again, the knee-jerk reaction was to increase the size and shape of composite attachments to attempt to urge teeth occlusally with bigger handles for the plastic to grip onto.47,48,62 In situations where extrusion was still not seen, the concept of aligners itself was blamed for the “lag” without thinking through the limitations and finding solutions.
If the intent is to extrude an incisor, then certainly adequate retention of the plastic is required to produce the extrusive force. We cannot expect a blade-shaped tooth to track occlusally as if magically attracted to plastic above it. But, is just a more prominent attachment simply enough?
Tight interproximal contacts or significant collisions (even without overlap or crowding) can stop extrusion; therefore, adequate space must be produced prior to attempting extrusion. This is especially important when considering the shape of teeth adjacent to one that is to be “pulledup.” Incisors or canines are often narrower gingivally and taper to a wider dimension incisally. In those instances, extrusion is an obvious non-starter without space creation. Finally, the application of root tip or
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torque along with labiolingual positioning also play intimate roles in extrusion. Sequential tooth movement to achieve the desired vertical change is much more complex than simply communicating “an order” with a lab tech to just pull the tooth occlusally.
Adjunctive Forces The addition of adjunctive forces to enhance clear aligners is not a new idea.6 For example, the application of orthodontic elastics to enhance or facilitate various biomechanics were some of the earliest and most obvious additions. But, attempting to improve aligner “tracking” and reduce the so-called “lag” for certain teeth has been the most perpetual dilemma affecting predictable results. Much like using intermaxillary elastics with braces to increase intercuspation when producing a solid final occlusion, a combination of bonded buttons and elastics can also be used with clear aligners. In these scenarios, buttons bonded on the buccal surfaces of posterior teeth can be used to apply “triangle, box, or up-and-down” elastics to seat teeth into the aligners and into the final occlusion prescribed in the set-up.15,63 Therefore, the application of orthodontic elastics that are connected to the aligner plastic, bonded buttons on teeth, or to miniscrew anchors have increased the variety of
19
achievable movements possible.
Boot-strapping for Predictability The predictability of single tooth extrusion and/or rotation can be enhanced using orthodontic elastics in a so-called “boot-strap” arrangement.64 Bootstrap mechanics involve placing bonded buttons at the gingival margin on the facial, lingual, or both surfaces of a tooth that is lagging.15,63 An orthodontic elastic is then stretched across the occlusal surface of the seated clear aligner to attach to buttons and/or notches cut into the tray on opposite sides. The gingival margin of the aligner plastic must be relieved to avoid the buttons so as to permit the tray to seat fully (FIG. 5,6).
Insert Fig. 5
Insert Fig. 6
Intrusion of a segment of teeth can also be supported using elastics supported by miniscrew anchorage. The possibilities include applying the orthodontic elastics or chain from buccal and/or palatal elastics to
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buttons bonded to teeth (while relieving plastic from the trays to avoid those attachments).15,63 As an example, bonded buttons can be placed at the gingival margin of all incisors (for a patient with a deep overbite) or of molars (for hypererupted posteriors). Orthodontic elastics from those buttons are stretched to miniscrews inserted interradicularly apical to the teeth to be intruded.15
Posterior intrusion to enhance the closure of anterior openbites is another option with elastics and miniscrews. Sequential intrusion of posterior teeth combined with some extrusion of anterior teeth to increase the curve of Spee22,23 can be reinforced with bootstrap elastics. In this scenario, elastics are stretched across the occlusal of seated aligners from miniscrews to buttons, notches in trays, or to another miniscrew in bone on the other side of the alveolus.15
Skeletal Discrepancies Addressing skeletal discrepancies were originally seen to be an anathema to be avoided with aligners.3,4 Only limited changes in over or underjet were attempted with aligners, while expecting little if any anteroposterior correction.65,66 Later, maxillary posterior distalization was attempted,
21
but without consideration of anchorage concerns. This led to the application of intermaxillary Class II elastics to make-shift notches cut into the plastic trays, then to buttons bonded to the trays, and finally to buttons directly bonded to the teeth (or some combination of those options).20 Once elastics were found to improve the chance of orthodontic movement for Class IIs, then employing sequential molar distalization supported by elastics20 was a logical strategy that has proven successful with aligners.67 Currently, elastics on aligners have also found their way into correcting Class IIs, IIIs, and openbites; especially for growing individuals.
Other options for improving the predictability of skeletal correction have included “pre-cursor” adjuncts used prior to beginning clear aligners. These have included the use of the Carrière arm (Carrière Distalizer, Ortho Organizers, Henry Schein, Melville, NY) to hold Class II elastics to distalize posterior segments14,68,69 or typical, stand-alone distalizers, such as the Pendulum or Distal Jet. Any of these devices may be supplemented with mini-screw anchorage to avoid reciprocal adverse effects.
22
For example, the Horseshoe Jet (Specialty Labs, Atlanta, GA), is a laboratory modification of the Distal Jet (American Orthodontics, Sheboygan, WI), supported by miniscrews. These appliances are used to produce molar distalization14,70-73 prior to initiating clear aligner treatment. After posterior teeth are moved into a Class I occlusion, clear aligners are prescribed to hold the molars in place while the remaining anterior teeth are retracted; just as they would be with fixed braces. This retraction is often supported with Class II elastics or even Class I intramaxillary elastics from mini-screws to reduce the possibility of anchorage loss.14,51,72
Finally, the introduction of a variety of methods intended to accelerate the rate of tooth movement have arisen in recent years. For example, the use of a device to produce microvibrations or perturbations (AcceleDent, OrthoAccel, Bellaire, TX) with the intent of stimulating tooth movement has been anecdotally described to reduce treatment time with clear aligners, warranting further research. 16,44,74
Predicting the Future of Clear Aligners Technology has continued to improve clear aligner treatments since their
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inception. Replacing impressions with increasingly more accurate intraoral scanners, 3D printing instead of plaster models, individual tooth movement and staging in virtual set-ups, and improvements in aligner materials and attachments are all examples of recent advancements. Simultaneously, general practitioners with limited training are providing more “social six” treatments (disregarding the fit and function of teeth) and nearly every orthodontic lab (in or out of the orthodontist’s practice) is offering some type of clear plastic product. Can all options (specialist or general dentist; scanner with computer setup versus bench model set-up) provide equal or similar results? Clearly, aligner treatments are here to stay, but are we all equally trained to serve the best interests of our patients using these devices or are some depending upon lab techs to provide the diagnosis and prescribe the treatment plan? All practitioners simply need to be as honest with ourselves as we are with those that choose to have us treat them. Products noted in the manuscript: (Clear Collection pliers, Hu-Friedy, Chicago, IL) (Invisalign®, Align Technology, San Jose, CA) (Chewies Aligner Tray Seaters , Dentsply Raintree Essix, York, PA) (Carrière Distalizer, Ortho Organizers, Henry Schein, Melville, NY) (Horseshoe Jet, Specialty Labs, Atlanta, GA)
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(Distal Jet, American Orthodontics, Sheboygan, WI) (AcceleDent, OrthoAccel, Houston, TX)
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Corresponding Author:
Dr. S. Jay Bowman Kalamazoo Orthodontics 1314 West Milham Avenue Portage MI 49024 269-344-2466 [email protected]
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FIGURE LEGENDS FIG 1. Ineffectual movement of a crowded and rotated central incisor due to insufficient space. Additional space must be created by either IPR and/or expanding to open visible space on either side of the affected tooth; otherwise, no movement is possible. Once space is created, a rotational couple is necessary to produce the desired change. Holding or preventing movement of the teeth adjacent to the rotated tooth in question (by staging in the set-up) and using accent pliers to place indents on opposite sides of the plastic (The Vertical pliers, Clear Collection, Hu-Friedy, Chicago, 15,51
IL) may enhance the intended biomechanical effect.
FIG 2. Torque ridge indentations to increase the force applied to the crown to enhance a rotational couple with intent to change the angulation of a tooth. These ridges can be prescribed in some commercial appliances such as Invisalign® or added using The Horizontal pliers (Clear 15,51
Collection, Hu-Friedy, Chicago, IL).
FIG 3. During the Invisalign Teen Study
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torque ridge indentations were added to aligner trays
with the intent to affect a rotation couple to improve the predictability of producing desired angulation of upper incisors. Although the “torque” amounts reasonably matched the planned “set-ups,” prescribing over-correction of this root movement may be required in order to achieve intended goals.
FIG 4. The American Board of Orthodontics determined that the most common error noted in case reports failing the ABO examination were inappropriate buccolingual inclination or torque of posterior teeth.
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Attention to detail in preparing virtual set-ups (e.g., adding buccal root torque)
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15,63
FIG 5. Preparing bootstrap biomechanics.
Tear Drop pliers are used to cut notches in mesial
and distal embrasures of the aligner plastic. Hole Punch pliers clear the aligner plastic to allow the addition of a bonded button. Orthodontic elastics are stretched from the button to the teardrop notches or, alternatively, to another button bonded on the opposite side of the tooth. (Clear Collection pliers, Hu-Friedy, Chicago, IL).
FIG 6. Bootstrap mechanics
15,63
to forcibly erupt “lagging” teeth using orthodontic elastics.
Adult female with Class II Division 2 malocclusion, deep overbite, crowding, rotations, and uneven maxillary anterior teeth. Initially, Invisalign ® treatment featured angulated, horizontal beveled attachments bonded to the facial of the lateral incisors.
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After limited improvement
noted in the overbite, torque of central incisors, and extrusion/rotation of the lateral incisors, a bootstrap set-up was created at “refinement.” Clear buttons were bonded at gingival margin on the facial and metal buttons on the lingual of both upper laterals. Plastic was relieved from the aligners to accommodate those buttons using The Hole Punch pliers.
63
Orthodontic elastics were
stretched from the lingual to the palatal buttons across the seated plastic aligners. The virtual setup featured arch expansion with visible space created adjacent to the laterals, forced extrusion of the laterals, intrusion and torque for central incisors, intrusion of lower anterior teeth, and overrotation for the lateral incisors.
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