- Email: [email protected]
Design of the Invisalign system performance
Author’s Accepted Manuscript Design of the invisalign system performance John Morton, Mitra Derakhshan, Srini Kaza, Chunhua Li, Victor Chen
www.elsevier.com/locate/ysodo
PII: DOI: Reference:
S1073-8746(16)30057-3 http://dx.doi.org/10.1053/j.sodo.2016.10.001 YSODO478
To appear in: Seminars in Orthodontics Cite this article as: John Morton, Mitra Derakhshan, Srini Kaza, Chunhua Li and Victor Chen, Design of the invisalign system performance, Seminars in Orthodontics, http://dx.doi.org/10.1053/j.sodo.2016.10.001 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.
DESIGN OF THE INVISALIGN SYSTEM PERFORMANCE
John Morton Sr. Technology Fellow, Product Innovation Align Technology San Jose, CA
Dr. Mitra Derakhshan, DDS, MS Sr. Director, Global Clinical Align Technology San Jose, CA
Srini Kaza MS, MBA Vice President, Product Innovation Align Technology San Jose, CA
Chunhua Li, PhD Senior Director, Materials and Biomechanics Align Technology San Jose, CA
Victor Chen Director, Clinical Affairs Align Technology San Jose, CA
Corresponding Author: John Morton Align Technology 2560 Orchard Parkway San Jose, CA 95131
[email protected] Tel
(408) 470-1205
Fax
(408) 914-1399
Introduction Innovation defines and drives Align Technology. Arguably, Invisalign is still at the forefront of clear aligner treatment. Incessant innovation and advancement built into the clear aligner has expanded its application beyond the early days of only limited minor tooth movement solutions. Principally, improvements to aligner material, force systems, staging of tooth movements, and treatment planning have enabled the Invisalign System to treat complex malocclusions with precise and predictable outcomes. Growth of Invisalign In 1997, Invisalign was created with the vision of providing an aesthetic and comfortable aligner appliance with which doctors could treat their patients. Collaboration between orthodontists and the researchers at Align throughout the past eighteen years has greatly increased the capabilities of the appliance. The ability to treat complex cases and acceptance by the profession has resulted in phenomenal growth. To date, Invisalign is available in over 90 countries worldwide and practitioners have used it to treat over 3.5 million patients. Doctors certified to treat with the Invisalign System
number 112,000. The 3D printing of molds for aligner fabrication is the largest in the world with 175,000 unique high precision aligners produced every day. Align researchers are continuously innovating to improve treatment outcomes with Invisalign. More than $500 million has been dedicated to Research and Development in areas that include biomaterials, biomechanics, 3D software, and doctor communication portals. More doctors are treating more cases as their confidence in achieving an excellent treatment outcome increases with each new innovation. (Figure 1) Looking toward the future, Align researchers will continue to innovate in areas of appliance performance and software, scanning, doctor communication and interface, and workflow. (Figure 2)
Although Invisalign has undergone tremendous growth, even larger opportunities lie ahead. Align analyses show there are 100 million adult consumers in the United States alone that are looking for a better smile and more than 2.5 million teenage consumers are looking for a better smile. Overall, 60-70% of the population of the United States feel they have crooked teeth and the worldwide numbers are of similar size. All of this data points to tremendous opportunities for practitioners treating patients with Invisalign System.
Overview of Principles used in the Invisalign System Thermoformed appliances have been used to move teeth since the 1940’s. These movements were limited, being restricted to tipping of the crowns and minor rotations to align the teeth. The understanding of the mechanics by which these appliances elicited tooth movement was limited as well. The innovations built into the Invisalign System over the past two decades, facilitate control of both the crown and the root movements. Indeed, the system is designed to proportionally move the root with respect to the crown. With this characteristic, the appliance can control the entire range of individual tooth movements, intrusion, extrusion, rotation, simple tipping, controlled tipping, translation (bodily movement) and root movement. Invisalign aligner employs the fundamental principles of biomechanics to control tooth movement.
The necessary tooth movement required for treatment determines the design of the force systems to be applied to the tooth. Subsequently, the appliance is designed to deliver these pre-determined forces. Undoubtedly, if the force system applied to the tooth is favorable to the desired movement, the probability of achieving the movement is greater. This principle holds true, be the appliance a bracket, a wire or an aligner. Engineers and scientists at Align experiment in controlling tooth movement with many different approaches. The shape of the aligner is altered, attachments are applied to modify the shape of the tooth, and the movement of the tooth is programmed along different paths (movement staging). The force systems delivered by the aligner are then measured and compared. The design which delivers the best force system is predicted to be the best design for clinical use. The aligner shape can be changed by another algorithm to increase the contact pressure at specific locations on the surface of the attachment or surface of the tooth. Examples of this are the Power Ridge feature to control anterior torque, a Pressure Point to deliver force to a specific location on the tooth surface, and a Pressure Area to deliver force along a specific surface of the tooth. Pressure Surfaces are used to improve anterior intrusion by directing the force along the long axis of the tooth through the apex. Algorithms (computer code) which define Optimized Attachment shapes are increasing in complexity and proving clinically to better control tooth
movements. Optimized attachments are automatically placed by the software and control the point of application of the force on the tooth, the direction of the force, and the amount of force applied. All Optimized attachments have an “active surface.” This is a flat surface on the attachment which receives the force from the aligner. This surface is placed by the algorithm at a specific location on the tooth, oriented with respect to the tooth axes and engaged by the aligner. (Figure 3)
The aligner does not contact any of the other surfaces of the optimized attachment to ensure no unwanted force is produced. These innovations are in a group of features referred to as SmartForce Features. Figure 4 shows the effectiveness of SmartForce features to improve control of tooth movements specifically, the control of root tip with root control attachments. These data, support attachments are necessary and needed to control certain tooth movements.
Determining the “best” force system to be applied for a movement has several clinical factors to consider. First, force systems are measured about an assumed center of resistance of a tooth. Excellent tooth movement is achieved with a simple definition of the center of resistance for each tooth. The location of the center of resistance will become more accurate as more information regarding root size and shape is provided by the cone beam radiographic images, and should lead to better control. Second, Invisalign aligners are designed to apply low constant force for optimal biological response as commonly defined in the literature. Third, the physiology of mature adults may be slow to respond to orthodontic forces, hence optimized attachments are designed to be engaged by the first three aligners in the series even If the tooth does “not” move. This design allows time for typical patient physiological response and provides for improved tracking from the beginning of treatment. The control of tooth movement with this advanced Invisalign System goes well beyond control of individual teeth. Orthodontic treatment requires control of different groups of teeth with respect to one another. The treatment of openbite, for example, often requires extrusion of the anterior dentition as a group or unit. The software identifies openbite treatment requiring extrusion of the four upper or lower anteriors, places optimized extrusion attachments and “activates” the aligner to produce similar force on all anteriors. Activation is a change in aligner shape to control the amount of force at the contact of the aligner and attachment. Controlling the interference of the aligner and the attachment controls the magnitude of the force delivered to the tooth. Stretching the aligner further delivers more force just as when stretching an elastic. In the process, the aligner is not moving and is in equilibrium; hence, the extrusive force on the anteriors must be accompanied by intrusive forces elsewhere on the aligner. These counteracting intrusive forces are delivered to the posterior dentition and often aid in the treatment of openbite. Similar mechanics is used in the treatment of deepbite. SmartForce features control the magnitude and balance of the intrusive forces on the anteriors and deliver the extrusive reaction forces to the posterior dentition. The lower bicuspids can receive forces to effectively extrude them, in effect, leveling the curve of Spee which may be of benefit in this type of treatment. Practitioners sometimes prefer Invisalign bite ramps in the treatment of deepbite or if they simply wish to dis-occlude the posterior teeth. Bite ramps are a change in the shape of the aligner on the lingual aspect of the upper anteriors. When biting, the ramps come in contact with the mandibular dentition and tooth movement may result.
Invisalign G5 introduced a new generation of bite ramps that change position with each stage of treatment if it is necessary to maintain contact with the mandibular incisor. It their current application, aligners show good control of balance of force systems between different groups of teeth. This performance characteristic is of great benefit in the treatment of extraction space closure. Invisalign G6 is designed to deliver force systems to accomplish maximum anchorage first bicuspid extraction treatment plan. Practitioners have proposed that an Invisalign aligner produces better anchorage than most fixed appliances. Perhaps the extensive contact on the buccal, lingual and occlusal surfaces of the tooth is responsible for better anchorage. The principle of differential moments is used in Invisalign G6 to provide for anterior retraction with maximum posterior anchorage. To control the balance of the “differential moments” the tooth positions used to fabricate the aligner are altered. The tooth positions are adjusted in six dimensions, three linear and three rotations. The aligner shape is then activated to precisely control the forces delivered to the attachments and tooth surfaces. This technique of controlling aligner shape and the force system delivered is referred to as SmartStage. SmartStage is an advanced algorithm that determines the shape of the aligner at every stage of treatment so that the aligner engages with the tooth and the active surface of the attachments to apply the necessary force system. This approach to controlling tooth movements will be the core of many Invisalign innovations to improve treatment outcomes in the future.
SmartTrack Material Practitioners would not be able to achieve excellent clinical results with this advanced Invisalign System if the aligners were comprised of a typical off-the-shelf aligner material. Scientists s at Align developed a clear multi-layer polymeric material with the performance characteristics necessary for excellent control of tooth movement and treatment overall. Among the criteria for appliance performance are clarity, load/deflection rate, resilience and shape recovery, activation, insertion force, working range, force magnitude, patient comfort, and tooth/aligner contact control. SmartTrack material is vital to the use of sophisticated techniques like SmartForce and SmartStage. SmartTrack is a proprietary highly elastic material. Conventional aligner materials undergo stress relaxation as their molecules rearrange and lose a substantial amount of force in the initial days of aligner wear. SmartTrack maintains more constant force over the time the patient wears the aligners to elicit excellent biological response for orthodontic movement. (Figure 5) The elasticity of the material achieves more tooth movement with each aligner than other aligner materials. The result is improved tracking throughout treatment. SmartTrack material more precisely conforms to tooth morphology, attachments, and interproximal spaces, hence stabilizing the contacts between the aligner and teeth providing for better control of tooth movement throughout treatment. (Figure 6) Invisalign Treatment Planning As with any orthodontic treatment plan, achieving an excellent clinical and aesthetic end result is the ultimate goal. Accordingly, it should be said: the end-goal guides treatment plan. In traditional orthodontics adjustments toward meeting that end goal must be made at each patient visit. In the world of digital treatment planning, the anticipated end result can be visualized by both clinician and patient using 3D computerized technology. This digital computerization allows visualization of the treatment plan not only at beginning and end, but also step by step, aligner by aligner throughout the treatment. Digital treatment planning has revolutionized how practitioners plan for orthodontic and interdisciplinary treatments. It provides a simple decision making process between treatment alternatives because each approach can be visualized and then a comparison made. For example, in a severe crowding case, there
can be multiple ways to treat, non-extraction or extraction. Final non-extraction tooth positions can be assessed virtually ahead of time and improvements to tooth position can be made if the desired result is not acceptable based on the decision not to extract. In this virtual space, one can add IPR to avoid advancing or proclining the incisors excessively forward, taking into consideration the periodontium of the patient. Should the visualized tooth positions be unacceptable, the decision would be made to extract.
Visualization of Treatment Alternatives Visualization of alternative plans also becomes helpful in cases presenting with small upper laterals and the decision whether to restore or not. One can digitally compare the esthetics and occlusion between maintaining the laterals as they are in the arch, or if the occlusion outcome would be superior if the laterals were to be bonded post-restorative. In a case with congenitally missing teeth, one could explore whether to position the canine to camouflage the lateral or open space to restore the lateral. There are many options to treat the case. One can digitally explore how to treat the missing tooth and resolve the lower crowding and still obtain acceptable overjet and overbite. Digital treatment planning allows more accurate measurements between teeth. Hence, digital planning can lead to more accurate plans for restorative procedures in conjunction with orthodontics. In the past treatment planning on plaster models was at the millimeter scale. Digital calipers provided greater accuracy. Today’s digital treatment planning is on the hundredths of millimeter scale and some even better. Precision has improved to the extent that it can be difficult to see with the eye. Final Position and Staging Align Technology was the first company in orthodontics to create algorithms to automatically set the initial bite position, the desired final position of the teeth in the arch, and staging of tooth movements based on a prescription from the clinician. These algorithms continue to be improved and adjusted based on learnings from a database of tooth movements, clinical preferences, and treatment outcomes, gleaned from 3.5 million patients. The initial bite is set-up by the AutoBite algorithm in maximum intercuspation (centric occlusion) by the proprietary planning simulation software known as “Treat.” Information on AP can be input by the technician. The technician checks the occlusal contacts in Treat against occlusal marks on the photos. Therefore, to optimize occlusal accuracy, the clinician should take occlusal photos with articulating paper marks. This practice allows the technician to set the bite and then verify the contact points against the intraoral photos. These photos are also available with the ClinCheck treatment plan and can be used to verify that the initial bite is correct by the clinician. The use of an intra-oral scanner allows the scan data to be used directly with no need for the auto-bite set tool. Additionally, if the clinician desires to have the teeth in centric relation, they may, by providing an intra-oral scan of the teeth in centric relation. Once the initial bite is set, algorithms that are set to orthodontic norms determine the final position. Changes based on inputs from the prescription form provide input for the position of the teeth in the arch (example: IPR allowed or not, is expansion allowed or not). Once the final position is set staging algorithms are used to move the teeth from initial to final position. Staging is the step-by-step progression of tooth movements from the initial position to the final position. In the current paradigm, tooth movements are simultaneous, with the caveat that the tooth that has the longest trajectory leads the number of stages based on a velocity matrix using a maximum of 0.25mm/stage or 2 degrees per rotation or 1 degree for lingual root torque. The tooth trajectory can be made utilizing multiple points on the tooth crown, the root and even the center of resistance. Invisalign’s algorithms are unique in shifting velocity reference points not only along the crown, but towards the root of the tooth. This provides the maximum velocity to the portion of the tooth that is moving the most, be it the root or the crown. The virtual root projection used today is based on GV Black’s reference of the
average length of roots. In the future, as CBCT becomes more widely accepted and used, using the position and length of the real roots in the plan will result in more accurate digital planning. The combining of data learned from the database of tooth movements, from understanding mechanics of clear aligner tooth movement, and the experience of clinicians, are destined in the future, to change the staging algorithms of how the teeth are moved. Rather than being based on the single dimension of velocities, they should take into account movement types, direction of movement, type of tooth being moved, age of the patient, bone biology and perhaps even compliance of the patient. Continual innovation and improvement in the Invisalign system is evident in the adjustments of staging patterns. Currently, the simultaneous staging of all tooth movements is no longer used. Instead, teeth are moved sequentially. For example, in a case with retroclined incisors, as in Class II division 2, the algorithm stages tooth movement to procline first, then intrude the teeth and then retract the upper incisors. This type of staging pattern seems to be more predictable then attempting to make all these movements together. This “retroclined tooth staging pattern” is automatically used when any of the upper incisors need to change inclination of at least 10 degrees. Another example is with Class II cases, where multiple staging patterns are available to be used. One such pattern is “sequential distalization” in which one tooth is moved at a time, and no more than two teeth moving at one time. Preservation of anchorage is critical during distalization. We had learned from the database of treated cases, that the use of elastics support the anchorage while the A-P correction takes place. Sequential distalization has proven to be effective despite the longer treatment time. To reduce treatment times in A-P correction cases with growing patients, elastic simulation can be programmed into the ClinCheck treatment plan. The Class II correction is simulated virtually using a onetime bite jump rather than the movement being displayed as a small amount on each stage of the treatment. The Class II correction is determined by elastic wear and the amount of time of the elastic wear will need to be determined by the clinician. Orthodontic knowledge and not the number of stages indicated in the ClinCheck plan should be used to determine the length of Class II wear. Precision Cuts were introduced by Align to accommodate the use of elastics. This is now a default for A-P correction. Clinicians can change the configuration of hooks or button cutouts using the precision cuts controls in ClinCheck Pro software. ClinCheck software is the virtual treatment planning tool used to communicate a treatment plan to Align Technology. It communicates the way in which Align Technology has staged the progression of tooth movements for the case. However, the practitioners knowledge is critical evaluating the tooth positions and movements required to achieve the expected outcome. Clinicians have different experiences and different plans and ClinCheck software is the means by which changes can be made in planning the treatment and final approval of the plan communicated to Align. With typical modifications, written comments are used to communicate with a technician. It is important to be precise with the direction of tooth movement, the amount of tooth movement and the location of tooth movement. This helps the technician in setting and interpreting the small nuances of the desired final position. With ClinCheck Pro, 3D modifications, the clinician has control to set the final position that are often difficult to convey in words or written instruction. By using the 3D controls, the clinician has more control over the treatment planning process. Real-time feedback is provided on the modifications that are being made and the impact of those changes. What sets ClinCheck Pro software apart from other software is the real-time visualization of planned movements and their impact on adjacent teeth and the arch can be seen during modifications. Clinicians also have the ability to add/remove precision cuts, adjust IPR, increase or decrease space with input from a Bolton analysis, or adjust conventional attachments for retention. Additional capabilities the clinicians have are verification of occlusal contacts, tooth size discrepancy check, all tools to fine tune the final occlusion. One exception may be with optimized attachments. These attachments were specifically designed with force driven properties and advantages as compared to conventional attachments. Whilst many things are possible, what is not currently possible with ClinCheck Pro, are real-time adjustments of staging of tooth
movements. This must be done by the technicians. A list of considerations to aid in treatment planning is provided.
Considerations in treatment planning 1. Biology of tooth movement should be kept in mind. For example, distalizing a tooth 10mm may not be possible dentally without the proper anchorage reinforcements such as a temporary anchorage device. 2. Have orthodontic principles govern and dictate the movements. For example, distalizing an entire arch as a unit and seeing it displaced in the software is possible, however, again without extraoral forces, this is unlikely to happen. 3. When setting up the final overjet, consider not leaving a tight overjet, to accommodate the thickness of the aligner in determining the tooth position. Align default currently is to leave the overjet at 0.5mm to allow for the thickness of the aligner in the overjet position. 4. Optimized Attachments and aligner features are placed based on software algorithms to apply the optimal forces needed in the direction needed for the programmed tooth movement. Consider not replacing them or removing them to experience their improved effectiveness on tooth movements. 5. If the treatment plan has IPR, consider not removing IPR unless there is the ability to procline or expand the teeth in the arch form and the periodontium is stable. Leverage the superimposition tool in the software to see the amount of tooth movement from initial to final and superimposed over the grid can give you more precise measurements. For the most precise measurements, the tooth movement table provides changes for both the crown and root. 6. For A-P correction cases, anchorage control must be maintained with the use of elastics. Check for Precisions Cuts for Class II or Class III correction and leverage the tool in the software 7. Remember to take into consideration the treatment time needed to correct the Class II or Class III even when a virtual elastic simulation or Bite jump at the end of treatment is shown. The one stage jump is a simulation of the A-P correction and the expectation is that the A-P correction is occurring with the use of elastics or in some instances with surgery. Understanding the virtual elastic simulation will become increasing important as Align looks at future offerings in the A-P correction space. 8. Virtual simulations or bite jumps also occur in other dimensions such as vertica. Remember these are virtual, and keep orthodontic principles in mind when correcting an openbite for example. Having the 3D model virtual jump close an openbite will require some form of tooth movement to remove interferences and facilitate the auto-rotation of the mandible. 9. Precision Bite Ramps do not extend beyond 3mm and therefore will not be in occlusion with overjets more than 3mm. If larger biteramps would be needed, consider placing bite ramps on the canines and then switching to Precision Bite Ramps when the overjet is 3mm or less. 10. Leverage the occlusal contacts tool, to look for premature contacts as well as to finalize the occlusion. In some instances leaving heavy occlusal contacts may be desired such as in deepbite cases to overcorrect for posterior extrusion. Be certain to inform the technician the intent is to keep the heavy occlusal contacts, otherwise they will be removed. At the same time, it is important to check the occlusal contacts, and identify any premature or heavy interference and have a plan during or at the end of treatment to eliminate them, whether thru equilibration or restorative procedures. 11. Overcorrection is prescribed by some clinicians to compensate for the lag of tooth movements accomplished in the aligners. Keep in mind especially when presenting to patients that with overcorrection the final position of the model is accentuated.
Tips for monitoring treatments 1. IP regions are defined through a mathematical algorithm and there are some assumptions of shape that are made that can lead to inaccuracies of treatment. Check contacts during treatment for binding with floss and if tight, loosen the tight contacts with a fine IPR strip. 2. Check patients before delivering overcorrection aligners, especially virtual C-Chain. The intent of virtual C-Chain is to tighten contacts during a space closure case like elastic chain to close residual space in a traditional bracket and wire case. If the contacts are already closed or tight, overcorrection aligners are not needed. Adding overcorrection aligners with already tight contact can lead to residual overlap and/or residual crowding. Similar to leaving elastic chain for increased period of time and contacts overlap in brackets and wires. 3. Monitor tooth movements that are lagging behind or not tracking as they may be the one causing interferences. One common effect of posterior tooth tipping or lack of upper expansion may manifest as a posterior open bite. 4. Residual posterior open bite has many root causes, and identifying the root cause is critical to its solution. Look for anterior inferences due to anterior bite deepening or lack of deepbite correct, or retroclined incisors or undertorqued incisors. In some instances, as seen with some expansion cases, check to see that the upper lingual cusps are not causing the posterior openbite. A posterior openbite of less than 1mm usually will settle when there is no other root cause identified, but in generally, there is another cause for the posterior openbite. Monitor for interferences whether anterior or posterior during treatment. 5. During treatment, there will be some tooth mobility especially with teeth with recession or reduced periodontal support. Like any orthodontic treatment, mobility is transitory. Conclusion Clear aligner treatment has progressed immensely since its inception. Invisalign innovations based on fundamental biomechanics, biomaterials, and orthodontic knowledge and experience have enabled practitioners to treat highly complex cases with excellent clinical results. Since 2010, doctors have used Invisalign aligners to treat over 750,000 patients with “complex” malocclusions, including Class II, open bite, bicuspid extraction, and deep bite. Many examples of these cases can be seen on Align Technology’s website https://global.invisaligngallery.com As technology incorporates “orthodontic smartness” into the Invisalign aligner and improves protocols and tools for treatment planning, aligner therapy moves closer and closer to becoming the true state-ofthe-art treatment.
www.elsevier.com/locate/ysodo
PII: DOI: Reference:
S1073-8746(16)30057-3 http://dx.doi.org/10.1053/j.sodo.2016.10.001 YSODO478
To appear in: Seminars in Orthodontics Cite this article as: John Morton, Mitra Derakhshan, Srini Kaza, Chunhua Li and Victor Chen, Design of the invisalign system performance, Seminars in Orthodontics, http://dx.doi.org/10.1053/j.sodo.2016.10.001 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.
DESIGN OF THE INVISALIGN SYSTEM PERFORMANCE
John Morton Sr. Technology Fellow, Product Innovation Align Technology San Jose, CA
Dr. Mitra Derakhshan, DDS, MS Sr. Director, Global Clinical Align Technology San Jose, CA
Srini Kaza MS, MBA Vice President, Product Innovation Align Technology San Jose, CA
Chunhua Li, PhD Senior Director, Materials and Biomechanics Align Technology San Jose, CA
Victor Chen Director, Clinical Affairs Align Technology San Jose, CA
Corresponding Author: John Morton Align Technology 2560 Orchard Parkway San Jose, CA 95131
[email protected] Tel
(408) 470-1205
Fax
(408) 914-1399
Introduction Innovation defines and drives Align Technology. Arguably, Invisalign is still at the forefront of clear aligner treatment. Incessant innovation and advancement built into the clear aligner has expanded its application beyond the early days of only limited minor tooth movement solutions. Principally, improvements to aligner material, force systems, staging of tooth movements, and treatment planning have enabled the Invisalign System to treat complex malocclusions with precise and predictable outcomes. Growth of Invisalign In 1997, Invisalign was created with the vision of providing an aesthetic and comfortable aligner appliance with which doctors could treat their patients. Collaboration between orthodontists and the researchers at Align throughout the past eighteen years has greatly increased the capabilities of the appliance. The ability to treat complex cases and acceptance by the profession has resulted in phenomenal growth. To date, Invisalign is available in over 90 countries worldwide and practitioners have used it to treat over 3.5 million patients. Doctors certified to treat with the Invisalign System
number 112,000. The 3D printing of molds for aligner fabrication is the largest in the world with 175,000 unique high precision aligners produced every day. Align researchers are continuously innovating to improve treatment outcomes with Invisalign. More than $500 million has been dedicated to Research and Development in areas that include biomaterials, biomechanics, 3D software, and doctor communication portals. More doctors are treating more cases as their confidence in achieving an excellent treatment outcome increases with each new innovation. (Figure 1) Looking toward the future, Align researchers will continue to innovate in areas of appliance performance and software, scanning, doctor communication and interface, and workflow. (Figure 2)
Although Invisalign has undergone tremendous growth, even larger opportunities lie ahead. Align analyses show there are 100 million adult consumers in the United States alone that are looking for a better smile and more than 2.5 million teenage consumers are looking for a better smile. Overall, 60-70% of the population of the United States feel they have crooked teeth and the worldwide numbers are of similar size. All of this data points to tremendous opportunities for practitioners treating patients with Invisalign System.
Overview of Principles used in the Invisalign System Thermoformed appliances have been used to move teeth since the 1940’s. These movements were limited, being restricted to tipping of the crowns and minor rotations to align the teeth. The understanding of the mechanics by which these appliances elicited tooth movement was limited as well. The innovations built into the Invisalign System over the past two decades, facilitate control of both the crown and the root movements. Indeed, the system is designed to proportionally move the root with respect to the crown. With this characteristic, the appliance can control the entire range of individual tooth movements, intrusion, extrusion, rotation, simple tipping, controlled tipping, translation (bodily movement) and root movement. Invisalign aligner employs the fundamental principles of biomechanics to control tooth movement.
The necessary tooth movement required for treatment determines the design of the force systems to be applied to the tooth. Subsequently, the appliance is designed to deliver these pre-determined forces. Undoubtedly, if the force system applied to the tooth is favorable to the desired movement, the probability of achieving the movement is greater. This principle holds true, be the appliance a bracket, a wire or an aligner. Engineers and scientists at Align experiment in controlling tooth movement with many different approaches. The shape of the aligner is altered, attachments are applied to modify the shape of the tooth, and the movement of the tooth is programmed along different paths (movement staging). The force systems delivered by the aligner are then measured and compared. The design which delivers the best force system is predicted to be the best design for clinical use. The aligner shape can be changed by another algorithm to increase the contact pressure at specific locations on the surface of the attachment or surface of the tooth. Examples of this are the Power Ridge feature to control anterior torque, a Pressure Point to deliver force to a specific location on the tooth surface, and a Pressure Area to deliver force along a specific surface of the tooth. Pressure Surfaces are used to improve anterior intrusion by directing the force along the long axis of the tooth through the apex. Algorithms (computer code) which define Optimized Attachment shapes are increasing in complexity and proving clinically to better control tooth
movements. Optimized attachments are automatically placed by the software and control the point of application of the force on the tooth, the direction of the force, and the amount of force applied. All Optimized attachments have an “active surface.” This is a flat surface on the attachment which receives the force from the aligner. This surface is placed by the algorithm at a specific location on the tooth, oriented with respect to the tooth axes and engaged by the aligner. (Figure 3)
The aligner does not contact any of the other surfaces of the optimized attachment to ensure no unwanted force is produced. These innovations are in a group of features referred to as SmartForce Features. Figure 4 shows the effectiveness of SmartForce features to improve control of tooth movements specifically, the control of root tip with root control attachments. These data, support attachments are necessary and needed to control certain tooth movements.
Determining the “best” force system to be applied for a movement has several clinical factors to consider. First, force systems are measured about an assumed center of resistance of a tooth. Excellent tooth movement is achieved with a simple definition of the center of resistance for each tooth. The location of the center of resistance will become more accurate as more information regarding root size and shape is provided by the cone beam radiographic images, and should lead to better control. Second, Invisalign aligners are designed to apply low constant force for optimal biological response as commonly defined in the literature. Third, the physiology of mature adults may be slow to respond to orthodontic forces, hence optimized attachments are designed to be engaged by the first three aligners in the series even If the tooth does “not” move. This design allows time for typical patient physiological response and provides for improved tracking from the beginning of treatment. The control of tooth movement with this advanced Invisalign System goes well beyond control of individual teeth. Orthodontic treatment requires control of different groups of teeth with respect to one another. The treatment of openbite, for example, often requires extrusion of the anterior dentition as a group or unit. The software identifies openbite treatment requiring extrusion of the four upper or lower anteriors, places optimized extrusion attachments and “activates” the aligner to produce similar force on all anteriors. Activation is a change in aligner shape to control the amount of force at the contact of the aligner and attachment. Controlling the interference of the aligner and the attachment controls the magnitude of the force delivered to the tooth. Stretching the aligner further delivers more force just as when stretching an elastic. In the process, the aligner is not moving and is in equilibrium; hence, the extrusive force on the anteriors must be accompanied by intrusive forces elsewhere on the aligner. These counteracting intrusive forces are delivered to the posterior dentition and often aid in the treatment of openbite. Similar mechanics is used in the treatment of deepbite. SmartForce features control the magnitude and balance of the intrusive forces on the anteriors and deliver the extrusive reaction forces to the posterior dentition. The lower bicuspids can receive forces to effectively extrude them, in effect, leveling the curve of Spee which may be of benefit in this type of treatment. Practitioners sometimes prefer Invisalign bite ramps in the treatment of deepbite or if they simply wish to dis-occlude the posterior teeth. Bite ramps are a change in the shape of the aligner on the lingual aspect of the upper anteriors. When biting, the ramps come in contact with the mandibular dentition and tooth movement may result.
Invisalign G5 introduced a new generation of bite ramps that change position with each stage of treatment if it is necessary to maintain contact with the mandibular incisor. It their current application, aligners show good control of balance of force systems between different groups of teeth. This performance characteristic is of great benefit in the treatment of extraction space closure. Invisalign G6 is designed to deliver force systems to accomplish maximum anchorage first bicuspid extraction treatment plan. Practitioners have proposed that an Invisalign aligner produces better anchorage than most fixed appliances. Perhaps the extensive contact on the buccal, lingual and occlusal surfaces of the tooth is responsible for better anchorage. The principle of differential moments is used in Invisalign G6 to provide for anterior retraction with maximum posterior anchorage. To control the balance of the “differential moments” the tooth positions used to fabricate the aligner are altered. The tooth positions are adjusted in six dimensions, three linear and three rotations. The aligner shape is then activated to precisely control the forces delivered to the attachments and tooth surfaces. This technique of controlling aligner shape and the force system delivered is referred to as SmartStage. SmartStage is an advanced algorithm that determines the shape of the aligner at every stage of treatment so that the aligner engages with the tooth and the active surface of the attachments to apply the necessary force system. This approach to controlling tooth movements will be the core of many Invisalign innovations to improve treatment outcomes in the future.
SmartTrack Material Practitioners would not be able to achieve excellent clinical results with this advanced Invisalign System if the aligners were comprised of a typical off-the-shelf aligner material. Scientists s at Align developed a clear multi-layer polymeric material with the performance characteristics necessary for excellent control of tooth movement and treatment overall. Among the criteria for appliance performance are clarity, load/deflection rate, resilience and shape recovery, activation, insertion force, working range, force magnitude, patient comfort, and tooth/aligner contact control. SmartTrack material is vital to the use of sophisticated techniques like SmartForce and SmartStage. SmartTrack is a proprietary highly elastic material. Conventional aligner materials undergo stress relaxation as their molecules rearrange and lose a substantial amount of force in the initial days of aligner wear. SmartTrack maintains more constant force over the time the patient wears the aligners to elicit excellent biological response for orthodontic movement. (Figure 5) The elasticity of the material achieves more tooth movement with each aligner than other aligner materials. The result is improved tracking throughout treatment. SmartTrack material more precisely conforms to tooth morphology, attachments, and interproximal spaces, hence stabilizing the contacts between the aligner and teeth providing for better control of tooth movement throughout treatment. (Figure 6) Invisalign Treatment Planning As with any orthodontic treatment plan, achieving an excellent clinical and aesthetic end result is the ultimate goal. Accordingly, it should be said: the end-goal guides treatment plan. In traditional orthodontics adjustments toward meeting that end goal must be made at each patient visit. In the world of digital treatment planning, the anticipated end result can be visualized by both clinician and patient using 3D computerized technology. This digital computerization allows visualization of the treatment plan not only at beginning and end, but also step by step, aligner by aligner throughout the treatment. Digital treatment planning has revolutionized how practitioners plan for orthodontic and interdisciplinary treatments. It provides a simple decision making process between treatment alternatives because each approach can be visualized and then a comparison made. For example, in a severe crowding case, there
can be multiple ways to treat, non-extraction or extraction. Final non-extraction tooth positions can be assessed virtually ahead of time and improvements to tooth position can be made if the desired result is not acceptable based on the decision not to extract. In this virtual space, one can add IPR to avoid advancing or proclining the incisors excessively forward, taking into consideration the periodontium of the patient. Should the visualized tooth positions be unacceptable, the decision would be made to extract.
Visualization of Treatment Alternatives Visualization of alternative plans also becomes helpful in cases presenting with small upper laterals and the decision whether to restore or not. One can digitally compare the esthetics and occlusion between maintaining the laterals as they are in the arch, or if the occlusion outcome would be superior if the laterals were to be bonded post-restorative. In a case with congenitally missing teeth, one could explore whether to position the canine to camouflage the lateral or open space to restore the lateral. There are many options to treat the case. One can digitally explore how to treat the missing tooth and resolve the lower crowding and still obtain acceptable overjet and overbite. Digital treatment planning allows more accurate measurements between teeth. Hence, digital planning can lead to more accurate plans for restorative procedures in conjunction with orthodontics. In the past treatment planning on plaster models was at the millimeter scale. Digital calipers provided greater accuracy. Today’s digital treatment planning is on the hundredths of millimeter scale and some even better. Precision has improved to the extent that it can be difficult to see with the eye. Final Position and Staging Align Technology was the first company in orthodontics to create algorithms to automatically set the initial bite position, the desired final position of the teeth in the arch, and staging of tooth movements based on a prescription from the clinician. These algorithms continue to be improved and adjusted based on learnings from a database of tooth movements, clinical preferences, and treatment outcomes, gleaned from 3.5 million patients. The initial bite is set-up by the AutoBite algorithm in maximum intercuspation (centric occlusion) by the proprietary planning simulation software known as “Treat.” Information on AP can be input by the technician. The technician checks the occlusal contacts in Treat against occlusal marks on the photos. Therefore, to optimize occlusal accuracy, the clinician should take occlusal photos with articulating paper marks. This practice allows the technician to set the bite and then verify the contact points against the intraoral photos. These photos are also available with the ClinCheck treatment plan and can be used to verify that the initial bite is correct by the clinician. The use of an intra-oral scanner allows the scan data to be used directly with no need for the auto-bite set tool. Additionally, if the clinician desires to have the teeth in centric relation, they may, by providing an intra-oral scan of the teeth in centric relation. Once the initial bite is set, algorithms that are set to orthodontic norms determine the final position. Changes based on inputs from the prescription form provide input for the position of the teeth in the arch (example: IPR allowed or not, is expansion allowed or not). Once the final position is set staging algorithms are used to move the teeth from initial to final position. Staging is the step-by-step progression of tooth movements from the initial position to the final position. In the current paradigm, tooth movements are simultaneous, with the caveat that the tooth that has the longest trajectory leads the number of stages based on a velocity matrix using a maximum of 0.25mm/stage or 2 degrees per rotation or 1 degree for lingual root torque. The tooth trajectory can be made utilizing multiple points on the tooth crown, the root and even the center of resistance. Invisalign’s algorithms are unique in shifting velocity reference points not only along the crown, but towards the root of the tooth. This provides the maximum velocity to the portion of the tooth that is moving the most, be it the root or the crown. The virtual root projection used today is based on GV Black’s reference of the
average length of roots. In the future, as CBCT becomes more widely accepted and used, using the position and length of the real roots in the plan will result in more accurate digital planning. The combining of data learned from the database of tooth movements, from understanding mechanics of clear aligner tooth movement, and the experience of clinicians, are destined in the future, to change the staging algorithms of how the teeth are moved. Rather than being based on the single dimension of velocities, they should take into account movement types, direction of movement, type of tooth being moved, age of the patient, bone biology and perhaps even compliance of the patient. Continual innovation and improvement in the Invisalign system is evident in the adjustments of staging patterns. Currently, the simultaneous staging of all tooth movements is no longer used. Instead, teeth are moved sequentially. For example, in a case with retroclined incisors, as in Class II division 2, the algorithm stages tooth movement to procline first, then intrude the teeth and then retract the upper incisors. This type of staging pattern seems to be more predictable then attempting to make all these movements together. This “retroclined tooth staging pattern” is automatically used when any of the upper incisors need to change inclination of at least 10 degrees. Another example is with Class II cases, where multiple staging patterns are available to be used. One such pattern is “sequential distalization” in which one tooth is moved at a time, and no more than two teeth moving at one time. Preservation of anchorage is critical during distalization. We had learned from the database of treated cases, that the use of elastics support the anchorage while the A-P correction takes place. Sequential distalization has proven to be effective despite the longer treatment time. To reduce treatment times in A-P correction cases with growing patients, elastic simulation can be programmed into the ClinCheck treatment plan. The Class II correction is simulated virtually using a onetime bite jump rather than the movement being displayed as a small amount on each stage of the treatment. The Class II correction is determined by elastic wear and the amount of time of the elastic wear will need to be determined by the clinician. Orthodontic knowledge and not the number of stages indicated in the ClinCheck plan should be used to determine the length of Class II wear. Precision Cuts were introduced by Align to accommodate the use of elastics. This is now a default for A-P correction. Clinicians can change the configuration of hooks or button cutouts using the precision cuts controls in ClinCheck Pro software. ClinCheck software is the virtual treatment planning tool used to communicate a treatment plan to Align Technology. It communicates the way in which Align Technology has staged the progression of tooth movements for the case. However, the practitioners knowledge is critical evaluating the tooth positions and movements required to achieve the expected outcome. Clinicians have different experiences and different plans and ClinCheck software is the means by which changes can be made in planning the treatment and final approval of the plan communicated to Align. With typical modifications, written comments are used to communicate with a technician. It is important to be precise with the direction of tooth movement, the amount of tooth movement and the location of tooth movement. This helps the technician in setting and interpreting the small nuances of the desired final position. With ClinCheck Pro, 3D modifications, the clinician has control to set the final position that are often difficult to convey in words or written instruction. By using the 3D controls, the clinician has more control over the treatment planning process. Real-time feedback is provided on the modifications that are being made and the impact of those changes. What sets ClinCheck Pro software apart from other software is the real-time visualization of planned movements and their impact on adjacent teeth and the arch can be seen during modifications. Clinicians also have the ability to add/remove precision cuts, adjust IPR, increase or decrease space with input from a Bolton analysis, or adjust conventional attachments for retention. Additional capabilities the clinicians have are verification of occlusal contacts, tooth size discrepancy check, all tools to fine tune the final occlusion. One exception may be with optimized attachments. These attachments were specifically designed with force driven properties and advantages as compared to conventional attachments. Whilst many things are possible, what is not currently possible with ClinCheck Pro, are real-time adjustments of staging of tooth
movements. This must be done by the technicians. A list of considerations to aid in treatment planning is provided.
Considerations in treatment planning 1. Biology of tooth movement should be kept in mind. For example, distalizing a tooth 10mm may not be possible dentally without the proper anchorage reinforcements such as a temporary anchorage device. 2. Have orthodontic principles govern and dictate the movements. For example, distalizing an entire arch as a unit and seeing it displaced in the software is possible, however, again without extraoral forces, this is unlikely to happen. 3. When setting up the final overjet, consider not leaving a tight overjet, to accommodate the thickness of the aligner in determining the tooth position. Align default currently is to leave the overjet at 0.5mm to allow for the thickness of the aligner in the overjet position. 4. Optimized Attachments and aligner features are placed based on software algorithms to apply the optimal forces needed in the direction needed for the programmed tooth movement. Consider not replacing them or removing them to experience their improved effectiveness on tooth movements. 5. If the treatment plan has IPR, consider not removing IPR unless there is the ability to procline or expand the teeth in the arch form and the periodontium is stable. Leverage the superimposition tool in the software to see the amount of tooth movement from initial to final and superimposed over the grid can give you more precise measurements. For the most precise measurements, the tooth movement table provides changes for both the crown and root. 6. For A-P correction cases, anchorage control must be maintained with the use of elastics. Check for Precisions Cuts for Class II or Class III correction and leverage the tool in the software 7. Remember to take into consideration the treatment time needed to correct the Class II or Class III even when a virtual elastic simulation or Bite jump at the end of treatment is shown. The one stage jump is a simulation of the A-P correction and the expectation is that the A-P correction is occurring with the use of elastics or in some instances with surgery. Understanding the virtual elastic simulation will become increasing important as Align looks at future offerings in the A-P correction space. 8. Virtual simulations or bite jumps also occur in other dimensions such as vertica. Remember these are virtual, and keep orthodontic principles in mind when correcting an openbite for example. Having the 3D model virtual jump close an openbite will require some form of tooth movement to remove interferences and facilitate the auto-rotation of the mandible. 9. Precision Bite Ramps do not extend beyond 3mm and therefore will not be in occlusion with overjets more than 3mm. If larger biteramps would be needed, consider placing bite ramps on the canines and then switching to Precision Bite Ramps when the overjet is 3mm or less. 10. Leverage the occlusal contacts tool, to look for premature contacts as well as to finalize the occlusion. In some instances leaving heavy occlusal contacts may be desired such as in deepbite cases to overcorrect for posterior extrusion. Be certain to inform the technician the intent is to keep the heavy occlusal contacts, otherwise they will be removed. At the same time, it is important to check the occlusal contacts, and identify any premature or heavy interference and have a plan during or at the end of treatment to eliminate them, whether thru equilibration or restorative procedures. 11. Overcorrection is prescribed by some clinicians to compensate for the lag of tooth movements accomplished in the aligners. Keep in mind especially when presenting to patients that with overcorrection the final position of the model is accentuated.
Tips for monitoring treatments 1. IP regions are defined through a mathematical algorithm and there are some assumptions of shape that are made that can lead to inaccuracies of treatment. Check contacts during treatment for binding with floss and if tight, loosen the tight contacts with a fine IPR strip. 2. Check patients before delivering overcorrection aligners, especially virtual C-Chain. The intent of virtual C-Chain is to tighten contacts during a space closure case like elastic chain to close residual space in a traditional bracket and wire case. If the contacts are already closed or tight, overcorrection aligners are not needed. Adding overcorrection aligners with already tight contact can lead to residual overlap and/or residual crowding. Similar to leaving elastic chain for increased period of time and contacts overlap in brackets and wires. 3. Monitor tooth movements that are lagging behind or not tracking as they may be the one causing interferences. One common effect of posterior tooth tipping or lack of upper expansion may manifest as a posterior open bite. 4. Residual posterior open bite has many root causes, and identifying the root cause is critical to its solution. Look for anterior inferences due to anterior bite deepening or lack of deepbite correct, or retroclined incisors or undertorqued incisors. In some instances, as seen with some expansion cases, check to see that the upper lingual cusps are not causing the posterior openbite. A posterior openbite of less than 1mm usually will settle when there is no other root cause identified, but in generally, there is another cause for the posterior openbite. Monitor for interferences whether anterior or posterior during treatment. 5. During treatment, there will be some tooth mobility especially with teeth with recession or reduced periodontal support. Like any orthodontic treatment, mobility is transitory. Conclusion Clear aligner treatment has progressed immensely since its inception. Invisalign innovations based on fundamental biomechanics, biomaterials, and orthodontic knowledge and experience have enabled practitioners to treat highly complex cases with excellent clinical results. Since 2010, doctors have used Invisalign aligners to treat over 750,000 patients with “complex” malocclusions, including Class II, open bite, bicuspid extraction, and deep bite. Many examples of these cases can be seen on Align Technology’s website https://global.invisaligngallery.com As technology incorporates “orthodontic smartness” into the Invisalign aligner and improves protocols and tools for treatment planning, aligner therapy moves closer and closer to becoming the true state-ofthe-art treatment.