UOP

Clinical Evolution of the Invisalign Appliance

Vicki Vlaskalic, BDSc, MDSc, and Robert L. Boyd, DDS, MEd

Copyright 2002 Journal of the California Dental Association.

Authors

Vicki Vlaskalic, BDSc, MDSc, is an assistant professor in the Department of Orthodontics at the University of the Pacific School of Dentistry.

Robert L. Boyd, DDS, MEd, is professor and chairperson of the Department of Orthodontics at UOP School of Dentistry.

Disclosures

The study described in this article was supported by a grant from Align Technology, Inc., Santa Clara, Calif.

Dr. Boyd has a financial interest in Align Technology, Inc.

The Invisalign System of tooth movement has been available to orthodontists since 1999 and has now become available to the entire dental profession. This paper explores the role of this system within the dental armamentarium and describes the clinical evolution of the appliance, based on a feasibility study initiated at the University of the Pacific in 1997.

Documented use of vacuformed removable appliances to move teeth has been available since the 1940s, and their use in the dental office may have occurred even earlier.^1-7 Kesling first described the movement of teeth via a tooth positioner, which is often used today to refine the occlusion after fixed appliance treatment.¹ In 1964, Nahoum published a thorough article describing his "vacuum formed dental contour appliance."² Unlike his colleagues who used home vacuum cleaning systems to create an appliance over a modified study cast, Nahoum used a laboratory vacuum system to create appliances that were subsequently used to treat significant malocclusions. He applied elastics and utilized attachment systems that are still being used today in what is erroneously considered to be a revolutionarily new tooth movement system. In the 1990s, Sheridan popularized the "Essix" overlay appliance as a retainer and an active orthodontic appliance.^3-5

The main feature that distinguishes the Invisalign System from those before it is 3-D computer software, which dramatically increases the ability to manipulate teeth via a series of precise, small, directional movements. As a result, the treatment of significant malocclusions requiring a greater magnitude of change is viable.⁸ Other notable departures from the traditional active, vacuformed appliance systems include the loss of direct control by the clinician in creating these movements, as well as the highly commercial nature of Align Technology, the company that produces the Invisalign System. Although standard orthodontic consultation and treatment planning is required to use this system to provide optimal occlusal outcome, many clinicians believe that the transformation of the clinician’s treatment plan to a 3-D computer image by employees of the company means a loss of control over treatment. While direct-to-consumer marketing is commonplace in the medical scene, it is relatively new to the world of orthodontics. These characteristics combine to embody this novel and controversial appliance, whose ultimate place within the dental armamentarium is at this stage unclear.

The development of the Invisalign System began in 1996, with the formation of a small team of computer engineers skilled in 3-D digital technology. By 1997, a crude technique had been established that would allow the fabrication of vacuformed "aligners," based on a stereolithographic resin model created from a computer-programmed laser. Align Technology approached the University of the Pacific to conduct a feasibility study to test and suggest improvements for its system.

Materials and Methods

The University of the Pacific contracted with Align Technology to conduct a feasibility study that would test the system in vivo and suggest any improvements that might lead to increased clinical efficiency. The protocol involved the recruitment of 40 subjects, who were added in groups according to malocclusion severity. More than 120 subjects were screened, many of them dental students. Subject selection criteria included fully erupted permanent dentition discounting third molars, dental health with no immediate need for restorations, availability for evening appointments, and a desire to comply with orthodontic treatment. The subject age range was 14 to 52 years. Some subjects had been treated previously with fixed appliances.

The first 10 subjects fell into the "mild" malocclusion category. These categories were selected via subjective determination. These were largely incisor crowding, spacing, and alignment cases. Once evidence of feasibility was established, two groups of 15 subjects with successive malocclusion severity were recruited. These subjects included those with crossbites, posterior rotations, severe crowding, and antero-posterior discrepancies. All subjects were offered $200 for their participation and the guarantee of treatment with fixed appliances if they were unhappy with the clinical results obtained with the system. Of the 40 subjects recruited, two patients in Phase III did not proceed with treatment, resulting in a final sample size of 38.

Records collected included patient history, intra- and extraoral photographs, panoramic or FMX radiographs, and lateral cephalometric radiographs. In addition, each subject completed a patient questionnaire while in treatment. Patients were reviewed on a four-week schedule. Progress records including intra- and extraoral photographs were taken at approximately six-month intervals. End-of-treatment records included study casts, intra- and extraoral photographs, and panoramic and lateral cephalometric films.

Results

The results of this feasibility study are represented by the clinical advancements that occurred during treatment of the subjects. These are summarized in Table 1.

The Phase I subjects were diagnosed with Class I mild crowding or spacing malocclusions. The average treatment time was 20 months. This was measured as the time from the first polyvinylsiloxane impression to the time of delivery of the retainer. This treatment time is excessive considering the mild malocclusions within this group. These subjects, however, were initiated in late 1998 and early 1999 when the manufacturing process was unrefined. It took an average of 4.8 months from the date of impression taking to the delivery of the aligners to the patient. In addition, the aligners delivered did not always fit the patient, or additional impressions were taken due to treatment plan change or case refinement. At this early stage, the optimum duration of wear of each aligner was unknown, and many patients changed aligners weekly. This rapid pace combined with programmed increments of tooth movement that were larger than the current 0.25 mm maximum in many cases led to ill-fitting aligners. This meant that in most cases, new impressions were required to continue treatment. Actual active treatment time in this sample is similar to that of fixed appliances, as physiology rather than the appliance system largely dictates the speed of tooth movement.

Figures 1a through j depict an example of one of the first cases treated in this study. The subject was diagnosed with Class 1 mild upper and lower anterior spacing. This spacing was consolidated by the treatment. The patient wore 15 maxillary and 11 mandibular aligners, with a total treatment time (including inactive treatment) of 22 months. Since this patient was treated, tooth movement increments have become automated within the software program used to create aligners.

A finding that should have been anticipated was the creation of a posterior open bite in some patients. This was pronounced in patients who had thicker aligners, who were clenchers, or who had little freeway space. Because many of the patients in the study tested aligners of different material and thickness, not all developed this open bite. The study utilized aligners of thickness ranging from 0.030 inch to 0.040 inch. Currently, aligners are 0.030 inch thick, and this dimension seems to cause a posterior open bite less frequently than was originally observed. Patients who developed open bites wore final aligners trimmed to cover the anterior teeth only, so that in a few days the posterior teeth re-erupted.

The need for overcorrection in the virtual 3-D treatment plan, or "Clincheck," also became apparent. This is likely due to the amount of elastic "give" in the aligner material, so that there is a slight difference between the position of a tooth on the 3-D setup and the clinical position of the tooth. This became most obvious when treating incisor rotations, though overcorrection is currently recommended not only for rotations but also for extrusion and root position.

The sophistication of the 3-D Clincheck software, which allows the clinician to view the setup and each increment of movement, has greatly improved during the progression of this study. Initially, the teeth were poorly defined, and the ability of the clinician to accurately assess alignment and occlusion was limited. The software has evolved not only in resolution, but also in terms of additional diagnostic features such as the use of calibrated grids and superimposition of dental changes. Future advancements under investigation by the team of the craniofacial research laboratory at the University of the Pacific include the addition of individualized root form and condyle position and accurate determination of dental change within the jaws, made possible by the latest digital radiographic scanning techniques.

The Phase II subjects were diagnosed with malocclusions comprising Class I moderate crowding, posterior crossbite, and lingually impacted mandibular premolars, as well as Class III malocclusions with mild to moderate crowding and anterior crossbite. The average treatment time was 27.2 months, with an average time of 3.6 months between initial PVS impression and aligner insertion.

For three Phase II patients, the treatment plan called for mandibular incisor extraction. This treatment required significant control of the root position from the appliance system. As the clinicians struggled to obtain this control, the demands on the properties of the aligner material increased. The material of choice was EX 30-30, a polyurethane sheet of 0.030 inch thickness. This material provides better control and comfort than others that were tested. In a continued effort to obtain root control, composite buttons, or attachments, were bonded onto the labial surface of teeth requiring movements that were less predictable to achieve. These movements included rotation of cylindrical shaped teeth, extrusion, and intrusion. They were also introduced on teeth adjacent to extraction sites. The shape and size of these attachments appeared to influence clinical efficiency, so many designs were tested.

It was also unclear whether the PVS impression should be taken before or after the clinical extraction of a tooth, as it was possible to do either. Allowing the computer programmers to virtually extract the tooth before the clinical extraction took place turned out to be the method of choice, although both options currently exist. The benefit to patients of a virtual extraction was that they would not have to wait for aligners while retaining the edentulous space for an extended period of time.

Figures 2a through p depict a Class I moderate maxillary and mandibular crowding case. The 33-year-old subject presented with a chief complaint of crooked teeth. She had no previous orthodontic treatment and was not interested in extraction treatment, despite a protrusive profile, gingival recession, and lack of adequate attached gingiva effecting the mandibular dentition. The treatment plan included resolution of crowding via dental arch expansion and interproximal reduction. Total treatment time (including inactive treatment) was 37 months, with a total of 41 maxillary and 43 mandibular aligners. Two sets of PVS impressions were needed to complete her treatment. Note the posterior open bite in the immediate post-treatment photographs. The patient’s maxillary aligners were cut distal to the first bicuspids to allow re-eruption of the posterior teeth. Cephalometric superimposition showed slight proclination of the maxillary and mandibular incisors, with no accompanying increase of the mandibular plane angle. Subjective determination of vertical control appears favorable with this system, likely related to the control of tooth extrusion and posterior bite-block effect.

The Phase III patients were diagnosed with malocclusions including Class I and II severe crowding. As a result, the treatment plan for most of these patients called for extraction therapy and, later, buccal segment correction via distalization of the maxillary buccal segments. The average treatment time of those subjects who have completed treatment was 31.5 months, with an average time of 3.3 months between PVS and aligner insertion.

The biggest clinical challenge with this group of patients was bodily extraction space closure. Initially, they experienced tooth tipping because they either had no attachments or had small suboptimal ones. Currently, UOP clinicians use at least one 5 x 1 x 1 mm long vertical attachment for root control, and they overcorrect root position on the virtual set-up. Esthetics was also a major concern for these bicuspid extraction patients. In those cases, the virtual pontic space that is created where the extracted tooth existed is now filled with a tooth-colored PVS material that sticks to the aligner. The virtual pontic system also aids controlled movement of adjacent teeth as it provides a wall of semi-rigid material for the teeth to move against, rather than a thinner span of plastic that tended to flex in the extraction space.

When surveyed, 100 percent of the subjects claimed that they would select the Invisalign system over regular fixed appliances. Even early in the study when subjects often spent more time waiting for aligners than wearing them, after multiple PVS impressions and testing many different materials, the subjects tolerated the aligners well and compliance was not an issue. Sample decay occurred (5 percent), particularly in the first phase of subjects since these were largely mild malocclusions, the system of treatment was not as refined as it is today, and many of the subjects graduated dental school and moved away. Loss and breakage were not the problem that was anticipated, perhaps due to the largely adult population of the group and relatively short duration (10 to 14 days) of wear of each aligner. When asked whether oral hygiene was easy to maintain while using the system, 100 percent of subjects replied that it was. This highlights the benefit of sustainable periodontal health to the patient.¹⁰

Discussion

This study was designed to determine the feasibility of abstracting data from a patient, transferring it to a 3-D computer format, manipulating the image according to the clinician’s treatment plan, and manufacturing a series of therapeutic, custom-made appliances based on the resulting 3-D program. At the advent of the study, many of the systems in place were still under development. Phase I of this study demonstrated that the cumulative error involved in the entire process was within clinically acceptable limits. Phase II demonstrated that judicious planning was required of the clinician to ensure the system of appliances would create the desired result. Considerations such as overcorrections and bodily control of individual teeth, as well as strategies for retention and less predictable movements such as extrusion needed to be addressed before appliance manufacturing. In this study, the clinician had the luxury of taking new impressions and refining treatment to achieve an ideal result and to test novel approaches. This creates longer overall treatment duration but allows the achievement of results comparable to fixed appliance treatment.

Phase III patients in this study are those with malocclusions that fall largely outside current commercial guidelines for the system. While most of these cases have finished with good clinical results, buccal segment distalization and large edentulous space closure remain less predictable. This study will continue to investigate methods to make these movements more predicable. The use of occlusal indices, such as the PAR Index, to objectively determine subject groupings via malocclusion severity would not have enhanced the resulting data of this feasibility study. However, future studies investigating the clinical efficiency of this system utilizing larger sample size would enhance their value from the application of such measurement tools.

The likely outcome of the availability of the system to other dental disciplines will be the use of the system to perform other, more limited tooth movement needs such as augmenting bridge, implant, or veneer preparation. This system has been used in orthognathic surgical cases to replace the pre- and postsurgical fixed appliance phase of the treatment plan. It should be stressed that these cases are not routine, and their outcome in comparison to traditional techniques is largely unknown.

It is clear that that many of the questions and concerns that the specialty and the entire dental profession raise regarding this appliance system have not yet and will not be addressed in a definitive manner by this continuing feasibility study alone. Thousands of patients are currently being treated in practices across the United States, Europe, and Asia with this system; and resulting case reports are providing valuable anecdotal clinical information. Although this study has helped refine and improve the system that is now commercially available, outstanding issues such as the clinical efficacy, occlusal quality, and iatrogenic effect compared to the "gold standard" of fixed appliances will require prospective, controlled clinical trials with adequate sample size, increased record base, and the use of objective assessment methods such as occlusal indices to evaluate outcome. As the Invisalign System has undergone rapid and continual evolution and refinement, it has been impractical to undertake such a controlled study up to this time.

The paucity of such objective information on this new product has induced justified clinician frustration and suspicion. As a profession, it is prudent to treat new products in such a manner and to embrace them with caution in an effort to protect patients from inexpedient treatment. While this system has provided an alternative to the mechanically more complicated fixed appliance system, it has not replaced the need for thorough dentofacial diagnosis and treatment-planning expertise. For those clinicians who choose to utilize this current system, sound knowledge in orthodontic treatment planning and biomechanics as well as experience in new skills such as manipulation and diagnosis via 3-D images are imperative to provide patients with quality outcomes. The orthodontists treating patients in this study experienced a steep learning curve with this system, so that it is advisable for the novice to begin with mild cases.

It is apparent that the combination of fixed appliance therapy and the Invisalign System is often a prudent treatment alternative to offer patients. To date, three of the 38 subjects in this study had some degree of fixed appliance and Invisalign combination to complete orthodontic treatment to an appropriate standard. The Invisalign System is an attractive alternative to traditional orthodontic appliances for the adult population due to its removable, esthetic nature, allowing high standards of oral hygiene during treatment. As such, it has attracted many patients who would not have otherwise sought treatment. However, it remains to be determined exactly how the dental profession will ultimately incorporate this system, with the introduction to the rest of the profession being a relatively recent occurrence. What is clear is that the role of the Invisalign System should be based on sound clinical results and not directed by consumer demand.

Conclusion

The Invisalign System of orthodontic tooth movement is a feasible alternative to traditional fixed and removable appliance therapy in select cases. The system has experienced a rapid clinical evolution and will likely continue to do so. Clinical results from this study and other sources suggest that permanent dentition patients with mild to moderate malocclusions may benefit from carefully planned orthodontic treatment using this system. The ultimate clinical potential of this product remains unclear without further investigation.

References

1. Kesling HD, The philosophy of the tooth positioning appliance. Am J Orthod 31:297-304, 1945.

2. Nahoum H, NY State Dent J 30; 9:385-90, 1964.

3. Sheridan, JJ, Ledoux W, McMinn R, Essix retainers: Fabrication and supervision for permanent retention. J Clin Orthod 27:37-45, 1993.

4. Rinchuse DJ, Rinchuse DJ, Active tooth movement with essix based appliances. J Clin Orthod 31:109-12, 1997.

5. Lindauer SJ, Shoff RC, Comparison of Essix and Hawley retainers. J Clin Orthod 32:95-7, 1998.

6. Ponitz RJ, Invisible retainers. Am J Orthod 59:266-72, 1971.

7. McNamara JA, Kramer KL, Juenker JP, Invisible retainers. J Clin Orthod 19:570-8, 1985.

8. Boyd RL, Miller RJ, Vlaskalic V, The Invisalign System in adult orthodontics: mild crowding and space closure. J Clin Orthod 34:203-13, 2000.

9. Vlaskalic V, Boyd RL, Orthodontic treatment of a mildly crowded malocclusion using the Invisalign System. Case Report Aust Orthod J 17: March 2001.

10. Boyd et al, Periodontal implications of orthodontic treatment in adults with reduced or normal periodontal tissues vs adolescents. Am J Orthod 96:191-8, 1989.

To request a printed copy of this article, please contact/Vicki Vlaskalic, BDSc, MDSc, UOP School of Dentistry, 2155 Webster St., Room 130, San Francisco, Calif, 94115.

Legends

Figure 1a. Phase 1 case example: Class I mild maxillary and mandibular spacing. Pretreatment frontal occlusion.

Figure 1b. Pretreatment right buccal occlusion.

Figure 1c. Pretreatment left buccal occlusion.

Figure 1d. Pretreatment maxillary occlusal.

Figure 1e. Pretreatment mandibular occlusal.

Figure 1f. Posttreatment frontal occlusion.

Figure 1g. Posttreatment right buccal occlusion.

Figure 1h. Posttreatment left buccal occlusion.

Figure 1i. Posttreatment maxillary occlusal.

Figure 1j. Posttreatment mandibular occlusal.

Figure 2a. Phase II case example: Class I moderate maxillary and mandibular crowding. Pretreatment extraoral smile.

Figure 2b. Pretreatment extraoral profile.

Figure 2c. Pretreatment extraoral profile smile.

Figure 2d. Pretreatment frontal occlusion.

Figure 2e. Pretreatment right buccal occlusion.

Figure 2f. Pretreatment left buccal occlusion.

Figure 2g. Pretreatment maxillary occlusal.

Figure 2h. Pretreatment mandibular occlusal.

Figure 2i. Pretreatment overbite -- overjet.

Figure 2j. Posttreatment frontal occlusion.

Figure 2k. Posttreatment right buccal occlusion.

Figure 2l. Posttreatment left buccal occlusion.

Figure 2m. Posttreatment maxillary occlusal.

Figure 2n. Posttreatment mandibular occlusal.

Figure 2o. Posttreatment overbite -- overjet.

Figure 2p. Cephalometric superimposition.