The Proliferation of Clear Aligner Orthodontics: Workflows, Materials, and Designs
Melissa D. Shotell, DMD, MS
The evolution and progress of orthodontic treatment with clear aligners has enabled clinicians and laboratory technicians to achieve remarkable clinical results. While many advances have resulted in what is known today as clear aligner orthodontics, the manufacturing process has seen tremendous developments in recent years. Early methods were a laborious process of resetting the teeth on a plaster model and creating vacuum-formed aligners. This was followed in the 1990s by mass commercial customization of clear aligners, ultimately leading to today's upsurge in in-office aligner production utilizing 3D printing, which has generated great interest in clear aligner workflows. Additionally, with the burgeoning of many material and software advancements clinicians are able to treat increasingly complex cases with clear aligner therapy, such that it is becoming a preferred orthodontic treatment for many practitioners.
Clear Aligner Processes and Commercialization
The first report of clear aligner therapy was the advent of the dental contour appliance in 1964 by Dr. Nahoum.1-3 He described the process of creating his first vacuum-formed appliances using a plaster model with the teeth reset to the desired position and a self-made vacuum-forming machine he fabricated with a steel drum, a household clothes iron, and a vacuum cleaner. The process of producing clear aligners essentially remained in this arduous state-creating reset plaster models and vacuum forms-for more than 30 years.
In 1997, a commercial clear aligner laboratory was founded (Align Technology, Inc.) and became the first lab to create mass-customized clear aligners for clinicians. This new laboratory used digital models to create the reset teeth models and large-scale commercial 3D printing to produce the models. This became the conventional workflow for clinicians to take clear aligner records and send them to the singular laboratory available. Over the next 20 years Invisalign® continued to develop more advanced workflows allowing for an entirely digital records process that eliminated conventional impressions in exchange for intraoral digital scans. Manufacturing advances led to multiple revisions of the aligner attachment system, new aligner designs, and enhanced materials. The Invisalign trade name became synonymous with clear aligner orthodontics.
Several small laboratories began offering aligners using this digital model workflow in the years following the founding of Align Technology, and in 2003 the Essix system was developed as an in-office solution for producing clear aligners without having to recreate a reset stone model. The Essix system utilized either a series of heated pliers or bonding composite directly on the teeth to customize the fit of passive clear aligners to facilitate tooth movement. This system was the first in-office aligner solution; however, the need for constant customization to the clear aligners and new impressions were generally considered a drawback with regard to clinical time.
In 2017, a surge of new commercial laboratories in the dental space emerged as existing patents held by Align Technology protecting the intellectual property of aligner planning began to expire. A wide range of customization and varying options have materialized, allowing aligner laboratories to offer a broader array of aligner designs and features. A push for in-office production has prompted the development of innovative and lower-cost software and 3D printing technologies. Today as technology, software, and systems have become increasingly available, clinicians are now able to control the entire clear aligner production process within the dental office.
Aligner Characteristics and Design
A clear aligner can have a variety of characteristics and be comprised of different materials. The combination of materials and aligner design affects the retention of the aligner on the teeth and, thus, tooth movement.4 Among the most debated elements of aligner design are the trimline, attachments or engagers, and cut-outs. The combination of these aligner characteristics is what distinguishes an aligner product's design.
Trimline-The trimline typically is either scalloped or straight. While the scalloped trimline has long been a characteristic of aligner design, research has indicated that a straight trimline reduces the flexibility of the aligner, provides improved retention of the aligner, and can aid in facilitating complex tooth movements.5
Attachments-Use of attachments or engagers has been widely accepted to aid in difficult tooth movements, however there is little research regarding attachment design. Some researchers have advocated that attachments be positioned near the gingival margin and beveled toward the gingival surface to increase aligner retention.4
Cut-outs-Aligner laboratories can place cut-outs in the clear aligners to accommodate the use of elastics secured directly to the aligner or to attachment buttons bonded directly to the teeth. The cut-out design is a property unique to the aligner manufacturer. Cut-outs can be a convenient feature for clinicians, allowing them to possibly avoid having to modify the aligner chairside.
Additional characteristics that clinicians may want incorporated into a clear aligner, depending on specific patient needs, include bite ramps, power ridges, and virtual power chains. Again, these features may differ among aligner brands.
Aligner Materials
Aligner thermoforming materials have advanced dramatically since the inception of clear aligner orthodontics. Aligner materials have gone from single-layer or monophasic plastic to a second gen-eration composed of polyurethane, to today's third generation of aligner materials consisting of polyurethane-like components with multilayer and multiphasic properties. The combination of hard and soft layers of materials allows for elastic deformation to adapt around and over the teeth when seating, and the hard layers increase strength and durability.Offering improved flexibility and stain resistance, these newer multilayer materials allow for excellent aligner adaptation to the teeth and permit complex tooth movements that are achieved when the elastic middle layer of material allows for distortion of the aligner to adapt over the teeth during insertion; after insertion the more ridged inner and outer layers recover their initial shape and will direct orthodontic tooth movement.6-9
In-Office Aligner Workflows and Integration Into General Practice
The development of workflows for in-office clear aligner production has resulted in significant interest by clinicians and smaller dental laboratories to produce branded in-office aligners for patients. Maturation of intraoral scanning technology, development of nonproprietary commercial software for producing tooth movement, and lower-cost desktop 3D printers permit rapid production of clear aligner models.
Historically a proprietary process, recent advancements in technology permits optical scans generated from intraoral scanners to be readily imported into commercially available software that can segment models into individual teeth and allow for tooth movement and manipulation into an ideal treatment set-up. The software is then able to breakdown the movements of the teeth into individual models based on the parameters set by the clinician for tooth movement. The software produces a series of files ready for 3D printing; when printed the files create models for vacuum-forming the clear aligners based on the sequential and gradual tooth movement.
After production of the 3D-printed models, vacuum-forming aligners made with contemporary multilayer thermoplastics can then be created on these models utilizing the clinician's preferred design characteristics for the aligner. This gives the clinician ultimate control of treatment plan set-up, aligner design, and clear aligner fabrication.
With the advances in clear aligner laboratories and in-office clear aligners the digital workflow has become increasingly available to general dentists. As clear aligner therapy becomes more widely used in general dentistry the benefits of pre-restorative tooth alignment may lead to more favorable treatment outcomes.
Future Progress of Clear Aligners
As the science and technology of clear aligner therapy progresses, the range of treatment options will continue to expand beyond traditional bracket and wire orthodontics. Further improvements in 3D printing speed, aligner material physical properties, and, ultimately, the ability to 3D print aligners directly and eliminate the model vacuum-forming process will foster an even more simplified manufacturing process with a smaller environmental footprint.
Recent advances in software and 3D printing technology have created an environment in which clinicians have access to both the software to design the clear aligners and the in-office 3D printing to enable a full in-office workflow production. These technological developments give clinicians the control to create a treatment set-up and design aligners based on the characteristics necessary for optimal tooth movement.
About the Author
Melissa D. Shotell, DMD, MS
Private Practice specializing in Orthodontics and Dentofacial Orthopedics, Sonora, California; Diplomate, American Board of Orthodontics
References
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5. Cowley DP, Mah J, O'Toole BJ. The effect of gingival-margin design on the retention of thermoformed aligners. J Clin Orthod.2012;46(11):697-702.
6. Zendura Dental. Zendura FLX Properties. https://www.zenduradental.com/pages/whats-zendura-flx-all-about. Accessed May 4, 2020.
7. Stewart RF, Lahlouh J, inventors; Bay Materials, LLC, assignee. Dual shell dental appliance and material constructions. US patent application 20190105881 A1. April 11, 2019.
8. Great Lakes Dental Technologies. Great Lakes ComfortTrack™ Clear Aligner Material. https://www.greatlakesdentaltech.com/comforttrack-clear-aligner-material-.76mm-125mm-round-10-pkg_021-130.html. Accessed May 4, 2020.
9. Li C, Chen Y, Pudleiner H, et al, inventors; Align Technology, Inc, assignee. Multilayer dental appliances and related methods and systems. US patent 9,655,693 B2. May 23, 2017.
