September 14, 2020
by Tiantong Lou, DMD, MSc, FRCDC (Ortho); Anthony Mair DDS, MClD
Although the clear aligners may seem like a relatively new modality of treatment in orthodontics, the initial concept dates as far back as the early 20th century. It began with the “Flex-O-Tite” appliance by Remensnyder2, from this, Kesling4 in 1945 created a rubber-based tooth positioner appliance and proposed the concept of using them in successive series for incremental tooth movements. It wasn’t until the 1960s that Nahoum3 would introduce the first clear thermoplastic appliance capable of orthodontic tooth movement. Based on his idea, Ponitz developed the first “invisible retainer”5 in the 1970s, which was then later refined by McNamara in the 1980s. A similar appliance known as the Essix retainer was developed by Sheridan in 1993.6
With the rise of the digital age of the 21st century, we have since been able to integrate modern technology with the these earlier fundamental principles to create a variety of contemporary clear aligner systems that allow for a more comprehensive approach to orthodontic treatment.7,8
Remensnyder’s Dental Massage Device
In 1925, Orrin Remensnyder developed the dental massage device, with the intended use of exercising and stimulating the gingiva, such as in treatment of periodontitis.2 It was a device made out of soft rubber, covering the clinical crowns, the marginal gingiva and marketed as the “Flex-O-Tite” gum-massaging appliance. In the subsequent years, he reported observations of minor tooth movement occurring with the use of this appliance.9
Kesling’s The Tooth Positioning Appliance
The fundamental concepts of modern clear aligner therapy can be traced back to Herald Dean Kesling in 1945.4 The desire for this type of treatment was driven by Kesling’s vision of a simple appliance that would guide the movement of all teeth into their ideal positions with relation to one another, without the interferences from any traditional bands or wires. This led to the conception of a device known as the “Tooth Positioning Appliance”. It is an active orthodontic appliance used for final artistic positioning of teeth as well as serving as an effective retainer. As a finishing appliance, the positioner took advantage of the fact that most teeth are still unstable and mobile from the on-going treatment and should respond readily to its influence. A modern-day version of the Tooth Positioner Appliance is still available from TP Orthodontics, Inc., an orthodontic supply company founded by Kesling.
The positioner appliance is originally fabricated from a one-piece pliable rubber material from a wax set up for which it can be patterned over. It is designed to completely fill the freeway space, as well as covering the labial and lingual surfaces of the maxillary and mandibular dentition. The appliance was intended for the correction of mild dental discrepancies, such as spacing, residual overbites, and mesial-distal or buccal-lingual relationships. Although its scope of possible tooth movements was limited, Kesling made a remark that was far ahead of his time, in that:
“Major tooth movements could be accomplished with a series of positioners by changing the teeth on the setup slightly as treatment progresses. At present this type of treatment does not seem to be practical. It remains a possibility, however, and the technique for its practical application might be developed in the future.”
– H.D. Kesling, 1945
However, as with all new appliances, the tooth positioner also had its share drawbacks.10 This includes reliance on patient compliance, the foul taste of the rubber material, deepening of the overbite, lack of proper inter-digitation and poor settling of the occlusion.11-15
Nahoum’s Dental Contour Appliance
The first documented clear thermoplastic appliance for the use in dentistry was developed by Henry Isaac Nahoum in 1959,3 fabricated using an industrial-grade vacuum former.16 It was known as the dental contour appliance as it was originally designed to maintain or change contours.17 The process could accommodate various materials, including acetates, vinyl, styrene, polyethylene, and butyrate, with translucent, clear or colored sheets.1 For its fabrication, an altered cast is formed by using a jeweler’s saw to section the teeth and baseplate wax is used to hold them in their new position. From here, the contour appliance is formed over the model. The resiliency of the appliance material will exert pressure until the teeth have attained their predetermined positions.
Nahoum postulated that this appliance could be used in orthodontics both as a retainer and for achieving minor orthodontic tooth movements, such as minor rotations and space closure. He built on Kesling’s idea of using a series of appliances in an incremental fashion for progressively achieving a desired tooth movement. This concept was developed with the realization that some tooth movements were too great to be corrected in one step. He made progressive adjustments to the teeth on the altered cast by gradually moving them through the wax and fabricated a new vacuum-formed retainer for each step. This method was recommended for usage predominantly in the anterior dentition. The auxiliary elements used in today’s clear aligner therapy also had origins in Nahoum’s methodology. For example, when both arches are treated, he suggested the use of acrylic buttons on the appliance for the attachment of interarch elastics.
In addition to its function as an orthodontic positioner and retainer, Nahoum also proposed the use of the contour appliance for various other aspects of dentistry.18 Such as splints, bite plates, surgical pack holders, medicament carriers and provisional crowns.19-21
Ponitz’s Invisible Retainers
In 1971, Robert John Ponitz proposed a vacuum-formed clear plastic appliance that can be used for finishing and retention of orthodontic cases.5 The material for these appliances was proposed to be made out of cellulose acetate butyrate, polyurethane, polyvinylacetate-polyethylene polymer, polycarbonate-cycolac, and latex. The fabrication procedure involved preheating a clear plastic material in an oven and using a vacuum unit to form the material to the shape of the dental arch from a cast.
Ponitz proposed that teeth can be moved and repositioned on the cast using wax prior to the formation of the retainer, thus allowing for the patient’s teeth to be moved to new positions by the means of the appliance. Moreover, acrylic bite planes can be formed over or under the appliance and secured with self-curing acrylic liquid. In cases involving edentulous regions, denture teeth can also be attached to the retainer by the same method.
The main advantages of these clear invisible retainers included: ease of fabrication; speed of insertion; minimal chair side adjustment; as well as reparability via heat guns. These appliances, at the time, were also used as holders for periodontal dressing, surgical splints, temporary partial dentures, as well as splints for occlusal trauma and bruxism.22,23
McNamara’s Invisible Retainers
Ponitz’s technique for fabricating invisible retainers for retention and final detailing were later refined by James A. McNamara in 1985.24 He reported the fabrication of these appliances using 1 mm thick BiocrylTM polymers with a Biostar forming machine. Rather than the vacuum pressure technique described by Ponitz, the Biostar machine used positive air pressure to adapt the thermoplastic BiocrylTM to the surface of the cast. This appliance was reported to be used in 80% of his private practice cases.20 McNamara ultimately concluded that although clear removable retainers had their advantages, they did not have the same long-term durability of traditional acrylic or bonded retainers.
Sheridan’s Essix Retainers
In 1993, John J. Sheridan introduced his variation of to the family of thermoplastic appliances, known as the Essix retainer, designed to function both as a retainer and positioner.6 It was fabricated using a 0.030” sheet of thermoplastic copolyester from Raintree Products. He advocated the use of a positive air pressure method for the thermoforming process, which will reduce the thickness of the sheet to 0.015” after completion.
In contrast to Nahoum’s idea of using serial appliances for successive movements, the fundamental principle of the Essix system is based on the use of a single appliance for in-course adjustments to achieve treatment goals. The two primary methods of creating tooth movement in the Essix system are via alterations in the aligner or the tooth surface. The first method involves spot-thermoforming the aligners via Hilliard thermopliers. The second method, known as mounding, involves alterations to create projections on the tooth surface, such that a force will be exerted as the resiliency of the aligner material presses against it. This is usually achieved by bonding composite materials, in the shape of a mound.
Contemporary Clear Aligners
Although the concept of using aligners in orthodontics has existed for many decades, the planning and fabrication processes were done manually, through tedious and laborious procedures such as sequential wax set-ups.8 The major limitation of these manual fabrication processes is at the aligners are limited to only a small subset, and thus cannot be used for comprehensive orthodontic treatments. The recent advancements in computer-aided design & computer-aided manufacturing (CAD/CAM) and rapid prototyping techniques has allowed for an industrial approach8 to the treatment planning and manufacturing of clear thermoplastic aligners.25,26
Contemporary aligners of the 21st century combines the principles pioneered by Remensnyder9, Kesling4, Nahoum3 and others5,6,24 and integrates them with the modern CAD/CAM technology. Today’s aligners are made using transparent and thermoplastic polymeric materials, custom fabricated to the patients’ individual dental arches.27 This approach achieve orthodontic tooth movement through the usage of a plurality of successive aligners, where each aligner incrementally moves teeth by a predetermined amount. The force system of aligners is generated when there is a pre-established geometric mismatch between the shape of the aligner tray and the dental arch.8 The force system of aligners can vary by the mechanical properties of the thermoplastic material, thickness of the aligners, amount of activation as well as the addition of auxiliary elements.
The CAD-based process involves multiple steps, beginning from the 3-dimensional reconstruction of the patients’ oral anatomy to the manufacturing of the aligners. The digital reconstructions are performed through either intra-oral scanning or digital scanning of a study model.28 The computer algorithm will then segment the individual clinical crowns from the rest of the digitized 3-dimensional model. The orthodontic treatment plan is then developed and partitioned into a sequence of smaller movements by the CAD software.126 The manufacturing of the physical molds of the dentition at each stage of treatment is performed using the rapid prototyping technique.25,26 The customized aligners are then produced using a thermoforming process and trimmed to the final configurations.8
The InvisalignTM System
The idea of introducing a mainstream and contemporary CAD/CAM based clear aligner system to the mass market was first conceived by Zia Chishti, an MBA student from Stanford University. After completion of his orthodontic treatment, he did not consistently wear his clear removable retainer as prescribed by his orthodontist and not surprisingly, he experienced relapse of crowding of his lower anterior teeth. Chishti attempted to use his current retainer to realign his teeth but was frustrated with the progress. This inspired him to develop a computer-aided system that designed a series of these clear appliances to incrementally move teeth. From this concept, Chishti and Kelsey Wirth, another Stanford MBA student, along with two orthodontists founded Align Technology in 1997 in a garage in Palo Alto. The InvisalignTM system was soon developed by Align Technology thereafter.31 It first came to the market in 1999 with initial availability limited to orthodontists but later expanded to general practitioners.31 The Invisalign appliance consists of a series of clear thermoplastic aligners that are worn for 1-2-weeks each. Each aligner was staged to achieve approximately 0.25-0.30 mm of orthodontic tooth movement per tray.
The first iteration of Invisalign worked behind a displacement-driven system34, where they were solely dependent on its shape to achieve results.35 No auxiliary elements were incorporated at that time. Limited research is available on efficacy of tooth movement by first generation aligners, with the only study done by Djeu et al. in 2005.36 The second generation of Invisalign began the use of various auxiliary elements for the purposes of enhancing the efficacy of orthodontic tooth movement.35 These included the use of attachments, incorporation of composite buttons and the use of inter-maxillary elastics. The third generation of Invisalign further enhanced this concept by introducing optimized attachments that can be placed automatically by manufacturer’s software.35 They are intended to improve control of tooth movements by adapting their shape with consideration to the individual tooth morphology.70
Categories of Contemporary Clear Aligners
There are currently four major categories of clear aligner products, classified based on their clinical applicability and method of delivery to the patient. These aligner systems range from those available direct to the consumer to comprehensive systems designed to treat more complex malocclusions.
Direct-to-Consumer Aligner Systems
Direct-to-consumer aligner systems are marketed directly to patients, designed for patients to treat themselves “at home”. These systems require the patient to take photographic records and make their own impressions of their dental arches. The resulting aligners are subsequently fabricated and delivered to the patient, without the direct supervision of a dental professional. As more of these “do-it-yourself” or over-the-counter aligner products arise, some researchers have expressed concern over the patients’ safety without a health care provider supervising their treatment.30 According to a recent consumer alert by AAO, patients may be subjecting themselves to increased risk of damage to their teeth, periodontium, and even adverse events, such as allergenic reactions, some of which could be life-threatening.
Minor Tooth Movement (MTM) Aligner Systems
MTM aligners are designed to provide limited clinical treatment, such as single arch or anterior alignment only. These systems have been marketed as a less expensive and faster alternative to comprehensive clear aligner therapy.
In-House Fabricated Aligner Systems
In-house fabricated aligners are created by companies that provide the 3D treatment planning software to the orthodontist’s office. This software can be integrated with 3D scanners and printers to allow the orthodontist to fabricate their own aligners directly.
Comprehensive Aligner Systems
Comprehensive aligner systems allow for 3D interactive treatment planning via incorporation of 3D CAD CAM tooth movements. They include usage of various auxiliary elements, such as bonded resin attachments, and slits for elastics. These systems provide various additional features that allow for more complex tooth movements in all planes of space and more comprehensive treatment than the previous options.
Clear aligner therapy has evolved immensely since its initial conception in the in the early 1900s. Many of its fundamental concepts, such as attachments, auxiliaries and the progressive use of aligners were derived from the ideas of Kesling, Nahoum, Sheridan and others. The arrival of the digital age has facilitated mass production of aligners and improved its availability to the general public. Its rapid rise in popularity among prospective new patients has made clear aligners a mainstay in contemporary orthodontics.
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