Clear Aligner Evolution: History And Generations

Clear Aligner Evolution: History And Generations

“Clear Aligner Evolution: History And Generations” – XDENT’s today blog is referenced from James Andrews’s published article: “Advances in orthodontic clear aligner materials”. Hope you will enjoy it!

Harold D. Kesling in 1945, first advocated the use of rubber-based tooth positioners, fabricated from wax setups of patient’s dentition, and demonstrated that these applicanes not only helped in detailing orthodontically treated cases but could also sequentially reposition misaligned teeth, thereby pioneering the fundametal concepts of modern CAT with the possibility of performing significant orthodontic tooth movements with thermoplastic materials. Kesling foresaw that moere ambious tooth movements could be realized with a series of aligners while recognizing the limitations of the technology available to him at that time: “Major tooth movements could be accomplished with a series of positioners by changing the teeth on the set-up slightly as treatment progresses. At present, this type of treatment does seem to be practical. It remains a possibility, however, and the technique for its practical application might be developed in the future.

Henry Nahoum in 1964, then refined Kesling’s technique by developing a vacuum-based appliance that displayed firm adaption to a patient’s cast. This technique involved the fabrication of a plaster cast on which rotated anterior teeth were sectioned from the cast using a fissure bur or a goldsmith’s saw the sectioned teeth were then re-arranged with wax on the cast per the desired outcome. A variety of thermoplastic materials, including acestates, vinyl, styrene, poly-ethylene, and butyrate sheets could then be vacuum formed over the designed cast to fabricate a dental contour appliance that could move teeth.

In 1971, Ponitz utilized Biocryl (polymethyl methacrylate), which was made out of cellulose acetate butyrate, polyurethane (PU), polyvinyl acetate-polyethylene polymer, polycarbonate-cycolac, and latex, to introduce an “Invisible retainer”. This vacuum-formed clear plastic appliance was initially fabricated for finishing and retention of orthodontic cases using base plate wax on the prepositioned cast and could obtain limited orthodontic tooth movements, chiefly by tipping crowns and could be used for subsequent retention.

In 1985, McNamara modified Ponitz’s technique and used 1 mm thick Biocryl polymers with a Biostar vacuum forming machine to fabricate invisible retainers for retention and final detailing. The difference between the Biostar machine and the previous vacuum former was that it used positive air pressure to adapt the thermoplastic Biocryl to the surface of the model rather than the vacuum pressure. However, the author admitted that the long-term durability of this clear removeable retainer was not comparable to the conventional acrylic of bonded retainers.

Finally, Jack Sheridan 1993 further modified the process of fabricating clear appliances, utilizing polypropylene, a 0.030’’ sheet of thermoplastic copolyester from Raintree Products, and coined the word ‘Essix appliance’ to describe an esthetic method for the alignment of anterior teeth by combining clear appliances with interproximal teeth reduction. He also recommended using a positive air pressure method for the thermoforming process like McNamara’s to decrease the thickness of the sheet almost by half after fabrication.

However, throughout these advances, the basic principle of producing minor tooth movements with individual clear appliances did not change. Fabricating appliances by making impressions, pouring casts, sectioning individual teeth, re-arranging them into proper alignment to obtain a final cast and repeating this process at every clinical appoiment was an extremely laborious and time-consuming process. The solution to which arrived in the form of clear aligner system named Invisalign, which involved a series of removable polyurethane aligners created digitally by two Stanford graduates – Zia Chishti and Kelsey Wirth, launched in 1998 by Align Technology (Santa Clara, CA). This was the first orthodontic appliance to be fabricated using transparent and thermoplastic polymeric materials, with the aid of modern CAD/CAM technology. 

This system involved a multiple-step process, beginning with the three-dimensional (3D) reconstruction of patients’s dentition, obtained either from an intra-oral scan or digital scanning of the study models, followed by segmentation of the individual crown with the help of a computer algorithm, and the development of treatment plans presented visually as a sequence of incremental tooth movement by a predetermined amount using the software. The next step comprised manufacturing the physical models for each stage of tooth movement using the rapid protopying technique. Finally, the despoke aligners were produced using a thermoforming process and trimmed to the final configuations.

Clear Aligner Evolution: History And Generations

Clear aligners have undergone multiple transformation over the years in the quest for improved clinical effiency to treat various malocclusion more aesthetically, comfortably, and effectively. These transformations of clear aligners in general and Invisalign in particular are highly representative and have been summarized and reviewed as eight generations of clear aligners by Hennessy and AI-Awadhi, Ganta, Moshiri, and Wajekar as follows.

Clear aligner evolution: first-generation

The earliest forms of clear aligner systems were sorely reliant on the thermoformed plastic aligner material to achieve intended results without any auxiliary elements being incorporated into the aligner system. The first case report was published in 2000 using 1st generation Invisalign to treat mild crowding and space closure cases. The material used to fabricate Invisalign aligners before September 2001 was a polymer mixture and the brand name was Proceed30 (PC 30), which failed to meet all the physical, chemical, and clinical requirements for orthodontic tooth movement. Clinicians encountered some difficulties and limitations with the usage of these clear aligners.

Clear aligner evolution: second-generation

With advances in aligner systems, manufacturers incorporated the use of attachments to provide better control of planned tooth movement. Clinicians could also request for composite button to be bonded on the teeth and utilized inter-maxillary elastics. Other features, including SmartForce attachments, Power Ridge, Velocity Optimization, and interproximal reduction (IPR) became universal in the Invisalign system. A single layered polymer material Exceed 30 (EX 30), an implantable medical-grade polymer made of polyurethane methylene dipheyl diisocyanate 1.6-hexanediol, tested for safety and biocompatibility by the United States Pharmacopeia, Class IV, was used to fabricate the aligners. The EX30 material provided 1.5 times greater elasticity and 4 times more adaptability than PC 30 and facilitiated easier insertion and removal of the trays for the patient.

Clear aligner evolution: third-generation

In 2010, the third-generation aligners included SmartForce features, such as optimized attachments, designed and placed automatically by commercial software as well as indentations in the polyurethane plastic that placed increased pressure on specified points on the crown to produce a moment of a couple and root torque (Power Ridge). Further, the clinician could also prescribe non-precision attachments to be placed on the teeth wherever needed, to improve movements such as derotation and extrusion. New precision cuts were introduced to help with Class II and Class III interarch elastics.

Clear aligner evolution: fourth-generation

In 2011, G4 attachments were released to facilitate the clinical outcomes in open bite cases with improved optimized extrusion attachments on multiple teeth. New multi-plane movement features were available for upper laterals to enhance extrusion along with introduced for better mesiodistal root control of canines and central incisors.

Since 2013, EX 30 has been replaced by a new multi-layer aromatic thermoplastic polyurethane/co-polyester material, called SmartTrack (LD 30). This new material provides better performance, such as a gentle and more constant force, more long-term action and improved adhesion, which eases the use for patients. Compared with EX30, LD30 has greater consistency of application of orthodontic forces, grater elasticity, chemical stablility and an even more precise and comfortable aligner fit.

Clear aligner evolution: fifth-generation

In late 2013, fifth-generation enhancement improved the predictability of deep bite correction by introducing pressure areas on the lingual of the upper and lower anterior teeth, precision bite ramps on the lingual of the upper incisors, and bevelled dome-shaped retention attachments on the premolars. However, a recent study by Blundellet al. found that the use of precision bite ramps does not appear to significantly improve the ability of SmartTrack material to predictably open the bite compared with EX30 materials.

Clear aligner evolution: six-generation

In late 2014, sixth-generation clinical innovation for orthodontic treatment of first premolar extractions was introduced using new SmartStage technology and SmartForce features to provide vertical control and root parallelism that optimize the progression of tooth movements for extraction treatment planned for maximum anchorage.

Clear aligner evolution: seventh-generation

Invialign G7, a set of features designed to deliver greater control of tooth movements and improved treatment outcomes was released in 2016, particularly for teenage patients. It aimed to deliver better upper lateral incisor control and improve root control and features to address the prevention of posterior open bites.

Clear aligner evolution: eighth-generation

Around late 2020, the eighth-generation enhancements were anounced aiming to further improve the predictability of deep-bite correction with SmartForce aligner activation for anterior instrusion and improvements in the ClinCheck virtual proprietary software setup to level the Curve of Spee. G8 also minizes unwanted crown tipping during posterior arch expansion with optimized expansion support and rotation attachments to reduce the potential for buccal crown tipping.

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