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2. Historical perspective

2.1. Origin and evolution

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 appliances not only helped in detailing orthodontically treated cases but could also sequentially reposition misaligned teeth, thereby pioneering the fundamental concepts of modern CAT with the possibility of performing significant orthodontic tooth movements with thermoplastic materials [11]. Kesling foresaw that more ambitious 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 not seem to be practical. It remains a possibility, however, and the technique for its practical application might be developed in the future [12].

Henry Nahoum in 1964 [13], then refined Kesling's technique by developing a vacuum-based appliance that displayed firm adaptation 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 and the sectioned teeth were then re-arranged with wax on the cast as per the desired outcome. A variety of thermoplastic materials, including acetates, 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), polyvi nyl 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 ortho dontic 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 [14].

In 1985, McNamara [15] 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 dif ference between the Biostar machine and the previous vacuum former was that it used positive air pressure to adapt the thermoplastic Bio crylTM to the surface of the model rather than the vacuum pressure. However, the author admitted that the long-term durability of this clear removable retainer was not comparable to the conventional acrylic or bonded retainers.

Finally, Jack Sheridan 1993 further modified the process of fabri cating 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 thick ness of the sheet almost by half after fabrication [16].

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However, throughout these advances, the basic principle of pro ducing 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 appoint

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ment was an extremely laborious and time-consuming process. The so lution to which arrived in the form of a 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 [17]. This system involved a multiple-step process, beginning with the three-dimensional (3D) reconstruction of patients' 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 treat ment 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 prototyping technique [18,19]. Finally, the bespoke aligners were produced using a thermoforming process and trimmed to the final configurations.