Orthodontic treatment has long been synonymous to a mouth full of “unattractive” wires. The stigma extended beyond outer appearance, with people perceiving those with metal braces to have a lower intellect.[1] Introduction of clear aligners thus came as a respite to the orthodontic patient, for their esthetic superiority made them more attractive to metal and ceramic brackets.[2] These are the active clear plastic trays fitting snugly onto the teeth, each worn for 2 weeks on an average, and changed sequentially to accomplish the incorporated tooth movements. From being used for mild malocclusion cases at the outset to treating vast variety of cases contemporarily, from minimal crowding to bicuspid extractions, clear aligners have come a long way and still continue to evolve.

Contrary to popular belief, clear aligners are not a new find. The chronology leading up to their present-day use dates to 1945, when Dr. Harold D. Kesling first advocated a rubber appliance for moving teeth.[3] In 1964, Nahoum fabricated the dental contour appliance in what was to be the first thermoformed plastic sheet for moving teeth.[4] Sheridan modified the same in 1993, calling it the Essix Appliance.[5] However, fabricating the appliance was a laborious process requiring impression-making at every successive appointment. The solution arrived in the form of a clear aligner system created digitally by two Stanford graduates – Zia Chishti and Kelsey Wirth. Called Invisalign®, it was launched in 1997 by Align Technology© (Santa Clara, CA) and is credited with bringing a comfortable alternative to braces to mainstream orthodontics. In 2005, a competitor emerged in the form of OrthoClear® – a clear aligner system developed independently by one of the original founders Zia Chishti. However, it was withdrawn a year later due to lawsuits alleging patent infringement. Many practicing clear aligner users suffered as a result, one of whom was Dr. Willis Pumphrey whose 400 patients were stranded mid-treatment. He responded by developing ClearCorrect (ClearCorrect, Round Rock, TX, USA), an Invisalign® alternative that quickly gained popularity. An onslaught of clear aligner systems has since followed that are being used to correct minor to complex malocclusions.[6]

CAT works on two basic principles:[7]

Creating space: Through arch expansion, tooth extraction, or interproximal reduction (IPR).

The mechanism of tooth movement with clear aligners is centered around two systems:[8,9]

Displacement-driven system: Controls tipping and minor rotations. Aligners are fabricated as per next staged position; the tooth continues to move till the aligner becomes passive. No root movement elicited.

Clear aligner materials have evolved from a single layered or monophasic plastic to the 2^nd generation polyurethane material, to the currently used 3^rd generation multilayered polyurethane-like material that comprises of hard and soft layers. While the soft layer imparts the property of elastic deformation allowing smooth seating of the aligner, the hard layer ensures strength and durability.[10] Polyethylene terephthalate glycol modified (PET-G) remains the commonly used material. Other materials include polypropylene, polycarbonate, thermoplastic polyurethanes, and ethylene vinyl acetate.[11] Table 1 enlists popular clear aligner brands and the material used for aligner fabrication.[12]

Clear aligner should seat completely on insertion and be properly anchored to the dental arch without any dislodgement or vertical lifting. A decline in aligner forces at gum line is seen due to flexibility of gingival margins, thus impeding the ability to produce certain tooth movements. The possible margin designs are scalloped, straight cut at the gingival zenith, and straight cut 2 mm above the gingival zenith (most retentive) [Figure 1].[28]

Wearing an aligner for 22 h/day for 2 weeks is mandated to ensure maximum expression of aligner prescription.[29] A decrease in daily wear time likely decreases the efficacy of certain incisor movements, reportedly labial inclination and buccolingual translation of mandibular incisors, rotation of maxillary central incisors, as well as buccolingual translation and intrusion of maxillary lateral incisors.[30] However, most incorporated movements occur within the 1^st week, thus casting doubt over usefulness of the additional 1 week of aligner wear.[9,31]

Replacing braces with clear polyurethane trays for orthodontic treatment have raised questions regarding its efficiency in moving the teeth. Tooth movement obtained most accurately with clear aligners is retrusion, followed by rotation, fan-type expansion, and protrusion, respectively.[32] Acceptable results can be achieved for the buccolingual inclination of maxillary and mandibular incisors in mild-to-moderate malocclusions.[33]

CAT is recommended in non-extraction cases with mild-to-moderate malocclusions in non-growing patients.[34] In extraction cases, proper root angulations can be attained with use of suitable attachments and adequate knowledge of the system.[35] Maxillary molar distalization (2.5 mm) and premolar extraction space closure (7 mm) are the most predictable and controlled movements that can be obtained.[36] To its demerit, as occlusal contacts are lost due to thickness of aligners, problems with the final settling of the occlusal plane can arise.[37]

While fixed orthodontic appliances cause root resorption as a side effect of heavy orthodontic forces, there is no clear consensus on root resorption with CAT. A lower incidence and severity of root resorption has been reported, with incisors being the most affected teeth.[38] The incidence of root resorption was seen to be comparable between aligners and light orthodontic forces.[39] Another study revealed a lower severity and prevalence of root resorption with aligners.[40]

Clear aligner treatment has flourished since its introduction into orthodontics. Continued developments on the technological front are being adapted to improve its efficiency, especially in complex cases. Being a removable appliance, patient compliance is the foremost criteria for success of treatment with aligners and patient motivation is indispensable to avail the planned treatment outcomes. Arrays of companies are manufacturing aligners today, providing them directly to consumers, or routing through the dentists; it falls beyond the scope of this article to study the individual characteristics of each system.