Properties of Orthodontic Clear Aligner Materials - A Review

Abstract

BACKGROUND Clear aligners are orthodontic devices that are transparent, a plastic used to correct malaligned teeth. Here patient wears a series of customized clear, removable aligners that gradually move the teeth to the desired position. The clear aligner system is a modern adaptation of the systems described since the middle of the 20th century, therefore there were different devices and philosophies that have led to its creation and the system has evolved a lot over the decades. Clear aligner therapy has been a part of the orthodontic practice for years, but, popularity was increased since the introduction of Invisalign appliances (Align Technology) in 1998. There are almost 27 different clear aligner products currently on offer for orthodontic treatment. Nowadays, more people prefer clear aligner treatment because it is aesthetically superior to brackets and lingual orthodontics. The superiority of clear aligners lies in their aesthetics. The optical properties of the clear aligner material play a major role in aesthetics. The rising demand among adult patients for “invisible” orthodontic treatment has led to an exponential growth in the clear aligner market. Indeed, these aligners have a low aesthetic impact, as well as being able to effectively and progressively guide the teeth into their programmed positions. They are also removable and therefore do not hamper oral hygiene maintenance, in turn reducing the risk of white spots, caries, gingivitis and periodontal disease. All the materials do not possess the same chemical composition. The properties change before and after wear. In this article, we bring out the different materials used for the manufacture of clear aligners and their various properties. KEY WORDS Clear Aligners, Optical Properties, Thermoplastic Material, Mechanical Properties

Advances in orthodontic clear aligner materials

“…Thermoplastic materials in general follow Hooke's law when subjected to forces of short duration, indicating that within the elastic limit of the stress-strain curve, when the stress is proportional to strain; the material exhibits an elastic behaviour with a return to its original size and shape on the removal of applied stress [ 81 ]. The yield strength of a material can be described as the stress beyond which the strain is no longer proportional to stress and the material will no longer be elastic and instead behave as plastic.…”

“…If the material used for aligner fabrication exhibits a very high modulus of elasticity (or high stiffness), then the resultant aligner would be very stiff leading to difficulty in aligner placement and removal for the patient. Likewise, a material with low stiffness will not be able to generate adequate forces required to move teeth [ 81 ].…”

“…Fig. 4 illustrates a relative comparison of the stress-strain curves of different orthodontic materials [ 52 , 81 , 85 , 86 ].

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“…4 An illustration of the relative comparison of the stress-strain curves of different orthodontic materials does not reflect absolute values. It is based on the reference data from the literature [ 52 , 81 , 85 , 86 ]. SS, stainless steel.…”

“…When thermoplastic materials are subjected to long-term forces, they do not follow Hooke's law, and instead, exhibit ‘viscoelastic’ properties showing a time-dependent relationship between stress and strain [ 81 ]. Clear aligner materials are ‘viscoelastic’ in nature, with their properties constituting the middle ground between the properties of purely viscous and purely elastic materials.…”