Effect of Dental Thermoplastic Materials on the Clinical Effectiveness of Clear Aligner

Review Article

Austin J Dent. 2021; 8(1): 1151.

Effect of Dental Thermoplastic Materials on the Clinical Effectiveness of Clear Aligner

Hallmann L¹* and Gerngroß MD²

¹School of Dentistry, Kiel University, Germany

²Institute of Material Science, Faculty of Engineering, Kiel University, Germany

*Corresponding author: Lubica Hallmann, School of Dentistry, Kiel University, Arnold-Heller-Str.16, D-24105 Kiel, Germany

Received: February 13, 2021; Accepted: March 05, 2021; Published: March 12, 2021


Objective: The aim of this review is to evaluate the influence of chemical and mechanical material properties properties on the effectiveness of the clinical treatment with a clear aligner.

Methods: A systematic literature study of recent and most relevant publications in this field has been performed to identify the most important material effects on the clinical effectiveness of clear aligners.

Results: Clear aligner devices have found a wide application in orthodontics. The development of new materials which are metal-free and the CAD/CAM engineering technique make it possible to treat the various kinds of malocclusion. Patients prefer clear aligners because of their aesthetics and comfort. The thermoplastic materials have inherent advantages: excellent aesthetic appearance, simple to be used and superior formability. Various thermoplastic materials are being developed for manufacturing of clear aligners. The mechanical properties of polymers play a crucial role in the effectiveness of the therapeutic treatments. Therefore, the elasticity, dimension and environment stability as well as biocompatibility are the main conditions, which the polymer must meet for being used in clear aligners.

Conclusions: The main advantages of clear aligner in comparison to metal or ceramic devices is their excellence aesthetic with better patient acceptance and a general better quality of life. From a clinical point of view, the treatment with clear aligner causes less pain compared to a traditional treatment and also reduce the damage to the gingiva and perodontal tissue. The clinical success of the clear aligner therapy strongly depends on the properties of the used aligner materials.

Significance: The use of thermoplastic materials for orthodontic tooth movements requires a better understanding of the material properties for selecting the optimal material and design.


The increasing aesthetic need of patients for orthodontic devises have lead to the development of materials for aligners [1-11]. Kesling was the first who developed the concept of clear orthodontic devices [11]. In 1997, Align Technology developed Invisalign technique to treat malocclusions, such minor crowding and space closure [10-13]. Clear Aligners (CA) have fond extend application in orthodontics with in the last 15 years through new technologies and new materials to widen the range of tooth movements. The main advantages of CA are better aesthetic, higher patient acceptance and better life quality. CA treatment results in less pain for patients compared to a traditional fixed treatment [4-12,14-29].

Clear thermoplastic removable aligners are produced by the CAD/CAM technology making possible to produce the movements required for a comprehensive orthodontic treatment and more esthetic acceptance in comparision to the traditional stainless steel or aesthethic bracket or wire systems [25,29-32]. The clear aligners production is based on transparent thermoplastic materials offering suitable physical, chemical and mechanical properties intended application [29,33]. The mechanical properties of polymers play a crucial role in the effectiveness of the theraupetic treatment. Low stiffness, good defomability, dimensional and environment stability as well as high biocomptability are the conditions that must met by the polymers to be used. The invisible orthodontic force, derived from the deformation and elasticity of aligners, depends on the chemical composition, arrangement and entanglement of the polymer chains, thickness, and the clinical protocol [1-3,5,17,32-38]. The ideal clear aligner material must deliver sufficient machanical force to induce an orthodontic movement in a highly controlled manner and simultaneously preventing a damage of the surrounding perodontal Tissues [37].

Polyethylene Terephthalate Glycol (Petg), Polycarbonate (PC) And Thermoplastic Polyurethane (TPU) are mostly used to modify the properties of aligners [5,29,38-39]. PETG, a non-crystalline amorphous co-polymer of Polyethylene Terephthalate (PET), exhibit good mechanical properties, high fatigues resistance, dimensional stability, and solvent resistance [14,18,20,29,33,40-42]. A Glygol Group (G) is added to the backbone of the copolymerizing agent that is a consisting of 31% mol 1,4-cyclohexylenedimethylene terephthalate (PCT) and 69 mol% PET [3]. PETG has almost the same glass transition temperature (Tg), deformation behavior and optical properties as PET, but does not exhibit the strain-induced crystallization behavior of PET at the production temperature [10,16,30,39].

PC offers an excellent mechanical strength, low water absorption, and transparency making this material very suitable for orthodontic applications [38]. Its properties are very close to the one of Polymethyl Methacrylate (PMMA), but PC offer a higher mechanical strength and is usable in a wider temperature range [33]. PC also has a high transparency in visible light spectrum and provides a higher light transmission behavior than many kinds of glasses [33,38].

Polyuretane (PU) is another elastic polymer that is resistant to abrasion and oil. Its properties strongly depend on the raw materials used for production [43-50]. Thermoplastic Polyurethane (TPU) is one of the most versatile engineering thermoplastics with elastomer properties. TPU exhibit excellent physical properties, chemical resistance, abrasion resistance, adhesion properties and ease of processing [33,51].

The first aligners developed by Align Technology, consisted of a single-layer rigid polyurethane produced from methylene diphenyl diisocyanate and 1,6-hexanediol. To improve its flexibility, fracture resistance, and transparency Smart Track (Align Technology, 2012) developed a new thermoplastic polyurethane [1-2,10,46-48]. The market demand increase of clear aligners has lead to the development of further thermoplastic materials for clear aligners by other companies; such as e. g. Invisaligh, Duran, Biolon, Zendura, Erkodur, Kombiplast, Imprelon, ClearCorrect, Erkoflex 95, Erkoloc pro, etc. [19, 49-50].

Therefore, the aim of this study was to study the influence of materials on the theraupetic treatment with clear aligners.

Material Properties of Aligners

Water absorption

The mechanical properties of dental polymers show a multitude of influence factors, such as structural factors (molecular and crystal structures etc.) and enviromental factors, (temperature, humidity etc.) [3,29]. The employed polymers are typically either amorphous or semi-crystalline. Amorphous polymers usually appear clear because visible light can pass through, while many semi-crystalline and crystalline polymers are opaque because they consist of mixture of crystalline and amorphous phases with different refractive indexes [3, 29]. The crystalline phase in semi-crystalline polymers can serve as a reinforcement gridleading to an improvement in the mechanical properties over a wide range of applications. Low crystallinity of polymers typically means: high flexibility, high elasticity, welladaptable to the tooth morphology, but on the other hand they providing low tensile strength, low chemical resistance and stability [29]. From a clinical point of view, polymers with high flexibility and elasticity are more comfortable for patients to insert or remove the aligners. Furthermore, they adapt better to the complexity of the tooth morphology, adhere perfectly to any surface. Compared to aligners of rigid materials they also ensure a continuity of force expression during the orthodontic treatment [29].

Ryokawa et al. investigated the properties of dental thermoplastic materials for clear aligner [3]. The polymeric materials for orthodontic applications used in this study are listed in (Table 1).