Mechanical Characterization of Thermoplastic Aligner Materials: Recommendations for Test Parameter Standardization

Abstract

Background. Understanding of the different mechanical properties of thermoplastic materials is essential for a successful aligner treatment and further developments. However, data of previous material testing studies are scarcely comparable. Aim of the current study was to evaluate the different test parameters to lay the foundations for guidelines for future, more standardized three-point-bending aligner material tests. Materials and Methods. Several parameters concerning the specimen preparation and experimental three-point-bending setup were varied. The specimens were collected from polyethylene terephthalate glycol (PET-G) Duran® foils with different thicknesses. Both raw foils and foils thermoformed using different geometrical forms were investigated. The three-point-bending tests were performed using span lengths of 8 and 16 mm and variable deflection ranges between 0.1 and 0.2 mm. The influence of water storage on the bending forces was studied using unloaded and loaded specimens. Experimental results were validated using a beam cantilever mathematical model. Results. Local macroscopic cracks after long-term loading could be avoided by keeping the deflections within a thickness-dependent individual range. The mathematical calculations confirmed that these individual deflection ranges lead to local stresses between 14 and 18 MPa. Constantly loaded specimens immersed for 24 hours in water showed a decrease of the bending force by 50%. This reduction was much smaller for the unloaded specimens (14%). Conclusion. During clinical aligner therapy, very small bending deflections are combined with small distances between the tooth surface regions supporting the aligner. In vitro aligner material testing by three-point bending should consider these geometrical aspects, while keeping the material stresses in a range between 14 and 18 MPa to avoid local microcracks. Considering these aspects, thickness-dependent deflections were established for three-point bending of the PET-G specimen for a span length of 8 mm. We recommend the application of these test parameters in future aligner material studies to achieve valid and comparable test results.

Variation of the modulus of elasticity of aligner foil sheet materials due to thermoforming

“…Several studies reported that mechanical and physical properties of orthodontic aligner materials may change after thermoforming, cyclic mechanical loading and/or thermocycling processes as well as after clinical use [3]. Thus mechanical properties should be determined after thermoforming and under the influence of thermocycling and cyclic loading [2,8,10,21].…”

“…The influence of test geometry, the amount of thickness reduction and the change of material parameters due to drawing over a three-dimensional (3D) object were to be analyzed as well. Test geometries described by Elkholy et al [8] and Kwon et al [14] were used.…”

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

“…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][5][6][7][8][9][10][11][12][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”

Mechanical Behavior of PET-G Tooth Aligners Under Cyclic Loading

“…In particular, it is widely used in many applications such as displays, vending machine windows, food packaging, cosmetics and electronics. This material is an amorphous copolymer of PET but, unlike this one, it is not able to undergo strain-induced crystallization independently of the stretching rates and temperatures used during processing (Dupaixa and Boyce, 2005;Elkholy et al, 2019). Due to the favorable combination of excellent esthetic characteristics, formability, high mechanical strength, excellent flexibility and high impact resistance (Sheridan et al, 1994;Sheridan and Armbruster, 1999;Oh and Wang, 2007;Lloyd et al, 2001), this material has gained great interest in the last decade in the medical device industry.…”

“…Other experimental studies report results from tensile tests or uniaxial and plane strain compression tests conducted over a wide range of temperatures and strain rates (Eliades and Bourauel, 2005;Kwon et al, 2008;Pascual et al, 2010;Zhang et al, 2011;Elkholy et al, 2019).…”