Orthodontic clear aligners as an alternative to traditional braces have become increasingly ubiquitous in the last decade, thanks in part due to more cost-effective manufacturing and material advances. Combined with the recent expiration of numerous clear aligner design and manufacturing patents in 2017, there has since been an increase in new aligner varieties entering the market. With this trend comes new challenges, one of which is benchmarking their relative performance, as pre-established and standardized measures do not yet exist. The current literature on orthodontic clear aligners has, to date, a strong focus on qualitative properties such as optical clarity, stain resistance, and patient comfort levels to name a few. Of the studies that do pertain to mechanical properties, however, the intrinsic mechanical properties of the aligners are not the central focus. Moreover, the effects of the intra-oral environment such as temperature and moisture have often been overlooked.
In the current study, our main objective is to establish a novel set of testing parameters for orthodontic clear aligners by applying testing methods from the fields of material science and mechanical engineering. The mechanical tests include 1) microhardness (n=6), 2) crack-resistance (n=3), 3) stress-strain (n=6), and 4) stress-relaxation (n=3). Two different aligner materials were chosen to represent the current available selection on the market, namely polyethylene terephthalate (PETG) and thermoplastic urethane (TPU). We hypothesize that our testing methods will be able to adequately distinguish between 1) the two selected material types, 2) varying material thickness (0.625mm vs 0.750mm), 3) heat treatment via thermoforming, and 3) exposure to simulated intra-oral conditions (temperature and humidity). Lastly, the resulting modulus from the stress-strain testing will be used as an input parameter for establishing an aligner mesh model for biomechanical simulation using the finite element method (FEM).
The selected mechanical testing methodologies have proven to be valuable in distinguishing between aligner material type, thickness, status of heat treatment, and soaking. With microhardness testing, both differences in heat formation and material selection could be detected, but it is best applied in the Dry state. For crack or impact resistance testing, the ability to distinguish between material types was not a difficult task, although it had issues with thickness differences and soaked samples. With stress-strain testing, distinguishing between materials was not an issue either in the Dry state at 0.625mm. Furthermore, it is promising since for TPU in all conditions it could distinguish for soaked and thickness status. Similarly, stress-strain results indicate a faster rate of relaxation for TPU overall. Lastly, a functional finite element model was successfully developed in the current study for future use in order to better model the physical behavior of a clear aligner given assigned material properties.
