Can wear time, material composition, and manufacturing processes affect microplastic release from clear orthodontic aligners? A multidisciplinary in vitro study

Objective: Plastics are widely used, including in medical applications. In dentistry, clear aligners (CAs) offer an alternative to fixed appliances, but their potential microplastics (MP) release raises health concerns. This study aims to investigate the detachment of MPs from CAs by analyzing the correlations between wearing time, manufacturing methods, and material composition. Methods: The following CAs were tested: Alleo (AL), F22 Aligner (F22), FlexiLigner (FL), Graphy (GP), Invisalign (INV), KeySplint (KS), Lineo (LIN), LuxCreo (LC), Spark (SP), and SureSmile (SS). One pair of CAs per group was immersed in artificial saliva, and the solution was stirred for 5 hours per day for 7 (T1) or 14 (T2) days to simulate mechanical friction. Saliva samples were subsequently vacuum-filtered through 1.6-µm pore-size membranes. Attenuated total reflectance-Fourier transform infrared spectroscopy was used to characterize the CA materials, and Raman microspectroscopy and scanning electron microscopy were used to assess the chemical composition, number, size, and shape of the MPs. The experiment was conducted in triplicate. Results: The release of MPs increased significantly from T1 to T2 (P < 0.05), especially those with a diameter < 5 µm. The mean size of the MPs did not significantly differ at either time point. MP release was associated with material composition at T1 and T2 (P < 0.0001). No significant correlation was observed with the manufacturing process, although GP, KS, and LC, which are three-dimensionally printed (3D-printed) CAs, released more uniform and spherical MPs. Conclusions: Replacing CAs after 7 days may help limit MP release, particularly smaller fragments; however, the clinical significance of this finding remains unclear and requires further in vivo investigation.