Effect of Surface Wettability and Energy on Bacterial Adhesion to Dental Aligners: A Comparative In Vitro Study

The use of orthodontic aligners has increased significantly due to their convenience and esthetic advantages. However, understanding their microbiological behavior and cytotoxicity is essential. This study aimed to evaluate the metabolic activity (MA) and proliferation of different bacterial strains-assessed through colony-forming unit (CFU) counts-as well as the cytotoxicity of three widely used aligner systems: Spark, Invisalign, and Smile. Wettability and surface free energy (both dispersive and polar components) were determined using the sessile drop technique. The bacterial strains <i>Streptococcus oralis</i>, <i>Actinomyces viscosus</i>, <i>Streptococcus gordonii</i>, <i>Enterococcus faecalis</i>, and <i>Porphyromonas gingivalis</i> were cultured, and their behavior on the aligner surfaces was assessed under simulated oral cavity conditions in both aerobic and anaerobic environments using a bioreactor. Cytocompatibility was evaluated with HFF-1 human fibroblasts. Distinct strain-specific behaviors were observed. For Spark aligners, the contact angle was 70.5°, Invisalign 80.6°, and Smile 91.2°, and the surface free energy was 60.8, 66.7, and 74. 2 mJ/m², respectively, highlighting the high polar component of the Spark aligner of 31.9 mJ/m² compared to 19.3 and 20.2 mJ/m² for Invisalign and Smile, respectively. The Spark aligner exhibited the lowest metabolic activity for <i>Streptococcus oralis</i> (23.1%), <i>Actinomyces viscosus</i> (43.2%), <i>Porphyromonas gingivalis</i> (17.7%), and biofilm formation (2.4%), likely due to its higher hydrophilicity. The Smile aligner showed the lowest metabolic activity for <i>Streptococcus gordonii</i> (23.6%) and <i>Enterococcus faecalis</i> (51.1%), attributed to its low polar surface free energy component. CFU counts were minimal for all aligners and bacterial strains, including biofilm. All aligners demonstrated cytocompatibility above 70% (Spark: 71.0%, Invisalign: 75.7%, and Smile: 75.6%). These findings highlight the importance of considering aligner material properties in clinical practice and underscore the need for proper oral hygiene and aligner maintenance.