The clinical effectiveness of clear aligners hinges on how accurately forces are transmitted through the interface between aligners and attachments. This study aimed to investigate the microscopic fit between two types of aligner materials—Duran+ and Zendura FLX—and two attachment designs, rectangular and optimized, using scanning electron microscopy (SEM). Fifty-six attachment–aligner samples were prepared and divided evenly into four groups: rectangular attachments with Duran+ (n = 14), rectangular with Zendura FLX (n = 14), optimized with Duran+ (n = 14), and optimized with Zendura FLX (n = 14). Attachments were bonded to bovine incisors following a standardized protocol, and aligners were thermoformed at 220 °C for 40 seconds. Cross-sections were examined under SEM at 250× magnification, and gaps between aligner and attachment were measured at multiple points (seven for rectangular, five for optimized). Gap sizes ranged from 14.75 ± 1.41 µm to 91.07 ± 3.11 µm. Zendura FLX exhibited significantly closer adaptation compared with Duran+ in rectangular attachments (42.10 ± 1.07 µm vs. 44.52 ± 1.51 µm, p < 0.001). Overall, optimized attachments achieved better microscopic fit than rectangular designs. Across all groups, the largest gaps were observed at gingival margins (67.18–91.07 µm), whereas the smallest were found on flat buccal surfaces (14.75–20.98 µm). Perfect contact between aligners and attachments was not observed for any combination. Both material type and attachment geometry play crucial roles in microscopic adaptation, with multi-layered aligner materials and optimized designs demonstrating superior conformity. These results provide insight into the mechanical reasons behind certain limitations in clinical clear aligner performance.
