# Comparative In Vitro Analysis of Composite Resins Used in Clear Aligner Attachments
## 4 Discussion
The hybrid nanocomposite AC consistently outperformed CF in terms of stiffness, load-bearing capacity, and structural integrity, showing nearly double the maximum indentation load and significantly higher elastic modulus values. Its high inorganic filler content contributed not only to increased mechanical robustness but also to greater dimensional stability under compressive loading. These properties make AC highly suitable for fabricating precise and long-lasting orthodontic attachments, where mechanical demands are substantial and patient compliance must be optimized. As a matter of fact, instrumented flat indentation testing revealed that AC reached a maximum load of 586.47 N \(\pm\) 25.02 at a penetration depth of 0.3 mm, which was significantly higher than the 291.76 N \(\pm\) 7.37 recorded for CF. This indicates greater resistance to localized deformation, a critical property for preserving attachment integrity under forces generated by aligners and daily oral functions such as chewing [19]. With regard to tribological performance, CF exhibited superior behavior--including lower friction coefficients, reduced wear volume, and greater stress dissipation--highlighting its value in applications requiring flexibility, adaptive stress response, or sliding contact. Moreover, the more substantial reduction in both Ra and Rt for the AC sample supports the hypothesis of greater material removal by an abrasive phenomenon, which was also evident from the stereomicroscopic analysis. In contrast, the CF sample retained a more stable surface profile, consistent with its improved wear resistance. Overall, the roughness data corroborate the mechanical observations and provide quantitative insights into the differing wear mechanisms and material responses of the two samples. Despite its lower strength, CF's enhanced surface durability suggests that it may be preferable in areas prone to micromovement or repeated mechanical friction, such as aligner contact zones, interproximal spaces, or undercut regions. AC, on the other hand, exhibited a higher coefficient of friction (0.62 \(\pm\) 0.060) compared to CF (0.55 \(\pm\) 0.095), suggesting increased interfacial friction when in contact with aligner materials. However, wear testing showed no significant difference in surface wear between the two materials, indicating a comparable wear pattern despite AC's higher friction. This suggests that while AC may generate slightly more friction, it does not compromise wear resistance and may influence aligner seating over time. Supporting this interpretation, [PERSON] et al. [20] found no differences in wear volume between nanohybrid flowable and nanofilled sculptable composites after repeated aligner insertion/removal over 24 weeks and also reported no significant effects of composite type on mean retentive force. Similarly, [PERSON] et al. [21] observed comparable clinical performance between flowable and conventional composites after a 3 year follow-up. These findings reinforce the importance of integrating thermal, mechanical, and tribological evaluations in the selection of orthodontic composite materials, ensuring optimal treatment effectiveness and durability over time.