# Comparative In Vitro Analysis of Composite Resins Used in Clear Aligner Attachments
## 4 Discussion  
Attachment features represent one of the key components in clear aligner treatment (CAT), as they ensure proper aligner fit and precise control of planned movements [1, 2, 5]. Therefore, their accurate reproduction depends on the suitable selection of composite resins, which guarantee integrity and shape stability of these features throughout the treatment course [3, 4, 14]. Flowable and conventional resins are commonly used for attachment fabrication, despite differences in filler volume and viscosity. Although several authors [1, 2, 16] have reported superior performance of conventional resins, flowable materials reveal easier adaptation to the attachment template [16]. Recently, the introduction of hybrid composites--such as Aligner Connect (AC)--has enabled one to combine handling benefits of flowable materials with the mechanical strength of conventional composites, providing an alternative that may optimize both placement and long-term stability of attachments. Thus, this study aimed to evaluate and compare the mechanical, thermal, and tribological properties of two nanocomposite materials, Aligner Connect (AC) and Connect Flow (CF), characterized by different composition and viscosity. The investigation provided a thorough comparative analysis of these advanced light-cured nanocomposites using a tailored multi-scale testing protocol developed specifically for this type of application. The results emphasized the clinical relevance of composite selection to optimize the clinical performance of orthodontic attachments, as the materials' behavior under stress, wear, and temperature changes can significantly influence the predictability of tooth movement and aligner fit. Choosing an inappropriate composite may compromise the integrity of the attachment surface, leading to premature debonding, reduced aligner retention, or ineffective force transmission. This highlights the importance of characterizing not only the mechanical performance but also the thermal and tribological properties, which together determine the long-term clinical success of CAT. Overall, AC demonstrated better durability and structural reliability due to its greater mechanical strength, more stable stress relaxation, and increased filler content. All these characteristics represent key factors for achieving effective and long-lasting orthodontic outcomes. In particular, the higher filler content of AC (61%), as measured during the combustion test, appears to be one of the main contributors to its superior mechanical performance. Consistent with our findings, previous studies [16, 17] have shown that low-viscosity composites with high filler loading exhibit enhanced mechanical properties. Increased filler content is generally associated with improved stiffness, enhanced load-bearing capacity, and improved wear resistance, also confirmed by the indentation test results. Thermal analysis further elucidated the structural differences between the materials [18]. Specifically, the AC composite, characterized by a higher inorganic filler content, exhibits greater thermal stability and a more rigid network architecture, as reflected in its broader glass transition and lower heat flow. In contrast, the CF composite demonstrates increased thermal activity, a sharper and lower Tg, and a more flexible polymer matrix, consistent with reduced filler loading. These differences highlight the critical role of filler content and matrix-filler interactions in influencing the thermal response, processability, and long-term stability of these composites. The ability of AC to maintain a stable structure under thermal fluctuations suggests a potential advantage in clinical conditions, where temperature changes from food and beverages may affect the integrity of attachments. CF, while less thermally stable, might benefit from a certain degree of flexibility which could reduce stress accumulation at the composite-tooth interface.