# Comparative In Vitro Analysis of Composite Resins Used in Clear Aligner Attachments
## 1 Introduction  
Nowadays, nanocomposite materials are increasingly used in orthodontic treatments for their favorable mechanical performance, aesthetics, and adaptability to various clinical demands [1,2]. In clear aligner treatment (CAT), they play a crucial role in the reproduction of attachments, which are essential for transmitting orthodontic forces to the teeth and controlling complex movements [1, 2, 3, 4]. The interaction between the aligner surface and attachments enhances aligner fit and consequently improves the control of planned orthodontic forces [5]. Since all aligners are designed and manufactured based on the attachment positions of the initial template, any inaccuracies during the first bonding phase can lead to improper tooth movements, which subsequent aligners are unable to compensate for throughout the treatment [6]. In a recent investigation, [PERSON] et al. [6] concluded that the bonding protocol may influence the precision of attachments, suggesting the use of low-viscosity composites or a two-phase procedure to achieve more accurate results. Furthermore, the shape, configuration, and positioning of attachments should be carefully preserved during treatment to ensure their intended function is fulfilled. In clinical practice, several factors may lead to a premature attachment failure, including detachment, shape deformation, and reduced aligner adaptation. Accordingly, their performance is also influenced by physico-chemical and mechanical characteristics of the nanocomposite materials which they are made from [1, 7, 8, 9]. The selection of a material with optimal properties is undoubtedly crucial for securing both durability and functional performance of the mentioned auxiliary elements during treatment [10]. Resin composites consist mainly of the following three components: an organic resin matrix, inorganic filler particles, and a coupling agent. Flowable composites share the same chemical structure but contain a lower amount of filler compared to conventional ones. Numerous studies [4, 5, 10, 11] have evaluated the effectiveness of various resin-based composites used for this purpose. According to [PERSON] et al. [4], the viscosity of composite materials does not affect the shape or volume of the attachments. By contrast, more recent findings by [PERSON] et al. [2], who compared two nanocomposite resins differing in viscosity and filler content, revealed that conventional composites with higher viscosity exhibit superior wear resistance, making them more suitable for clinical use. Recent advances in dental material science have led to the introduction of more innovative resin-based nanocomposites specifically formulated for use with clear aligner systems [11, 12]. Despite the wide adoption of attachments in CAT, few studies have systematically analyzed how differences in composite composition--particularly between flowable and hybrid nanocomposites--impact the mechanical integrity and long-term retention of attachments [2, 4, 11, 12]. Conventional and flowable nanocomposites currently represent the most frequently used resins for attachment reproduction [1, 2, 13, 14, 15], despite their differences in filler volume and viscosity. The composition of flowable resins allows for easier distribution of the composite within the attachment template. However, they exhibit lower wear resistance and stability when compared with conventional materials [2, 3]. [PERSON] et al. [2] highlighted the significantly superior performance and mechanical resistance of conventional nanocomposites over flowable resins following mechanical testing. More recently a new composite, Aligner Connect (AC), has been introduced, featuring intermediate characteristics between flowable and conventional resins with the aim of combining ease of handling with improved mechanical performance. AC exhibits mechanical properties comparable to those of conventional resins, while presenting an intermediate viscosity level.