## 1 Introduction  
The increase in demand for aesthetic orthodontic treatments has made clear aligners very popular over the last two decades. These appliances are used for treating mild to moderate malocclusions [1]. For this, a sequence of custom-made, transparent, and removable aligners able to progressively guide the patient's tooth or teeth into their corrected position are produced [2]. The original concept for clear aligners began with the Tooth Positioner developed by Kesling in 1946 [3], which involved a long and tedious hand-made process. Later in 1997, Align Technology expanded on this concept by utilizing computer-aidedtechnology for the introduction of Invisalign, and with this, the modern concept of clear aligner therapy (CAT) was born [1]. Clear aligners are typically made from materials such as polyethylene terephthalate (PET), polyethylene terephthalate glycol (PET-G), polypropylene (PP), thermoplastic polyurethane (TPU), or even a combination of two different materials [4]. The manufacturing process involves heating a thermoplastic sheet until pliable and then shaping it over dental models using force--typically vacuum, pressure, or both. This process is now commonly integrated with computer-aided design and manufacturing (CAD-CAM) technologies, where dental models are digitally created and 3D-printed, serving as the base for thermoforming individual aligners [5]. In addition, with the rapid advancements in 3D printing, clear aligners are now increasingly being produced directly using this technology. When combined with features such as attachments, this approach has further expanded the range of tooth movements that can be effectively achieved with clear aligner therapy [6]. Despite ongoing technological advancements and increased popularity, uncertainty remains regarding the ability of a clear aligner to produce tooth movement that closely matches the planned outcome, within the expected timeframe, and with minimal need for refinements (clinical effectiveness), especially when compared to fixed orthodontic appliances [6]. For CAT to be regarded as a valid and effective alternative to fixed appliance therapy (FAT), treatment outcomes must be both accurate and predictable. However, the accuracy of actual tooth movement achieved with clear aligners can differ significantly from the predicted movement [7], with some studies reporting discrepancies as high as 50% [8]. As a result, many orthodontic cases treated with clear aligners require mid- or post-treatment refinements [6; 9]. Several systematic reviews in recent years have highlighted a lack of high-quality studies evaluating the effectiveness of clear aligners [6; 10]. These reviews have not identified well-established reasons for the discrepancies between predicted and actual treatment outcomes. While patient- and tooth-specific factors can influence results with CAT [11; 12; 13], the mechanical properties of the aligner material also play a significant role, particularly given that aligners must exert continuous, controlled, orthodontic forces capable of producing tooth movement [14]. Desirable properties for orthodontic materials include large spring-back, low stiffness, good formability, and the ability to store and release energy effectively [15]. To achieve effective tooth movement, clear aligner materials must be sufficiently stiff and possess a high enough yield strength to deliver forces within their elastic range. The sometimes limited treatment efficacy of CAT may, in part, be attributed to the mechanical limitations of the aligner materials. The field of clear aligner therapy is rapidly evolving, making it difficult to systematically assess the effectiveness of different aligner materials, especially given the time required to conduct high-quality scientific studies. Nevertheless, a strong evidence base should support all areas of dental practice, including material selection.