## 5 Discussion  
Aesthetic brackets were introduced in response to patients' aesthetic demands. According to the study by [PERSON] et al., the better aesthetics of ceramic and polycarbonate brackets compared with conventional stainless-steel brackets is not only well accepted by patients, especially adults, but also positively sought [5]. The majority of commercially available ceramic brackets are based on alumina either of monocrystalline or polycrystalline structure. Despite the superior aesthetic properties, clinicians report often bracket wing fractures during practice, since most ceramic materials are strong but at the same time brittle in nature [26]. Despite offering aesthetic benefits, ceramic brackets present elevated friction resistance (FR) and diminished fracture toughness as compared to stainless-steel brackets. The differences between ceramic and metallic brackets, in addition to their aesthetic appearance, are the greater residual adhesive after debonding resulting in more difficult removal of resin remnants and greater bracket-wire friction, involving less efficient dental movements. On the other hand, bracket failure should be considered, as reported by the study of [PERSON] et al. which compares the failure rates of ceramic and metal brackets in a 12-month clinical study [27]. According to this study, ceramic brackets showed higher failure rates. Patients should be aware that orthodontic treatment with ceramic brackets may involve delays and inconvenience due to the higher failure rate compared to metal brackets [27]. To address these limitations, various innovative approaches have been proposed to develop advanced orthodontic brackets, with zirconia emerging as a promising material for enhancing mechanical properties owing to its remarkable toughness [12]. While aesthetic bracket options have been limited to commercially fabricated polymeric or ceramic brackets in the past, emerging digital workflows and additive manufacturing technologies present the potential to enable the direct design and fabrication of brackets which combine both aesthetic and biomechanical properties taking into consideration the customization for each patient [17]. Considering the high aesthetic demands, the colour stability of these brackets is often analysed in the literature, but yet there is no study comparing 3D-printed brackets. Although the colour characteristics of ceramic brackets are their main advantage over metal brackets, colour stability is still a cause of disagreement. The present study compared the colour stability of four commercially available aesthetic brackets with IH3 DBs. The present study was conducted by simulating 2-year brushing, with a 16.250 cyclic number of repetitions, in artificial saliva with colouring agents [23; 24; 25; 28], measuring changes in chromaticity with a spectrophotometer. The spectrophotometer is a widely used instrument for measuring surface colour because of its reliability, precision and accuracy [28]. Colour changes in aesthetic brackets have a multifactorial origin. According to some studies, the colour of ceramic brackets changes over time when exposed to potentially colouring solutions commonly found in people's diets, such as red wine and products high in caffeine. Moreover, the colouration is cumulative: it increases with increasing exposure time to the colouring elements [29]. On the other hand, discolouration of dental materials can be the result of intrinsic factors such as water absorption, incomplete polymerization of adhesives or resins, material matrix composition, content and size of reinforcement particles and branding or extrinsic factors such as contact with food or beverages containing pigments, use of mouthwashes, saliva, nicotine, lipstick and heat [28; 29; 30]. An evaluation of brackets using a scanning electron microscope (SEM) showed that discolouration is mainly due to stain absorption and sub-surface stain absorption occurring between the dye solution and ceramic brackets [30].