## 1 Introduction  
Aesthetics represents an essential aspect of labial orthodontic treatment.  
The desire to improve dental aesthetics is the principal motivation that guides patients to undergo treatment [1]. Patients' growing demand for the most invisible appliance [2] has led to an increasing use of alternative solutions to traditional labial metal brackets, such as aesthetic brackets [3]. Stainless steel brackets are widely employed as labial orthodontic treatment [4] even if they have poor aesthetic properties.  
Plastic brackets took over in the early 1970s. Initially, they were constructed from acrylic and later polycarbonate, but inherent problems were soon noticed, including staining and odours, but, more importantly, their lack of strength and stiffness resulted in bonding problems, tie wing fractures and permanent deformation.  
To compensate for these deficits, polymer brackets were also developed with different filler materials, e.g., ceramic or fibreglass, or reinforced with metal slots [5, 6].  
Ceramic brackets were introduced in the 1980s, offering many advantages such as higher strength, more resistance to wear and deformation, better colour stability and, most importantly, superior aesthetics.  
On the other hand, ceramic brackets, despite their superior optical properties, have several mechanical disadvantages compared to metal ones: an increasing enamel damage risk during debonding [5, 7, 8], the inability of plastic deformation, a major risk of wing fracture [5, 7, 8] and a higher frictional resistance [9, 10, 11]. The colour stability of aesthetic brackets has always been a topic of study because of their great aesthetic need. However, studies report colouration changes related to the brackets' material themselves and to extrinsic factors such as smoking, dyes in food and frequent consumption of colour-intensive beverages [12; 13].  
In order to satisfy both patient demands and biomechanical properties it would be desirable to use a bracket material that combines both the biomechanical and aesthetic requirements. In addition, it would be a further advantage of future bracket systems to turn away from conventional, standardised bracket designs towards a patient-specific, customised bracket system [14].  
The current increasing rate of three-dimensional printing applications in dentistry disciplines leads to the development of in-house 3D-printed resin brackets (IH3 DBs) which can overcome the limits of the latter [15]. Thanks to the relatively long history of computer-aided design/computer-aided manufacturing (CAD-CAM) technology in dentistry and the advances in 3D imaging and modelling technologies, such as intraoral scanning and Cone Beam Computed Tomography (CBCT), the 3D printing technology has found wide applications. This has led to an increase in 3D printing techniques but research has demonstrated that stereolithography (SLA) and digital light processing (DLP) printers have achieved the best accuracy [16]. A digital workflow could enable clinicians to design and fabricate brackets of desirable colour and translucency in customised shapes and prescriptions to meet the individual needs of their patients' tooth anatomy. The customisation of bracket shape and prescription, and the optimal bracket placement, could lead to greater efficiency and minimisation of finishing bends at the end of the treatment [17].  
CAD-CAM bracket production is practical, economical and customised; additionally, IH3 DBs have better biomechanical properties such as less force loss and better sliding mechanics than ceramic ones, in some cases equivalent to metal brackets [14; 18; 19; 20].  
The aim of the study is to evaluate colour stability in artificial saliva by mechanically simulating brushing between in-house 3D-printed resin brackets (IH3 DBs) and commercially available brackets.