# PEEK Retainers without CAD-CAM: Simple Solutions for Everyday Challenges
## 1 Introduction  
Orthodontic retention is important for maintaining post-treatment outcomes, as it prevents teeth from returning to their original positions and opposes the natural tendency of teeth to shift towards the centre. The traditional clinical approach in orthodontic retention provides for a fixed orthodontic retainer, which should have some ideal characteristics related to mechanical properties and biocompatibility [1]. Also, the adhesion properties are fairly good as currently metal detachments became infrequent, thanks to the adhesive systems currently in use, ensuring reliable retention of orthodontic appliances. Metal alloys are usually employed [1; 2; 3] for this purpose since they show excellent elastic properties [4]. Usually, a stainless-steel braided wire is chosen and placed on the lingual surface of the inferior teeth between the two amines. The positioning of this wire is a very delicate procedure, as it must be shaped to adhere closely to the lingual surfaces of the teeth, ensuring it is passive and does not apply forces that could cause tooth movement [5]. Unfortunately, all metal alloys have recently generated issues due to aesthetics and complications such as metal allergies since several of the employed wires release considerable amounts of Nickel (Ni), exposure to which may have biological implications [6]. Ni has been suggested to be a potent allergen that induces an immune response, and it is present in orthodontic appliances, especially wires. Mechanical and chemical injuries in the oral environment cause the release of Ni from the appliance, and this effect might be enhanced in orthodontic fixed retainers that remain in the oral cavity for long periods. Unfortunately, mechanical loading and changes in pH have been suggested to stimulate Ni release from conventional and manganese-steel \"Ni-free\" wires, then prevents the use of these wires for long intervals due to their possible biological implications [7, 8]. Moreover, currently, demand for non-metallic orthodontic materials has increased because they interfered with magnetic resonance imaging (MRI) [9, 10, 11]. The need to perform occasional or continuous MRI exams and the interference with metal orthodontic appliances might be important but of a secondary role during orthodontic treatment since the latter is performed for a limited time. Nevertheless, this aspect becomes relevant and takes on a primary role during retention since the retention period is significantly longer than the orthodontic treatment itself. Recent evidence has recommended the removal of metal fixed orthodontic retainers when MRI needs to be performed due to the artefacts that might affect diagnosis, especially with the recently increased power of the magnetic field of the new generation MRI machines such as 3T and 5T [12]. The previously discussed reasons have encouraged clinicians and researchers to investigate non-metallic materials in orthodontics, especially fixed retention. However, non-metal alternatives to orthodontic wires for fixed retainers should have comparable properties to metal wires in mechanical behaviour, strength, resistance, ductility and adhesion properties, and biocompatibility. Several materials have been proposed, and the lack of ideal characteristics prevented their spread and use in clinical routines. Several non-metallic materials have been proposed as a potential alternatives for fixed retainers in orthodontics [13], such as glass fibre-reinforced plastic (GFRP). The fibres, usually glass, carbon, or polyamide, are completely submerged in a resin layer. These materials have demonstrated high flexural strength and good aesthetic properties, as their colour closely matches the natural colour of teeth [14]. Unfortunately, they also exhibited considerable bending and distortion that causes the disruption of the fibre-polymer interface and decreases their mechanical properties [15, 16].