# PEEK Retainers without CAD-CAM: Simple Solutions for Everyday Challenges
## 1 Introduction
Moreover, GFRP has shown significantly lower bond strengths when compared to conventional stainless-steel wires, which may result in higher bond failure, regardless of the bonding procedure [17]. The biocompatibility of GFRP is still also questionable since it is not recommended in patients at high risk of dental caries related to the cytotoxicity exhibited by human oral fibroblasts [18] and, according to recent findings, it has been shown to significantly reduced cell viability and increase DNA damage and oxidative stress production in human epithelial cells [19]. Other non-metallic materials have been investigated and among them polyphenyl-lene polymers have been developed into transparent orthodontic wires that can deliver forces equivalent to the beta-titanium and Ni-Ti wires commonly used in orthodontic practice [20, 21, 22]. Unfortunately, currently, the lack of materials completely comparable with metals in fixed orthodontic retention prevents clinicians from completely avoiding metal fixed retainers, which are still largely used in clinical practice. In recent years, plastic materials have been developed in other fields of medicine to provide good mechanical and thermal properties. They are known as'super engineering plastics' (SEPs), which might have several ideal characteristics for their application in general dentistry and orthodontics. Among these SEPs, polyetheretherketone (PEEK) is a synthetic, tooth-coloured polymeric semicrystalline material that has been used in orthopaedics and maxillofacial surgery as a bone substitute for many years [23]. It has good strength properties combined with a lower Young's modulus of 3-4 GPa, like human bone, which makes it a sensational material. In addition, this material might also be modified by incorporating other materials, such as carbon fibres, that might increase the elastic modulus up to 18 GPa [21, 23]. Its flexibility modulus ranges from 140 to 170 MPa, a value very close to enamel and dentin. These characteristics and its considerable resistance and resilience make this material suitable for supporting the normal forces developing in the oral cavity during swallowing, chewing, and phonation [24]. Moreover, it can be easily sterilised since it has been revealed to be resistant to thermal (melting above 300 \({}^{\circ}\)C), mechanical, and chemical attacks. In addition to being sterilizable, the smooth surface of this material retains less plaque, with an index equal to or lower than that of zirconia [25]. This property facilitates easier cleaning for the patient, thereby reducing the risk of periodontal problems. SEPs, particularly PEEK, offer improved aesthetics compared to metal retainers due to their colour closely matching that of natural teeth. Moreover, these materials are not completely opaque and can reveal the underlying dental matrix, allowing them to integrate harmoniously with dental structures. These materials are also highly biocompatible, making them an excellent alternative for patients allergic to nickel, which is commonly released into the oral cavity from traditional metal retainers [26]. Like other SEPs, it might be available in different shapes and sizes for different purposes, and it might be used directly from the manufactory or employed in milling machines and 3D printing processes. One of its uses is to provide a range of high-technology tubing for angioplasty shafts and inner members, vascular and endoscopic catheters, chemotherapy, and drug delivery tubing because of its high biocompatibility. The possibility of independently designing the required shape makes it possible to have custom-made devices for each patient. This feature is precious for retainers because achieving precise passivity is crucial for maintaining the stability of the orthodontic result and ensuring patient comfort. PEEK was recently proposed in general dental practice [27] for its excellent mechanical properties with application in prosthodontics [28] and orthodontic aesthetic and metal-free wires [21; 24; 25]. [PERSON] et al.