## 1 Introduction  
Traditionally, a removable partial denture (RPD) consists of a metal framework that provides rigid support to a dental prosthesis. The most prevalent technique to produce RPD metal frameworks is the conventional lost-wax casting method [1, 2]. This technique involves multiple laboratory procedures and is susceptible to errors and material distortion that may compromise the mechanical properties and fitting, thus is considered a highly sensitive technique [1, 3, 4]. More recently, computer-aided design and manufacturing (CAD-CAM) technologies have been used to produce RPD frameworks, overcoming some limitations of the conventional approaches [4, 5]. High reproducibility and predictability, with simplified workflows and reduced laboratory time, are essential advantages of digital methods [2, 5, 6, 7], along with new materials and techniques [6, 8, 9]. CAD-CAM technologies involve three distinct phases: image acquisition (scanning), image analysis and manipulation (CAD) and manufacturing (CAM). During image acquisition, digital scanners measure superficial points of an object to define its topography, converting this information into a universal (standard tessellation language (STL)) file. This file is then integrated into CAD software that allows the different components of the RPD to be planned and designed [10]. For computer-aided manufacture (CAM), two strategies can be used--subtractive (milling) or additive (printing)--depending on whether it involves the subtraction of material from a prefabricated block or the additive deposition of material in successive layers, respectively. Compared with the conventional methods, the milling technique has some advantages, such as decreased microporosity and improved mechanical properties. This technique, however, is ill-suited to produce metal frameworks due to the high wear of cutting tools [2; 11; 12]. The recently introduced soft-milled Co-Cr alloys have demonstrated interesting in vitro results in complete-arch implant-supported fixed dental prostheses, but their clinical performance is yet to be assessed [13]. On the contrary, 3D printing has shown promise for metal manufacturing [14]. The additive selective laser melting (SLM) technique with local melting and rapid material solidification enables less material waste and, according to some authors, allows the production of fitting RPDs with excellent mechanical properties [2; 15; 16; 17; 12]. Cobalt-chromium (Co-Cr) metal alloys remain the most frequently used material to produce RPD frameworks due to their high elastic limit, excellent wear and corrosion resistance, biocompatibility and low cost [13; 18; 19]. Co and Cr are the most abundant elements of these alloys, but other metallic and non-metallic elements are present, such as molybdenum (Mo) and carbon (C), respectively. Each element contributes to particular properties of the alloy, with the C content influencing the wear resistance [13]. These alloys, however, may be associated with hypersensitivity reactions, a metallic taste sensation, osteolysis around abutment teeth, increased weight of the denture, aesthetic impairment and, consequently, patient discomfort and dissatisfaction [6; 20; 21]. For both aesthetic and biocompatibility reasons, non-metallic materials have been developed, such as polyether ether ketone (PEEK) [21; 22]. Based on the literature, PEEK is considered more aesthetic than metal-based materials, which, combined with its low weight and biocompatibility, provides increased patient comfort and satisfaction [6; 23; 24]. Additional beneficial properties of this material are its adequate mechanical properties, good stability to chemical and physical agents, low affinity to bacterial plaque and low stress transmission to abutment teeth [23; 25]. Also, the emergence of new European laws regulating the use of Co-Cr due to the inclusion of cobalt in the list of carcinogenic, mutagenic, and toxic substances for reproduction reinforces the need for further research on alternative metal-free materials [26].