## 3 3D Printing Technologies for Orthodontic Appliances  
Orthodontic appliances with skeletal anchorage require a metal framework and are usually manufactured by selective laser melting and sintering (SLM/SLS). This additive manufacturing is a kind of solid freeform fabrication and belongs to the powder bed fusion techniques. Since the invention of selective laser sintering (SLS) in 1989 by [PERSON], there has been constant progress in the further development of this technique. Using different sources of energy and powdered materials, SLS and SLM can be applied to almost every material melted by laser radiation which solidifies while cooling down, thus making it one of the most versatile additive manufacturing techniques [48, 50]. In this technique, a laser emitter moves over a bed of metal powder (e.g., CrCoW) which is heated minimally below its melting point. By tracing the modulated laser over the compacted powder, heat-fusible materials selectively melt in layers. Upon completion of a layer, the powder bed  
Figure 4: (**A**) Superimposed lateral ceph radiograph for the planning of implant placement. (**B**) CBCT can aid implant placement planning in more complex cases (software: Blender, Blender Foundation). containing the structures being printed is lowered and new material is provided by rolling a thin coating of powder on top (Figure 5). Modern SLS can produce objects in layers with a thickness of 30-200 \(\upmu\)m, depending on the particle size of the powder [49; 50]. Once a structure is finished, the remaining powder must be brushed or blown off. Depending on the laser's power and the used material, the particles are sintered or fully melted together. In SLS, the emitted laser melts the grains' surface, while the core stays unaffected and solid. This partial melting allows for binding between the grains. Employing higher-powered lasers melts the grains fully to the core, resulting in homogenous structures, which solidify by cooling down [48]. Equipped with highly powered precision lasers, SLM improved many disadvantages of SLS, such as rough surfaces with high porosities and large shrink rates of the produced parts [48; 49; 50]. On the other hand, complete melting leads to higher surface tensions and therefore requires a smaller thickness of layers [48]. SLM-produced objects demonstrated high accuracy and precision in prosthodonic research, satisfying clinical requirements, sometimes surpassing other methods such as casting or milling [51; 52; 53; 54]. Metal powders used for three-dimensional printing must fulfill all requirements regarding biocompatibility, thus allowing a safe mid- and long-term use in the oral environment. Therefore, alloys certified for oral application are commonly utilized for orthodontic purposes, among which, chromium-cobalt alloys [6; 8; 51; 54; 55] and titanium alloys for submucosal placements [48; 50; 55] are the most popular in orthodontics. In the post-processing, the surface must be milled to remove all supporting structures as well as the rough outer oxide layer. Since SLS/SLM requires fewer supporting structures, post-processing is eased compared to other additive manufacturing procedures [49] (Figure 6A). If needed, preformed parts such as orthodontic screws, hooks for extraoral gears, or tubes can be added to the device by welding them to the framework (Figure 6B,C). Appliances designed with direct screws also require the matching thread to be welded onto the framework, because its co-axial construction is too complex to be printed. Figure 5: Scheme of SLM fabrication process. Auxiliary devices such as insertion guides or models are usually manufactured using printing resin materials. Depending on the purpose and the chosen resin, printing technologies can be divided into light curing and fused deposition modeling (FDM) [6; 55]. Photosensitive resins can be printed using specific light-curing techniques called stereolithography (SLA), digital light processing (DLP), and photo jet (PJ).