# Surface Morphology of Three-Dimensionally Printed Replicas of Upper Dental Arches
## 1 Introduction  
Three-dimensional printing is the process of creating a 3D model of any shape from a digital model by using selective addition of material. Three-dimensional printing or rapid prototyping started in 1981, when [PERSON] prepared a 3D print prototype, using ultraviolet light in the process of layering of special material to construct a three-dimensional plastic model. The term stereolithography (SLA) was used for the first time by [PERSON] in his patent submission in 1984 (U.S. patent No. 4575330). The SLA technology is based on layer-by-layer laser beam curing of photosensitive polymers. When the layer of resin is completely polymerized, the liftingplatform moves a one-layer thickness in the vertical direction and cures the next layer and then the process is repeated many thousands of times to form a 3D physical object. The precision of printing in dentistry ranges approximately from 25 to 100 \(\upmu\)m and printing time depends mainly on the number of layers [1]. Three-dimensional printing in dentistry and orthodontics is based mainly on intraoral scanners which contain a handheld camera, computer and software. All devices prepare a reconstruction of the dental arch with three-dimensional geometry [2]. The 3D printing result is a digital format, standard tessellation language (STL), which can be used in systems applicable in clinical practice: CEREC Omnicam (Dentsply Sirona), CEREC Bluecam (Dentsply Sirona), Planmeca Planscan (Planmeca USA), Cadent iFero (Align Technology), Carestream 3500 (Carestream Dental), 3 Shape Trios 3 (3 Shape North America) and 3 Shape D800 model scanner (3 Shape North America) [3]. The main advantages of those systems are in the data collection and treatment which is based on a virtual model and following 3D printing [4]. Trueness and precision of scanners depend also on the substrate itself and it was proved that dentin as substrate gives the most accurate scans in contrast to enamel [5]. In addition, data acquisition using an intraoral 3D scanner has shown that object rotation is an important factor which can increase or reduce distances from 0.9% to 1.4% [6]. For dentistry and orthodontics, four types of printers are the most popular: fused-deposition modeling (FDM), polyjet technology (PJ), SLA and selective laser sintering (SLS). These methods are not the only methods, where additive manufacturing plays important role. The cement-based approaches studied by [PERSON] [7] with his Wurzburg team aimed to develop a dual-setting biocement system based on a brushite-forming calcium phosphate cement and a tetraethyl orthosilicate based silica gel, which may be beneficial mainly in clinical applications, where we expect continuous releasing of medicaments (for example vancomycin) or support of osseointegration of implants of substances used to cure bone defects for example in maxilla-facial surgery; [PERSON] with his team [8] published in 2014 research, where selective-laser-melted NiTi parts were confirmed as possible way of manufacturing of medical implants and exhibits ultra-high mechanical damping properties. The additive manufacturing using ceramic [9] and metallic materials allows to use hard materials for teeth replacement now, too. Fused-deposition modeling (FDM) is a method of 3D printing where the final object is composed of thin layers of material which come to the printer in the form of filaments of thermoplastic material [10]. The head of the printer moves usually in one plane to create one layer of the object and is controlled digitally. As a result of this, we can usually see the layers on the surface. Polyjet (PJ) is another kind of 3D printing technology, which uses drops of liquid photopolymer which solidify after exposure to UV light. This produces objects with a very smooth surface and thin layers of material [11]. Stereolithography (SLA) technology uses light curing of monomers, which causes polymerization and creation of a 3D object. The advantage of this method is speed of printing; the products often require machining after the priming time [10].