# Creation of Anatomically Correct and Optimized for 3D Printing Human Bones Models
## 2 General Workflow
### Printing a 3D Model  
Special types of 3D printing technologies have been developed with different functions. This section provides an overview of the 3D printing technologies, but only some techniques are widely applied in the medical industry and most commonly used to create anatomical models. The main reasons are the specific fabrication process and raw material to meet the high-quality requirements [22]. Several researchers have been conducted aiming at studying the accumulation of residual stresses and strains during the material build-up at the end or during the fabrication process in these technologies [23]. For example, the potential advantage of fused deposition modeling (FDM) is that it offers fabricating prototypes, tooling, and functional parts without geometrical complexity limitations, but there are observed significant dimensional deviations of the model surfaces [24, 25].  
According to _\"The American Society for Testing and Materials\"_ in the US (ISO/ASTM 52900), methods of 3D printing were standardized into seven groups, including the binding jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photopolymerization [26, 27]. The selection of the method will depend on the use and visual appearance of the replica, the properties of the materials, and the possibilities of the printer (e.g., cost, settings, print time, and volume) [28].  
There are four common additive manufacturing techniques (extrusion-based printing [29], vat polymerization-based printing [30], droplet-based printing [31], and powder-based printing [32]) when it comes to printing anatomical models.  
Extrusion-based printing is commonly referred to as fused deposition modeling (FDM) or fused filament fabrication (FFF) [33]. FDM is a mature technology that is based on the extrusion of thermoplastic or composite materials drawn through the hot extrusion head with one or multiple extrusion nozzles [34]. For fabricating complex devices with functional parts such as valves, lenses, and fluidic interconnects, in FDM is a widely used vat-polymerization-based printing technique [35]. It is based on light-curing resin material and light-selective hardening polymerization molding. The process, where droplets of liquid materials are ejected and polymerized throughout hundreds of jets, is a material jetting technology. By directed UV for designed structures, the polymerization occurs only selectively [36]. It includes aerosol jet printing (AJP), binder jet printing (BJP), and poly jet printing (PJP).  
Powder-based 3D printing is a technique with excellent ability for customized fabrication with a variety of external shapes, internal structures, and porosities. Four common powder-based printing techniques are selective laser sintering (SLS), selective laser melting (SLM), direct metal laser sintering (DMLS), and electron beam melting (EBM) [37]. There exist differences in the printing process and product characters among these printing techniques that are based on localized heating to generate melted metallic powder. It would be used to fabricate the customized products.  
These different 3D printing technologies have their benefits and disadvantages.  
The most widespread is extrusion technology. The main disadvantage of this method is its low precision; in its turn, the main advantage is its low printing costs (necessaryequipment, maintenance, and material costs) as well as being the most user-friendly 3D-printing technology. Researchers have tested and concluded that several process parameters (layer thickness, extruding temperature, printing speed, retraction distance, etc.) have to be carefully tuned to accomplish the desired and successful 3D printing results, using the lower end of desktop fused filament fabrication (FFF) 3D printers, and there exist a lot of challenges [38].  
In cases where high precision and quality are required, different photopolymerization technologies can be used.