## 4 Discussion  
The proposed method allows to fabricate a 1:1 scale model of the bony structures, providing a detailed and realistic representation of the anatomical region of interest, facilitating preoperative planning and subsequent surgery, contributing to increase surgical precision and reduce operative time. In recent years, research focused on the uses of 3D printing technology for surgery [6]. However, most of these applications require expensive and professional 3D printers that conflict with the worldwide contraction of the financial resources spent on healthcare. In clinical practice, rapid prototypes are typically obtained through a third-party vendor. These companies use state-of-the-art machines to construct high-resolution models [7], cutting guides, metal plates, and implantable prostheses [8]; the cost of such technology can reach $500,000 or more. Therefore, the cost of a commercially produced sterilizable bony model, or patient-specific cutting guide varied on a case-by-case basis but was estimated by vendors to exceed 300-500 Euros. The charges for virtual surgical planning services also started at approximately 300-400 Euros. Vendor model turnaround time is generally 3 business days if 1-day shipping is used. If a virtual planning session and customized sterile cutting guides are added to the order, an additional 2 to 3 business days were required. This interval is unsuitable in some situations such as emergency. Our workflow allows a very low cost, in-house 3D printed model production, since it relies on open source software and low cost, \"entry level\" 3D printers. In our practice a finished, sterilized 3D printed product was available 10 to 16 h after CT scans acquisition and it cost approximately 130 Euros per model. Moreover, the choice of a low-cost 3D printer was dictated by the fact that the 3D printer can become an integral part of the CT process. From this perspective, a hospital can afford several low budget 3D printing stations to handle more cases at the same time with greater ease. Furthermore, by implementing a low-cost 3D printer in the process described, even hospitals and emergency services with fewer resources could be able to benefit from the advantages of patient specific anatomical models. For this reason, Stereolithography technology has been rejected since the set-up costs are much higher even than mid-range FDM 3D printers: The High Temperature resins available on the market are more expensive than HTPLA filament. For example, 1 It of High Temp resin by Formlabs costs 229 Euros while 1 kg of HTPLA by ProtoPasta costs 68 Euros. The higher cost of a SLA 3D printer and the SLA resin are unsuitable with emergency services or small hospitals with few resources. In [9] the authors pioneered the use of Fused Deposition Modeling (FDM) for 3D medical models but the 3D printer was a semi-professional end-user printer. In [10] the 3D model is printed in Acrylonitrile Butadiene Styrene (ABS), which is more resistant than PLA in terms of Rockwell Hardness: R105 to R110 for ABS, R70 to R90 for PLA [11]. However, ABS material presents more technical difficulties in printing. ABS 3D printers demand a heated bed and a closed printing chamber for two reasons: a controlled and forced cooling is required in order to avoid warping of bottom layers and to avoid toxic gas escape. In general, most common ABS filaments are printed at higher temperatures than commercially available PLA filaments: the ABS extruding temperature range is 220-240 \({}^{\circ}\)C, while PLA extruding temperature range is 180-220 \({}^{\circ}\)C. For these reasons, the use of ABS commits a more expensive equipment. The key feature of our 3D printing process is the material chosen for the printing. The 3D model is printed in HTPLA. The advantages of HTPLA are multiple. HTPLA can be printed at 200 \({}^{\circ}\)C or less and without heated bed, so it doesn't require expensive equipment.