# An Innovative and Cost-Advantage CAD Solution for Cubitus Varus Surgical Planning in Children
## 3 Results and Discussion  
The methodology described has led to the creation of a surgical guide that adapts perfectly to the patient's bone model. This favors its positioning in the operative phase and guarantees a precise cut of the bone that takes place exactly along the predetermined planes during the simulation (Figure 10). Figure 10: Perfect adhesion between the surgical guide and the bone model: (**a**) this favors its positioning in the operative phase and guarantees a precise cut of the bone that (**b**) takes place exactly along the predetermined planes during the simulation. Figure 9: Printing the surgical guide in HTPLA from different angles. The analysis of the direct costs incurred to produce the surgical guide concerned:  
* the cost of materials;
* the purchase of the 3D printer;
* the cost of the software used;
* the cost of qualified personnel. A crucial point of the whole study was to find a material for printing that was biocompatible and non-toxic, with high mechanical properties, printable with FDM technology and sterilizable in an autoclave. The choice fell on HTPLA. It is a material produced by the Proto-Pasta company and is obtained from heat-treated PLA. Like PLA, HTPLA is biocompatible, non-toxic and can be printed using the FDM technique. In addition, HTPLA has a higher strength and tensile modulus due to its semi-crystalline structure. The percentage of crystallinity in HTPLA can be improved by heat treatment to improve its mechanical properties. Figure 11 compares the thermal and mechanical properties of PLA with those of HTPLA. In particular, the maximum tensile strength and modulus for the PLA samples are respectively 65.75 MPa and 4.9 GPa at 250 \({}^{\circ}\)C. For the HTPLA samples, the maximum tensile strength and modulus were acquired respectively 67.4 MPa and 5.65 GPa at 250 \({}^{\circ}\)C. This increase in tensile strength and modulus is due to the heat treatment and recrystallization of the samples, which improved the mechanical properties [45; 46]. The model printed in HTPLA requires a complete annealing. The cooking phase takes place in a laboratory oven. The 3D printed model is heated for 30 min at 115 \({}^{\circ}\)C and remains at this temperature for one hour. Finally, it is left to cool for 30 min. After annealing, the HTPLA model can withstand up to 140 \({}^{\circ}\)C.  
Due to the ability of HTPLA to withstand higher temperatures, sterilization of the cutting guide in an autoclave is therefore possible. The cost of HTPLA is approximately 68.00 EUR/kg. 0.01 kg of material is required to make the surgical guide. The printer used in this study cost about 250 EUR and has a working life of approximately 2000 working hours. Table 3 shows a comparison between this procedure and the more common ones present in the literature for the case of cubitus varus deformity. This comparison concerns the cost of materials and printers used. Entry level printers, easily available on the market, were taken as a reference. The cheapest printing technology is the FDM procedure. Specifically, the printer used in this study is a Delta-type EZT3D and costs around \(\upxi\) 250. The  
Figure 11: Tensile modulus values (**a**) and tensile strength values; (**b**) in terms of printing temperature. most expensive printing technology is the SLS procedure, with prices ranging between 7000 and 145,000 EUR for a single printer. Cheaper materials include PLA, Acrylonitrile butadiene styrene (ABS) and Polyethylene Terephthalate Glycol-modified (PETG). However, these materials are not suitable for autoclaving, which is considered the safest method of sterilization. The most expensive material is medical resin. Raydent Surgical Guide (Zortrax) resin was also considered for cost analysis. Ultimately, the use of the FDM procedure and the choice of HTPLA as the printing material is the best, both qualitatively and economically. The innovative process described is in-house, so the cost of the location infrastructure has not been considered.