## 4 Discussion  
In this work, 3D models that reproduce the most frequent lesions in minor surgery have been modeled, manufactured and evaluated. These models are representative of realistic lesions, since they are intended to be used to train GPs in primary care centers. Manufactured training models cost less than commercial ones and faithfully reproduce injuries and lesions. In addition, they have been developed under an open-source license, so that anyone interested can download and print them from their home or primary care center. The models have been evaluated by a panel of experts in minor surgery. Four aspects were assessed: the level of aesthetic recreation, realism during infiltration of the anesthesia, realism during removal of the lesion, and realism during closure of the surgical wound. The panel of experts valued the level of recreation of the lesions positively, as well as the realism during the infiltration of the anesthesia. The removal of the lesion is more laborious than in real interventions, due to the density selected as the printing parameter and the qualities of the material used. It is not possible to close the surgical wound because the material is not elastic enough. From the results obtained, the following statements can be made:  
* Primary care centers can now access 3D printing. In hospitals, 3D-printed models have been used for several years to train surgeons [11; 19]. [PERSON] et al. [17] have found that the use of training models manufactured using 3D printing increases the success rate in major surgery operations and reduces the time spent on each intervention. Driven by this reality, the authors propose the use of this methodology in primary care centers, to carry out practical courses that serve to initiate GPs in minor surgery. * The cost of the proposed models is low. FDM 3D printing allows healthcare resources to be manufactured at a low cost [32]. The training models proposed in this work have a cost of 6 Euros, compared to 38 Euros for commercial training models. This detail is important in a public health system such as the Spanish one, or in developing countries [33]. In addition, the proposed process allows a user to customize the models, and to manufacture those that have interest in each geographic point [34]. * The proposed models have a high level of aesthetic recreation. In major surgery, when lesions have some complexity, images taken by computed tomography (CT) and magnetic resonance imaging \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Dermal Nevus** & **Seborrheic Keratosis** & **Epidermal Cyst** & **Ingrown Nail** \\ \hline Workpiece mass (kg) & 0.06 & 0.05 & 0.06 & 0.01 \\ Printing time (h) & 5.9 & 5.80 & 6.00 & 2.01 \\ Cost of plastic material (€) & 3.61 & 3.10 & 3.61 & 0.98 \\ Cost of electricity (€) & 0.44 & 0.44 & 0.45 & 0.15 \\ Equipment amortization cost (€) & 1.77 & 1.74 & 1.80 & 0.60 \\ Cost of failures (€) & 0.29 & 0.26 & 0.29 & 0.09 \\ Total (€) & 6.11 & 5.53 & 6.15 & 1.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cost of printing the training models manufactured (not including the cost of labor, as the printing could be supervised by internal doctors’ residents as part of their training process). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Cost of plastic (€/kg) & 60 \\ Cost of electrical energy (€/kWh) & 0.15 \\
3D Printer power (kW) & 0.500 \\ Cost of the 3D printer (€) & 599 \\ Amortization period (years) & 1 \\ Days active per year & 250 \\ Hours per day (h) & 8 \\ Failure rate (\%) & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameter values used during costing. (MRI) are used to model the lesion in 3D [35]. In this case, the models have been performed in parametric design software from photographs and drawings made by one of the authors, an expert in minor surgery. However, the panel of experts have positively assessed the level of aesthetic recreation in the models. * The models have been shared on Thingiverse.