# An Innovative and Cost-Advantage CAD Solution for Cubitus Varus Surgical Planning in Children
## 1 Introduction
### Cutting Guides for Surgery  
Below are some representative examples of custom-made orthopedic surgical guides taken from the literature, analyzing their peculiar characteristics. All the surgical guides analyzed were made for pediatric patients suffering from cubitus varus deformity. A study conducted by [PERSON] et al. [30] led to the fabrication of a real acrylic model of an osteotomy template [30]. A model of the patient's humerus was created by rapid prototyping using a 3D-plastic printing based on the CT scanner data. The guides were created by the contact of the prototypes with one unique possible position. The surgical guide made by [PERSON] et al. [31] was also made in acrylate resin and printed with the stereolithography rapid prototyping technique (SLA) (Figure 3a) [31]. Invented in the 1980s, it was the first 3D printing technology in the world and is still one of the most popular technologies on a professional level. Stereolithography uses a laser to polymerize the liquid resin into hardened plastic. The template made by [PERSON] et al. [28] was designed based on a preoperative three-dimensional computer simulation with use of commercially available software (Bone Viewer and Bone Simulator; Orthree, Osaka, Japan) and was produced as a plastic model with use of rapid prototyping technology and medical-grade resin. Two metal osteotomy slits and four metal sleeves are mounted on the template (Figure 3b) [28]. Figure 2: Deformity of the varus cubitus in the left elbow (**a**). The inclination angle is 15 varus (**b**) and 25 (**c**) respectively. The surgical guides made by [PERSON] et al. were printed using polylactic acid (PLA) (Figure 4a) [32,33]. The PLA guides are printed using thermoplastic material deposition technology (FDM-Fused Deposition Modelling or FFF-Fused filament fabrication), which is the most used method as it is economical, reproducible and versatile. FDM printers basically consist of a nozzle that heats up to 200 \({}^{\circ}\)C-250 \({}^{\circ}\)C depending on the material and extrudes a plastic filament depositing it layer by layer on a heated bed. This material has printing temperatures between 200 and 220\({}^{\circ}\) and it is not toxic. However, its glass transition temperature of 55-65 \({}^{\circ}\)C makes it deformable at high temperatures [33]. In fact, once the print is finished, the piece must undergo a sterilization process in order to be used in the operating room. There are several sterilization methods available in hospitals. The most common are sterilization in an autoclave, gas plasma and ethylene oxide (EtO). The sterilization in an autoclave is the most common method of sterilization because it is inexpensive and non-toxic. It involves temperatures between 121 and 134 \({}^{\circ}\)C under pressure. The higher temperatures used in the process could theoretically affect the structure of prints, especially those made from PLA with lower glass transition temperatures. Ethylene oxide is a gas frequently used for low temperature sterilization (54 \({}^{\circ}\)C), but due to its high toxicity it requires cycles of up to 14 h mainly for washing with air. The gas plasma uses hydrogen peroxide and temperatures of 37-44 \({}^{\circ}\)C. It represents one of the most advanced techniques for sterilization: it consists in the application of hydrogen peroxide in the gaseous state in the presence of a strong electric field. This brings the peroxide to the plasma state by stripping electrons and generating free radicals. The radicals have a high germicidal capacity and considerably damage cell membranes. However, bacterial growth was observed on cylinders printed in PLA using a 3D printer with a filling density of 12% and sterilized with hydrogen peroxide. Therefore, the use of plasma gas for the sterilization of PLA surgical guides is not recommended. Otherwise, in order to avoid the risk of contamination during the sterilization process, a filling density of 100% should be used when molding the surgical guides [34].