## 4 Discussion  
Limb salvage for pelvic tumor is a complex surgical treatment and represents a challenge for orthopaedic oncology surgeons. The primary goal in the removal of the tumor and replacement with an implant is to achieve an adequate margin and to preserve the relationship between the remaining tissues [5,8]. The recent advent of 3D printing technology, together with the use of CT and MRI, provides an important solution for these problems [7,14,15,38,42]. In the present study, an original procedure is defined, with all steps from these medical imaging to an accurate 3D anatomical--based model with the shape of the implant; this would be fundamental also for virtual pre-surgical planning, and the design of patient-specific cutting guides. The entire procedure is here shown by depicting the results in a single real case. These guides are fundamental, for the surgeons to achieve the result designed in the pre-operative planning, in particular the perfect intra-operative positioning and fixation of the prosthesis, which would result also in a safe margin for full tumor removal [3]. Knowing from the virtual models the exact shape of the bones, and the exact position and orientation of the cuts, together with the experience of the surgical access and overall room, the design of these cutting guides and their additive manufacturing with suitable material is straightforward. In the operating theatre, their correct position is usually checked with the help of drawings from pre-operative planning and also with sterile bone models from the case. Once the cutting guides are placed in the correct position, bone is cut with oscillating saw, and then bone and guides are removed. The anatomical 3D computer models include bone shapes from CT, and position and extension of the tumor and soft tissues from MRI. These models, in their digital format, \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **FIRST MODEL** & **SECOND MODEL** & **THIRD MODEL** \\  & **-** & **-** & **-** \\
**Affected Hemipelvis** & **Hemipelvis Section** & **-** \\
**Mean to Host the** & **Containing the** & **-** \\
**Implant** & **Tumor** & **Shape of the Implant** \\ \hline SECTION AREA & 2454.8 & 2454.8 & 2489.6 \\ (mm\({}^{2}\)) & 178,601.5 & 103,961.6 & 135,535.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Area of the cutting sections and volume of the final models obtained. are registered in space (Figure 3), i.e., positioned in corresponding locations; from this, a process starts for an initial shape of the personalized implant to be obtained (Figures 4-7, Table 1). The major originality of the present procedure is the definition of a best possible initial virtual model for the final personalized implant (Figure 6C). This is designed to restore skeletal anatomy as best as possible, by mirroring the corresponding contralateral part (Figure 4) and thus it matches well with the remaining bone to host the implant (Figure 6B) by definition. These two models do come from different hemipvelvis, but the cuts are performed once, when these hemipvelvis are mirrored and registered one over the other (Figures 4B and 5). In other words, the same virtual bone cuts (planes a-b-c-d in Figure 5) identify the bone-stock with the tumor (to be removed, in the affected hemipvelvis) and the corresponding contralateral part (for the replacement, in the unaffected hemipvelvis), respectively parts A and C in Figure 6. This guarantees a perfect match between these models, for a final successful replacement in the operating theatre, apart of course the anatomical asymmetry also caused by the tumor and the spatial registration error, which are expected however to be small. These are the only sources of the present very small difference (about 35 mm\({}^{2}\)--Table 1) of the matching faces in the corresponding resection areas, which is clinically acceptable. In any case, the very final design of the implant can cope and adjust this mismatch easily.