## 4 Discussion
These operations were all performed at the computer, using suitable software tools, but can also be replicated by using physical models, thanks to current accessible 3D printers, able to manufacture corresponding objects by the additive technology, now in a fast and cheap way [14]. These physical models are very important both for surgeons and bioengineers, to handle and check the virtual planning by using an exact replica of the case; final recommendations can be made, and refinements can still be performed to the design of the implant and of the treatment. In particular, the fixation elements of the implant and the bone cutting guides are to be checked carefully. This phase is also suitable for the preparation and training of the team, and in case for the explanation of the anatomical conditions and of the relevant surgery to the patient and relatives. The industry is also involved in this process, to assess the final production of the implant (for instance materials, surfaces, porosity, lattice, etc.). In this report, the ICP registration between the bone models was performed. These spatial registrations can be performed also via the so-called single value decomposition method [38], but in this case a repeatable procedure to identify corresponding anatomical landmarks must be developed and tested, besides the fact that generally the present ICP registration method resulted in a lower error value [38]. The overall computer models after spatial registration between CT- and MRI- based reconstruction of anatomical structures may also involve the soft tissue. Together with the pelvic bones and the tumor, also the muscle-tendon units and even blood vessels can be included, as their location can possibly have a very critical impact with the implant and the surgical instruments such as guides, drills, and saws. The overall quality of the planning can be checked preoperatively at the computer but also with the physical models. During surgery, additional checks shall be performed. Postoperatively, additional measurements can be taken to validate quantitatively the overall process of segmentation, modelling, registration between models, designing and implanting. A CT scan of the resected bone affected by the tumor can be performed, and its 3D computer model can be compared to the corresponding model defined in the virtual preoperative planning; after a spatial merging of the two models, a root mean square error would well represent the overall quality of the digital and surgical actions [30]. On the other hand, a larger CT scan of the entire operated pelvis can in theory reveal the quality of the entire plan and surgery, but the presence of large metal implants results in severe image artefacts, these being difficult to be removed. This study has limitations. First of all, the use of a single patient to better explain the procedure; other cases may correspond to more or less complex situations. This was also run by only one trained operator: the result of the present procedure was certainly influenced by this single implementation, in particular the overall process of segmentation and the identification of the tumor. This is by necessity a generalised procedure, because of the uniqueness of each single clinical case in this area; in other words, established standard procedures cannot be defined, as bone tumor location and extension cannot be known and catalogued precisely [2]. In this respect, the role and involvement of the surgeons are fundamental for tumor identification, location, and treatment, together for the definition of the cutting planes and the overall preoperative surgical planning. These resection planes could be better positioned by taking advantage of more cautious considerations and knowledge of the magnitude and direction of the hip joint contact forces; these can be taken generically from the literature, or even by patient-specific measurements, by using state-of-the-art gait analysis and musculo-skeletal models of the lower limbs.