## 6 Discussion  
This study originally presented the exploitation of 3D computer-based anatomical modelling and printing to improve correction surgery on a patient with severe cervical deformity, both for pre-surgical planning and for intra-operative support in terms of reference and guidance in relevant surgery. Compared with traditional assessments based on two-dimensional X-rays or CT images, the use of a 3D anatomical model, i.e., PRE-VIR, allows a more complete understanding of the deformity, as well as a virtual overview of the anatomical structures involved in the surgical treatment, as provided by PLA-VIR and POS-VIR models. Of note, the visibility of these structures before surgery and in the surgical planning received further enhancement thanks to the use of corresponding 3D-printed anatomical models (i.e., PRE-3 DP and PLA-3 DP, respectively), where bone structures were reproduced, also showing the critical vessel locations. Overall, the surgeon reported a positive experience for intra-operative use of a 3D-printed model during placement of rods and screws, following the safe locations as in PLA-3 DP. Relevant numerical analyses eventually confirmed the achievement of the surgical goal with respect to the pre-operative planning. In the present work, the known procedure from medical imaging to 3D bone models was enhanced also by original biomechanical analyses: the definition of bone-based anatomical reference frames for each vertebra, the calculation of bone-to-bone alignments based on established international recommendations [18], and the 3D matching and distance map analysis for bone model comparison. These observations concur with most of those reported in the relevant literature, though it has been difficult to find studies addressing exactly the same surgical issues on the same anatomical complex with the same thorough techniques used in the present work. [PERSON] et al. reported that biomodelling may assist the surgical team to understand better and to deal with complex anatomy and pathologies, in addition to medical imaging used for complex spinal surgery [20]. [PERSON] et al. presented a method to reproduce patient-specific spines by 3D printing from geometric features extracted from pre-operative radiographs, thus not CT-based, to be used for surgical planning in correction surgeries in scoliotic subjects [21]. In this study, an overall discrepancy smaller than 10\({}^{\circ}\) was found between the planned correction based on the 3D-printed model and that obtained surgically, which was considered by those authors to be encouraging. In general, by considering applications in paediatric [22] or oncological orthopaedic surgery [23], the implementation of 3D printing in anatomical modelling, as well as in device prototyping, is considered to be of huge importance for the achievement of more precise and safe surgeries [24; 25; 26]. For the present work, state-of-the-art software tools, 3 DP devices and materials, and biomechanical analyses were exploited, but many other different technologies can be used. The accuracy of the present virtual and physical models was taken from nominal information, which was considered well above the necessary level for the present exercise. The present exercise could not thoroughly address a cost-benefit analysis; however, roughly and practically, with a cost of a few tens of Euro for the bone physical model, many tens of minutes were saved in the operating theatre, and very potentially critical surgical situations were removed. We suggest working in a team, with the surgeon accompanied by a radiologist and bioengineer, for the multidisciplinary nature of these activities. The use of these techniques is thus expected to increase, although further in-depth evaluations are necessary [27].