## 3 3D Modelling, Printing, and Surgical Planning  
A high-resolution pre-operative computed tomography (**CT**) (GE Healthcare, Chicago, IL, USA; spiral CT system; 1 mm slice thickness) of the cervical spine was executed one month before surgery (Figure 2A). Relevant DICOM files were then imported in dedicated software (Mimics(r) Innovation Suite--Materialise(r), Leuven, Belgium) and segmented image-by-image using a semi-automatic procedure [7,8] (Figures 2B and 3). The pre-operative virtual 3D model (**PRE-VIR**) was thus obtained, both as a single aggregated model of all bones together, and as separated eight verteba-by-vertebra models, i.e., from C1 to T1 (Figures 2C and 3). These models were obtained from a state-of-the-art software tool by an experienced technical operator. Figure 2: 3D model reconstruction of the pre-operative cervical spine. DICOM images are imported in a dedicated software (**A**), where the bone vertebral bony contours, along with the internal bony structures, are identified and marked by the operator (**B**); all detected contours are then merged to create the 3D virtual model for the overall C1-to-T1 spine section (i.e., PRE-VIR) and for each single verteba ((**C**), each verteba was marked using a different colour). Afterwards, the reconstructed virtual model was manufactured via 3 DP (PRE-3 DP) and a plastic red wire mimicking the cervical artery was inserted in the relevant printed vessel course (**D**). Starting from the PRE-VIR model, a corresponding pre-operative 3D-printed model (**PRE-3 DP**) was manufactured in polymeric material (P430 ABS Plus) via fusion-deposition-material technology [12], with full density and 0.25 mm layer height (Dimension Elite(r) 3D Printer, Stratasys Ltd., Eden Prairie, MN, USA). After debris removal and polishing via a thermo-chemical treatment, and careful visual inspection, this PRE-3 DP mimicked exactly the external surface of the vertebral bones, including the spinal canal and the course of the main vessels, thus revealing the exact overall spine deformity of the patient (Figure 2D). These are established technologies and materials, used widely for this and similar scopes. The PRE-3 DP model was handled by the surgeon for pre-operative planning and surgical simulation, eventually obtaining a 3D physical model of the planned surgery (**PLA-3 DP**; Figure 4). At this stage, relevant information about possible optimal bending and positioning of two fixation rods, along with relevant screws, was set and extracted. In detail, this phase implied the following actions: drilling, tapping, screws placement, rod bending, and laminectomy. In particular, all the surgical steps implying screws insertion were tested on the PRE-3 DP, and surgical risks related to hardware implants, e.g., possible vertebral artery injury, were timely predicted, identified, and made evident on the model (Figure 4A); this allowed critical surgical manoeuvres in the operating room to be avoided, thus best preserving patient safety. Two standard cervical fixation rods were also bent over PLA-3 DP according to the degree of correction as set by the surgeon based on the morphology of this cervical spine, as in PRE-3 DP (Figure 4B, two views). By using the same medical imaging and reconstruction procedures reported above, the PLA-3 DP model, i.e., with the rod and screws still fixed in the final planned configuration, was then scanned again. These DICOM files were segmented to generate the virtual 3D model of the planning (**PLA-VIR**) (Figure 5), offering a reference to be used at the post-operative follow-up control. Figure 3: Screenshot from the segmentation software. The C1-to-T1 vertebrae were segmented independently of each other to obtain eight different 3D bone models. These can also be merged into a single model after an easy aggregation process. Subsequently, the two pre-bent rods were removed from the PLA-3 DP model and sterilized the day before surgery to be used by the surgeon during surgery.