## 5 Follow-Up Control  
Twenty months after surgery, the patient received clinical and radiological examinations. The MRI showed effective decompression of the spinal cord (Figure 8), and the patient could walk with one stick for about 50 m and reported a significant improvement in upper limb movements. By using the same medical imaging and 3D reconstruction procedures reported above, the post-operative virtual 3D model (**POS-VIR**) of the operated complex was obtained. Differently from the pre-operative computed tomography, the two fixations' rods, as inserted in their final position and configuration, were now also visible in the medical images, and thus segmented and reconstructed, along with all the vertebral bony structures. Comparisons between the POS-VIR and PLA-VIR were performed via distance mapping analysis (Figure 9) using a dedicated software package (Geomagic Studio, 3D Systems Inc., Rock Hill, SC, USA) [7; 17]. The analysis revealed average 3D deviations of 1.7 mm and 2.0 mm in model-to-model separation and interpenetration, respectively; the corresponding locally observed maximum values were 5.6 mm for both separation and interpenetration. The overall root-mean-square error was 2.1 mm for both, well-comparable to the CT axial slicing parameter. Figure 7: Post-operative radiological examination. In addition, likely local rotational and translational mismatch analysis was performed between these two models using international recommendations [18]. To this end, an original set of anatomical landmarks was investigated, based on recent observations [19], and here utilized to define anatomical reference frames for each single vertebra (Figure 10). This allowed us to extract quantitative information on pre- and post-operative verteb alignment, and on the amount of post-operative replication with respect to the preoperative planning based on the PLA-3 DP model (Table 1). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Planes** & **C7** & **C6** & **C5** & **C4** & **C3** & **C2** & **C1** \\ \cline{2-9}  & SAGITAL & \(14.3\) & \(14.2\) & \(14.3\) & \(-10.1\) & \(-48.5\) & \(-59.2\) & \(-2.7\) \\  & FRONTAL & \(-0.2\) & \(1.8\) & \(-0.8\) & \(-9.5\) & \(-28.3\) & \(-28.8\) & \(-39.2\) \\  & TRANSVERSE & \(-6.4\) & \(1.4\) & \(1.7\) & \(-8.0\) & \(0.1\) & \(28.4\) & \(-17.6\) \\ \hline  & **Planes** & **C7** & **C6** & **C5** & **C4** & **C3** & **C2** & **C1** \\ \cline{2-9}  & SAGITAL & \(10.8\) & \(6.8\) & \(7.7\) & \(-19.1\) & \(-46.8\) & \(-53.0\) & \(-1.4\) \\  & FRONTAL & \(-0.5\) & \(1.9\) & \(-0.6\) & \(-7.3\) & \(-15.9\) & \(-18.4\) & \(-31.4\) \\  & TRANSVERSE & \(-7.3\) & \(0.8\) & \(1.5\) & \(-0.3\) & \(0.5\) & \(23.3\) & \(-7.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pre- (PRE) and post-operative (POS) relative orientations of the cervical vertebrae calculated with respect to T1-based anatomical reference frame in PLA-VIR and POS-VIR. Figure 8: MRI examination at 20 months after surgery. The third (C3) and fifth (C5) cervical vertebrae are indicated by labels. Figure 10: Anatomical reference frames (anterior-posterior, medial-lateral, and proximal-distal axis in red, green, and blue, respectively) defined on each vertebra for inter-vertebra rotational assessment in the PLA-VIR (**left**) and POS-VIR (**right**). Figure 9: Software screenshots reporting the distance map analysis between PLA-VIR (target) and the POS-VIR (source); in red and blue were the areas of the model with the largest difference, in separation and interpenetration, respectively. In yellow and green were those with the smallest difference. In detail, the mean differences between PLA-VIR and POS-VIR model averaged over all vertebrae were \(-2.5\pm 5.6^{\circ}\), \(4.7\pm 5.3^{\circ}\), and \(1.6\pm 5.2^{\circ}\) on T1-based sagittal, frontal, and transverse plane (Table 1). By considering the relative orientation between distal-proximal vertebral couples, these values were lower, being 1.6 \(\pm\) 5.2\({}^{\circ}\), 0.9 \(\pm\) 3.2\({}^{\circ}\), and 1.1 \(\pm\) 4.0\({}^{\circ}\), respectively.