## 4 Discussion  
The main overall result of this study concerns the technique adopted, from the acquisition of CT scans in upright postures (i.e., using a modern device with the CBCT technology) to the calculation of the bone orientation and joint angles of foot bones. This overall technique was demonstrated to provide, for the first time, thorough alignment measurements of the foot bones in a natural weight-bearing condition. Therefore, the complete skeletal architecture of the flat foot before intervention and the relevant settlement after intervention can be quantified accurately, in 3D and in weight-bearing [13; 22], as frequently recommended by physicians [2; 8]. This technique overcomes the traditional radiographic measurements, which are less time-consuming and resource-intensive but can provide only bidimensional, mostly only sagittal, measurements of a few basic angles [16; 22]. Nevertheless, recent studies in the literature are now also addressing the comparison between traditional and modern radiographic techniques [23]. Accurate 3D bone models were reconstructed using state-of-the-art tools, from which anatomical long axes were defined via the established Principal Component Analysis [13; 22]. Clearly, the latter automatic calculation is independent of manual or other subjective actions or definitions, these being also sometimes affected by the position and orientation of the foot in the radiological device [16; 22]. Ultimately these 3D axes show the alignment of the bones in the three anatomical plane projections but also in a spatial view (the so-called 3D angle, Figure 2), exactly in the plane containing the axis, thus overcoming any issue associated with severe deformity or malpositioning of the foot. In fact, this 3D angle was always found to be different from any of its projections in the anatomical planes (Tables 1 and 2). In particular, this study shows that the modified Grice-Green surgical procedure can improve foot alignment at the talus by correcting its drop down and at the subtalar joint by reestablishing more physiological values of the angle between the talus and the calcaneus in all three anatomical planes (_TACA angles) and also between the talus and the first metatarsal bone (RL_TAM1), i.e., the Meary's angle. As expected by the surgeons, the 3D corrections observed were not only at the rearfoot where the arthrodesis was performed but also at the forefoot (Figure 1C); this of course cannot be represented by traditional radiography. Possibly because of the improvement in foot alignment, a significant improvement was also observed in the clinical outcomes, with patients showing a larger range of motion of the major foot joints and reporting a better foot function, lower discomfort, and less pain following surgery. In any case, the present encouraging results can now further support the surgeons for the present original surgical technique. The present work also contributes to the recent investigations of possible correlations between radiological and clinical observations, as recommended in the current literature [24]. As mentioned above, a variety of surgical procedures can correct alignment of the foot arches [4; 5; 6]. To the best of the authors' knowledge, the present study is among the few reporting clinical outcomes and bone alignments after surgery in a population with AAFD symptoms, but the same technique can be used for many other corrective surgical treatments on the foot. In a recent paper, normal values of bony relationships at the normal foot and ankle have been reported in 3D from weight-bearing CT [25]. The study used slightly different techniques for these measures, i.e., bone segmentation and axis calculations, but some of these measurements provide a valuable comparison with the present results.