## 4 Surface and Volumetric Measurements
### Centre of Rotation Assessment  
In a paper in Scientific Reports, [PERSON], [PERSON] et al. used a method called digital volume correlation (DVC) to identify the center of rotation of the subtalar joint in a series of healthy volunteers. The subjects were placed in a PedCAT bilateral WBCT device (CurvebeamAI, Hatfield, PA, USA) and asked to perform inversion and aversion of their subtalar joints. The authors report that the center of rotation of the subtalar joint was consistently found in the middle of the subtalar joint [75]. The principles of DVC are to reference one of the implicated bones and to measure the displacement using a vector field associated to the other bone, between the two considered positions.  
2.3 Description of Techniques for Advanced Clinical Applications Derived from WBCT Computerized Techniques  
As mentioned before, segmentation is the key to identifying individual bone contours, and once that has been achieved, multiple new techniques can be applied. One of these is the externalized manufacturing of custom 3D-printed surgical guides for osteotomy [18] or total joint replacement [77], in this particular case ankle replacement. However, it is not clear, in the absence of dedicated literature, whether automatic segmentation software would be reliable enough to provide fully automatic surgical guide manufacturing. Indeed, human intervention by biomechanical engineers is still required to ensure the quality of the fit, according to the patient's specific bone surface characteristics and the quality of the alignment correction on the three planes of space. Therefore, whoever provides the segmentation software, implant manufacturers still must provide human resources to secure these steps, hence an increased cost to include customized bone cut guides, which may not be accessible to all patients, surgeons and healthcare systems. However, in the future, solutions should be found for widespread adoption, taking into account the multiple advantages of customized surgery, such as specific planning of the correction on natural stance WBCT datasets, faster learning curves, operative times and improved accuracy and reliability of axial corrections.  
Although customized surgical guides existed before WBCT was implemented, it is important to distinguish what the added value of WBCT is in this case. Here, the segmentation and surface matching of the guide does not require weight-bearing; it is the planning for alignment correction that does. In that case, as mentioned earlier, the rapid acquisition of natural stance 3D-WBCT is definitely an improvement as compared to the traditional 2 DXR-MDCT sequence. One often-heard argument here is that the correction is performed surgically on a patient who is lying down, so what is the point of planning on standing images? It is important to debunk this argument: first, planning is carried out standing because the alignment only makes sense standing. Also, even in the traditional sequence, the planning of corrections is made using standing 2D images (for total ankles, anterior-posterior and lateral 2 DXR, including the knee), which are less reliable than 3D-WBCT. In that sequence, it is only the surface fitting of the guide which is performed using the MDCT: the cut itself and the corrections are planned using, again, the alignment measurement setup, in the former case, 2 DXR.  
Similarly, we anticipate that WBCT data will soon be implemented for use in robotic orthopedic surgery, although, to the best of our knowledge, no record of such a procedure has been recorded in the literature at the time this work is being written.