# Three-Dimensional (3D) Model-Based Lower Limb Stump Automatic Orientation
## 4 Discussion  
Based on the results obtained, it can be assumed that the developed algorithm provides with relatively good agreement with the expert assessment typically within 10 degrees along the vertical axis in any direction, as revealed by comparison of the fitting line obtained by the automatic algorithm and a similar line fitted for the model oriented manually by the expert. The most pronounced deviations could be observed in the third section for the fifth patient that could be attributed to the specific anatomic features of the stump. As indicated in Table 1, no relevant results were obtained for the second patient, as attempted analysis resulted in the inverted positioning of the 3D model. A possible reason for this result is that the angle of inclination of the 3D-model cross section plane above the knee to the virtual anatomical axis of the femur was untypically sharp, resulting in the input requirements not being fulfilled.  
The limitations of the proposed approach include limited automation, as the expert has to define the position of the proximal edge of the stump to determine the location of the upper section. We also have to note that the algorithm in its present implementation relies upon the assumption that the virtual anatomic axis is orthogonal to the sagittal plane, which is not exactly, but only approximately fulfilled especially for longer models, see also Figure 6 for visual reference.  
In our opinion, the above algorithm, taking into account all its inherent limitations, can nevertheless be used as a decision support tool for the prosthetic expert and 3D designer considerably reducing the workload of qualified personnel. Among relevant analogues, the work [28] should be mentioned, where 2D cross-sections of the upper and lower limbs 3D-model were used to create a \"bone structure\". The sections were created by placement of any number of parallel planes inside the 3D-model and calculating corresponding intersections between the 3D-model and the plane. This \"bone structure\" can be used to determine the orientation of the scanned model in 3D space in order to create additional cross-section images, as well as obtaining basic information about the shape of the hand or foot or missing fingers, which can be used to create a 3D-model of the prosthesis. However, getting accurate dimensional information about the 3D-model being scanned will require additional input data from the user to determine the scale of the 3D-model, which in our opinion appears to be a certain limitation of this approach, as preprocessing of the 3D-model by an expert is required increasing the workload and potentially reducing the objectivity of the first approach to the model orientation.  
As an outlook, we believe that further optimization of the algorithm could make the dependence of the performance on the number of polygons in the 3D-model less pronounced. For that, an algorithm module that allows orientation of the 3D-model in a horizontal plane using the quaternion method for rotating an array of 3D-model points sounds a perspective solution. Since recent technological advancement has enabled the estimation and simulation of the distribution of stresses applied to the residual limb tissues, as well as the volume fluctuations affecting the stump over time and the temperature variations influencing the residual tissues for different variants of socket design, a more automated functional socket design may be developed in the near future. Moreover, recent advancements in prosthetic technologies suggest that future directions could be associated with advanced socket designs capable of self-adaptation to the complex interplay of factors affecting the stump, under both static and dynamic loads, as a replacement for the current fixed socket-orientation scenario [18].