## 4 Discussion
The lines of force flow will change due to the type of loading scheme, but will be almost identical when changing only the values of forces. In this way, based on the space trajectories distribution presented in Figure 9, it is possible to determine the orthotic area responsible for performing the orthosis' correction function. Other regions perform a supportive role, for example, stabilising the orthosis against the body. In Figure 11a, areas of the orthosis structure that require less effort and have a supporting role are shown in the dark colour, while areas of the brace structure that require less effort and have a supporting role are shown in the light colour. The location of the correction region will be similar to that specified in the optimisation routine [13]. Figure 10: Stress distribution in section A–A of the front bracing wall (the upper side of diagram B corresponds to the inner side of the orthosis surface): superposition of stresses due to bending acting on moment \(M\) and force \(F\). Nevertheless, in comparison with the approach of minimising the elastic energy in the method of topological optimisation, it appears the method presented above for analysing the performance of a structure is more comprehensible in the context of stress distribution analysis. There are no studies in the literature to date that have analysed principal stress for orthoses. On the basis of the numerical model analysis results shown previously, it is possible to propose methods for practical use. Based on the research findings, the weight of the orthosis can be reduced with no significant change in the mechanical performance of the orthosis, especially the orthosis stiffness in regions bearing correction loading. A simple solution may be to make the orthosis from thinner material by using an additional stiffener at the dark-coloured region in Figure 11a, which carries the corrective loads. The orthosis can be more rigid in this region by incorporating thin layers of composite with carbon or glass fibre reinforcement, orientated alongside the principal stress trajectory illustrated in Figure 11b. A comparison of Figures 7-9 clearly shows that the most effective overlap would be on the inner face at the front of the orthosis, as the inner face is subjected to a much higher loading in this region than the outer face. In the vicinity of the correction force points, the situation is reversed: the outer face bears considerably more load than the inner face, so overlays should be applied on the exterior, as shown in Figure 11c. In a similar way, locations for the overlays to be applied to other regions bearing correctional loads can be determined. It can be seen from the above discussion that, for a three-point force system, it is fairly easy to predict the trajectory of the force flow line in the orthosis, which serves as the foundation for determining the regions that need strengthening. At its most basic,  
Figure 11: Boston brace: (**a**) area of the brace’s structure exerting the greatest effort, performing a corrective role (dark colour), and the remaining area, playing an auxiliary role (light colour), (**b**) stiffening by integrating its surface with thin composite layers made of polypropylene reinforced with glass or carbon fibres that are arranged by the principal stress trajectories, (**c**) a cross-section showing the brace overlay positions; dashed lines—orientation of reinforcement fibres. the orthosis corrects the curvature using the principle of three-point compression, which consists of fixing below, above and on top of the curvature [28]. Selected experimental configurations mimic the physiological load as closely as possible. In difficult cases, the experimental stand and the numerical modelling need to consider the corresponding more complicated arrangements. However, it appears that the procedure described can be used to optimise orthosis design in these more complex cases as well. However, further numerical calculations are needed to verify that hypothesis.