## 4 Discussion  
A series of tests were carried out to differentiate the values of the applied external load in the three-point system of forces, without changing their points of application. The results of measurements of the displacement distribution were of a similar nature; therefore, the article presents a representative example of a three-point load case. This case of force distribution corresponds to a given variant of scoliosis. In the event of using a brace by the patient, the values of the forces exerted by the orthosis on the torso undergo certain changes, but the general nature of their distribution does not change because the patient's variant of scoliosis does not change. As a result, the nature of the distribution of strains and stresses in the brace does not change significantly. In the case of the three-point pressure principle, the results of the application of forces are unambiguous and immediate, which is confirmed by studies using ESPI and FEM. Figure 9: Principal tensile stress vector distribution with plotted trajectories of principal stress. Places where forces are applied: \(A\) and \(C-\)forces acting on the thoracic cage, \(B-\)force applied to the pelvis. The results of the model verification tests, presented in Figure 6, indicate the clear similarity between the distribution of displacement contour lines in the FEM image and the pattern of ESPI correlation fringes. A quantitative comparison of the obtained results of the numerical simulations with the experimental results shows that the relative difference in displacements in the central part of the brace's front wall, determined according to both methods, is small and amounts to approximately 0.6%. Despite the fact that only out-of-plane displacements of the brace were measured, their good compliance with the results of FEM calculations on the entire front surface, covering almost half of the brace, allows the conclusion that this method of model verification is sufficient. By using the linear-elastic relations, it allows one to conclude on the values of strains and stresses related to the displacement field. In uniaxial tension (compression), [PERSON]'s law has the form (Equation (2))  
\[\sigma=E\varepsilon \tag{2}\]  
where: \(\sigma\)--normal stress, \(\varepsilon\)--strain, \(E\)--Young's modulus. It can therefore be accepted that the proposed method of describing the orthosis geometry, the applied values of the mechanical parameters of the materials used for modelling and, above all, the density of the mesh and type of finite elements used for FEM description, allow the development of a model with high correspondence between its mechanical characteristics and the characteristics of the actual object. As a result, it appears that using the FEM model for additional testing, using different variants and values of loading in distributions similar to those seen in real life, could provide a foundation to draw conclusions about the work of a real life brace. Taking an approach in the direction defined in this way, a conventional analysis method of the FEM modelling results output was used, consisting of the calculation of the von Mises stress distribution at loads equal in order to the magnitude of the common correction loads, imposed in accordance with an elementary \"three-point system\". Figure 7 shows that the chosen thickness of the polypropylene shell provides the orthosis with the required strength parameters. Von Mises Equivalent stress is determined on the basis of strength hypotheses. Equivalent stress represents the action of all the component stresses in a loaded body and its value must be less than or equal to the critical stress depending on material yield strength and the assumed factor of safety. The von Mises stress values for this material are significantly below the allowable stress (\(F_{a}\)) values. Apart from the force application and support locations, the stress values in other parts of the orthosis were less than 4.4 MPa, consistent with the linear stress-strain curve characteristics of polypropylene [25].