# Development of a Generic Numerical Transtibial Model for Limb-Prosthesis System Evaluation
## 4 Discussion
### Numerical Analysis of the Generic Transtibial Model
The introduction of ISO 10328 into the virtual environment, in combination with the generic model, enables a numerical evaluation of the socket-liner system in more complex and realistic loading conditions than simulating only a stance. The proposed approach allows a static numerical simulation of a gait and, thus, unifies the boundary conditions so that different studies can compare results relative to each other. In order to present the usability of the results from the realistic gait cycle, the heel strike (P5 I) and the push-off (P5 II) were analysed regarding the settling and proof test force. From the results, it can be seen that, under push-off conditions, a higher maximum contact pressure occurred for both rectification methods (Figure 7). A rare severe event, such as a trip or fall, was represented by the proof test force. This loading condition resulted in a significantly higher contact pressure: 318 kPa and 257 kPa for the PTB and TSB sockets at heel strike and 575 kPa and 506 kPa for PTB and TSB sockets at push-off. The pressure distribution also varied depending on the load case and rectification method, as shown in Figure 9. Such high contact pressures could cause damage to the biological tissue and should therefore be avoided, or, if not possible, minimised by providing a stable prosthesis. It is challenging to relate the biomechanical results to the subjective sensation. According to a previous study, contact pressure can be a parameter for evaluating a pain pressure threshold and pain pressure tolerance [34]. The stability of the prosthesis can be assessed by analysing the relative deformation of the socket. Until the pain pressure threshold is reached, minimal deformation of the socket is desirable to ensure maximum stability. On the other hand, controlled deformation could reduce the pressure concentration and, hence, increase the comfort of the socket. The numerical results show that the composite socket proves to be very stiff, deforming only 2.18 mm at the settling test force and thus providing stability during gait (Figure 11). Due to the high stiffness of the socket, most of the deformation occurs in the soft tissue and silicone liner, which helps to alleviate stress concentrations in the residual limb that could lead to pain or discomfort. Since we cannot change the material properties of the biological tissue, the liner is the main component that can be designed to mitigate contact pressure further. Although the numerical model provides a realistic biomechanical response, some limitations should be addressed. The geometry of the model is generic, and the simulation results cannot be extrapolated to a specific case but rather serve as a relative comparison of the different socket and liner designs. Additionally, the material models are simplified and are time-independent, so muscle activation is not taken into account. All load cases were static, regardless of the fact that walking is a dynamic process and that inertia has an influence on the results. All simplifications have been introduced in order to ensure numerical convergence and minimise computational costs. In addition to the already performed validation presented in this paper, the developed numerical model should also be validated by experimental tests. A possible solution is to design an experiment with the 3D-printed or moulded silicone stump to approximate a bulk soft tissue behaviour. A dynamic simulation should be considered to analyse the entire gait cycle according to ISO 10328. A more detailed residual limb model should be investigated, including muscle activation, tendons, skin, meniscus, etc. Using the developed model, different socket and liner designs should be analysed considering different materials. Future efforts should be made to link biomechanical parameters with subjective sensations.