# Development of a Generic Numerical Transtibial Model for Limb-Prosthesis System Evaluation
## 4 Discussion
### Numerical Analysis of the Generic Transtibial Model  
Currently, the model represents the size of a 50 th percentile male transtibial limb but could easily be scaled and remodelled using the AnyBody Model Repository to provide sizes for various age, gender, weight, or ethnicity groups. Additionally, the size and shape of the limb could be modified by cutting the bones at different lengths and reshaping the soft tissue according to the anthropometric measurements of a given target group of patients. The same approach can be applied to transfemoral limbs. In this study, a PTB and a TSB socket were analysed, which gave significantly different results in terms of pressure distribution and pressure intensity (Figures 8 and 9). The Prosthetist can use the contact pressure results on the limb surface to understand better the effects of the different rectification methods. The definitive composite socket is much stiffer than the soft tissue; hence, the residual limb takes the form of the socket when donned, while the deformation of the socket remains negligible (maximum deformation of 0.42 mm for the PTB and 0.57 mm for the TSB socket). The rectification of the socket has a significant impact on the level and distribution of the initial contact pressure. As intended, after donning, a PTB socket produced the maximum contact pressure at the PT and a TSB socket at PA, which are the regions that can withstand higher loads. The preloading of the limb in this step gives the user a sensation of stability and security. However, on the other hand, the loads must be distributed to the preferred areas to avoid discomfort, or even pain. Following the donning, a single-leg stance was simulated by applying an equal-weight vertical force to the femoral cavity and fixing the attachment plane of the adapter. Similar loading conditions have been used frequently in previous numerical and experimental studies and are, therefore, a fruitful source for validating the model. This step generated a peak contact pressure of 106 kPa and 100 kPa for the PTB and TSB sockets, respectively. In contrast to the initial stress state, a higher contact pressure occurred for the PTB socket, indicating that TSB rectification distributes the pressure more evenly over the limb surface. The locations where the maximum pressure occurred were similar to the donning load case for both rectification methods, as shown in Figure 8.  
While the loading cases of donning and single- or double-leg stance are easy to interpret and standardise, this is not the case with gait analysis due to the complex boundary conditions that occur during walking. In the static simulation of the gait cycle, a particular instance, e.g., heel strike or heel-off, is evaluated, and these conditions can vary significantly from case to case. One of the main objectives of our study was to establish and define a standard numerical loading case of a gait and a severe event for the evaluation of prostheses. Therefore, the boundary conditions for FE analysis were proposed according to ISO 10328. The standard is essentially intended for the physical testing of various prosthetic components, except the prosthetic socket. However, since there is no specific standard for socket evaluation, the above standard has also been used for testing the socket physically in previous studies [6; 37; 38; 39; 40]. A major drawback of this standard for the experimental evaluation of prosthetic sockets is that a foam or sponge filling and plaster of Paris or various polymers are typically used to replace a residual limb in the socket, which hardly reflects the biomechanical response of the tissue [41; 42]. While this is not problematic for other prosthetic components, the nonlinear behaviour of biological tissue affects the stress-strain results significantly by deforming and, thus, mitigating stress concentrations. Hence, more realistic conditions can be achieved using a generic numerical limb model and an approximation of the bulk soft tissue response.