# Development of a Generic Numerical Transtibial Model for Limb-Prosthesis System Evaluation
## 4 Discussion
### Biomechanical Validation  
A validation of the numerical model of the residual limb in the sense of traditional verification, as is known from classical mechanics, is currently not possible due to the complex geometry and the rate-dependent, nonlinear material properties, which can vary from site to site and also from day-to-day [26]. The material models of biological tissue determined in vitro are only time-independent approximations and do not define the actual biomechanical properties of the tissue fully. Another challenge is controlling the boundary conditions of the in vivo tests so that they are repeatable. In order to extend the point further, anatomical structures and their topological characteristics vary significantly between lower limb amputees, which influences the results in terms of contact pressure, stresses, strains, and deformations. All the above factors should be taken into account, and therefore numerical results should only be evaluated in relative context to each other. In order to validate the generic numerical model in biomechanical terms, we used material models from the literature that have been shown to provide valid numerical results for bulk soft tissue, silicone liners, and composite sockets [27; 28; 29]. The geometry of the numerical model was developed using the verified TEM 2.1 skeleton, and this was scaled and modified further following the surgical guidelines for lower limb amputation [3]. In addition, the CoF between the limb-liner and liner-socket was specified based on previous studies [5; 25]. A mesh analysis of the model was performed to obtain mesh-independent results and ensure numerical convergence. With the exception of the static gait analysis according to ISO 10328, the frequently used loading conditions of donning and single-leg standing were used, allowing for relative comparison to the previous results with similar load cases. In light of the aforementioned points, the numerical model should be considered validated in the biomechanical sense. In order to validate the numerical results further, the peak contact pressures reported in previous studies were compared with the results of the generic model developed in the current study (Figure 12). Only studies with similar definitions were considered due to the wide range of reported results, which can be attributed to differences in prosthesis design, loading levels, the definition of boundary conditions, and the use of material models. Two commonly stressed regions on the residual limb were analysed, namely, the patellar tendon (PT) and the posterior area (PA). For the donning load case, the contact pressure ranged from 60 to 96 kPa and 15 to 147 kPa for PT and PA, respectively. The results of the current study for the PTB socket provide a peak contact pressure of 67 kPa at the PT region, while the TSB socket gives a 90 kPa at the PA region. Although the contact pressure is higher for the TSB socket compared to the PTB, both results were in line with those found in the literature. The influence of the socket rectification is notable in this step and determines how the pressure is distributed over the residual limb, as seen in Figure 8.  
The most frequently analysed load case is the single- or double-legged stance (middle sector in Figure 12). In the current study, the single-leg stance generated a maximum pressure of 106 kPa and 100 kPa for the PTB and TSB sockets, respectively. The reported results can be divided into older and newer studies due to the significant differences in peak contact pressure. The results from older studies ranged from 100 to 250 kPa for PT and 41 to 210 kPa for PA. More recent studies reported a range of 75 to 100 kPa, which fits better with the results obtained in the current study [8; 28]. The higher peak pressure reported in older studies can be attributed to the fact that a foam liner or even no liner was used to alleviate the pressure between the limb and socket [30; 32; 33; 14; 34].