## 4 Discussion  
In the present study, differences in knee joint kinematics for MB TKRs with different design variants were presented deploying a musculoskeletal multibody model of the lower right extremity during a squat motion. Regarding the selection of these designs from a surgeon's point of view, it is important to understand the design-specific elements of MB TKR's and the biomechanical behavior in terms of the knee joint parameters (femoral translation or internal-external rotation). In this context, the computational prediction of these knee joint parameters is useful [11; 24; 25; 29; 31]. We intended to show the feasibility to integrate diverse MB design variants into a musculoskeletal multibody model and to systematically analyze the effects of different MB design variants on knee joints kinematics and kinetics. The results revealed differences in the kinematics of the knee joint for the various MB TKR design variants. Compared to the commercially available MB design variant I, the other design variants showed differences in terms of the tibial insert internal-external rotation. As expected, constraining the insert rotation mechanically by end stops is mandatory when a limited rotational range of motion is in demand. Although the tibial insert rotation was mechanically constrained by end stops, design variant I showed almost the identical femoral anterior-posterior translation compared to variant II. This leads to the assumption that the tibial insert internal-external rotation does not directly influence the femoral translation. In the case of the design variant III, according to the previous studies [8; 9], a higher range of motion should be enabled due to a facilitated increased femoral rollback when a functioning posterior cruciate ligament remains, since the femoral component is sliding on the tibial insert. Our results revealed that this design variant (III) had more than 4 mm higher femoral posterior translation compared to the other MB design variants. Our data correspond well with previous studies [8; 9]. During flexion, the femoral rollback is an important characteristic as it significantly affects patellofemoral and tibofemoral contact forces [50]; therefore, it should not be neglected. Furthermore, the range of motion in terms of the medio-lateral translation was also higher. TKR with MB design poses various challenges to computational modeling. For instance, one challenge is to generate a realistic and stable model with an appropriate representation of the additional articulating interface between the tibial insert and the tibial try [51]. Previous studies revealed that there are different modeling strategies to consider MB TKR model design variants within the numerical models [6; 11; 21; 24; 25; 29; 30; 52]. The most frequently used modeling strategy is a six DoF joint between the tibial insert and try [11; 24; 25; 29; 34; 35; 52; 33]. In this study, we proposed a new modeling strategy as a two DoF joint with one-dimensional end stops in both DoFs. During the squat motion, only slight differences were observed between the kinematics of the six DoF baseline and the two DoF joint, which could be a result of either the pure contact definitions between the tibial insert and tray or numerical effects in the interaction of the tibofemoral contact [47]. This strategy is computationally more efficient compared to the six DoF baseline and is able to map the investigated MB design variants. On the one hand, our data showed the versatility of the six DoF modeling strategy, which can be applied to all currently available MB design variants. On the other hand, this strategy resulted in considerably more computation time compared to the proposed two DoF joint with end stops, which makes large-scale parameter studies less efficient. Therefore, the modeling strategy with six DoF baseline is the least favorable for multibody modeling. Additionally, for further preclinical testing, computational time is an important factor that should be considered.