## 1 Introduction  
Total knee replacement (TKR) is a well-established surgical procedure for the treatment of advanced osteoarthritis of the human knee joint [1, 2]. Despite excellent survival rates of total knee implants, patient dissatisfaction after primary TKR is about 15% to 20% higher than in comparable orthopedic treatments, e.g., total hip replacement [1, 2, 3, 4, 5].  
Currently, cruciate-retaining TKR's are widely used, with available fixed-bearing (FB) and mobile-bearing (MB) options [6, 7, 8, 9]. In contrast to the FB designs, i.e., the tibial insert is fixed to the tibial tray, MB designs enable rotation or multidirectional movement of the tibial insert relative to the tibial tray, i.e., anterior-posterior translation as well as internal-external rotation [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. MB TKRs are considered forgiving in terms of the intraoperative implant component malpositioning [6; 20; 21]. Clinically, the superiority of FB versus MB designs is still controversially discussed [22; 23]. Additionally, knowledge of the biomechanical behavior of MB TKRs is not fully understood so far [24; 25; 26]. It is difficult to obtain the kinematic behavior of the tibial insert [10], especially during weight-bearing activities [26]. Numerical simulations, such as finite element analysis (FEA) and multibody simulation, have been frequently used to describe and analyze the biomechanical behavior of MB TKR designs [21; 24; 25; 27; 28; 29; 30; 31].  
From the modeling perspective, the question arises how an MB TKR can be modeled and to what extent this influences the results on total knee joint dynamics. The FB insert can be assumed to be fixed on the tibial component with zero degrees-of-freedom (DoF). Contrarily, MB designs enable rotation around a rotation pin or translation and rotation on the tibial tray depending on the design principle [14; 32; 33]. To our knowledge, several strategies for modeling MB designs are possible. Most commonly, the MB interface is modeled by a six DoF joint with a contact interface between the tibial insert and tibial tray; therefore, the motion of the tibial insert relative to the tibial tray is constrained by the respective design features of the bearing geometry [14; 24; 29; 34; 35]. Using FEA, [PERSON] et al. [24] investigated the effect of the femoral component malalignment in the case of an MB TKR on the total knee joint dynamics. In their study, the MB TKR was modeled deploying a friction contact condition between the tibial insert and tibial tray considering six DoF [24]. Using a multibody analysis, [PERSON] et al. [29] modeled MB TKRs through a compliant contact algorithm described by [PERSON] et al. [36]. The next possible strategy would be to kinematically constrain some of these DoF, while the remaining DoF should allow for motion compatible with the modeled geometry. This approach aims to increase numerical stability and reduce computation time by maintaining the accuracy of numerical results. For MB TKR with a frictionless uniaxial articulation between the tibial insert and tray, the interface may be modeled as an idealized revolute joint, as shown in previous studies [6; 21; 30; 37].  
However, few comprehensive studies have been conducted to investigate the influence of different MB design variants on the total knee joint dynamics. Therefore, the objective of our computational study is to compare MB TKRs with different design principles by deploying a validated musculoskeletal multibody model of the lower right extremity during an active knee squat movement [38]. Additionally, the influence of two different modeling strategies on the kinematic behavior of the knee joint was investigated for the MB designs.