## 4 Discussion  
Soft tissue structures and implant design play important roles in TKA functionality. In particular, it is known that the sagittal curvature of the femoral component influences TKA kinematics [12; 14; 15], however, the influence of this design parameter on the functions of knee ligaments is not yet completely understood. This study combined video-fluoroscopy and multibody modelling of the knee to estimate the elongation patterns of the PCL, MCL, and LCL and compare them between gradually reducing femoral radius and dual-radii femoral component implants. The results indicated that the dual-radii implant design induced slightly larger ligament strains compared to the gradually reducing design, especially at the mid-flexion range. Clear phase distinction was observed in the elongation patterns of the collateral ligaments along the complete cycle of gait activities. The bundles showed relatively isometric behaviour during the first 60% of the GC, likely due to the small knee joint range of motion. With the transition to the swing phase, the ligaments experienced considerable length changes (relative to their reference lengths), associated with the knee exhibiting larger flexion angles. Similar flexion-dependent strain patterns were observed in the sit-to-stand tasks, indicating very small strains when the knee was close to full extension, but considerable length changes at large flexion angles (Figure 7). The MCL length increased with higher flexion angles and reached its maximum at a 40-60\({}^{\circ}\) flexion angle, independent of the performed task, however, the strain was non-uniformly distributed between the MCL bundles (based on their location in the AP direction). The MCL strain patterns of the two implant designs were generally consistent with those measured in the healthy knee [41] and TKA knees [26; 27; 42], indicating the role of the anterior fibres in post-TKA joint stability. Moreover, the present data show that the knee ligaments experience large elongations in the unloaded swing phases of level walking and stair descending, as was also reported in the previous literature [20; 27; 41]. These findings indicate the importance of assessing ligament function throughout the entire activity cycle. The LCL continuously slackened with higher flexion angles during the studied activities, as has been consistently reported in the literature [26; 27; 43; 44]. Among the three activities studied, the LCL was slackest during the sit-stand-sit activity, followed by stair descent and then level walking. As a major restraint to the lateral opening of the knee joint, the LCL remained constantly lax, since no remarkable valgus stress occurred during the studied tasks [45; 46]. Increased slackening with higher flexion angles can further be explained by the posterior position of the femoral attachment site relative to the centre of the knee rotation on the lateral condyle. These findings correspond to recently reported observations that the knee flexion angle drives post-TKA ligament elongation [26; 27]. Soft tissue balancing is important for achieving satisfactory outcomes after TKA. While ligament laxity can lead to knee instability, highly tensioned ligaments may impair the knee's range of motion [47]. Furthermore, unbalanced soft tissues promote polyethylene wear rates, leading to early TKA failure [48; 49]. Normal length change patterns, specifically mirroring the strain patterns of ligaments in the natural, healthy knee, are a prerequisite for the PCL in achieving optimal functionality in PCR-TKA, aligning with its intended design [50; 51]. Our results for the two studied implants reveal a slackening of the ligament as the knee flexed, with the shortest lengths of the PCLal and PCLpm occurring at around a 30-40\({}^{\circ}\) knee flexion before the ligament started lengthening (Figure 7). These patterns generally align with the elongation patterns of the natural PCL bundles [52; 53; 54], but with slight differences, as the PCLal in the healthy knee starts lengthening from early knee flexion.