## 4 Discussion
Having both ligaments slack at early knee flexion angles may contribute to mid-flexion instability and paradoxical anterior femoral translation, as observed in some PCR-TKA implants [11; 12; 13; 14; 15; 55; 56]. Furthermore, our findings fall within the range of ligament strains reported in a prior study investigating PCL elongation in TKA [20], although they may differ from a few others showing continuous lengthening of the ligament with knee flexion starting from full extension [30; 57]. The substantial variations in reported data from theliterature may partially originate from differences in measurement techniques, but are mainly related to the diverse implant designs studied. The most striking differences between the GRAD and the DUAL implants were related to the strain patterns of the AL and PM bundles of the PCL. Statistical analysis revealed significant differences in the elongation patterns between the implant designs, which were consistent for all studied activities. In general, subjects with DUAL implants exhibited consistently higher elongation than those with GRAD implants. Interestingly, with higher flexion angles (>70\({}^{\circ}\)), the lengthening patterns of the PCL bundles in both groups tended to align (Figures 7, 9, 12 and 13), corresponding with the similar AP translation patterns of the two implants previously reported at these larger flexion angles [12]. The literature states that the shift from the larger anterior radius to the smaller posterior radius at around 30\({}^{\circ}\) flexion contributes to mid-flexion instability [13; 14; 15]. Our results showed statistically significant differences between the two implant designs in the elongation patterns of the PCLal at 30\({}^{\circ}\) flexion across all studied activities (Figures 4, 5, 7, 9, 12 and 13). In particular, the larger strains observed in PCLal in the DUAL design, compared to the GRAD design, could potentially overload the ligament bundle, leading to abnormal anteroposterior translation. This abnormal motion can manifest as a sensation of instability, as the knee fails to maintain proper alignment during dynamic activities. It is also important to note that the ligament reference strains reported in this study are relative to the strain at full knee extension. In practice, ligament fibres may experience some degree of pre-strain after intraoperative soft tissue balancing, which could result in higher strain peaks during functional activities and, in extreme cases, ligament overloading. Therefore, special attention should be paid to the design of the sagittal curvature, particularly in PCR-TKA implants, to ensure more natural ligament lengthening patterns. This can prevent the risks of over-slackening or over-stensioning of the ligaments, which can lead to instability and compromised functionality in the replaced joint. Some limitations of the current study should be considered while interpreting the results. First, the data used in this study were originally collected as part of a different research project, and as such, we did not conduct an a priori sample size calculation specific to this study. Moreover, subject-specific multibody models were created by scaling a generic model according to the skin marker positions obtained during the standing trial and the implant dimensions. As a result, the exact individual femoral and tibial anatomy could not be represented within our multibody modelling pipeline. Moreover, the generic model consisted of a mechanically aligned TKA. Although a mechanical alignment was targeted for the studied subjects, the exact subject-specific implant alignments may deviate from those represented by the model. However, our study was designed to mainly focus on inter-implant and inter-activity comparisons, rather than reporting the subject-specific ligament elongation patterns, thus, the general findings of the current study remain valid.