## 1 Introduction  
Total knee arthroplasty (TKA) is one of the most successful joint replacement surgeries [1, 2]. However, a significant number of patients remain dissatisfied with the surgery outcomes. Only half of patients report satisfaction with their ability to resume routine daily activities after joint replacement [3], and even lower satisfaction rates are reported in regard to returning to sport [4]. Among several factors, abnormal kinematics (defined as deviations from the natural movement patterns of a healthy knee joint) and postoperative knee instability are major reasons for patient dissatisfaction after TKA [5, 6]. TKA procedures can lead to different kinematics compared to those of intact knees [7, 8, 9, 10]. Unnatural knee kinematics, such as the paradoxical femoral anterior motion associated with cruciate-retaining (CR) TKAs [11, 12, 13, 14, 15], may originate from differences between the articular geometries of the implants compared with the natural knee [16, 17]. A previous modelling study emphasised the influence of condylar geometry on AP translation during a simulated squat movement [18]. They suggested that paradoxical anterior motion is caused by the dual-radii femoral condyle design, since it occurs at the transition from the distal to the posterior radius. This finding was further supported by several other studies reporting less sudden anterior translation in gradually reducing radius implants [12, 13, 14, 15]. In addition to TKA design, improper ligament balancing has been consistently mentioned as a factor that may contribute to abnormal knee kinematics and postoperative joint instability [19; 20]. In particular, excessive laxity of the posterior cruciate ligament (PCL) has been linked to suboptimal kinematic outcomes and joint instability, while a tight PCL may lead to overloading of the polyethylene inlay, potentially shortening the lifespan of the implant [19; 21; 22; 23; 24]. In the context of cruciate-retaining total knee arthroplasties (CR-TKAs), it is crucial to uphold a harmonious interplay between the implant's articular geometry and ligament strain patterns. This balance ensures the proper functionality of the posterior cruciate ligament (PCL), essential for controlling anteroposterior translation, while also preventing potential issues related to ligament over-tensioning that could adversely affect the overall performance of TKAs [6]. However, there is a limited understanding of the intricate relationship between implant design, ligament strain patterns, and their collective impact on TKA biomechanics. Technical difficulties and ethical considerations hinder proper assessments of ligament laxity/tension in vivo. As a result, the majority of our knowledge about ligament strain patterns after TKA comes from in vitro cadaveric investigations, where the cadaveric specimen is mounted onto a mechanical rig and the ligament strain/force during loaded or unloaded knee flexion is measured by implantable sensors. Using such an approach, [PERSON] and co-workers [25] investigated the relationship between implant conformity and collateral ligament strains. They noted that augmenting implant constraints has the potential to decrease the strain on the medial collateral ligament (MCL), consequently reducing frontal plane laxity in the knee. However, owing to challenges associated with its anatomical position, they refrained from measuring PCL strain. Furthermore, in vitro cadaveric test setups are typically limited in their ability to simulate dynamic functional activities. Recent advances in combining video-fluoroscopy and multibody modelling of the knee appear to overcome these limitations by estimating ligament strain patterns based on the relative movement of the ligament attachment sites during complete cycles of dynamic activities [26; 27; 28]. This technique has been employed to evaluate the elongation patterns of the native knee ligaments [27; 28; 29], as well as ligament strain patterns post-TKA [27; 30].