## 4 Discussion  
This is the first report that provides a systematic description of the broad available preclinical and clinical literature on BCR-TKA. It underlines the current interest in improving kinematics and clinical outcome based on a more anatomical TKA design. Indeed, a recent survey amongst 346 experienced knee surgeons confirmed that there is a strong interest to perform BCR-TKA and the percentage of potentially eligible patients is high [54]. In our view, the current systematic review is especially insightful, as for the first time, the extensive preclinical models can be compared to the available clinical data. The importance of implant design with adequate tibial fixation to prevent aseptic loosening was highlighted. In addition, because the rationale behind BCR-TKA aims at native kinematics, intraoperative precise joint line restoration and soft tissue balancing during BCR-TKA, as well as adequate PTS, are important factors to consider during surgery to minimize increased loading and impingement of the preserved ACL. An important challenge to overcome for the implant design is the difference in laxity that is observed between the stable medial side and the more mobile lateral side in the native knee. Indeed, preclinical data suggest that using a medial constrained insert and a lateral insert with a posterior slope of +3\({}^{\circ}\) may aid in this challenge [36; 37]. Nevertheless, the preclinical data that compare the BCR-TKA design to the CR-TKA design are conflicting, and optimal ligament balancing for an ACL- and PCL-preserved knee has yet to be defined. Thus, while these data suggest ACL retention to have theoretical kinematic advantages in TKA, the heterogeneity in the described models creates difficulty in optimal implant design. What is more, newer anatomical bearing designs with more constrained (concave) medial and convex lateral surfaces may be as important for anterior-posterior stability and kinematics as ACL retention. Unfortunately, while such designs have been described, their exact influence on kinematics is yet to be determined [25]. Several clinical studies suggest BCR-TKA may provide kinematic improvements over contemporary implants with superior movement of the lateral condyle, medial condyle and tibial internal rotation through a full range of flexion, as well as reduce AP laxity. Again, while newer-generation BCR-TKAs seem to show promising results, there are no clinical data that show superior results compared to other designs. This may be attributed to several patient, implant, surgical and methodological factors. For example, the implant which has been explored in the most clinical trials to date, i.e., the Vanguard XP, showed similar clinical and RSA results compared to a CR design in a single-blinded randomized controlled trial (n = 50) at 2 years [22].This has been confirmed by several smaller (retrospective) cohort studies in the short term up to three years (Table 1). However, at an intermediate follow-up of 5 years, a large retrospective cohort study including 1253 patients raised concerns regarding the performance of BCR-TKA, as a higher revision rate for aseptic loosening was found compared to an AS CR-TKA design of 9.7% vs. 0.1%, respectively [21]. While surgical experience was suggested to be a confounder, a later report showed a revision-free survival of 88% at 3 years, which is lower than traditional TKA designs [19]. Furthermore, lower postoperative range of motion and stiffness have been reported [20; 55]. Here, it is important to stress that these findings need to be interpreted with care, as the majority of these studies are of low (level IV) methodological quality which is prone to patient and surgeon bias. Interestingly, to the best of our knowledge, the rational of improving propriocepsis due to ACL retention has yet to be confirmed in the literature. Similarly, while the young and active patient would have a theoretical advantage in the BCR-TKA design, no comparative clinical data between the older (>65 years) and younger (<65 years) were identified.