## 3 Results
### Preclinical Kinematics  
Kinematics of the human knee are a result of an interconnected system of bones, soft tissue structures and muscles resulting in a motion in all six degrees of freedom, with a complex set of translations and rotations. BCR-TKA aims to restore pain-free motion of the joint, based on these physiological kinematics [33]. To achieve this goal, optimal anatomic design, implant position and use of native proprioception are key factors. Several studies have aimed to assess the kinematics of BCR-TKA to better understand how to achieve its theoretical improvement in kinematics. Indeed, during simulated deep knee bend, chair-sit and walking, a biomimetic BCR implant showed activity-dependent kinematics similar to healthy knees in vivo [34]. Restoring native knee geometry together with ACL preservation may thus provide these desired kinematics over contemporary ACL-preserving and ACL-sacrificing implants. Indeed, in 14 cadaveric knee specimens tested under passive conditions with and without external loads, the width of the varus/valgus and internal/external laxity envelope for the native knee and the bicruxiate-retaining knee were almost the same. However, in the native knee, a difference in laxity was seen between the stable medial side and the more mobile lateral side [35]. With an image-free navigation system in six fresh-frozen whole-body cadavers, one study found that the amount of tibial internal rotation throughout knee flexion was more similar to that in a native knee if a medial constrained insert was used compared to a flat insert [36]. Others showed that lateral joint laxity was potentially decreased in BCR-TKA, limiting native flexion kinematics. The loss of internal rotation in deep flexion may be compensated by using a lateral insert with a posterior slope of +3\({}^{\circ}\)[37]. Although the importance of retaining the ACL was underlined within the BCR-TKA group, no difference was found in contact forces, rotations, translations and ligament forces when BCR-TKA was compared to CR-TKA during a normal walking pattern [38]. Furthermore, [PERSON] et al. found that the preserved ACL in the BCR-TKA cadaveric knees had a 2-fold to 6-fold higher strain than that in intact knees at 0\({}^{\circ}\), 15\({}^{\circ}\), 90\({}^{\circ}\) and 120\({}^{\circ}\) of passive motion [39]. Computer modeling has also suggested that restoring native knee geometry together with ACL preservation may provide kinematic superiority over contemporary ACL-sacrificing implants [34]. More precisely, computational simulation results suggested that BCR-TKA improves the posterior movement of the lateral condyle, medial condyle and tibial internal rotation through a full range of flexion when compared to CR-TKA and PS-TKA [40]. However, one computer model suggests that surgeons using symmetric BCR-TKA may consider using a kinematic alignment method to achieve sufficient ligament laxity throughout knee flexion to prevent a conflict between the retained ACL and the replaced joint surface [41]. Moreover, the loss of the conforming anatomy of menisci and tibial cartilage and replacement by a relatively flat polyethylene insert may account for the loss of tibial internal rotation and the slight paradoxical anterior motion of the medial femoral condyle [42]. In a cadaveric model, [PERSON] et al. found BCR-TKA (UnityKneeTM 77, Corin Ltd., Cirencester, UK) reduced AP laxity versus CR-TKA, and may improve knee stability without using conforming geometry in the implant design. When compared to an implant design with a bridge between the two tibial bearing trays, the use of two components akin as used in UKA prevented tibial avulsion fractures in extension in this model [43]. Still, a cadaveric model showed that the rotational kinematics may be lost once the tibial replacement is performed [44]. In Vivo (Clinical) Kinematics  
A variety of small clinical studies have analyzed in vivo kinematics after BCR-TKA to better understand the advantages of this design.