**4. Kinematic**

Numerous prostheses have been developed to improve the durability and function of TKR and kinematic principles that have been the focus of current implant designs, have been to replicate the native knee's femoral rollback during flexion and the external rotation of the femoral component relative to the tibial component.

In the normal knee, a rolling motion predominates for the first 20° of flexion, which produces a posterior translation of the femoral contact position on the tibia. After 30° flexion, sliding becomes predominant but the net motion remains in the posterior direction [11] and the PCL performs several functions: It guides the rollback of the femoral condyles on the tibial plates during flexion, thus allowing the back portion of the condyles to clear the posterior surface of the tibia at high bending degrees and improving the mechanical efficiency of extensor apparatus; from the point of view of stability, it prevents the posterior subluxation of the tibia on the femur during flexion, with a critical secondary role in stability in varus and valgus with knee flexion [12].

Several studies about CR-TKR evaluated in the past the kinematic of this type of implant: Stiehl et al. in a fluoroscopic analysis showed that CR-TKR "…did not reproduce normal knee Kinematics in any case, but showed a starting point posterior to the tibial midline which translated anteriorly with flexion…" and "…Physiological roll-back has not been demonstrated and its absence is likely to reflect anterior cruciate deficiency, alteration of the normal joint line, or other subtle changes which modify kinematics…" [11]. Similar results were obtained by Dennis et al. in 1998: they found that "…a lack of customary posterior femoral rollback in posterior cruciate-retaining designs, and conversely showing an average anterior femoral translation with knee flexion. Posterior femoral rollback, less than in normal knees, routinely was observed in posterior cruciate substituting total knee arthroplasty, attributed to engagement of the femoral component cam with the tibial post" [13]. However, the same author in a study of 2002, using a standardized technique and a specific new generation implant, found that "the subjects…experienced consistent posterior femoral rollback of the posterior cruciate-retaining total knee arthroplasty" and concluded that "having asymmetric femoral condyles may lead to PFR with increasing knee flexion" [14].

A more recent study by Banks et al. also demonstrated that "…greater axial rotations were associated with complete preservation of the PCL insertion." Definitely, in this study, it was shown a normal axial rotation and a normal condylar translation in CR group, while "cruciate-substituting or post/cam substitution of PCL… exhibited lower ranges of axial rotation and condylar translation than the implant was designed to accommodate."

The key point is that the joint surfaces should be designed to be compatible with normal femoral rollback. Components with different curvature radius of the two condyles allow the femoral component to roll back much more laterally than medially, as on the normal knee. On the tibial side, a slightly flattened design in the sagittal plane makes the best use of the PCL preservation, allowing the femur to roll backward and rotate in a relatively normal way. At the same time, a significant congruity in the frontal plane helps to minimize stress on polyethylene, thus reducing long-term wear [12].
