Purpose of the study: Kinetic studies of total knee arthroplasty (TKA) in vivo have provided divergent data but have agreed on one point: knee kinetics is abnormal after TKA. Restitution of a normal kinetics is thus the goal to reach to improve functional outcome after TKA. The Journey^® TKA is specifically designed to induce automatic medial rotation of the tibia during flexion. This would align the extensor system during flexion and would reduce mediolateral shear forces applied to the patellofemoral joint. Fluoroscopic dynamic studies have been conducted in vivo to confirm the reality of the femorotibal kinematics but to date there has been no study of the patellofemoral kinematics. Magnetic resonance imaging (MRI) is the gold standard for exploring the knee. The important artefacts caused by metal implants made of chromium-cobalt alloys make it difficult or impossible to interpret the images in patients with TKA. Oxinium^® implants are weakly ferromagnetic, allowing the development of a specific MRI sequence which can be used to explore a TKA.

Material and methods: We used this technique in vivo for a 3D exploration of the patellofemoral kinematics of six Jouney^® TKA in comparison with five Genesis II^® TKA with preservation of the posterior cruciate ligament and with 13 normal knees. We analysed: patellofemoral surface area of contact, patellar translation and shift during weight-bearing flexion.

Results: The results showed that the patellofemoral kinematics of the Journey^® TKA are close to that observed in normal knees and that the patellofemoral pressures of the posterior cruciate ligament TKA are significantly higher than with the Journey^® TKA.

Discussion: These findings confirm our initial hypothesis and allow hop for better functional outcome and reduced wear of the patellar implant with the Journey^® TKA.