Purpose of the study: Knowledge of the normal kinematics of the knee joint, and particularly the femoropatellar joint, is indispensable for evaluating prosthetic implants. Accurate measurements are however necessary, especially for patellar tracking. The purpose of this study was to propose a new experimental set up for analysis of the knee joint and to validate its pertinence in terms of accuracy and incertitude.

Materials and Methods: Eight anatomic specimens of non-embalmed healthy knees were tested on the new setup with a fixed femur and a tibia left free to move. The flexion-extension movement was created by applying force to the quadriceps tendon and resistance to the distal end of the tibia. The femorotibial and femoropatellar kinematics were monitored with an infrared optoelectronic tracking system after acquisition of the bone geometry and the position of the markers on stereoradiographs coupled with a specific 3D reconstruction software. The landmarks used to interpret the kinematic measurements were calculated from the reconstructions of anatomic specimens. Incertitude linked to the determination of these landmarks was assessed as was its impact on the kinematic measurements.

Results: Trials were run on eight knees to validate the experimental setup and study knee kinematics during flexion-extension movements. Method-related measurement incertitude was less than 0.2° in rotation (1 SD) and less than 0.9 mm in translation (1 SD) for the tibia and less than 0.2° in rotation (1 SD) and 0.6 mm in translation (1 SD) for the patella. Quantitative analysis was completed by an animation to visualise any anomalies under different angles.

Discussion: This protocol which couples 3D imaging with a kinematic analysis enables real time tracking of the bone pieces during the experimental trials. This in vitro setup produces femoropatellar and tibial kinematics in agreement with data in the literature. Observations will enable better understanding of femoropatellar function and provide objective data on potential kinematic anomalies.

Conclusion: This experimental evaluation combining bone geometry and kinematic monitoring specifically designed for the knee joint should enable objective evaluation of implants and a validation of personalised finite elements models of the knee.