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.

Purpose of the study: The issue of patellar kinematics remains a difficult problem for patellar resurfacing during conventional or computer-assisted knee surgery, yet adequate knowledge is required for appropriate orientation of the patellar cut and insert positioning. The purpose of this study was to develop a non-invasive tool for in vivo kinematic analysis of the patellar tract and to compare results with the gold-standard invasive method.

Material and methods: A special experimental set-up designed for this study enabled experimental simulation of load-bearing flexion-extension cycles of the knee joint. Range of motion from 0 to 102° was imposed with a computer-controlled motor. The analysis was conduced on 14 complete lower limb cadaver specimens. Patellar kinematics was analyzed for each knee simultaneously with two systems: a non-invasive method using a low-dose stereoradiographic scan linked to a 3D reconstruction software; and the reference system using tripodes implanted on the patella and radio-opaque spherical markers. Six degrees of freedom were considered: three translations and three rotations. Sequential kinematic recordings were made by calculating the position of a patellar landmark in relation to a femoral landmark.

Results: The mean difference between the results obtained with the two systems was less than 1 mm for anteroposterior and vertical translations, greater for mediolateral translations. It was less than 2° for patellar flexion-extension, to the order of the motion itself for abduction-adduction, and to the order of 5° for horizontal tilt.

Discussion and conclusion: The non-invasive technique proposed here appears to be reliable for patellar translations and flexion, but need further improvement for tilt and adduction-abduction. This is particularly true for the 45° to 90° range of motion because of the difficult problem of determining the contours of the patella. Further developments for this tool are under way.