Real-time measurement of patellofemoral kinematics in asymptomatic subjects

The aims of this study were to determine whether patellofemoral kinematics in the anteroposterior (AP), mediolateral (ML) and proximal–distal (PD) directions could be measured in real time with high repeatability using a three-dimensional magnetic tracking system, and to evaluate the symmetry between sides. Data was collected from both knees of 40 asymptomatic subjects while performing knee flexion from 0° to 60°. Measurement of patellar displacement along the three anatomical axes (AP, ML, and PD) with respect to an anatomical coordinate system was collected using the Fastrak system. Data from three trials of each knee was plotted against knee angle and the repeatability of the data determined by examining the similarity of the movement–time curves. Symmetry was determined for maximal displacement and kinematic pattern, and a paired t-test performed to evaluate differences between sides. The data was found to be highly repeatable with mean r values for the three directions over 0.85 for both sides. The pattern of patellar displacement and maximal displacement was comparable to that reported in the literature. Results of paired t-test revealed no significant differences between the knees when comparing maximal displacement in any direction (p>0.05). Overall, there was a highly symmetrical kinematic pattern for ML and PD displacement and a small degree of asymmetry for AP displacement. However, wide variations among the subjects were noted between the left and right patellae in all directions. The results of this study have demonstrated that the use of magnetic tracking sensors provides repeatable and relevant three-dimensional kinematic data of in vivo patella tracking from 0° to 60° of knee flexion.