Evaluating the accuracy of knee kinematics measured in six degrees of freedom using surface markers

Injury to the anterior cruciate ligament (ACL) of the knee can result in joint instability even following reconstruction. This instability may be quantified by measuring in vivo knee kinematics in six degrees of freedom. Motion capture systems have been used for measuring kinematics but they are limited by system inaccuracies and error resulting from skin movement. Therefore, the overall objective of this thesis was to determine the level of accuracy that a motion capture system using surface markers provides for measuring knee kinematics. There are three specific aims in this thesis. The first specific aim was to mathematically investigate the effect of random errors in marker locations on the accuracy of knee kinematics calculated using the point cluster technique (PCT), triad and Helen Hayes marker sets. The results indicated that the PCT marker set had the greatest potential for accurately measuring knee kinematics. The second aim was to determine how inaccuracies of the motion capture system contribute to errors of joint kinematics measured with the PCT. Despite its high accuracy, the average errors of joint kinematics attributed to the system were up to 1° and 2 mm. The final specific aim was to investigate the efficacy of an algorithm called the interval deformation technique (IDT) for reducing errors of knee kinematics resulting from skin movement. The IDT reduced the errors of kinematics by 90% for an activity with skin movement simulating muscle contraction but was unable to reduce the errors resulting from skin movement at heel strike of gait. The overall errors of knee kinematics resulting from system inaccuracies and skin movement were estimated to be 2° and 4 mm. While this technique may be useful for measuring the changes in knee kinematics that result from ACL injury, this accuracy may not be sufficient to discern the small differences in knee kinematics between ACL intact and reconstructed subjects or for predicting ligament forces. Thus, further research is suggested in order to better quantify skin movement and provide data for improving the accuracy of kinematics measured with surface markers