In biomechanics of the knee joints, the definition of the cruciate ligament adhesion areas is discussed for the cruciate ligament reconstruction. Effective information can be expected to be fed back to the cruciate ligament reconstruction if accurate movement of the natural knee joints can be analyzed. Therefore, this study performed in vivo six-degree-of-freedom (6-DOF) motion analysis for the natural knee joints with high precision using an image matching technique. The image matching technique was performed determining the strength of correlation between the X-ray images from a single-plane flat panel detector (FPD) and the computational simulated image from three-dimensional gray-scale model reconstructed using computed tomography (CT). Average root mean square errors of this technique were 0.84 mm for the in-plane translation, 0.17 mm for the out-of-plane translation, and 0.29 degrees for the rotation. We report analysis results for the natural knee joints in squat activity using this technique. Subjects were the natural knee joints of four healthy men. We assessed the four items for the motion pattern and the functionality of cruciate ligament. The roll-back motion of subluxation in high-flexion, and the importance of anteromedial (AM) bundle and posterolateral (PL) bundle in the anterior cruciate ligament (ACL) could be confirmed from the results. From these, this highly accurate technique can be expected to become a valuable tool for the medical device development and the diagnostic in field of orthopedics.
