 Radiostereometric Analysis, or RSA, is an accurate in vivo measurement technique using two simultaneous radiographs. It provides researchers with three dimensional motion analyses to look not only at routine flexionextension, but also other rotational and translational changes. The measurement accuracy offered by this technique far exceeds the manual techniques currently used. Utilization of the RSA technology at the IHP and the United States is in its infancy. The investigators have placed much effort over the past 2-3 years to put the necessary dedicated resources and personnel in place. This has been a collaborative effort between the Department of Orthopedic Surgery and the Imaging Core at IHP. Preliminary work on the accuracy, reliability, and surgical planning for use of RSA has been completed and presented at national conferences. 