Arlington, Virginia.An Instructional Course Lecture, The American Academy of Orthopaedic Surgeons.*Printed with permission of The American Academy of Orthopaedic Surgeons. This article will appear in Instructional Course Lectures, Volume 47, The American Academy of Orthopaedic Surgeons, Rosemont, Illinois, March 1998.**No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.(Engh, Ammeen) Anderson Orthopaedic Research Institute, 2445 Army Navy Drive, Arlington, Virginia 22206.