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 45, The American Academy of Orthopaedic Surgeons, Rosemont, Illinois, March 1996.** 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.*** Yale University School of Medicine, Box 208071, New Haven, Connecticut 06520.**** Department of Orthopaedic Surgery, Vanderbilt University Medical Center, 21st Avenue, Nashville, Tennessee 37232.***** Medical University of South Carolina, 171 Ashley Avenue, Charleston, South Carolina 29425.****** Cornell University Medical College, The Hospital for Special Surgery, 535 East 70th Street, New York, N.Y. 10021.