1Department of Orthopacdics, University of Washington, Seattle, WA 98195, U.S.A.
2Orthopaedic Research Laboratories, Massachusetts General Hospital, Roston, MA 02114, U.S.A.
3Deparment of Orthopaedics, Loyola University Stritch School of Medicine, Maywood, IL 60153, U.S.A.
4Department of Mechanical Engineering, University of Washington, Seatle, WA 98195, U.S.A.