1Department of Mechanical Engineering, Massachusetts Institute of Technology. Cambridge, MA 02139, U.S.A.
2Department of Orthopedic Research, Harvard Medical School at the Children’s Hospital Medical Center, Boston, MA 02115, U.S.A.
3Department of Metallurgy and Materials Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
4Present address: Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada.
5Present address: Department of Orthopaedic Surgery, Princess Margaret Rose HospitaI, Edinburgh, EH43L2, Scotland.