1Orthopedic Research Laboratories, Harvard Medical School, 300 Longwood Avenue, Boston, Massachusetts 02115, U.S.A.
2Department of Mechanical Engineering, M. I. T., 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, U.S.A.
3Department of Metallurgy and Materials Sciences, M. I. T., 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, U.S.A.