1Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Columbia University, New York, New York 10032, U.S.A.
2Department of Mathematical Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, U.S.A.
3Orthopaedic Research Laboratory, Departments of Orthopaedic Surgery and Mechanical Engineering, Columbia University, New York, New York 10032, U.S.A.