1. McKay Orthopaedic Laboratory, University of Pennsylvania, Philadelphia, PA 19104
2. McKay Orthopaedic Laboratory, University of Pennsylvania, Philadelphia, PA 19104;
3. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138;
4. Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02115
5. McKay Orthopaedic Laboratory, University of Pennsylvania, Stemmler Hall, 3450 Hamilton Walk, Philadelphia, PA 19104 e-mail: