1. McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, 424 Stemmler Hall, 36th Street and Hamilton Walk, Philadelphia, PA 19104 e-mail:
2. McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, 424A Stemmler Hall, 36th Street and Hamilton Walk, Philadelphia, PA 19104 e-mail:
3. McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, 426C Stemmler Hall, 36th Street and Hamilton Walk, Philadelphia, PA 19104 e-mail: