1 Aesculap AG Research and Development, 78532 Tuttlingen, Germany ; Department of Orthopaedic Surgery, University Hospital of Munich (LMU), Campus Grosshadern, 81377 Munich, Germany.
2 Department of Orthopaedic Surgery, University Hospital of Munich (LMU), Campus Grosshadern, 81377 Munich, Germany.
3 Section of Tribology, Rush University Medical Center, Chicago, IL 60612, USA.