1. Orthopedic Research Laboratory, University of Zurich, Forchstrasse 340, Balgrist, CH-8008 Zurich, Switzerland. Fax: +41 44 386 1109; Tel: +41 44 386 3755
2. Institute for Biomechanics, ETH Zurich, Zurich, Switzerland
3. RMS Foundation, Bettlach, Switzerland
4. Laboratory of Cell Biophysics, Ecole Polytechnique Fédérale de Lausanne, Switzerland