**8. Acknowledgements**

This study was supported by Internal Grant Agency Ministry of Health, Czech Republic (NT/11049-5), the Sigrid Juselius Foundation, by the Helsinki University Central Hospital evo-funds, Finska Läkaresällskapet, Wilhelm och Else Stockmanns Stiftelse, ORTON Orthopaedic Hospital of the Invalid Foundation, National Doctoral Programme of Musculoskeletal Disorders and Biomaterials, Danish Council for Strategic Research, European Science Foundation "Regenerative Medicine" RNP, and by Grant 1R01AR055650- 04 from the National Institute of Health and the French Granting Agency E.F.M.C. N°1. We apologize to all authors whose work could not be cited due to space limitations.
