1. Institute of Clinical Medicine, University of Eastern Finland, Kuopio, Finland
2. Department of Physical and Rehabilitation Medicine, Kuopio University Hospital, Kuopio, Finland
3. Department of Surgery, Mutual Insurance Company Etera, Helsinki, Finland
4. Department of Neurosurgery, Helsinki University Hospital, Helsinki, Finland
5. Department of Physical and Rehabilitation Medicine, Helsinki University Hospital, Helsinki, Finland
6. Department of Surgery, University of Helsinki, Helsinki, Finland