Prof. Siebert was the previous Medical Director of Vitos Orthopedic Center Kassel and Chief of Adult Hip and Knee Reconstruction from 1994 to 2019. Vitos Orthopedic Center Kassel, led by Professor Siebert, has grown into one of the largest arthroplasty centers in Germany. Besides, Professor Siebert has been ranked Germany's top hip and knee surgeons for eight consecutive years in "FOCUS" list.
• 2018: Congress President of German Orthopedic and Trauma Society (DKOU)
• Since 2018: President Elect of the German Orthopedic Society (DGOOC)
• 2017: President Elect of the German Knee Society (DKG)
• 2014-2016: President of the European Hip Society (EHS)
• Chief Editor in of the German Orthopedic Journal OUP