A Classification System of the Extent of Femoral Head Osteonecrosis Using MRI - Kyung-Hoi Koo(구경회); In-Oak Ahn(안인옥); Hae-Ryong Song(송해룡); Soon-Taek Jeong(정순택); Hyung-Bin Park(박형빈); Sanjay Dhar(신자이 다르); Se-Hyun Cho(조세현) (The journal of the Korean Orthopaedic Association, 1998)