1. Department of Orthopaedic Surgery, Seoul National University Children’s Hospital, 28 Yeongeon-dong Jongno-gu, Seoul 110-744, Korea.
2. Department of Orthopaedic Surgery, Seoul National University, Bundang Hospital, 300 Gumi-Dong, Bundang-gu, Seongnam-si, Gyeonggi-do, 463-707, Korea.
3. Department of Statistics, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea.