aDepartment of Orthopaedic Surgery, Keio University School of Medicine, Tokyo, Japan

bJapanese Multicenter Research Organization for Ossification of the Spinal Ligament, Japan

cDepartment of Orthopedic Surgery, Tokyo Medical and Dental University, Tokyo, Japan

dDepartment of Orthopedic Surgery, Saiseikai Kawaguchi General Hospital, Saitama, Japan

eDepartment of Orthopedic Surgery, Kudanzaka Hospital, Chiyadaku, Japan

fDepartment of Orthopaedic Surgery, Wakayama Medical University, Wakayama, Japan

gDepartment of Orthopedic Surgery, Hirosaki University Graduate School of Medicine, Aomori, Japan

hDepartment of Orthopedic Surgery, Niigata University Medical and Dental General Hospital, Niigata, Japan

iDepartment of Orthopedic Surgery, Faculty of Medicine, University of Tsukuba, Tsukuba, Japan

jDepartment of Orthopedics, Jichi Medical University, Tochigi, Japan

kDepartment of Orthopedic Surgery, Chiba University Graduate School of Medicine, Chiba, Japan

lDepartment of Orthopedic Surgery, Yamaguchi University Graduate School of Medicine, Yamaguchi, Japan

mDepartment of Orthopedic Surgery, Osaka Rosai Hospital, Osaka, Japan

nDepartment of Orthopaedic Surgery, Faculty of Medicine, The University of Tokyo, Tokyo, Japan

oDepartment of Orthopedic Surgery, Nagoya University Graduate School of Medicine, Nagoya, Aichi, Japan

pDepartment of Orthopaedic Surgery, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, Sapporo, Japan

qDepartment of Orthopaedic Surgery, Shiga University of Medical Science, Tsukinowa-cho, Shiga, Japan

rDepartment of Orthopaedics and Rehabilitation Medicine, Faculty of Medical Sciences University of Fukui, Fukui, Japan

sDepartment of Orthopedic Surgery, Tokyo Medical University, Tokyo, Japan

tDepartment of Orthopedic Surgery, Kitasato Universiy Hospital, Kanagawa, Japan

uDepartment of Orthopedic Surgery, Graduate School of Medicine, Osaka UniversityOsaka, Japan

vDepartment of Orthopaedic Surgery, Kurume University School of Medicine, Fukuoka, Japan

wDepartment of Orthopedic Surgery, Hamamatsu University School of Medicine, Shizuoka, Japan

xDepartment of Orthopaedic Surgery, Graduate School of Medical Sciences, Kanazawa University, Kanazawa, Japan

yDepartment of Orthopedic Surgery, University of Yamanashi, Yamanashi, Japan

zDepartment of Orthopaedic Surgery, Dokkyo Medical University School of Medicine, Tochigi, Japan

aaDepartment of Orthopaedic Surgery, Graduate School of Medicine, Kyoto University, Kyoto, Japan

bbDepartment of Orthopedic Surgery, Surgical Science, Tokai University School of Medicine, Kanagawa, Japan

ccDepartment of Orthopaedic Surgery, Tohoku University School of Medicine, Miyagi, Japan

ddDepartment of Orthopedic Surgery, Faculty of Medicine, University of Toyama, Toyama, Japan

This study was approved by each institutional review board.

This study was supported by Japan Agency for Medical Research and Development and Japanese Health Labor Sciences Research Grant.

The authors report no conflicts of interest.

Address correspondence and reprint requests to Narihito Nagoshi, MD, PhD, Department of Orthopaedic Surgery, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan; E-mail: [email protected]