1. Copyright © 2015 Lippincott Williams & Wilkins. Unauthorized commercial reproduction of this slide is prohibited Supplemental PowerPoint Slides Novel two radiographic measurements for atlantoaxial instability in Down syndrome children Naoyuki Nakamura, MD1), Yutaka Inaba, MD, PhD2), Masatoshi Oba, MD1), Yoichi Aota, MD, PhD3), Yogen Morikawa, MD1), Yurika Ata, MD1), Jiro Machida, MD, PhD1), Tomoyuki Saito, MD, PhD2) 1) Department of Orthopedic Surgery, Kanagawa Children's Medical Center, 2) Department of Orthopedic Surgery, School of Medicine, Yokohama City University, 3) Department of Spine and Spinal Cord Surgery, Yokohama Stroke and Brain Center

2. C1 inclination angle C1/4 SAC ratio On lateral radiographs of the cervical spine in the neutral position

3. Correlation between the indices was observed without age factors taken into consideration (r  0.521, p  0.01). Moreover, almost all the patients in the surgical group satisfy both a C1 inclination angle  10  and a C1/4 SAC ratio  0.9.