Fig. 2

shows the analysis marking points. Four metal balls on pliable wires attached to a pair of glasses served as external markers for occiput (C0), the central areas of the medullary cavities of the anterior and posterior arch were marked on atlas (C1). Two points in proximity to the inferior vertebral plate were marked on axis (C2). The third to the sixth cervical vertebrae (C3-C6) were marked with 4 points in proximity to the vertebral plates. The seventh cervical vertebra (C7) was marked with two points in proximity to the superior vertebral plate. The mid-planes were calculated from the marking points, and joint angles were further calculated between mid-planes. The author XW created the skeletal illustrations