Measurements of the craniocervical angulation. S, sella point; N, nasion point; CV2tp, the most posterior point of the odontoid process; CV2ip, the most inferior and posterior point of the body of the second cervical vertebra; CV4ip, the most inferior and posterior point of the body of the fourth cervical vertebra; NSL, the plane passing through the S point and the nasion point; OPT, the plane tangent to the odontoid process (CV2tp point) passing through the CV2ip point; CVT, the plane tangent to the odontoid process (CV2tp point) passing through the CV4ip point.