The intervertebral flexion-extension kinematics of the neck are widely studied, but most of the range of motion analyses do not quantify the neutral neck position. Therefore, the influence that the neutral position might have on the kinematics is usually ignored. To improve the understanding and the prediction of intervertebral motion, this study was designed to measure the intervertebral range of motion with 3D geometries obtained from biplanar X-rays, and to explore the impact of the neutral posture on maximal flexion and extension.

The subject-specific 3D geometries of the cervical spines (C1-C7) of thirteen male and female volunteers were studied in full flexion, neutral position, and full extension. Biplanar X-ray images were obtained with an EOS low-dose system. The intervertebral values were measured as the angles between the transversal vertebral body midplanes for each vertebral level.

The intervertebral neutral position correlated with C3-C7 full flexion and full extension. The cervical levels exhibiting greater extension in the neutral position achieved a lower range of motion in full extension (   ). Similarly, higher flexion in the neutral position resulted in a reduced intervertebral range of motion in full flexion (   ).

Variations in the neutral position may affect full flexion and extension range of motion. We believe it is the first time that these linear relationships considering the neutral neck position are presented in the literature. Considering the neutral position can help more accurately predict kinematics at the end range of the motion.