Normal intervertebral segment rotation of the subaxial cervical spine: An in vivo study of dynamic neck motions

Abstract

Background: Accurate knowledge of the intervertebral center of rotation (COR) and its corresponding range of motion (ROM) can help understand development of cervical pathology and guide surgical treatment.

Methods: Ten asymptomatic subjects were imaged using MRI and dual fluoroscopic imaging techniques during dynamic extension-flexion-extension (EFE) and axial left-right-left (LRL) rotation. The intervertebral segment CORs and ROMs were measured from C34 to C67, as the correlations between two variables were analyzed as well.

Results: During the EFE motion, the CORs were located at 32.4 ± 20.6%, -2.4 ± 11.7%, 21.8 ± 12.5% and 32.3 ± 25.5% posteriorly, and the corresponding ROMs were 13.8 ± 4.3°, 15.1 ± 5.1°, 14.4 ± 7.0° and 9.2 ± 4.3° from C34 to C67. The ROM of C67 was significantly smaller than other segments. The ROMs were not shown to significantly correlate to COR locations (r = -0.243, p = 0.132). During the LRL rotation cycle, the average CORs were at 85.6 ± 18.2%, 32.3 ± 25.3%, 15.7 ± 12.3% and 82.4 ± 31.3% posteriorly, and the corresponding ROMs were 3.5 ± 1.7°, 6.9 ± 3.8°, 9.6 ± 4.1° and 2.6 ± 2.5° from C34 to C67. The ROMs of C34 and C67 was significantly smaller than those of C45 and C56. A more posterior COR was associated with a less ROM during the neck rotation (r = -0.583, p < 0.001). The ROMs during EFE were significantly larger than those during LRL in each intervertebral level.

Conclusion: The CORs and ROMs of the subaxial cervical intervertebral segments were segment level- and neck motion-dependent during the in-vivo neck motions.

The translational potential of this article: Our study indicates that the subaxial cervical intervertebral CORs and ROMs were segment level- and neck motion-dependent. This may help to improve the artificial disc design as well as surgical technique by which the neck functional motion is restored following the cervical arthroplasty.

Keywords: Center of rotation; Cervical spine; In-vivo spine kinematics; Intervertebral segments; Range of motion; Total disc replacement.

Figure 1

(A) The 3D cervical vertebral models and invivo cervical spine motion showing extension-flexion-extension (EFE) motion; (B) the 3D cervical vertebral models and in-vivo cervical spine motion showing axial left-right-left (LRL) rotation.

Figure 2

(A) The average intervertebral segment COR locations are calculated using the endplate coordinate systems. The red spheres represent the geometric centres of the intervertebral segments, the green spheres represent the average CORs during the flexion-extension motion and the purple spheres represent the average CORs during the left-right rotation of each intervertebral segment; (B) the coordinate system of an endplate. The upper endplate surface of the inferior vertebra was used as a reference for calculation of the relative motion of the lower endplate of the superior vertebra at each intervertebral segment.

Figure 3

(A) The correlations between the COR locations (%) and corresponding ROMs (°) during the EFE motion of the neck; (B) the correlations between the COR locations (%) and corresponding ROMs (○) during the LRL rotation of the neck. The “0” on the COR axis represents the location of disc centre. The −50% represents the anterior edge of disc, whereas the +50% represents the posterior edge of disc. EFE = extension-flexion-extension; LRL = left-right-left; ROM = range of motion.