In Vivo 3D Intervertebral Motion Analysis of the Cervical Spine in Lateral Bending using 3D-MRI

INTRODUCTION In vivo 3D motion of the cervical spine has not been documented until now, since they are too complicated to follow with conventional radiographs or CT. Although in vitro studies using cadaver specimens reported data on the cervical 3D motion, it might not actually reflect physiological motion due to the lack of tonus of musculature. Moreover, understanding 3D motions with only a numerical combination of rotations and translations was difficult. To overcome these problems, we developed a quite unique in vivo 3D motion analysis system using 3D-MRI, which can make the 3D animation of the motion, and reported the kinematics of the cervical spine in rotation using this system [1,2]. The purpose of this study is to demonstrate for the first time in vivo 3D intervertebral motions of the cervical spine in lateral bending. METHODS Twelve healthy volunteers underwent 3D-MRI of the cervical spine in 7 positions with 10° increments during lateral bending using a 1.0-T imager. Relative motions of the cervical spine were calculated by automatically superimposing a segmented 3D-MRI of the vertebra in the neutral position over images of each position using volume registration, which is a method to determine relative position between two volume images by means of superimposing two 3D images to make each voxel value coincide each other maximally, and correlation coefficient was used as similarity measure. Threedimensional motions of adjacent vertebrae were represented with six degrees of freedoms by Euler angles and translations on the coordinate system defined by Panjabi, and visualized in animations using surface bone models reconstructed with marching cubes algorithm in the Visualization Toolkit (VTK). As we have already declared, the accuracy of this system was 0.24° for flexion-extension, 0.31° for lateral bending, and 0.43° for axial rotation. Mean absolute translational error was 0.52 mm for superoinferior translation, 0.51 mm for anteroposterior translation, and 0.41 mm for lateral translation. [1]. RESULTS AND DISCUSSION Mean maximum lateral bending of the cervical spine to one side in maximum head lateral bending (30.3°) was 1.9° at Oc/C1, 1.6° at C1/2, 3.7° at C2/3, 3.5° at C3/4, 3.3° at C4/5, 4.3° at C5/6, 5.7° at C6/7, and 4.1° at C7/T1. C6/7 showed the larger lateral bending than other levels (P < 0.05). Coupled axial rotation in the opposite direction to that of lateral Figure 1: Cervical motion in left and right lateral bending