Kinematics of the subaxial cervical spine in rotation in vivo three-dimensional analysis.

STUDY DESIGN: Three-dimensional intervertebral motions of the subaxial cervical spine during head rotation were investigated in healthy volunteers using three-dimensional magnetic resonance imaging (MRI).

OBJECTIVES: To document intervertebral coupled motions of the subaxial cervical spine during rotation.

SUMMARY OF BACKGROUND DATA: In vivo three-dimensional kinematics of the subaxial cervical spine in rotation have not previously been well described, since they are too complicated to follow using conventional radiography or computed tomography techniques.

METHODS: Ten healthy volunteers underwent three-dimensional MRI of the cervical spine in 11 positions with 15 degrees increments during head rotation using a 1.0-T imager. Relative motions of the subaxial cervical spine were calculated by automatically superimposing a segmented three-dimensional MRI of the vertebra in the neutral position over images of each position using volume registration. Three-dimensional motions of adjacent vertebrae were represented with 6 df (6 degrees of freedoms) by Euler angles and translations on the coordinate system defined by Panjabi, then visualized in animations using surface bone models.

RESULTS: Mean axial rotation of the subaxial cervical spine in maximum head rotation (69.5 degrees ) was 2.2 degrees at C2-C3, 4.5 degrees at C3-C4, 4.6 degrees at C4-C5, 4.0 degrees at C5-C6, 1.6 degrees at C6-C7, and 1.5 degrees at C7-T1. Coupled lateral bending with axial rotation was observed in the same direction as axial rotation at all levels (C2-C3, 3.6 degrees ; C3-C4, 5.4 degrees; C4-C5, 5.0 degrees ; C5-C6, 5.3 degrees ; C6-C7, 4.9 degrees ; C7-T1, 1.2 degrees ). Coupled extension with axial rotation occurred in the middle cervical region (C2-C3, 1.4 degrees ; C3-C4, 2.3 degrees ; C4-C5, 1.5 degrees ), while in the lower cervical region, flexion was coupled with axial rotation (C5-C6, 0.9 degrees ; C6-C7, 2.4 degrees ; C7-T1, 3.0 degrees ).

CONCLUSIONS: We investigated intervertebral motions of the subaxial cervical spine during head rotation using a three-dimensional imaging system, and obtained the first accurate depictions of in vivo coupled motion. These findings will be helpful as the basis for understanding abnormal conditions.