Kinematic analysis of the relationship between sagittal alignment and disc degeneration in the cervical spine

Retrospective analysis using kinetic magnetic resonance images (MRIs). To investigate the relationship of changes in the sagittal alignment of the cervical spine on the kinematics of the functional motion unit and disc degeneration. Normal lordotic alignment is one of the most important factors contributing to effective motion and function of the cervical spine. Loss of normal lordotic alignment may induce pathologic changes in the kinematics and accelerate degeneration of the functional motion unit. However, the relationship of altered alignment on kinematics and degeneration has not been evaluated. Kinetic MRIs in flexion, neutral, and extension were performed. Study participants were classified into 5 groups based on the C1-C7 Cobb angle of sagittal alignment--Group A: Kyphosis (n = 19), Group B: Straight (n = 29), Group C: Hypolordosis (n = 38), Group D: Normal (n = 63), and Group E: Hyperlordosis (n = 52).Intervertebral disc degeneration was graded (Grades 1-5), and the kinematics of the functional spinal unit were obtained. When the alignment shifted from normal to less lordotic, the translational motion and angular variation tended to decrease at all levels. The contribution of the C1-C2, C2-C3, and C3-C4 levels to total angular mobility tended to be higher in Group C than Group D. However, the contribution of the C4-C5, C5-C6, and C6-C7 levels tended to be lower in Group C than in Group D. The grade of disc degeneration associated with loss of lordosis tended to be higher than that associated with normal alignment at the C2-C3 and C3-C4 levels. The present study demonstrated that the changes in sagittal alignment of the cervical spine affect the kinematics. Consequently, it may cause changes in the segment subjected to maximum load for overall motion and accelerate its degeneration.