Cervical coupling during lateral head translations creates an S-configuration

Objective. To determine cervical coupling during the posture of lateral head translation relative to a fixed thoracic cage. Design. Digitized measurements from anteroposterior cervical radiographs of 20 volunteers were obtained in neutral, left, and right lateral translation posture of the head compared to a fixed thorax. Background data. Clinically, lateral translation of the head is a common posture. Ranges of motion and spinal coupling have not been reported for this movement. Methods. Vertebral body corners, mid-lateral articular pillars and the superior spinous–lamina junction of C3–T4 were digitized on 60 radiographs. Using the orthogonal axis of positive x-direction to the left, vertical as positive y and anterior as positive z, digitized points were used to measure projected segmental z-axis rotation, y-axis rotation, and segmental lateral translations of each vertebra. Results. Subjects translated their heads laterally a mean of 51 mm. The major coupled motion was lateral bending (z-axis rotation), which changed direction at the C4–C5 disc space creating an S-shape. Upper cervical (C3–C4) lateral bending was contralateral to the main motion of head translation direction. Lower cervical and upper thoracic lateral bending were ipsilateral. Other segmental motions averaged less than 1 mm and 1°. Conclusions. Lateral head translations (x-axis) compared to a fixed thoracic cage can be large with a mean of 51 mm to one side. The major spinal coupling was lateral bending which changed direction at C4–C5 resulting in an S-configuration. This might have application in side impacts. All other segmental movements were small, less than 1 mm and 1°.