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 degrees 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 degrees Relevance The clinically common posture of lateral head translation results in an S-shaped cervical spine and may occur in side impact trauma. This posture has not been studied for cervical coupling patterns or range of motion (ROM). (C) 2000 Elsevier Science Ltd. All rights reserved.