Instantaneous axes of rotation of the typical cervical motion segments: a study in normal volunteers.

The locations of the instantaneous axes of rotation for the cervical motion segments C2-3 to C6-7 were determined from flexion-extension radiographs of 40 normal subjects using a modified overlay technique. The biological variation of the instantaneous axes of rotation was small, as was the technical error associated with the technique used. The data obtained enabled the formal definition of the normal range of locations for the instantaneous axes of rotation of the typical cervical motion segments.