Instantaneous axes of rotation of the typical cervical motion segments: II. optimization of technical errors.

The reliability of a modified protocol for plotting the instantaneous axes of sagittal rotation for the cervical spine was evaluated by measuring the observer differences when the process was performed separately by two observers, and by a single observer, on two separate occasions. Small observer differences were found both for individual steps in the process and for the process as a whole. These differences were substantially less than those found using the conventional technique for plotting the instantaneous axes of rotation. The improvement in the technique was resultant from the use of stricter criteria for recognizing and tracing vertebral landmarks.