Instantaneous axes of rotation of the typical cervical motion segments: I. an empirical study of technical errors.

The reliability of the conventional technique for determining the instantaneous axes of rotation of the cervical motion segments was evaluated by comparing the determinations made by two observers and the same observer on two separate occasions. Different observers found the same mean location for the instantaneous axis at each segment in a test population of 17 normal subjects, but for any given subject the interobserver and intraobserver differences were large, with a relative variation of up to 35%. Thus, while reliable for determining the mean location of the instantaneous axis of rotation in a population, the technique was not reliable for determining the axis in a given subject. Qualitative analysis suggested that the identification of the radiographic images of the vertebrae and their tracing is the greatest source of error for this technique.