Study Design Three-dimensional flexibility changes due to the application of an external fixator at C4-C5 were studied in cervical spine specimens. Objectives to evaluate the biomechanical effects of applying a cervical external fixator to a patient using an in vitro model. Summary of Background Data There is controversy regarding the relationship between the changes in spinal motion and clinical instability. Methods Using fresh cadaveric C4-C7 specimens, multidirectional flexibility was measured at all vertebral levels, before and after the fixator application at C4-C5, C5-C6, and C4-C6. Results The average ranges of motion for flexion, extension, lateral bending, and axial rotation were 8.3° 7.2°, 5.3°, and 5.6°, which descreased by 40%, 27%, 32%, and 58%, repectively, because of the fixator application. The corresponding neutral zones were 3.4°, 3.4°, 3.0°, and 2.0°, which decreased by 76%, 76%, 54%, and 69%, respectively. The decreases with the fixation at C4-C5 were similar to those for fixation at C5-C6. Conclusions This in vitro study documented that the application of an external fixator to the cervical spine decreases the intervertebral motion in general, and decreases flexion, extension and torsional neutral zones in particular. The findings help explain the clincal instability of the spine and support the hypothesis that the neutral zone is more closely associated with the clinical instability than is the range of motion.