ON THE UNDERSTANDING OF CLINICAL INSTABILITY

Study Design. Three-dimensional flexibility changes due to the applica tion of an external fixator at C4-C5 were studied in cervical spine sp ecimens. Objectives. To evaluate the biomechanical effects of applying a cervical external fixator to a patient using an in vitro model. Sum mary of Background Data. There is controversy regarding the relationsh ip between the changes in spinal motion and clinical instability. Meth ods. Using fresh cadaveric C4-C7 specimens, multidirectional flexibili ty 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 we re 8.3-degrees, 7.2-degrees, 5.3-degrees, and 5.6-degrees, which decre ased by 40%, 27%, 32%, and 58%, respectively, because of the fixator a pplication. The corresponding neutral zones were 3.4-degrees, 3.4-degr ees, 3.0-degrees, and 2.0-degrees, which decreased by 76%, 76%, 54%, a nd 69%, respectively. The decreases with the fixation at C4-C5 were si milar to those for fixation at C5-C6. Conclusions. This in vitro study documented that the application of an external fixator to the cervica l spine decreases the intervertebral motion in general, and decreases flexion, extension and torsional neutral zones in particular. The find ings help explain the clinical instability of the spine and support th e hypothesis that the neutral zone is more closely associated with the clinical instability than is the range of motion.