[The unstable spine--an "in vitro" and "in vivo study" on better understanding of clinical instability].

In cases of suspected painful instability of a cervical segment, temporary external fixation by means of external fixator was applied. The segmental immobilization caused immediate relief of pain. The pain reoccurred after removal of the immobilization. The effect of immobilization by external fixation was investigated in biomechanical tests using fresh cadaveric C4-7 specimens. Multidirectional flexibility was measured before and after application of the fixator at C4/C5, C5/C6 and C4-6. We measured the reduction in motion between the different segments. In every situation the neutral zone decreased more than the range of motion. The findings are helpful to understand the clinical instability of the spine and support the hypothesis that the neutral zone is more closely associated with clinical instability than range of motion. The combination of clinical application and biomechanical investigation allowed us to establish a direct correlation between instability and pain.