Three-dimensional biomechanical properties of the human cervical spine in vitro. II. Analysis of instability after ligamentous injuries.

We wanted to determine the biomechanical properties of the human cervical spine with ligamentous injuries. The three-dimensional displacements under four pure moments--flexion, extension, left-right lateral bending and left-right axial torsion--were measured in 18 functional spinal units (FSUs) taken from 9 human cadaveric cervical spines. The experimentation was first performed with intact FSUs, then a series of ligamentous injuries were artificially created. The same measurements for the same FSUs were repeated after each step of ligamentous injury. For each mode of loading and each step of injury, three-dimensional load-displacement curves were plotted. From these curves, we calculated the following parameters for the principal motion: the neutral zone, the range of motion and the flexibility coefficients. A statistical analysis of these parameters was performed between intact FSUs and different ligamentous injury situations. The analysis of ligamentous injuries shows the consequences of different ligamentous injuries and will help us to assess certain assumptions about clinical stability.