The effect of the three columns of the spine on the instantaneous axis of rotation in flexion and extension.

Instrumentation designed for stabilization and correction of spinal deformities must limit the amount of motion in flexion and extension. In flexion or extension, the vertebral bodies move about a specific point called the instantaneous axis of rotation. The ability of the implant to limit this motion is a function of its relation to the axis of rotation of the spine. The goal of this study was threefold: 1) to define the instantaneous axis of rotation of the spine in flexion and extension; 2) to study the effect of the loss of the three columns of the spine on the location of the instantaneous axis of rotation; and 3) to determine how the above parameters relate to the choice of anterior or posterior instrumentation. Ten human cadaver spines were subjected to compressive loads in flexion and extension. The columns of the spine were then destroyed in sequence at L3. The instantaneous axis of rotation for each vertebral body was found by the method of Reuleaux, and the effect of the compromise of the columns on the location of the instantaneous axis of rotation was noted. Understanding the exact location of the instantaneous axis of rotation after a specific injury would allow the clinician to objectively choose the best surgical approach and the appropriate instrumentation.