Internal forces and moments in transpedicular spine instrumentation. The effect of pedicle screw angle and transfixation--the 4R-4bar linkage concept.

The three-dimensional components of force and moment within the plates and screws of a bilevel transpedicular spine implant construct subjected to different physiological loads were determined by experimental and finite element methods. The effect of pedicle screw angle and transfixation were studied. Untransfixed 0 degrees pedicle-to-pedicle (P-P) angle constructs with limited screw-bone torsional resistance are unstable 4R-4bar linkages. They will not resist lateral load or (when not in a rectangular position) axial load until the spinal column load shares. Untransfixed constructs with (0 degrees less than P-P angle less than 60 degrees) are structures. However, as P-P angle approaches 0 degrees, the structure becomes more flexible (unstable) and some internal force and moment components exponentially increase (starting at approximately a 30 degrees P-P angle). Transfixation eliminated the linkage instability and associated exponential increase in internal loads. These observations apply to all bilevel systems that allow no relative joint motion between pedicle screw and longitudinal member. If relative motion does exist, other types of linkage instability can occur.