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 physiolog-ical loads were determined by experimental and finite element methods. The effect of pedicle screw angle and transfixation were studied. Untransfixed 0° 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° < P-P angle < 60°) are structures. However, as P-P angle approaches 0°, the structure becomes more flexible (unstable) and some internal force and moment components exponentially increase (starting at approximately a 30° P-P angle). Transfixation eliminated the linkage instability and asso-ciated exponential increase in internal loads. These ob-servations 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. © Lippincott-Raven Publishers.