[Biomechanical analysis of lumbar spinal instrumentations]

Five different types of spinal instrumentation (Steffee, Luque, Zielke, A-O anterior screw and wiring, and A-O anterior and posterior screw and wiring) were tested for biomechanical performance (i.e., relative initial stiffness, the loosening generated in the bone-implant interface during cyclic loading, and the influence on bone calcium content on the first two) using porcine vertebrae. Steffee's instrumentation produced the stiffest construction, followed by A-O anterior and posterior screw and wiring. A positive correlation was noted between the initial stiffness and bone calcium content. With low calcium, cyclic loading caused early loosening in the Steffee's instrumentation while the A-O anterior and posterior screw and wiring maintained its stability. It was concluded that the stability of these instrumentations was dependent on the calcium content and for sustained stability, when the calcium was low, the A-O anterior and posterior screw and wiring was superior to Steffee's.