Biomechanical analysis of screw load sharing in pedicle fixation of the lumbar spine

Segmental fixation of the spine by means of rods or plates and pedicle screws posteriorly usually results in a stable and rigid construct. The magnitude of the stresses on the instrumentation particularly at the bone-screw and rod (or plate) interfaces will depend on the load distribution between the bone and the implant as well as the number of sites of bone purchase of the implant. If a fusion is to be obtained in the case of a degenerative spine, the role of the instrumentation is to prevent translation of the motion segments, thereby allowing the compressive loads to be transmitted through the vertebral bodies and the degenerated discs. In the case of a fracture, the instrumentation is made to withstand the bulk of the loads since the structural integrity of a motion segment may have been lost. This study was undertaken to evaluate the effect of different constructs on the stresses in Cotrel-Duboussett (C-D) pedicle (tulip) screws close to their junction with the rod bridging the motion segments to be immobilized. In order to minimize the effect of anatomical and material property variation between spines, adult porcine spines were used, thus providing a reproducible experimental model. Fresh 3-year-old porcine spines were potted in holders after the soft tissues were removed (with the exception of the ligamentous structures).(ABSTRACT TRUNCATED AT 250 WORDS)