Insertional torque and pull-out strengths of conical and cylindrical pedicle screws in cadaveric bone

Study Design. Insertion torque and pull-out strengths of conical and cylind rical pedicle screws were compared in human cadaveric vertebral bodies. Objectives. To compare the performance of the conical design with the cylin drical design, and to determine whether insertional torque correlates with pull-out strength. Summary of Background Data. A tapered pedicle screw design may lessen the l ikelihood of implant Failure. Its effect on thread purchase is not known. P revious studies of cylindrical designs on the relation between insertion to rque and pull-out strength have been conducted in bovine and synthetic bone . Methods. Seventy-eight pedicles were assigned randomly to one of the follow ing pedicle screw: Texas Scottish Rite Hospital (Sofamor-Danek, Memphis, TN ), Steffee VSP (Acromed, Cleveland, OH), Diapason (Dimso, Paris, France), A O Schanz (Synthes, Paoli, PA), or Synthes USS (Synthes, Paoli, PA). Pedicle screws were inserted with a torque screwdriver. Each screw was extracted a xially from the pedicle at a rate of 1.0 mm/sec until failure using an MTS machine (Bionix 858, Minneapolis, MN). Force data were recorded. Results. The conical design had the highest insertion torque. There were no significant differences in pull-out between any of the screw types. Correl ation between insertional torque and pull-out strength was statistically si gnificant only with the Texas Scottish Rite Hospital and Steffee VSP in L4 and AO Schanz in L5, Conclusions. A conical screw profile increases insertion torque, although i nsertional torque is not a reliable predictor of pull-out strength in cadav eric bone. Screw profile (with similar dimensions) has little effect on str aight axial pull-out strengths in cadaveric bone.