EFFECTS OF BONE-MINERAL DENSITY ON PEDICLE SCREW FIXATION

Study Design. In an attempt to evaluate the effects of bone mineral de nsity on the quality of fixation of pedicle screws in the lumbar spine , the axial pullout force was determined and compared in normal and os teoporotic human lumbar spines. Objectives. Four techniques of screw h ole preparation were evaluated. Two pedicle screw/offset laminar hook constructs also were evaluated to determine whether the adjunct fixati on of the laminar hooks would improve quality of fixation to a level s ufficient to allow their use in the osteoporotic lumbar spine.Methods. Pedicle screws were inserted by one of the listed techniques into fre sh frozen cadaveric human spines. The fixation strength then was evalu ated by pullout on a uniaxial testing frame. Results. Bone mineral den sity was a strong influence on axial pullout force. In normal bone, th e method of screw hole preparation did not significantly affect the qu ality of fixation. However, in the osteoporotic spine, either an untap ped screw hole or the tapping of a screw hole with a 5.5 mm tap improv ed the pullout force a statistically significant amount (P < 0.003). A lso, a pedicle screw with offset hooks at two adjacent levels improved the fixation significantly, increasing the pullout force to twice the expected value. Conclusion. Pedicle screw pullout strength was highly correlated with bone mineral density. A 5.5 mm tap or preparation wit h a ganglion knife improved pullout strength. Use of pedicle screws in conjunction with laminar hooks at two levels improved pullout strengt h.