PREDICTING THE INTEGRITY OF VERTEBRAL BONE SCREW FIXATION IN ANTERIORSPINAL INSTRUMENTATION

Study Design. Using 2-week-old calf lumbar vertebrace brae as a model for the human adolescent spine, the strength and rigidity of different methods of anterior spinal screw fixation and the use of noninvasive techniques for predicting bone screw stability before surgery and scre w insertional torque intraoperatively were investigated. Objectives. T he objectives were to determine what factors most effect the strength and rigidity of screw fixation, to determine the strongest and most ri gid type of screw fixation for anterior spinal instrumentation, and to determine if noninvasive measurements of bone density before surgery by dual energy x-ray absorptiometry or quantitative computed tomograph y and if intraoperative measurement of screw insertional torque can be used to predict the in vivo strength and rigidity of vertebral screw fixation. Summary of Background Data. Anterior spinal instrumentation is an efficient method to correct spinal deformities in the thoracolum bar and lumbar spine. Fewer vertebrae are instrumented and arthrodesed , allowing for greater spine mobility. The forces transmitted to each vertebra are higher, perhaps accounting for the clinical failure rate of 13-30% at the metal bone interface from screw cut out. Methods. Qua ntitative computed tomography and dual energy x-ray absorptiometry wer e used to assess the bone density of 24, 2-week-old calf lumbar verteb rae. Four different methods of vertebral screw fixation were evaluated : unicortical screw, bicortical screw, bicortical screw and washer, an d bicortical screws and staple. The maximal screw insertional torque w as measured for each specimen. Each vertebral body-screw construct fai led in a mode simulating in vivo screw cut out. The applied moment and rotatory displacement were recorded. Ash density was measured for eac h vertebral body after removing all hardware. Results. Noninvasive mea sures of bone density varied linearly with ash density (P < 0.01). Scr ew insertional torque varied linearly with bone mineral content and bo ne mineral density (r(2) = 0.50) acid was correlated with the yield mo ment for all types of fixation except the staple. Density measured by quantitative computed tomography did not affect rigidity or yield mome nt. Post hoc analysis showed that the screw-staple construct was the s trongest and most rigid form of fixation. Conclusions. Measurement of bone density before surgery using dual energy x-ray absorptiometry and intraoperative measurement of screw insertional torque can be used to assess the stability of anterior spinal in strumentation. A bicortica l screw inserted through a Dwyer-type staple provided the strongest an d most rigid form of fixation.