Predicting the integrity of vertebral bone screw fixation in anterior spinal instrumentation

Using 2-week-old calf lumbar vertebrae 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 screw insertional torque intraoperatively were investigated.

The objectives were to determine what factors most affect the strength and rigidity of screw fixation, to determine the strongest and most rigid 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 tomography and if intraoperative measurement of screw insertional torque can be used to predict the in vivo strength and rigidity of vertebral screw fixation.

Anterior spinal instrumentation is an efficient method to correct spinal deformities in the thoracolumbar 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.

Quantitative computed tomography and dual energy x-ray absorptiometry were used to assess the bone density of 24, 2-week-old calf lumbar vertebrae. Four different methods of vertebral screw fixation were evaluated: unicortical screw, bicortical screw, bicortical screw and washer, and bicortical screws and staple. The maximal screw insertional torque was measured for each specimen. Each vertebral body-screw construct failed in a mode simulating in vivo screw cut out. The applied moment and rotatory displacement were recorded. Ash density was measured for each vertebral body after removing all hardware.

Noninvasive measures of bone density varied linearly with ash density (P < 0.01). Screw insertional torque varied linearly with bone mineral content and bone mineral density (r2 = 0.50) and was correlated with the yield moment for all types of fixation except the staple. Density measured by quantitative computed tomography did not affect rigidity or yield moment. Post hoc analysis showed that the screw-staple construct was the strongest and most rigid form of fixation.

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 instrumentation. A bicortical screw inserted through a Dwyer-type staple provided the strongest and most rigid form of fixation.