A biomechanical study of anterior thoracolumbar screw fixation

Study Design. The pullout strength of unicortical and bicortical screws in thoracic and lumbar vertebral bodies was measured as a function of bone mineral density. Objectives. To determine the influence of bone mineral density and screw insertion technique on the stability of anterior thoracolumbar spine screw fixation. Summary of Background Data. No previous study has addressed the specific technique of screw insertion or stability of screw fixation in the anterior spine. Methods. Fifty-one human thoracic vertebral bodies were tested in pullout with 6.55-mm cancellous screws inserted using unicortical and bicortical techniques. Results. Pullout force increased exponentially with increasing bone mineral density for unicortical and bicortical screws. Bicortical screws were significantly stronger in resisting pullout than unicortical screws. Conclusion. Advancing an anterior vertebral body screw to engage the second cortex increases resistance to pullout by 25-44%, depending on vertebral bone mineral density. The difference in resistance between unicortical and bicortical techniques was smaller in specimens with low mineral densities.