A biomechanical study of anterior thoracolumbar screw fixation

Abstract

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.