Comparison of pullout strength of pedicle screws following revision using larger diameter screws

Pedicle screw fixation and fusion are the gold standard for the treatment of spinal instability. Screw failures such as pullout and breakages have been reported during the past several years of research and development in this field. Further, the rate of revision surgeries due to failed pedicle screws is around 2-12%. This creates unavoidable hardship to the patients. Improper screw size for revision surgery can lead to complications such as pedicle fractures, screw pullout, or reduced stability of the fusion construct. We performed pullout strength studies on five osteoporotic lumbar vertebra and a rigid polyurethane foam block to find the effect of the outer diameter of revision screws as per American Standards for Testing of Materials (ASTM) 543-07 protocol. The present study revealed that whereas the use of revision screws that were one millimeter greater in diameter than the original screws decreased the pullout strength by 79% in the foam model, the pullout strength increased by 121% when the original index screws were replaced with screws that were two millimeters greater in diameter. The effect of revision screw diameter on pullout strength was significant (p < 0.05). Cadaveric testing reveals a trend that agrees with the foam model tests.