Pedicle Screw Designs in Spinal Surgery: Is There a Difference?  A Biomechanical Study on Primary and Revision Pull-Out Strength. 
An experimental laboratory-based biomechanical study.  The objective of this study was to evaluate, in a synthetic bone model, the difference in primary and revision pull-out strength using pedicle screws of different thread designs.  Over the past few decades, there has been a growing interest in optimizing the screw pull-out strength using various screw designs (single-thread, mixed-thread, dual-thread).  Although primary pull-out strength has been studied previously, little is known about revision pull-out strength of different pedicle screws.  The pull-out strengths of three different pedicle screw designs (single-thread, mixed-thread, dual-thread) were tested in standardized polyurethane foam in three sequences.  Sequence 1: A 6.5 mm screw was inserted into the foam block and the primary pull-out strength measured.  Sequence 2: A 6.5 mm screw was inserted, removed, and then reinserted into the same foam block.  The revision pull-out strength was then measured.  Sequence 3: A 6.5 mm screw was inserted, removed and a 7.5-mm screw of the same thread design was reinserted.  The revision pull-out strength was then measured.  The primary pull-out strength was similar across screw designs, although dual-thread screws showed higher primary pull-out strength (2628.8 N) compared to single-thread screws (2184.4 N, P < 0.05).  For revision pull-out strength, the mixed-thread screws had significant reduction in revision pull-out strength of 18.6% (1890.2 N, P = 0.0173).  Revision with a larger diameter screw improved the pull-out strength back to baseline.  Single and dual-thread screws showed no significant reduction in revision pull-out strength.  The dual-threaded screws provided the strongest primary pull-out strength for spinal fixation.  The mixed-thread screws, however, had the poorest revision pull-out strength, decreasing by 18.6% compared to other screw designs.  In cases in which mixed-threaded screws have to be revised (at the index or revision surgery), surgeons should consider the use of larger diameter screws to improve the pull-out strength.  5.