• Wei Ren Daniel Seng, Siaw Meng Chou, Shiblee S Siddiqui, and Jacob Y L Oh.
• Department of Orthopaedic Surgery, Tan Tock Seng Hospital, National Health Group, Nanyang, Singapore.
• Spine. 2019 Feb 1; 44 (3): E144-E149.

Study DesignAn experimental laboratory-based biomechanical study.ObjectiveThe 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.Summary Of Background DataOver 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.MethodsThe 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.ResultsThe 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.ConclusionThe 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.Level Of Evidence5.

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