REVISION PEDICLE SCREWS - BIGGER, LONGER SHIMS - WHAT IS BEST

Study Design. To evaluate the effect of change in screw dimensions and hole augmentation in pedicle screw revisions, the insertional torque was determined, and results were compared with those in control specim ens in an in vitro study using cadaveric thoracolumbar spines. Objecti ves. To determine the best method of salvage for failed pedicle screws , by evaluating the insertional torque after placing a larger diameter or longer screw into a stripped hole. Use of a shim and use of larger and longer screws were also investigated. Finally, the effect on inse rtional torque of simply removing and replacing a pedicle screw in its original hole was investigated. Summary of Background Data. The effec ts of using bigger or longer screws and shims to salvage failed pedicl es have been studied. The interaction between how much larger, how muc h longer, and inserting with or without shims, has not been well studi ed. Optimizing reinsertional torque through the use of bigger screws r isks exceeding the pedicle capacity. Using longer screws risks violati on of the anterior vertebral body, thereby placing the great vessels a nd viscera at risk. By knowing the relative contribution of increase i n length and diameter, the surgeon can optimize the risk-benefit ratio . Methods. Eight cadaveric spines from T10 to S1 were harvested. The s pecimens underwent radiographic screening and bone densitometry. A mod ified Latin square randomization was designed to evaluate the screw di ameters and lengths. Each pedicle was its own control. A 35- x 6.5-mm screw was used as a control. Test screws were placed after pedicle scr ew hole failure was achieved and documented by stripping. For the test screws, the diameters were increased by 1 mm and 2 mm, the lengths we re increased by 5 mm and 10 mm. Shims were added randomly. The peak in sertional torque was measured for each control screw and test screw pl acement. In addition, during each screw placement, the screw was remov ed and replaced to determine the effect. Results. Insertional torque, after the pedicle screw is removed and replaced in the same hole, was decreased by 34% (P < 0.000005). Increasing the diameter of the salvag e screw by 2 mm caused the insertional torque to be increased by 8.4% of the original. Increasing the length of the screw did not improve th e salvage screw insertional torque. There was an interaction effect fo r the l-mm increase in diameter and the increase in length. At this di ameter, increasing the length had a significant effect (P = 0.009) on the salvage torque. Using a shim created no improvement in salvage ins ertional torque (P = 0.77). There was a poor linear correlation betwee n torque and bone mineral density (r = 0.18) in these osteoporotic spe cimens. Conclusions. Removing and replacing a pedicle screw in its ori ginal hole substantially decreases its mechanical fixation. For pedicl e salvage, increasing the diameter causes the greatest restoration of strength. Shims had no effect in pedicle salvage in osteoporotic speci mens.