Revision pedicle screws: Bigger, longer shims - What is best?

David W. Polly, Joseph R. Orchowski, Richard G. Ellenbogen

Research output: Contribution to journalArticlepeer-review

110 Scopus citations

Abstract

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 specimens in an in vitro study using cadaveric thoracolumbar species. Objectives. To determine the best method of salvage for failed pedicled screws, by evaluating the insertional torque after placing a larger diameter or longer screw into a stripped hole. Use of a shim and user of larger and longer screws were also investigated. Finally, the effect on insertional torque of simply removing and replacing a pedicle screw in its original hole was investigated. Summary of Background Data. The effects of using bigger or longer screws and shims to salvage failed pedicles have been studied. The interaction between how much lager, how much longer, and inserting with or without shims, has not been well studied. Optimizing reinsertional torque through the use of bigger screws risks exceeding the pedicle capacity. Using longer screws risks violation of the anterior vertebral body, thereby placing the great vessels and viscera at risk. By knowing the relative contribution of increase in length and diameter, the surgeon can optimize the risk-benefit ratio. Methods. Eight cadaveric spines from T10 to S1 were harvested. The specimens underwent radiographic screening and bone densitometry. A modified Latin square randomization was designed to evaluate the screw diameters and lengths. Each peptide was its own control. A 35- x 6.5-mm screw was used as a control. Test screws were placed after pedicle screw hole failure was achieved and documented by stripping. For the test screws, the diameters were increased by 1 mm and 2 mm, the lengths were increased by 5 mm and 10 mm. Shims were added randomly. The peak insertional torque was measured for each control screw and test screw placement. In addition, during each screw placement, the screw was removed 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 salvage screw by 2 mm caused the insertion torque to be increased by 8.4% of the original. Increasing the length of the screw did not improve the salvage screw insertional torque. There was an interaction effect for the 1-mm increase in diameter and the increase in length. At this diameter, increasing the length had a significant effect (P = 0.009) on the salvage torque. Using a shim created no improvement in salvage insertional torque (P = 0.77). There was a poor linear correlation between torque and bone mineral density (r = 0.18) in these osteoporotic specimens. Conclusions. Removing and replacing a pedicle screw in its original hole substantially decreases its mechanical fixation. For pedicle salvage, increasing the diameter causes the greatest restoration of strength. Shims had no effect in pedicle salvage in osteoporotic specimens.

Original languageEnglish (US)
Pages (from-to)1374-1379
Number of pages6
JournalSpine
Volume23
Issue number12
DOIs
StatePublished - Jun 15 1998
Externally publishedYes

Keywords

  • Bone mineral density
  • Insertional groups
  • Pedicle screw salvage

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