Quantitative analysis of misplaced pedicle screws in the thoracic spine: how much pullout strength is lost?

Presented at the 2009 Joint Spine Section Meeting 

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Object

The object of this study was to investigate the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of thoracic pedicle screws.

Methods

Forty human thoracic vertebrae (T6–11) from human cadavers were studied. Before pedicle screws were inserted, the specimens were separated into 4 groups according to the type of screw used: 1) standard pedicle screw (no cortical perforation); 2) screw with medial cortical perforation; 3) screw with lateral cortical perforation; and 4) “airball” screw (a screw that completely missed the vertebral body). Consistency among the groups for bone mineral density, pedicle diameter, and screw insertion depth was evaluated. Finally, each screw was pulled out at a constant displacement rate of 10 mm/minute while ultimate strength was recorded.

Results

Compared with well-placed pedicle screws, medially misplaced screws had 8% greater mean pullout strength (p = 0.482) and laterally misplaced screws had 21% less mean pullout strength (p = 0.059). The difference in mean pullout strength between screws with medial and lateral cortical perforations was significant (p = 0.013). Airball screws had only 66% of the mean pullout strength of well-placed screws (p = 0.009) and had 16% lower mean pullout strength than laterally misplaced screws (p = 0.395).

Conclusions

This in vitro study showed a significant difference in mean pullout strength between medial and lateral misplaced pedicle screws. Moreover, airball screws were associated with a significant loss of pullout strength.

Abbreviations used in this paper: BMD = bone mineral density; DEXA = dual energy x-ray absorptiometry; PMMA = polymethylmethacrylate.

Article Information

Address correspondence to: Neil R. Crawford, Ph.D., c/o Neuroscience Publications, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, Arizona 85013. email: neuropub@chw.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Radiographs depicting the trajectories in each configuration studied. A: Standard pedicle screw. B: Pedicle screw with a medial cortical perforation. Note the extensive cortical border along the path of the screw in the pedicle and spinal canal. C: Pedicle screw with a lateral cortical perforation. D: Airball screw. The dashed lines represent the ideal screw trajectory.

  • View in gallery

    Photograph of the test setup showing the appropriate alignment of the axis of the screw with the axis of the piston of the test machine.

  • View in gallery

    Bar graph showing mean ultimate pullout strength of each screw type. Values represent the peak of the load-deformation curve. Error bars show standard deviations.

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