Trajectory analysis and pullout strength of self-centering lumbar pedicle screws

Laboratory investigation

Restricted access

Object

An experiment was performed to study the limits of the ability of screws designed to center themselves in the pedicle during insertion, and to study whether straight-ahead versus inward screw insertion trajectories differ in their resistance to pullout.

Methods

Forty-nine human cadaveric lumbar vertebrae were studied. Pedicle screws were inserted in trajectories starting 0°, 10°, 20°, or 30° from the optimal trajectory, either medially or laterally misdirected. The surgeon then inserted the screw with forward thrust but without resisting the screw's tendency to reorient its own trajectory during insertion. On the opposite pedicle, a control screw was inserted with the more standard inward-angled anatomical trajectory and insertion point. Cortical wall violation during insertion was recorded. Screws were then pulled out at a constant displacement rate while ultimate strength was recorded.

Results

Lateral misdirection as small as 10° was likely to lead to cortical wall violation (3 of 7 violations). Conversely, medial misdirection usually resulted in safe screw insertion (1 of 21 violations for 10°, 20°, or 30° medial misdirection). The resistance to pullout of screws inserted in a straight-ahead trajectory did not differ significantly from that of screws inserted along an inward trajectory (p = 0.68).

Conclusions

Self-tapping, self-drilling pedicle screws can redirect themselves to a much greater extent during medial than during lateral misdirection. The cortical wall is more likely to be violated laterally than medially. The strength of straight-ahead and inward trajectories was equivalent.

Abbreviations used in this paper: ANOVA = analysis of variance; BMD = bone mineral density; PMMA = polymethylmethacrylate; VB = vertebral body.

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: neil.crawford@chw.edu.

Current affiliation for Dr. Yüksel: Kahramanmaraş Sütçü İmam University, Kahramanmaraş, Turkey.

Current affiliation for Dr. Doğan: Uludag University Faculty of Medicine, Bursa, Turkey.

Current affiliation for Dr. Soto-Barraza: Hospital Juárez de México, Juárez, Mexico.

Current affiliation for Dr. Villasana-Ramos: Hospital Angeles León, Guanajuato, Mexico.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Photograph of the self-drilling pedicle screw (Techtonix, Stryker Spine) used in this study; the screw is composed of titanium alloy and has a blunt tip.

  • View in gallery

    Photographs of the loading setup. Left: The piston of the servohydraulic test frame was attached to a uniaxial load cell, which in turn was attached to a connector rod. In this photograph, the connector is not yet interfaced with the head of the screw. Specimens were potted in a block of PMMA and positioned in an angle vise so that the axis of the screw was aligned with the piston of the servohydraulic test machine. The angle vise was clamped to the base of the servohydraulic test frame by using 2 C-clamps. Right: After alignment was confirmed, the screw head was locked into the connector rod and then loaded under tension to the point of failure.

  • View in gallery

    Bar graph showing the success rate of screw placement in each group. The screws were most often placed successfully when the forced deviation occurred toward the medial side of the pedicle.

  • View in gallery

    Bar graph showing the mean ultimate strength of control and study sides in each group of specimens. Error bars show the SD.

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