Pedicle screw insertion angle and pullout strength: comparison of 2 proposed strategies

Laboratory investigation

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Object

Minimally invasive pedicle screws inserted vertically (that is, dorsoventrally) through the pedicle, as opposed to the more common coaxial technique, offer potential advantages by minimizing soft-tissue stripping during screw placement. The screws are designed for insertion through a medial starting point with vertical trajectory through the pedicle and into the vertebral body. As such, no lateral dissection beyond the insertion point is necessary. However, the effects of this insertion technique on the screw biomechanical performance over a short- and long-term are unknown. The authors investigated the pullout strength and stiffness of these screws, with or without fatigue cycling, compared with comparably sized, traditional screws placed by coaxial technique.

Methods

Twenty-one lumbar vertebrae (L-3, L-4, and L-5) were tested. Each pedicle of each vertebra was instrumented with either a traditional, coaxial pedicle screw (Group A), placed through a standard starting point, or a vertically oriented, alternative-design screw (Group B), with a medial starting point and vertical trajectory. The specimens were divided into 2 groups for testing. One group was tested for direct pullout (10 specimens) while the other was subject to pullout after tangential (toggle) cyclic loading (11 specimens). The screws were cycled in displacement control (± 5 mm producing ~ 4-Nm moment) at a rate of 3 Hz for 5000 cycles. Pullout tests were performed at a rate of 1 mm/minute.

Results

Two-way ANOVA showed that Group B screws with a medial starting point (2541 ± 1090 N for cycled vs 2135 ± 1323 N for noncycled) had significantly higher pullout loads than Group A screws with a standard entry point (1585 ± 766 N for cycled vs 1417 ± 812 N noncycled) (p = 0.001). There was no significant effect of cycling or screw insertion type on pullout stiffness. Tangential stiffness of the Group B screws was significantly less than that of the Group A screws (p = 0.001). The stiffness of both screws in the toe region was significantly affected by cycling (p = 0.001).

Conclusions

The use of Group B screws inserted through a medial starting point showed greater pullout load than a Group A screw inserted through a standard starting point. The greater pullout strength in Group B screws may be due to screw thread design and increased cortical bone purchase at the medial starting point. Nevertheless, anatomical considerations of the medial starting point, that is, pedicle or lateral vertebral body cortex breach, may limit its application. The medial starting point of the Group B screw was frequently in the facet at the L-3 and L-4 pedicle entry points, which may have clinical importance.

Abbreviations used in this paper: BMD = bone mineral density; VB = vertebral body.

Article Information

Address correspondence to: Robert F. McLain, M.D., Department of Orthopaedic Surgery, Desk A41, Center for Spine Health, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, Ohio 44195. email: mclainr@ccf.org.

Please include this information when citing this paper: published online February 25, 2011; DOI: 10.3171/2010.11.SPINE09886.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    A: Lateral photograph demonstrating the ideal starting point of the Group B screw (upper inset). The Group B (Techtonix) screw (lower right inset) has a smaller ratio of inner to outer diameter and a narrower thread pitch compared with a Group A (Xia) screw (lower left inset). B and C: Illustrations of the screw entry points and trajectories. Standard pedicle screw with coaxial trajectory (Group A) inserted in the right pedicle (that is, the pedicle that appears on the left side of the drawings) is illustrated. On the left pedicle (that is, the pedicle that appears on the right side of the drawings), the screw hole with the medial starting point and vertical trajectory (Group B) is being prepared at the junction of the inferior aspect of the superior articular process and pars in this L-5 vertebra.

  • View in gallery

    Representative schematic view of the load-displacement curves, where the initial (dashed line) and last cycles (solid line) are superposed. Two stiffness calculations (dotted line) were made at the initial phase (toe) and terminal phase (elastic zone [EZ]) of the loading period.

  • View in gallery

    Two-way ANOVA testing showed a significant effect of screw (insertion) type on the pullout load with no significant effect of cycling. Pooled data and breakdown for cycled pullout (CP) and noncycled-pullout (nCP) are illustrated. *p = 0.001. Data are presented as the mean ± SD.

  • View in gallery

    Two-way ANOVA test showed no significant effect of screw (insertion) type or cycling on the pullout stiffness. Pooled data and breakdown for cycled pullout and noncycled pullout are illustrated. Data are presented as the mean ± SD.

  • View in gallery

    Two-way ANOVA test showed a significant effect of screw (insertion) type on the tangential stiffness in the elastic region of the loading curve with no significant effect of cycling. Pooled data and breakdown for initial and last cycles are illustrated. Data are presented as the mean ± SD. *p = 0.001.

  • View in gallery

    Two-way ANOVA test showed a significant effect of cycling on the tangential stiffness in the toe region of the loading curve with no significant effect of screw (insertion) type. Pooled data and breakdown for Group A and Group B are illustrated. Data are presented as the mean ± SD. *p = 0.001.

  • View in gallery

    Photograph showing the standard lower (L-3) lumbar pedicle screw with standard starting point (left) versus the Group B screw with the medial starting point (right). In the L-3 vertebrae, the Group B screw starting point was frequently in the facet.

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