Plate fixation in the cervical spine: traditional paramedian screw configuration compared with unique unilateral configuration

Laboratory investigation

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Object

This study compared the fixing strength and stability achieved by a unilateral plate and screw configuration against a standard cervical fixation plate using a single-level corpectomy and allograft strut graft model.

Methods

Multidirectional in vitro flexibility tests were performed using a robotic spine testing system. Human cadaveric spines were assessed for spinal stability after vertebral corpectomy and anterior instrumentation. Specimens were mounted cranially and caudally on custom jigs that were then attached to load cells on the robotic system's end effector and base pedestal. C2–T1 spine specimens (n = 6) were tested intact; then after C-5 corpectomy (the vertebral body was excised), allograft placement and anterior plate fixation were performed. The surgeons performed a uniform corpectomy and reconstruction of each specimen in a protocol fashion. Two plates were compared: a unilateral 4-hole cervical plate designed to obtain rigid fixation using 4 convergent fixation screws all placed unilateral to the vertebral midline, and a standard cervical plate with bilateral plate screw configuration. The plate testing sequence was selected at random to limit bias. Fixation screws were matched for length and diameter. Pure moments were applied under load control (maximum 1.8 Nm) in flexion, extension, left/right lateral bending, and left/right axial rotation. Vertebral motion was measured using an optoelectronic system. The mean relative range of motion between C-4 and C-6 was compared among groups using repeated-measures ANOVA (significance level of 0.05).

Results

In comparing the intact construct and 2 different plates in all planes of motion, only motion in extension (intact vs unilateral plate, p = 0.003; intact vs standard plate, p = 0.001) and left axial rotation (intact vs unilateral plate, p = 0.019) were significantly affected. In terms of immediate cervical stability after 1-level corpectomy and placement of an allograft reconstruction, the unilateral plate showed comparable stiffness to the standard plate in all 3 motion planes (flexion [p = 0.993], extension [p = 0.732], left lateral bending [p = 0.683], right lateral bending [p = 0.546], left axial rotation [p = 0.082], and right axial rotation [p = 0.489]). The unilateral plate showed a trend toward improved stiffness in axial rotation. In no direction did the unilateral configuration prove significantly less stiff than the traditional configuration.

Conclusions

The unilateral plate design proposed here requires minimal dissection and retraction beyond the midline of tissues susceptible to scar, postoperative pain, and swelling. The authors' study suggests that a unilateral plate can be configured to provide comparable fixation strength and torsional stiffness compared with traditional, widely accepted plate designs.

Abbreviation used in this paper:ROM = range of motion.

Article Information

Address correspondence to: Robert F. McLain, M.D., Cleveland Clinic Lerner College of Medicine, Cleveland Clinic Center for Spine Health, 9500 Euclid Avenue, Desk S40, Cleveland, Ohio 44195. email: mclainr@ccf.org.

Please include this information when citing this paper: published online April 19, 2013; DOI: 10.3171/2013.3.SPINE12949.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Left: Photograph showing a standard cervical fixation plate. Right: Illustration showing soft-tissue exposure and retraction for placement of the plate. Right panel reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2012. All rights reserved.

  • View in gallery

    Left: Photograph showing a unilateral cervical fixation plate. Right: Illustration showing soft-tissue exposure and retraction for placement of the plate. Right panel reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2012. All rights reserved.

  • View in gallery

    Photograph showing the robotic testing system.

  • View in gallery

    Graphs showing mean ROMs for flexion-extension (A), lateral bending (B), and axial rotation (C). wrt = with respect to.

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