Hybrid dynamic stabilization: a biomechanical assessment of adjacent and supraadjacent levels of the lumbar spine

Laboratory investigation

Restricted access

Object

The object of this study was to evaluate the effect of hybrid dynamic stabilization on adjacent levels of the lumbar spine.

Methods

Seven human spine specimens from T-12 to the sacrum were used. The following conditions were implemented: 1) intact spine; 2) fusion of L4–5 with bilateral pedicle screws and titanium rods; and 3) supplementation of the L4–5 fusion with pedicle screw dynamic stabilization constructs at L3–4, with the purpose of protecting the L3–4 level from excessive range of motion (ROM) and to create a smoother motion transition to the rest of the lumbar spine. An industrial robot was used to apply continuous pure moment (± 2 Nm) in flexion-extension with and without a follower load, lateral bending, and axial rotation. Intersegmental rotations of the fused, dynamically stabilized, and adjacent levels were measured and compared.

Results

In flexion-extension only, the rigid instrumentation at L4–5 caused a 78% decrease in the segment's ROM when compared with the intact specimen. To compensate, it caused an increase in motion at adjacent levels L1–2 (45.6%) and L2–3 (23.2%) only. The placement of the dynamic construct at L3–4 decreased the operated level's ROM by 80.4% (similar stability as the fusion at L4–5), when compared with the intact specimen, and caused a significant increase in motion at all tested adjacent levels. In flexion-extension with a follower load, instrumentation at L4–5 affected only a subadjacent level, L5–sacrum (52.0%), while causing a reduction in motion at the operated level (L4–5, −76.4%). The dynamic construct caused a significant increase in motion at the adjacent levels T12–L1 (44.9%), L1–2 (57.3%), and L5–sacrum (83.9%), while motion at the operated level (L3–4) was reduced by 76.7%. In lateral bending, instrumentation at L4–5 increased motion at only T12–L1 (22.8%). The dynamic construct at L3–4 caused an increase in motion at T12–L1 (69.9%), L1–2 (59.4%), L2–3 (44.7%), and L5–sacrum (43.7%). In axial rotation, only the placement of the dynamic construct at L3–4 caused a significant increase in motion of the adjacent levels L2–3 (25.1%) and L5–sacrum (31.4%).

Conclusions

The dynamic stabilization system displayed stability characteristics similar to a solid, all-metal construct. Its addition of the supraadjacent level (L3–4) to the fusion (L4–5) did protect the adjacent level from excessive motion. However, it essentially transformed a 1-level lumbar fusion into a 2-level lumbar fusion, with exponential transfer of motion to the fewer remaining discs.

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 July 27, 2012; DOI: 10.3171/2012.6.SPINE111054.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Photographs of the testing apparatus and constructs. A: Follower load fixtures attached to the lumbar spine. B: Construct A, a single-level rigid construct (outlined in green) with bilateral pedicle screws in the L-4 and L-5 vertebrae, with longitudinal titanium rods. C: Construct B, a 2-level hybrid construct (outlined in green) with rigid L4–5 implants extended by a semirigid longitudinal member connecting to bilateral L-3 pedicle screws. D: The robotic spine testing system.

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    Mean ROM for flexion-extension.

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    Adjacent-level effect (ALE) results for flexion-extension.

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    Mean ROM for flexion-extension with a follower load.

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    Adjacent-level effect results for flexion-extension with a follower load.

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    Mean ROM for lateral bending.

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    Adjacent-level effect results for lateral bending.

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    Mean ROM for axial rotation.

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    Adjacent-level effect results for axial rotation.

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