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

Abstract

<jats:sec> <jats:title>Object</jats:title> <jats:p>The object of this study was to evaluate the effect of hybrid dynamic stabilization on adjacent levels of the lumbar spine.</jats:p></jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p>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.</jats:p></jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>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%).</jats:p></jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p>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.</jats:p></jats:sec>

Hybrid Surgery Combined with Dynamic Stabilization System and Fusion for the Multilevel Degenerative Disease of the Lumbosacral Spine

“…4 Our biomechanical study using finite element analysis also revealed stiffness resulting from the Dynesys system which is the same as that from rigid fixation. 10 In contrast to the results of Mageswaran et al, Durani et al reported that a dynamic stabilization system could reduce the hypermobility caused by extended arthrodesis.…”

“…3 Hybrid surgery is not to prevent further degeneration of the asymptomatic adjacent segment but is used to replace fusion when treating symptomatic, degenerated adjacent segments. 4 Thus, a retrospective comparative study was done in patients with hybrid surgery or pure fusion surgery for lumbosacral degenerative disease involving two-segments.…”

Zero-profile hybrid fusion construct versus 2-level plate fixation to treat adjacent-level disease in the cervical spine

“…29,35 It is unique in its ability to efficiently combine both load control and displacement control for testing of multisegmental specimens. It also provides the flexibility to control both the loading and boundary conditions, desired for conducting biomechanical tests.…”

“…The mounting procedure and robotic test system have been described in detail in previous studies. 28,29,39 For this study, the robot was programmed to apply 3 continuous loading and unloading cycles of applied moment along each primary axis of motion to simulate flexion-extension (FE), lateral bending (LB), and axial rotation (AR). The specimens were preconditioned to eliminate any viscoelastic effects.…”

“…24 The extent to which alterations of adjacent-level biomechanics and intradiscal pressures accelerate the normal degenerative process remains unclear at this time. [12][13][14]20,26,[28][29][30][31]34,40 Fusing a level adjacent to a prior ACDF often calls for exposing and removing the previously placed plate and screws. This additional step has the potential to prolong operating time and exposure magnitude, which may correlate with outcomes and infection rates.…”