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Stephanus V. Viljoen, Nicole A. DeVries Watson, Nicole M. Grosland, James Torner, Brian Dalm, and Patrick W. Hitchon

Object

The objective of this study was to evaluate the biomechanical properties of lateral instrumentation compared with short- and long-segment pedicle screw constructs following an L-1 corpectomy and reconstruction with an expandable cage.

Methods

Eight human cadaveric T10–L4 spines underwent an L-1 corpectomy followed by placement of an expandable cage. The spines then underwent placement of lateral instrumentation consisting of 4 monoaxial screws and 2 rods with 2 cross-connectors, short-segment pedicle screw fixation involving 1 level above and below the corpectomy, and long-segment pedicle screw fixation (2 levels above and below). The order of instrumentation was randomized in the 8 specimens. Testing was conducted for each fixation technique. The spines were tested with a pure moment of 6 Nm in all 6 degrees of freedom (flexion, extension, right and left lateral bending, and right and left axial rotation).

Results

In flexion, extension, and left/right lateral bending, posterior long-segment instrumentation had significantly less motion compared with the intact state. Additionally, posterior long-segment instrumentation was significantly more rigid than short-segment and lateral instrumentation in flexion, extension, and left/right lateral bending. In axial rotation, the posterior long-segment construct as well as lateral instrumentation were not significantly more rigid than the intact state. The posterior long-segment construct was the most rigid in all 6 degrees of freedom.

Conclusions

In the setting of highly unstable fractures requiring anterior reconstruction, and involving all 3 columns, long-segment posterior pedicle screw constructs are the most rigid.

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Brian J. Park, Colin J. Gold, David Christianson, Nicole A. DeVries Watson, Kirill V. Nourski, Royce W. Woodroffe, and Patrick W. Hitchon

OBJECTIVE

Adjacent-segment disease (ASD) proximal to lumbosacral fusion is assumed to result from increased stress and motion that extends above or below the fusion construct. Sublaminar bands (SBs) have been shown to potentially mitigate stresses in deformity constructs. A similar application of SBs in lumbar fusions is not well described yet may potentially mitigate against ASD.

METHODS

Eight fresh-frozen human cadaveric spine specimens were instrumented with transforaminal lumbar interbody fusion (TLIF) cages at L3–4 and L4–5, and pedicle screws from L3 to S1. Bilateral SBs were applied at L2 and tightened around the rods extending above the L3 pedicle screws. After being mounted on a testing frame, the spines were loaded at L1 to 6 Nm in all 3 planes, i.e., flexion/extension, right and left lateral bending, and right and left axial rotation. Motion and intradiscal pressures (IDPs) at L2–3 were measured for 5 conditions: intact, instrumentation (L3–S1), band tension (BT) 30%, BT 50%, and BT 100%.

RESULTS

There was significant increase in motion at L2–3 with L3–S1 instrumentation compared with the intact spine in flexion/extension (median 8.78°, range 4.07°–10.81°, vs median 7.27°, range 1.63°–9.66°; p = 0.016). When compared with instrumentation, BT 100% reduced motion at L2–3 in flexion/extension (median 8.78°, range 4.07°–10.81°, vs median 3.61°, range 1.11°–9.39°; p < 0.001) and lateral bending (median 6.58°, range 3.67°–8.59°, vs median 5.62°, range 3.28°–6.74°; p = 0.001). BT 50% reduced motion at L2–3 only in flexion/extension when compared with instrumentation (median 8.78°, range 4.07°–10.81°, vs median 5.91°, range 2.54°–10.59°; p = 0.027). There was no significant increase of motion at L1–2 with banding when compared with instrumentation, although an increase was seen from the intact spine with BT 100% in flexion/extension (median 5.14°, range 2.47°–9.73°, vs median 7.34°, range 4.22°–9.89°; p = 0.005). BT 100% significantly reduced IDP at L2–3 from 25.07 psi (range 2.41–48.08 psi) before tensioning to 19.46 psi (range −2.35 to 29.55 psi) after tensioning (p = 0.016).

CONCLUSIONS

In this model, the addition of L2 SBs reduced motion and IDP at L2–3 after the L3–S1 instrumentation. There was no significant increase in motion at L1–2 in response to band tensioning compared with instrumentation alone. The application of SBs may have a clinical application in reducing the incidence of ASD.

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Vibhu K. Viswanathan, Ranjit Ganguly, Amy J. Minnema, Nicole A. DeVries Watson, Nicole M. Grosland, Douglas C. Fredericks, Andrew J. Grossbach, Stephanus V. Viljoen, and H. Francis Farhadi

OBJECTIVE

Proximal junctional kyphosis (PJK) and failure (PJF) are potentially catastrophic complications that result from abrupt changes in stress across rigid instrumented and mobile non-fused segments of the spine (transition zone) after adult spinal deformity surgery. Recently, data have indicated that extension (widening) of the transitional zone via use of proximal junctional (PJ) semi-rigid fixation can mitigate this complication. To assess the biomechanical effectiveness of 3 semi-rigid fixation constructs (compared to pedicle screw fixation alone), the authors performed cadaveric studies that measured the extent of PJ motion and intradiscal pressure changes (ΔIDP).

METHODS

To measure flexibility and ΔIDP at the PJ segments, moments in flexion, extension, lateral bending (LB), and torsion were conducted in 13 fresh-frozen human cadaveric specimens. Five testing cycles were conducted, including intact (INT), T10–L2 pedicle screw-rod fixation alone (PSF), supplemental hybrid T9 Mersilene tape insertion (MT), hybrid T9 sublaminar band insertion (SLB1), and hybrid T8/T9 sublaminar band insertion (SLB2).

RESULTS

Compared to PSF, SLB1 significantly reduced flexibility at the level rostral to the upper-instrumented vertebral level (UIV+1) under moments in 3 directions (flexion, LB, and torsion, p ≤ 0.01). SLB2 significantly reduced motion in all directions at UIV+1 (flexion, extension, LB, torsion, p < 0.05) and at UIV+2 (LB, torsion, p ≤ 0.03). MT only reduced flexibility in extension at UIV+1 (p = 0.02). All 3 constructs revealed significant reductions in ΔIDP at UIV+1 in flexion (MT, SLB1, SLB2, p ≤ 0.02) and torsion (MT, SLB1, SLB2, p ≤ 0.05), while SLB1 and SLB2 significantly reduced ΔIDP in extension (SLB1, SLB2, p ≤ 0.02) and SLB2 reduced ΔIDP in LB (p = 0.05). At UIV+2, SLB2 similarly significantly reduced ΔIDP in extension, LB, and torsion (p ≤ 0.05).

CONCLUSIONS

Compared to MT, the SLB1 and SLB2 constructs significantly reduced flexibility and ΔIDP in various directions through the application of robust anteroposterior force vectors at UIV+1 and UIV+2. These findings indicate that semi-rigid sublaminar banding can most effectively expand the transition zone and mitigate stresses at the PJ levels of long-segment thoracolumbar constructs.