Biomechanical assessment of proximal junctional semi-rigid fixation in long-segment thoracolumbar constructs

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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.

ABBREVIATIONS ASD = adult spinal deformity; BMD = bone mineral density; CT = computed tomography; INT = intact; LB = lateral bending; MT = Mersilene tape; PJ = proximal junctional; PJF = PJ failure; PJK = PJ kyphosis; PSF = pedicle screw fixation; SLB1 = sublaminar band at UIV+1; SLB2 = sublaminar band at UIV+1 and UIV+2; UIV = upper instrumented vertebra; ΔIDP = change in intradiscal pressure.

Article Information

Correspondence H. Francis Farhadi: The Ohio State University Wexner Medical Center, Columbus, OH. francis.farhadi@osumc.edu.

INCLUDE WHEN CITING Published online November 2, 2018; DOI: 10.3171/2018.7.SPINE18136.

Disclosures The authors report that research support was provided by Implanet America Inc. for the cadaver experiments performed at the University of Iowa and that Implanet had no role in data gathering, analysis, interpretation, or manuscript preparation. Dr. Grossbach reports a consultant relationshp with DePuy Synthes. Dr. Viljoen reports honorarium receipt from DePuy Synthes for speaking at spine deformity courses.

© AANS, except where prohibited by US copyright law.

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Figures

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    Photographs of 3 hybrid constructs. A: MT construct. B: SLB1 construct showing placement of bilateral sublaminar bands at the T9 level. C: SLB2 construct showing placement of bilateral sublaminar bands at the T8 and T9 levels.

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    Schematic diagram of PSF and the 3 hybrid constructs (MT, SLB1, and SLB2). The functional spinal units at the proximal junction exposed to high stress are shaded in gray. The MT, SLB1, and SLB2 vectors are depicted with arrows pointing in the direction of maximal applied force. Copyright The Ohio State University Wexner Medical Center. Published with permission.

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    Only SLB1 and SLB2 show reduced motion at T9–10 as compared to PSF in flexion. Left: Box plots showing median (bold line inside box) percentage of intact flexion motion values at the T7–8, T8–9, T9–10, T10–11, and T11–L2 levels for PSF and the 3 hybrid constructs (MT, SLB1, and SLB2). The boxes indicate IQRs and the error bars indicate the minimum and maximum values. Right: Line graphs of flexion (in normalized degrees) showing a less abrupt transition (reduced slope) between the fixed and mobile transitional segments for the SLB1 and SLB2 constructs.

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    Only MT and SLB2 show reduced motion at T9–10 as compared to PSF in extension. Left: Box plots showing median (bold line inside box) percentage of intact extension motion values at the T7–8, T8–9, T9–10, T10–11, and T11–L2 levels for PSF and the 3 hybrid constructs (MT, SLB1, and SLB2). The boxes indicate IQRs and the error bars indicate the minimum and maximum values. Right: Line graphs of extension (in normalized degrees) showing a less abrupt transition (reduced slope) between the fixed and mobile transitional segments for the MT and SLB2 constructs.

  • View in gallery

    Only SLB1 and SLB2 show reduced motion at T9–10 as compared to PSF in lateral bending. SLB2 further reduces motion in LB at T8–9 and T10–11. Left: Box plots showing median (bold line inside box) percentage of intact lateral bending (LB) motion values at the T7–8, T8–9, T9–10, T10–11, and T11–L2 levels for PSF and the 2 hybrid constructs (MT, SLB1, and SLB2). The boxes indicate IQRs and the error bars indicate the minimum and maximum values. Right: Line graphs of LB (in normalized degrees) showing a less abrupt transition (reduced slope) between the fixed and mobile transitional segments for the SLB1 and in particular SLB2 constructs.

  • View in gallery

    Only SLB1 and SLB2 show reduced motion at T9–10 and SLB2 further shows reduced motion at T8–9 as compared to PSF in torsion. Left: Box plots showing median (bold line inside box) percentage of intact torsion motion values at the T7–8, T8–9, T9–10, T10–11, and T11–L2 levels for PSF and the 3 hybrid constructs (MT, SLB1, and SLB2). The boxes indicate IQRs and the error bars indicate the minimum and maximum values. Right: Line graphs of torsion (in normalized degrees) showing a less abrupt transition (reduced slope) between the fixed and mobile transitional segments for the SLB1 and in particular SLB2 constructs.

  • View in gallery

    All 3 hybrid constructs show reduced ΔIDP in flexion and torsion at T9–10 as compared to PSF. SLB1 and SLB2 reduced ΔIDP at T9–10 in extension and SLB2 reduced ΔIDP in lateral bending (LB). SLB2 further reduced ΔIDP at T8–9 in extension, LB, and torsion. Box plots showing median (bold line inside box) percentage of intact flexion (A), extension (B), LB (C), and torsion (D) ΔIDP values at the T8–9, T9–10, and T10–11 levels for PSF and the 3 hybrid constructs (MT, SLB1, and SLB2). The boxes indicate IQRs and the error bars indicate the minimum and maximum values.

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