Impact of dual-headed pedicle screws on the biomechanics of lumbosacral junction multirod constructs

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  • 1 Department of Neurosurgery and
  • 2 Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona
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OBJECTIVE

The objective of this study was to evaluate a novel connector design and compare it with traditional side connectors, such as a fixed-angle connector (FAC) and a variable-angle connector (VAC), with respect to lumbosacral stability and instrumentation strain.

METHODS

Standard nondestructive flexibility tests (7.5 Nm) and compression tests (400 N) were performed using 7 human cadaveric specimens (L1–ilium) to compare range of motion (ROM) stability, posterior rod strain (RS), and sacral screw bending moment (SM). Directions of motion included flexion, extension, left and right lateral bending, left and right axial rotation, and compression. Conditions included 1) the standard 2-rod construct (2R); 2) the dual-tulip head (DTH) with 4-rod construct (4R); 3) FACs with 4R; and 4) VACs with 4R. Data were analyzed using repeated-measures ANOVA.

RESULTS

Overall, there were no statistically significant differences in ROM across the lumbosacral junction among conditions (p > 0.07). Compared with 2R, DTH and FAC significantly reduced RS in extension, left axial rotation, and compression (p ≤ 0.03). VAC significantly decreased RS compared with 2R in flexion, extension, left axial rotation, right axial rotation, and compression (p ≤ 0.03), and significantly decreased RS compared with DTH in extension (p = 0.02). DTH was associated with increased SM in left and right axial rotation compared with 2R (p ≤ 0.003) and in left and right lateral bending and left and right axial rotation compared with FAC and VAC (p ≤ 0.02). FAC and VAC were associated with decreased SM compared with 2R in right and left lateral bending (p ≤ 0.03).

CONCLUSIONS

RS across the lumbosacral junction can be high. Supplemental rod fixation with DTH is an effective strategy for reducing RS across the lumbosacral junction. However, the greatest reduction in RS and SM was achieved with a VAC that allowed for straight (uncontoured) accessory rod placement.

ABBREVIATIONS 2R = 2-rod construct; 4R = 4-rod construct; DEXA = dual-energy x-ray absorptiometry; DTH = dual-tulip head; FAC = fixed-angle connector; LAR = left axial rotation; LLB = left lateral bending; PSO = pedicle subtraction osteotomy; RAR = right axial rotation; RLB = right lateral bending; ROM = range of motion; RS = rod strain; S2AI = S2 alar iliac; SM = sacral screw bending moment; TLIF = transforaminal lumbar interbody fusion; VAC = variable-angle connector.

Supplementary Materials

    • Figures S1 and S2 (PDF 2.47 MB)

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Contributor Notes

Correspondence Jay D. Turner: c/o Neuroscience Publications, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ. neuropub@barrowneuro.org.

INCLUDE WHEN CITING Published online February 5, 2021; DOI: 10.3171/2020.8.SPINE191545.

Disclosures Dr. Turner reports consulting and conducting education and research for NuVasive; conducting research for Medtronic; and consulting for SeaSpine. Dr. Kelly reports conducting research for NuVasive and Medtronic. This study was supported in part with research grant funding through Medtronic.

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