Optimal tether configurations and preload tensioning to prevent proximal junctional kyphosis: a finite element analysis

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OBJECTIVE

Proximal junctional kyphosis (PJK) is, in part, due to altered segmental biomechanics at the junction of rigid instrumented spine and relatively hypermobile non-instrumented adjacent segments. Proper application of posteriorly anchored polyethylene tethers (i.e., optimal configuration and tension) may mitigate adjacent-segment stress and help prevent PJK. The purpose of this study was to investigate the impact of different tether configurations and tensioning (preloading) on junctional range-of-motion (ROM) and other biomechanical indices for PJK in long instrumented spine constructs.

METHODS

Using a validated finite element model of a T7–L5 spine segment, testing was performed on intact spine, a multilevel posterior screw-rod construct (PS construct; T11–L5) without tether, and 15 PS constructs with different tether configurations that varied according to 1) proximal tether fixation of upper instrumented vertebra +1 (UIV+1) and/or UIV+2; 2) distal tether fixation to UIV, to UIV−1, or to rods; and 3) use of a loop (single proximal fixation) or weave (UIV and/or UIV+1 fixation in addition to UIV+1 and/or UIV+2 proximal attachment) of the tether. Segmental ROM, intradiscal pressure (IDP), inter- and supraspinous ligament (ISL/SSL) forces, and screw loads were assessed under variable tether preload.

RESULTS

PS construct junctional ROM increased abruptly from 10% (T11–12) to 99% (T10–11) of baseline. After tethers were grouped by most cranial proximal fixation (UIV+1 vs UIV+2) and use of loop versus weave, UIV+2 Loop and/or Weave most effectively dampened junctional ROM and adjacent-segment stress. Different distal fixation and use of loop versus weave had minimal effect. The mean segmental ROM at T11–12, T10–11, and T9–10, respectively, was 6%, 40%, and 99% for UIV+1 Loop; 6%, 44%, and 99% for UIV+1 Weave; 5%, 23%, and 26% for UIV+2 Loop; and 5%, 24%, and 31% for UIV+2 Weave.

Tethers shared loads with posterior ligaments; consequently, increasing tether preload tension reduced ISL/SSL forces, but screw loads increased. Further attenuation of junctional ROM and IDP reversed above approximately 100 N tether preload, suggesting diminished benefit for biomechanical PJK prophylaxis at higher preload tensioning.

CONCLUSIONS

In this study, finite element analysis demonstrated UIV+2 Loop and/or Weave tether configurations most effectively mitigated adjacent-segment stress in long instrumented spine constructs. Tether preload dampened ligament forces at the expense of screw loads, and an inflection point (approximately 100 N) was demonstrated above which junctional ROM and IDP worsened (i.e., avoid over-tightening tethers). Results suggest tether configuration and tension influence PJK biomechanics and further clinical research is warranted.

ABBREVIATIONS ASD = adult spinal deformity; IDP = intradiscal pressure; ISL = interspinous ligament; PJK = proximal junctional kyphosis; PS = multilevel posterior screw–rod; ROM = range of motion; SSL = supraspinous ligament; UIV = upper instrumented vertebra.

Article Information

Correspondence Thomas J. Buell: University of Virginia Health System, Charlottesville, VA. tjb4p@hscmail.mcc.virginia.edu.

INCLUDE WHEN CITING Published online February 8, 2019; DOI: 10.3171/2018.10.SPINE18429.

Disclosures Dr. Bess reports receipt of research support from K2M, Nuvasive (including for present study), Medtronic, DePuy Synthes, Zimmer-Biomet, Allosource, Orthofix, EOS, and ISSGF; consultant relationships with K2M, Allosource, DePuy Synthes, Misonix, and EOS; and holding patents with K2M and Innovasis. Dr. Schwab reports consultant relationships with Zimmer-Biomet, K2M, Nuvasive, Medicrea, and MSD; receipt of research support from SRS, AOSpine, DePuy Synthes, NuVasive, K2M, and Stryker (paid through ISSGF); direct stock ownership in Nemaris Inc.; and speaking/teaching arrangements with Zimmer-Biomet, MSD, NuVasive, and K2M. Dr. Lafage reports consultant relationships with Nemaris Inc, NuVasive, Medicrea, and DePuy Synthes; direct stock ownership in Nemaris Inc.; receipt of non–study-related support from SRS, NASS, NuVasive, NIH, DePuy Synthes, K2M, and Stryker (paid through ISSGF); and speaking/teaching arrangements with AO Spine and DePuy Spine. Dr. Ames reports consultant relationships with Medtronic, DePuy Synthes, Stryker, Zimmer-Biomet, K2M, and Medicrea; receipt of royalties from Zimmer-Biomet and Stryker; receipt of non–study-related clinical or research support from Titan Spine, DePuy Synthes, and ISSG; receipt of grant funding from ISSG; being on the executive committee of ISSG; being director of Global Spinal Analytics; receipt of royalties from Biomet Zimmer, Stryker, DePuy Synthes, K2M, Next Spine, and Medicrea; and a patent holder relationship with Fish & Richardson PC. Dr. Shaffrey reports consultant relationships with Medtronic, Nuvasive, Zimmer-Biomet, EOS, and K2M; receipt of royalties from Medtronic, Nuvasive, and Zimmer-Biomet; direct stock ownership in NuVasive; and receipt of grants from NIH, DOD, and NACTN. Dr. Smith reports receipt of royalties from Zimmer-Biomet; consultant relationships with Zimmer-Biomet, Cerapedics, NuVasive, K2M, and AlloSource; receipt of honoraria from Zimmer-Biomet, NuVasive, and K2M; receipt of research support from DePuy Synthes, and ISSGF; and receipt of fellowship support from NREF and AOSpine.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Finite element spine model configurations. Posterior view of 17 spine models tested: intact spine, multilevel posterior screw–rod construct (PS; T11–L5) without tether, and 15 PS constructs with different tether configurations, grouped according to their most cranial proximal fixation point (upper instrumented vertebra +1 [UIV+1] vs UIV+2 spinous process) and use of a loop (continuous red line) versus weave (alternating red/yellow lines) of the tether when attaching to UIV+1 and/or UIV+2. A: Intact (non-instrumented) model: intact T7–L5 spine segment without instrumentation for baseline comparison. B: Pedicle screw (PS) construct without tether: bilateral segmental pedicle screws and 5.5-mm–diameter cobalt-chromium rods from T11 to L5. C: UIV+1 Loop Tethers: PS construct with proximal tether fixation at UIV+1. Distal tether fixation is indicated below. i) Distal tether fixation to UIV spinous process. ii) Distal tether fixation to UIV−1 spinous process. iii) Distal tether fixation to rods between UIV and UIV−1. iv) Distal tether fixation to rods between UIV−1 and UIV−2. D: UIV+1 Weave Tethers: PS construct with proximal tether fixation at UIV+1. Additional segmental weave attachment to UIV and distal tether fixation are indicated below. i) Segmental weave attachment to UIV and distal tether fixation to UIV−1 spinous process. ii) Segmental weave attachment to UIV and distal tether fixation to rods between UIV−1 and UIV−2. E: UIV+2 Loop Tethers: PS construct with proximal tether fixation at UIV+2. Distal tether fixation is indicated below. i) Distal tether fixation to UIV spinous process. ii) Distal tether fixation to UIV−1 spinous process. iii) Distal tether fixation to rods between UIV and UIV−1. iv) Distal tether fixation to rods between UIV−1 and UIV−2. F: UIV+2 Weave Tethers: PS construct with proximal tether fixation at UIV+2. Additional segmental weave attachment to UIV and/or UIV+1 and distal tether fixation are indicated below. i) Segmental weave attachment to UIV+1 and distal tether fixation to UIV spinous process. ii) Segmental weave attachment to UIV and UIV+1 and distal tether fixation to UIV−1 spinous process. iii) Segmental weave attachment to UIV+1 and distal tether fixation to rods between UIV and UIV−1. iv) Segmental weave attachment to UIV+1 and distal tether fixation to rods between UIV−1 and UIV−2. v) Segmental weave attachment to UIV and UIV+1 and distal tether fixation to rods between UIV−1 and UIV−2. Figure is available in color online only.

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    Segmental ROM. In comparison to non-instrumented (intact) spine, junctional ROM for the pedicle screw spine model without tether increased abruptly from 10% at T11–12 to 99% at T10–11. Tethers dampened junctional ROM and demonstrated more gradual transition adjacent to the proximal terminus of constructs, with UIV+2 (Loop or Weave) tether configurations performing best. Different distal tether fixation and use of a Loop versus Weave configuration had minimal effect. Mean segmental ROM at T11–12, T10–11, and T9–10 was 6% (± 0%), 40% (± 2%), and 99% (± 0%) for UIV+1 Loop; 6% (± 0%), 44% (± 2%), and 99% (± 0%) for UIV+1 Weave; 5% (± 1%), 23% (± 3%), and 26% (± 1%) for UIV+2 Loop; and 5% (± 0%), 24% (± 4%), and 31% (± 3%) for UIV+2 Weave, respectively. Figure is available in color online only.

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    Segmental ROM with preload tensioning. In comparison to conditions without preload tensioning, tethers with 50-N preload further dampened junctional ROM, thereby producing more gradual transition adjacent to the proximal terminus of constructs. However, increasing tether preload tension above approximately 100 N placed the proximally adjacent non-instrumented segments into extension, which created more abrupt transitions in junctional ROM. Figure is available in color online only.

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    Intradiscal pressure (IDP). In comparison to non-instrumented (intact) spine, IDP for the pedicle screw model without tether increased abruptly from 32% at T11–12 to 100% at T10–11. Tethers dampened IDP within and adjacent to the proximal terminus of constructs. UIV+2 Loop and/or Weave tether configurations attenuated IDP most effectively. Different distal tether fixation and use of a Loop vs Weave configuration had minimal effect on IDP. Mean IDP at T11–12, T10–11, and T9–10 was 28% (± 1%), 61% (± 2%), and 100% (± 1%) for UIV+1 Loop; 29% (± 1%), 62% (± 1%), and 99% (± 0%) for UIV+1 Weave; 27% (± 1%), 46% (± 3%), and 50% (± 0%) for UIV+2 Loop; and 28% (± 1%), 47% (± 3%), and 53% (± 2%) for UIV+2 Weave, respectively. Figure is available in color online only.

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    Intradiscal pressure with preload tensioning. In comparison to conditions without preload tensioning, tethers with 50-N and 100-N preload further dampened IDP. However, increasing preload tension beyond approximately 100 N caused an increase in IDP at the proximally adjacent tethered segments (A) T10–11 and (B) T9–10. Figure is available in color online only.

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    Posterior ligament complex forces. A: In comparison to non-instrumented (intact) spine, inter- and supraspinous ligament (ISL/SSL) forces for the pedicle screw model without tether increased abruptly from 2% at T11–12 to 100% at T10–11. All tether constructs demonstrated dampened ISL/SSL forces. UIV+2 Loop and/or Weave tether configurations dampened ISL/SSL forces most effectively. Different distal tether fixation and use of a Loop versus Weave did not appreciably alter ISL/SSL forces. For UIV+1 Loop tethers, mean ISL/SSL force increased from 1% (± 2%) at T11–12 to 27% (± 3%) at T10–11. For UIV+1 Weave tethers, mean ISL/SSL force increased from 1% (± 2%) at T11–12 to 29% (± 1%) at T10–11. For UIV+2 Loop tethers, mean ISL/SSL force increased from 2% (± 3%) at T11–12 to 16% (± 2%) at T10–11 and 27% (± 1%) at T9–10. For UIV+2 Weave tethers, mean ISL/SSL force increased from 2% (± 2%) at T11–12% to 16% (± 2%) at T10–11 and 30% (± 3%) at T9–T10. B: In comparison to conditions without preload tensioning, tethers with preload further dampened inter- and supraspinous ligament (ISL/SSL) forces. At the T10–11 segment, 100-N preload tension reduced ISL/SSL complex forces to 0 N for all tether constructs. C: At the T9–10 segment, 50-N preload tension reduced ISL/SSL complex forces to 0 N for UIV+2 Loop and/or Weave tether constructs. Figure is available in color online only.

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    Pedicle screw forces. A: No preload. The largest screw force was demonstrated in the nontethered pedicle screw (PS) construct at the UIV (T11) and was 54.7 N. The PS construct screw forces at UIV−1 (T12) and UIV−2 (L1) were 8.0 N and 4.1 N, respectively. Tethers reduced screw loads at the proximal terminus of constructs. Screw load attenuation was most pronounced at the UIV compared to more inferior levels (UIV−1 and UIV−2). UIV+2 Loop and/or Weave tether configurations demonstrated only slight advantage for attenuating UIV (T11) screw forces. Different distal tether fixation and use of a Loop vs Weave did not appreciably alter screw forces. Screw loads at T12 and L1 were comparable between all tether models. Pedicle screw loads at T11, T12, and L1, respectively, were 21.7 ± 3.5 N, 4.4 ± 0.8 N, and 2.7 ± 0.4 N for UIV+1 Loop; 22.1 ± 6.5 N, 5.6 ± 1.3 N, and 3.0 ± 0.4 N for UIV+1 Weave; 19.5 ± 2.1 N, 4.3 ± 0.7 N, and 2.5 ± 0.5 N for UIV+2 Loop; and 17.6 ± 0.5 N, 4.8 ± 1.2 N, and 2.8 ± 0.6 N for UIV+2 Weave. B: Preload conditions. In comparison to conditions without preload tensioning, tethers with preload increased UIV (T11) pedicle screw loads without apparent inflection points between 0 N and 150 N. Figure is available in color online only.

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