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.
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.
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.
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.
Correspondence Thomas J. Buell: University of Virginia Health System, Charlottesville, VA. email@example.com.
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.
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