The effect of posterior polyester tethers on the biomechanics of proximal junctional kyphosis: a finite element analysis

Abstract

<jats:sec> <jats:title>OBJECTIVE</jats:title> <jats:p>Proximal junctional kyphosis (PJK) remains problematic following multilevel instrumented spine surgery. Previous biomechanical studies indicate that providing less rigid fixation at the cranial aspect of a long posterior instrumented construct, via transition rods or hooks at the upper instrumented vertebra (UIV), may provide a gradual transition to normal motion and prevent PJK. The purpose of this study was to evaluate the ability of posterior anchored polyethylene tethers to distribute proximal motion segment stiffness in long instrumented spine constructs.</jats:p></jats:sec> <jats:sec> <jats:title>METHODS</jats:title> <jats:p>A finite element model of a T7–L5 spine segment was created to evaluate range of motion (ROM), intradiscal pressure, pedicle screw loads, and forces in the posterior ligament complex within and adjacent to the proximal terminus of an instrumented spine construct. Six models were tested: 1) intact spine; 2) bilateral, segmental pedicle screws (PS) at all levels from T-11 through L-5; 3) bilateral pedicle screws from T-12 to L-5 and transverse process hooks (TPH) at T-11 (the UIV); 4) pedicle screws from T-11 to L5 and 1-level tethers from T-10 to T-11 (TE-UIV+1); 5) pedicle screws from T-11 to L-5 and 2-level tethers from T-9 to T-11 (TE-UIV+2); and 6) pedicle screws and 3-level tethers from T-8 to T-11 (TE-UIV+3).</jats:p></jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>Proximal-segment range of motion (ROM) for the PS construct increased from 16% at UIV−1 to 91% at UIV. Proximal-segment ROM for the TPH construct increased from 27% at UIV−1 to 92% at UIV. Posterior tether constructs distributed ROM at the UIV and cranial adjacent segments most effectively; ROM for TE-UIV+1 was 14% of the intact model at UIV−1, 76% at UIV, and 98% at UIV+1. ROM for TE-UIV+2 was 10% at UIV−1, 51% at UIV, 69% at UIV+1, and 97% at UIV+2. ROM for TE-UIV+3 was 7% at UIV−1, 33% at UIV, 45% at UIV+1, and 64% at UIV+2. Proximal segment intradiscal pressures, pedicle screw loads, and ligament forces in the posterior ligament complex were progressively reduced with increasing number of posterior tethers used.</jats:p></jats:sec> <jats:sec> <jats:title>CONCLUSIONS</jats:title> <jats:p>Finite element analysis of long instrumented spine constructs demonstrated that posterior tethers created a more gradual transition in ROM and adjacent-segment stress from the instrumented to the noninstrumented spine compared with all PS and TPH constructs. Posterior tethers may limit the biomechanical risk factor for PJK; however, further clinical research is needed to evaluate clinical efficacy.</jats:p></jats:sec>

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

“…Such biomechanical alterations have been shown to be further aggravated by the removal of supraspinous and interspinous ligaments. 2,3 MT restricted motion only under extension loads and reduced DIDP only under flexion and torsion moments at the UIV+1 level. As shown in Fig.…”

“…These pathologic forces result in increases in angular displace- ment and intradiscal pressure that lead to PJK. [2][3][4] Based on these studies, local biomechanical factors have been identified that can favorably influence these parameters. These factors include preservation of soft tissue elements, the presence of an anchor at the UIV, and the use of a transitional rod, as well as rod curvature and diameter.…”

“…1,2,26,30 One proposed model involves the use of posterior tethers proximal to the UIV. 2 Specifically, augmentation of the posterior spinal ligamentous complex using either spinous process-anchored Mersilene tape (MT; Ethicon) 30 or polyester sublaminar bands 27 positioned at the PJ levels and fastened to the long-segment construct has been employed to generate semi-rigid fixation with distinct force vectors. These hybrid constructs are designed to provide a smoother transition and to reduce the stress across the proximal junction, thereby preventing proximal flexion failure.…”

Posterior Ligamentous Reinforcement of the Upper Instrumented Vertebrae +1 Does Not Decrease Proximal Junctional Kyphosis in Adult Spinal Deformity

“…A recent finite element analysis found that polyester tether reinforcement of the posterior elements was able to successfully reduce adjacent segment stress at levels above the upper instrumented vertebrae (UIV). 20 Other authors have described a semitendinosus graft to augment the posterior spinous ligament at the UIV. 21 In our practice, we utilize a surgical nylon tape (SNT) to reinforce the PLS at the UIV in select cases.…”

Establishment and Finite Element Analysis of a Three‐dimensional Dynamic Model of Upper Cervical Spine Instability

“…The dynamic positioning of X‐ray imaging of cervical spine flexion and extension can provide a good reference for the upper cervical spine, and it can also indicate that upper cervical spine instability occurs in the movement process of the cervical spine. However, the currently popular medical‐imaging methods cannot accurately capture and describe the overall process and dynamic characteristics of upper cervical spine instability.…”