Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P13. Changes in spinous process tether looping method significantly alters segmental biomechanics for proximal junctional kyphosis prophylaxis

BACKGROUND CONTEXT Spinous process (SP) tethering is gaining interest as a prophylactic technique to alleviate load concentrations and reinforce the posterior ligament complex (PLC) at the uppermost instrumented vertebra (UIV) to reduce the risk of developing proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgeries. Consensus has yet to be established on appropriate technique and there is a lack of biomechanical data to provide initial guidance. While multiple SP looping methods have recently been proposed, no study has evaluated the effect of tether looping method on segmental biomechanics. PURPOSE We sought to evaluate the effects of four tether looping methods on segmental biomechanics at UIV, UIV+1, and UIV+2. We hypothesized that the different looping methods would result in significant differences in flexion range-of-motion (ROM), intervertebral disc (IVD) pressures, and peak tether loop tensions. STUDY DESIGN/SETTING Biomechanical study. OUTCOME MEASURES Flexion ROM, IVD pressure, peak tether tension. METHODS Nine T1–T4 cadaveric motion segments were tested in flexion–extension bending to 5 Nm using a pure-moment testing machine. UIV was located at T3 using standard pedicle screws and rods. A rod crosslink (CL) was placed inferior to the pedicle screws. Motion of UIV, UIV+1, and UIV+2 was recorded using a motion capture system. IVD pressure was measured at UIV/UIV+1 and UIV+1/UIV+2 using custom pressure probes. A 5 mm braided polyester suture was used for tethering. The tether was looped under the CL at UIV, and through holes drilled at the base of UIV+1 and UIV+2 SPs. Tether tension was measured using custom in-line tension sensors for each independent loop. An untethered test was used for baseline values. Tethered tests included one single level (SL) and three double level (DL) tests: (1) SL, a single tether from CL at UIV to UIV+1 SP, (2) DL Common (CM), a tether from CL at UIV to UIV+1 SP and another tether from CL at UIV to UIV+2 SP, (3) DL Chained (CH), a tether from CL at UIV to UIV+1 SP and another tether from UIV+1 SP to UIV+2 SP, and (4) DL Figure-Eight (F8), a single tether from CL at UIV to UIV+1 SP and UIV+2 SP in a woven figure-eight loop. Loops were pretensioned to 22N before each test. RESULTS The SL method resulted in significant reductions in flexion ROM at UIV/UIV+1 (p=.001) but not at UIV+1/UIV+2 (p=.052). SL also yielded significant reductions in IVD pressure at both UIV/UIV+1 (p=.007) and UIV+1/UIV+2 (0.002). The choice of DL method had a significant univariate effect on flexion ROM reduction at UIV/UIV+1 (p=.004) but not at UIV+1/UIV+2 (0.14). The choice of DL method also had a significant univariate effect on IVD pressure reduction at UIV/UIV+1 (p CONCLUSIONS The use of different tether looping methods significantly alters the tether's effect on segmental biomechanics. double level techniques provide greater reductions in flexion ROM and IVD pressures; however, they yield greater tether tensions at the uppermost tethered level compared to SL. CM produced the best results for improved segmental stability. The transfer of peak SP loading to superjacent levels may provide benefits in protecting the UIV+1 SP and the PLC. New understanding in the effect of looping methods may provide improved ability to reduce PJK caused by ligamentous laxity.