Biomechanics of Prophylactic Tethering for Proximal Junctional Kyphosis: Comparison of Posterior Tether Looping Techniques

Abstract

Study DesignBiomechanical study. ObjectivesCompare effects of four spinous process (SP) tether looping methods on segmental flexion range of motion (ROM), intervertebral disc (IVD) pressures, and peak tether forces. Summary of Background DataSP tethering has been gaining interest as a prophylactic technique to prevent PJK caused by ligamentous laxity in ASD corrective surgery. Several SP tether looping methods have been proposed; however, there is no consensus on appropriate technique. No study has investigated the effect of the tether looping method on segmental biomechanics. MethodsNine T1–T4 cadaveric motion segments were tested to 5 Nm of flexion-extension. The uppermost instrumented vertebra (UIV) was located at T3 using standard pedicle screws and fusion rods. A crosslink (CL) was placed inferior to the pedicle screws. A 5-mm polyester tether was looped under the CL at UIV and through holes drilled at the base of UIV + 1 and UIV + 2 SPs. Biomechanical measurements included flexion ROM, IVD pressure, and peak tether forces at UIV/UIV + 1 and UIV + 1/UIV + 2. An untethered test was used for baseline values. Tethered tests included one single-level (SL) method and three double-level (DL) methods: common (CM), chained (CH), and figure-8 (F8). ResultsSL yielded significant reductions in flexion ROM at UIV/UIV + 1 (p = .001) and in IVD pressure at UIV/UIV + 1 (p = .007). Choice of DL method had a significant effect on flexion ROM at UIV/UIV + 1 (p = .004) but not at UIV + 1/UIV + 2 (p = .14). Choice of DL method also had a significant effect on IVD pressure at UIV/UIV + 1 (p < .001) but not at UIV + 1/UIV + 2 (p = .311). CM produced the greatest reductions in flexion ROM and IVD pressure, with the lowest peak tether forces among the DL methods. ConclusionTether looping method significantly alters segmental biomechanics. Tethering with the CM method to UIV + 2 allows for reductions in loads acting on the UIV + 1 SP and posterior ligaments. Level of EvidenceLevel V, biomechanical study.