Evaluating Motion Above Multi-Level Soft-Landing Constructs Proximal to Lumbar Deformity Constructs: A Cadaveric Biomechanical Study

Introduction: Proximal junctional kyphosis is a common consequence of long segment instrumented spinal fusion. Here, we compare motion adjacent to a fusion without a soft landing to motion adjacent to various multi-level soft-landings adjacent to a long lumbar fusion.

Methods: Seven (7) human cadaveric thoracolumbar spine segments were evaluated using a using a six degrees-of-freedom motion simulator (±7.5Nm) to track range of motion (ROM) in flexion/extension (FE), lateral bending (LB), and axial rotation (AR). Four soft-landings, placed at T8-T10, were tested sequentially: Transverse Process Hooks (TP), Untensioned Polyethylene-Terephthalate Cord (U-PET), Tensioned Polyethylene-Terephthalate Cord (T-PET), and a dynamic stabilization device (DSD). All specimens were fused from T10-Pelvis with posterior fixation. Relative motion adjacent to the fusion (T7-T8 for each soft-landing) was compared to a motion of the free disc adjacent to the fusion in the control with no soft landing.

Results: There was a significant difference in ROM, adjacent to the fusion, between groups, in LB (p = 0.010), but not FE (p = 0.061), or AR (p = 0.190). FE ROM of the TP group (+1.62°) was significantly less than DSD (+3.30°) (p = 0.010), which experienced the greatest FE ROM increase. TP experienced the smallest ROM increase in FE and LB. In LB and AR, TP (LB: +1.67°; AR: +2.88°) and U-PET (LB: +1.79°; AR +2.85°) behaved similarly. In LB, Control (+4.04°) had the greatest increase in ROM, followed by DSD (+3.05°). In AR, Control (+4.57°) and PET (+4.58°) had similar increases in ROM, with U-PET (+2.85°) experiencing the smallest increase.

Conclusion : Soft-landing techniques outperformed the control group, via limiting motion adjacent to a long-segment fusion, only in LB. Our data show PJK is still a concern with muti-level soft-landing constructs adjacent to a long-segment lumbar fusion. Therefore, surgeons, who perform long lumbar deformity surgery, should consider the implications of placing a multi-level soft-landing, as some do not limit motion compared to adjacent free levels.