Director of Research Musculoskeletal Education and Research Center Audubon, PA, US
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 single-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 (T8-T9 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, in LB (p = 0.047) and AR (p = 0.012), but not FE (p = 0.168). Of all groups, Control experienced the largest ROM increase (FE +3.00°; LB +4.04°; AR +4.57°). In FE, U-PET (+0.79°) and TP (+0.87°) had similar increases in ROM. In FE, U-PET’s ROM increase was significantly less than DSD (+1.87°) (p = 0.008). Of all groups, TP had the smallest LB ROM increase (+1.76°). LB ROM increase in the T-PET group (+2.69°) was significantly less than DSD (+3.66°) (p = 0.005). In AR, ROM increase in the Control group (+4.57°) was significantly greater than the U-PET group (+2.18°) (p = 0.035). Of all soft-landings, DSD experienced the greatest ROM increase, in all bending planes (FE: +1.87°; LB: +3.66°; AR +3.91°).
Conclusion : One-level soft-landings may be useful in reducing hypermobility of the disc adjacent to long lumbar fusion, compared to Control. Motion of the disc, adjacent to the soft-landings, was limited most in TP and U-PET groups, while DSD experienced the greatest motion, in all bending planes. Surgeons should consider risk of PJK may still exist, in long lumbar deformity constructs, even when a soft-landing technique is employed.