Comparing Motion of Discs Adjacent to One- and Two-Level Soft-Landing Techniques Proximal to Lumbar Deformity Constructs: A Cadaveric Biomechanical Study

Introduction: Proximal junctional kyphosis (PJK) is a common consequence of long segment spinal fusion. progression of PJK may be affected by hypermobility of the free levels adjacent to long fusion. This study compares the ability of single-level and multi-level proximal soft-landing techniques, to provide gradual transition of motion adjacent to the upper instrumented vertebra (UIV) of a long lumbar fusion.

Methods: The difference in motion between the upper instrumented disc (UID) and the adjacent level (UID+1) of seven human cadaveric spine segments (T5-Pelvis) was evaluated using a using a six degrees-of-freedom range of motion (ROM) simulator (±7.5Nm) in flexion/extension (FE), lateral bending (LB), and axial rotation (AR). Four soft-landing techniques were evaluated: Transverse Process Hooks (TP), Untensioned Polyethylene-Terephthalate Cord (U-PET), Tensioned Polyethylene-Terephthalate Cord (T-PET), and a dynamic stabilization device (DSD). Soft-landing constructs were placed across one (T9-T10) and two (T8-T10) levels. All specimens were fused from T10-Pelvis with posterior fixation.

Results: The ROM increase from UID to UID+1 is almost consistently higher in two-level soft-landing constructs with the exception of DSD (one-level: +3.66°; two-level: +3.52°) in LB. One-level U-PET had the smallest ROM increase, in FE (+0.79°) and AR (+2.18°). LB ROM increase was similar for U-PET one-level (+1.76°) and two-level (+1.79°). In LB, TP had the smallest ROM increase of one level (+1.44°) and two-level (+1.66°) constructs. Of all one-level soft-landing constructs, DSD had the greatest ROM increase, in all bending planes (FE: +1.87°; LB: +3.66°; AR: +3.91°). Of all two-level soft-landing constructs, DSD had the greatest ROM increase in FE (+3.29°) and LB (+3.52°); while T-PET had the greatest ROM increase in AR (+4.58°).

Conclusion : Adjacent to fusion, two-level soft-landing constructs may be more likely to induce greater ROM at the free adjacent discs, as compared to one-level soft-landing constructs. Further clinical analysis is needed to determine if there this translates to a greater risk of PJK adjacent to the fusion.