The Biomechanics of 1-level and 2-level Soft-Landing Techniques Below a Cervicothoracic Fusion Construct

Introduction: In multi-segmental cervicothoracic fusion (CTF), the mobile cervical spine meets the rigid upper thoracic segment. This alters biomechanics and increases motion in adjacent segments, which may lead to distal junctional kyphosis (DJK). Semi-rigid soft-landing techniques gradually translate stiffness between CTFs and non-rigid distal levels to help prevent DJK, but there is no consensus on technique or number of levels. This study investigated the biomechanical effects of single- and multi-level soft-landings on spinal range-of-motion (ROM) and intradiscal pressure (IDP).

Methods: Seven healthy fresh-frozen human cadaveric spines (O-T7) were tested for ROM and IDP during flexion-extension using a custom-built motion simulator (±2.5Nm). Specimens were treated with posterior fixation (C3-T2) and tested for ROM and IDP. One- and 2-level soft-landings were applied at T3-T4 and T3-T5, respectively, using intralaminar hooks and tensioned PET bands (100N). ROM and IDP were normalized to intact and compared at the lowest instrumented disc (LID) with rigid or semi-rigid fixation and the first two un-instrumented distal adjacent levels (LID-1 and LID-2).

Results: Following CTF, transition of motion was abrupt from LID-to-(LID-1) (+167% intact) and IDP decreased linearly from LID-to-(LID-2). Motion at the level below CTF (215% intact) dropped with 1-level hooks (83% intact) and tensioned bands (67% intact) but increased IDP (+35% and +203% intact). Motion at the 2nd level below CTF (213% intact) was less with 2-level hooks (67% intact) and bands (83% intact) than 1-level (179% and 224% intact) while IDP (140% intact) was greater with 2-level hooks (173% intact) and bands (312% intact) than 1-level (110% and 149% intact). One-level hooks and 2-level bands transitioned stiffness more gradually from LID-to-(LID-1) (+96% and +102% intact) than 2-level hooks and 1-level bands (+127% and +168% intact). Two-level bands transitioned stiffness more gradually from (LID-1)-to-(LID-2) (+28% intact) than 1- and 2- level hooks (+81% and +46% intact).

Conclusion : Two-level soft-landing via hooks or tensioned bands reduced motion distal to the CTF more than 1-level, but increased IDP. One-level tensioned bands did little to mitigate hypermobility and increased IDP the most. Two-level tensioned bands had less motion and a more gradual transition of stiffness than 1- and 2-level hooks, but greater IDP.