Biomechanical evaluation of transition rods and sublaminar hooks for proximal junctional kyphosis prevention: A finite element study.

Abstract

Proximal junctional kyphosis (PJK) is a common complication following long-segment spinal fusion, characterized by abnormal kyphotic angulation at the junctional level adjacent to the upper instrumented vertebra (UIV). Multiple risk factors, including construct rigidity and abrupt stiffness transitions have been implicated. Strategies such as sublaminar hooks and transition rods have been proposed to achieve a "soft landing" and mitigate PJK risk. A validated FE model of the T1-pelvis human spine was developed and instrumented with four posterior construct configurations: (1) uniform 6 mm rods; (2) transition rods tapering from 6 mm to 5 mm; (3) transition rods tapering from 6 mm to 4.5 mm; and (4) pedicle screws with a sublaminar hook at T10. Flexion-extension, lateral bending, and axial rotation were simulated. Range of motion (RoM), von Mises stress at the UIV and vertebra directly above the UIV (UIV + 1), and discal stresses were evaluated. All constructs reduced global RoM (≈54-73%) relative to intact model. Sublaminar hook constructs reduced UIV stress (13 MPa) and annular stress (29.4%) most effectively. Transition rods showed superior performance in lowering rod stress (≈69 MPa) and nucleus stress (≈40.5%). While transition rods are effective in reducing the rod and nucleus pulposus stress, hooks are better in reducing UIV and annular stress. The results emphasize that hooks and transition rods may be preferred in osteoporotic and sagittal imbalance cases, respectively.