Abstract
BACKGROUND: Spinal cord compression in patients with vertebral metastases often requires surgical decompression with spinal fixation. Recent studies reported increased implant failures due to mechanical complications, raising concerns about current clinical practices. Long-segment fixation (Lf) is commonly employed to enhance mechanical stability and reduce the severity of pedicle screw failure. The study investigates how the number of vertebral levels involved in fixation affects the loads on pedicle screw anchorages in a fatigue-related displacement domain. METHOD: Using a rigid-flexible multibody approach, a non-linear T12-S1 model was employed to simulate two fixation types following L3 posterior decompression surgery: Lf spanning two levels above and below the decompression site (L1, L2, L4, and L5) and a short-segment fixation (Sf) involving only adjacent vertebrae. Internal reactions at the rod-pedicle screw anchorages were estimated in terms of pullout, shear forces, and bending moments. The range of motion analysed (flexion: 22°, extension: 8°, lateral bending: 12°, axial rotation: 5°) was confined to the "Cone of Economy", representing a small-displacement volume where loads are assumed cyclically exchanged. RESULTS: Lf exhibited up to fivefold higher reactions than Sf, with a heterogeneous shear force distribution: middle screws appeared shielded, while extremity screws were overloaded (~400 N, comparable to experimental fatigue strength). Pullout forces remained within safe limits (< 150 N). CONCLUSIONS: The rigid-flexible multibody approach effectively estimated internal loads in the implant-spine constructs under dynamic conditions. The findings highlight the long-term implications of Lf, demonstrating that involving more vertebral levels triggers adverse loads on pedicle screws, potentially compromising implant durability.