Abstract Purpose To evaluate the biomechanical performance of a novel Bezier surface-smoothed transition rod, and to compare it to conventional and stepped rods, focusing on correction capability, spinal stabilization, instrumentation and spinal loading related to risk of proximal junctional kyphosis (PJK). Methods A spine finite element model with patient-specific 3D spinal geometry (severe sagittal imbalance from thoracolumbar kyphosis) was used. Surgical instrumentation with five rod types was simulated: (1) constant 6.0 mm diameter, (2) stepped 6.0 mm–5.0 mm diameter, (3) Bezier 6.0 mm–5.5 mm–5.0 mm diameter, (4) constant 5.5 mm diameter, and (5) Bezier 5.5 mm–5.0 mm–4.75 mm diameter. Gravitational forces and flexion movements were simulated to compare load transfer between the spine and instrumentation. Results All rod configurations achieved equivalent sagittal correction. Load distribution analysis showed that Bezier rods provided smoother load transitions and better offloading of proximal segments compared to constant diameter rods. The highest moment sustained by the segment adjacent to the instrumentation was observed with the constant 6 mm rod (9N.m), while the Bezier 5.5–5–4.75 mm rod showed the lowest moment (7.5Nm), indicating reduced stress of 16% on the upper adjacent vertebrae. Similarly, the Bezier rods were more effective in offloading pedicle screws up to 45% with respect to the stiffer rod construct, potentially reducing the risk of PJK. Conclusions The simulation analysis demonstrates Bezier rods offer promising biomechanical benefits particularly in load distribution and stress reduction at adjacent levels of long thoracolumbar instrumentation. Future efforts will focus on clinical validation and optimization of patient-specific designs.