<p>Proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) are common complications of adult spinal deformity (ASD) surgery. Hybrid fixation techniques have emerged as a potential solution to mitigate this risk. However, the biomechanical benefits of hybrid rod fixation in long-segment spinal surgery have not been fully explored or validated. This study aimed to evaluate the effect of hybrid fixation using both polyetheretherketone (PEEK) rods and titanium rods on the increased incidence of PJK and PJF after long-segment ASD surgery using finite element analysis. A three-dimensional finite element model of the adult spine (T6-S1) was developed to simulate the biomechanical effects of different fixation techniques. An intact spine (model A) and two internal fixation configurations were modeled: model B incorporated titanium rods from T10 to S1 and model C incorporated PEEK rods from T10 to T11 with titanium rods from T11 to S1. Then, the resulting stresses on the intervertebral discs, facet joints, pedicle screws, and fixation rods were analyzed. Under six physiological motion states, intervertebral mobility and maximum von Mises stress on the adjacent segment (T9-T10) intervertebral disc, endplate, and facet joint was greater in models B and C than model A. Furthermore, intervertebral mobility and maximum von Mises stress on the adjacent segment intervertebral disc and facet joint were greater in model B than model C. Under all physiological motion conditions except for extension, the maximum von Mises stress on the adjacent segment endplate was greater in model B than model C. Hybrid fixation using PEEK and titanium rods shows promise for reducing the risk of PJK and PJF after ASD surgery. By decreasing intervertebral mobility and concentration of stress on the adjacent segment, hybrid fixation could improve long-term surgical outcomes and reduce the need for revision surgery.</p>