Purpose: To investigate the effect of adjacent load transfer pre- and post-fusion surgery of lumbar scoliotic spines using FE models. Methods: Ten three-dimensional nonlinear FE models of the lumbosacral spine were created from pre- (Cobb angle: 28.1° ± 10.5°) and post-scoliosis surgery and healthy in vivo CT scans. During surgery, pedicle screws and rods were implanted at lumbar and sacral levels. A compressive load and six different moments (flexion, extension, lateral bending, and axial rotation) were applied to the top level of each model. Outcome measures were range of motion (RoM), intradiscal pressure (IDP), and facet joint forces (FJF). Results: Spinal fusion did alter the mechanical function of the scoliotic spine. Scoliotic spine presented abnormal and asymmetrical kinetic and kinematic behavior in comparison to a healthy spine. After the fusion surgery, RoM decreased by 91.88 %, IDP decreased by 46.87 %, and FJF decreased by 60.63 % at the fused level on average whereas a minor increase of RoM, IDP, or FJF was observed at the adjacent level. Compared to the healthy subjects, the pre-surgical scoliosis subjects have up to 8.03 % greater RoM, 20.04 % increased IDP, and 18.38 % increased FJF on average at the adjacent level. Conclusions: This study was the first to investigate the effect of adjacent load transfer before and after fusion surgery using in vivo CT scans of ten scoliotic spines. A posterior fusion has only a minor effect on mechanical behavior and large effect on pressure and forces at the adjacent level. As expected, a large effect on the kinematics and kinetics was found at the fused level.