General Session: Biomechanics - Hall F
Presented by: R. Haddas
R. Haddas(1), M. Xu(2), I. Lieberman(3), J. Yang(2)
(1) Texas Back Institute, Research Foundation, Plano, TX, United States
(2) Texas Tech University, Mechanical Engineering, Lubbock, TX, United States
(3) Texas Back Institute, Plano, TX, United States
Introduction: Adult degenerative scoliosis results from age related changes leading to segmental instability, deformity and stenosis. Spinal alignment surgery has proven to be successful when performed for the correct indication. Adjacent segment degeneration is a clinically relevant deterioration of adjacent vertebral segments after a fusion, and it is a well-recognized outcome after spinal fusion. It has been described as radiographic changes associates with new clinical symptoms, occurring after a symptom free interval, to the radiographic changes. These phenomena are often seen following a spinal fusion where either the proximal or distal adjacent segment can degenerate and lead to further symptoms.
Purpose: To investigate the effect of adjacent load transfer pre and post fusion surgery of lumbar scoliotic spines using FE models.
Methods: Ten patients (7 males and 3 females, age: 60.9 ± 15.0, Cobb angle: 28.1º ± 10.5º) pre- and post-scoliosis surgery CT scans were used to produced 20 3D nonlinear FE models of the scoliotic lumbosacral spine. Furthermore, five (3 males and 2 females, age: 41.4 ± 10.0) three-dimensional nonlinear FE models of the healthy lumbosacral spine were created based on CT scans of five healthy subjects 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). Repeated measurement ANOVA used to determine differences in RoM, IDP, and FJF in scoliosis pre- and post-surgery FE models.
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 top fused level on average whereas 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 is the first to investigate the biomechanical changes of adjacent load transfer before and after fusion surgery of lumbar scoliotic spines using FE models based on in vivo CT scans of scoliotic spines. A posterior fusion has only a minor effect on mechanical behavior and large effect on pressure and forces at adjacent level. Large effects in spine kinematics and kinetics were found at the fused level. The results will help define the variables contributing to adjacent level deterioration in the clinical environment. With this information a surgeon can plan the most appropriate surgical strategy pre-surgery to prevent mid and long term adverse outcomes associated with adjacent level deterioration.