Ram Haddas1, Ming Xu2, Isador Lieberman3, James Yang2. 1. Texas Back Institute Research Foundation, 6020 West Parker Road No. 200, Plano, TX 75093, USA. Electronic address: rhaddas@texasback.com. 2. Mechanical Engineering, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA. 3. Texas Back Institute, 6020 West Parker Road No. 200, Plano, TX 75093, USA.
Abstract
STUDY DESIGN: Pre-post cohort finite elements (FE). OBJECTIVES: To investigate the effect of adjacent load transfer pre and post fusion surgery of lumbar scoliotic spines using FE models. SUMMARY OF BACKGROUND DATA: Adult degenerative scoliosis (ADS) results from age-related changes, leading to segmental instability, deformity, and stenosis. FE study is capable of capturing the biomechanical parameters internal to the bones and connective soft tissues of the spine, which is difficult to measure by experimental approaches. Literature that describes the underlying mechanisms responsible for spinal fusion in scoliosis patients is limited, and FE study with larger subject sample size should be conducted. METHODS: Twenty three-dimensional nonlinear FE models of the lumbosacral spine were created from pre (Cobb angle: 28.1° ± 10.5°) and post scoliosis surgery 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, right lateral bending, left lateral bending, right axial rotation, left 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). Spinal fusion did alter the mechanical function of the scoliotic spine. RESULTS: Scoliotic spine presented abnormal and asymmetrical kinetic and kinematic behavior. RoM: At the adjacent level, spinal fusion surgery produced a statically significantly increased left and right later bending intersegmental rotation (p < .006) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced a statically significantly reduced intersegmental rotation in all the loading conditions (p = .001) in comparison to presurgical scoliosis models. IDP: At the fused level, spinal fusion surgery produced a much lower IDP in all of the loading conditions (p = .001). FJF: At the adjacent level, spinal fusion surgery produced a considerably larger left lateral rotation FJF (p = .001) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced considerably lower FJF in all the loading conditions (p = .001) in comparison to presurgical scoliosis models. 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 10 scoliotic spines. A posterior fusion has only a minor effect on mechanical behavior and a large effect on pressure and forces at the adjacent level. As expected, a large effect in the kinematics and kinetics was found at the fused level. LEVEL OF EVIDENCE: Level 3.
STUDY DESIGN: Pre-post cohort finite elements (FE). OBJECTIVES: To investigate the effect of adjacent load transfer pre and post fusion surgery of lumbar scoliotic spines using FE models. SUMMARY OF BACKGROUND DATA: Adult degenerative scoliosis (ADS) results from age-related changes, leading to segmental instability, deformity, and stenosis. FE study is capable of capturing the biomechanical parameters internal to the bones and connective soft tissues of the spine, which is difficult to measure by experimental approaches. Literature that describes the underlying mechanisms responsible for spinal fusion in scoliosispatients is limited, and FE study with larger subject sample size should be conducted. METHODS: Twenty three-dimensional nonlinear FE models of the lumbosacral spine were created from pre (Cobb angle: 28.1° ± 10.5°) and post scoliosis surgery 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, right lateral bending, left lateral bending, right axial rotation, left 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). Spinal fusion did alter the mechanical function of the scoliotic spine. RESULTS: Scoliotic spine presented abnormal and asymmetrical kinetic and kinematic behavior. RoM: At the adjacent level, spinal fusion surgery produced a statically significantly increased left and right later bending intersegmental rotation (p < .006) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced a statically significantly reduced intersegmental rotation in all the loading conditions (p = .001) in comparison to presurgical scoliosis models. IDP: At the fused level, spinal fusion surgery produced a much lower IDP in all of the loading conditions (p = .001). FJF: At the adjacent level, spinal fusion surgery produced a considerably larger left lateral rotation FJF (p = .001) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced considerably lower FJF in all the loading conditions (p = .001) in comparison to presurgical scoliosis models. 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 10 scoliotic spines. A posterior fusion has only a minor effect on mechanical behavior and a large effect on pressure and forces at the adjacent level. As expected, a large effect in the kinematics and kinetics was found at the fused level. LEVEL OF EVIDENCE: Level 3.