General Session: Biomechanics - Hall F
Presented by: W. Wang
W. Cho(1), W. Wang(2), B. Bucklen(2)
(1) Albert Einstein College of Medicine/Montefiore Medical Center, Department of Orthopaedic Surgery, Bronx, NY, United States
(2) Musculoskeletal Education of Research Center, Biomechanics, Audubon, PA, United States
Introduction: The failure rate of lumbopelvic fixation after long construct fusions in patients with adult spinal deformity has been reported as high as 11.9%. A retrospective study found that rod fractures occurred in 5.2% of patients (8/155) with 6 mm rod diameter and the use of iliac screws was identified as a risk factor. Pseudoarthrosis has been observed in 50% of patients with rod failure, necessitating revision surgery. With potentially high rates of implant failure, questions remain regarding which risk factors are dominant, and if there is any relationship between implant type, spinopelvic parameters, or failure to achieve fusion. The purpose of this study is to evaluate the biomechanics of long-segment posterior reconstructions as affected by 1) biological fusion versus pseudoarthrosis, and 2) major failure versus non-failure conditions.
Materials and Methods: Finite element models (T10-Pelvis) were created to match the average spinal-pelvic parameters (pelvic tilt, sacral slope, and lumbar lordosis) of two cohorts of patients, major-failure (defined as pseudoarthrosis or rod fracture above S1) and non-failure groups. Vertebral segments were modeled as three-dimensional solid elements. Intervertebral discs, including the nucleus and annulus, were structured as hyperelastic materials. The sacroiliac joint was modeled as articular cartilage contacts surrounded by six types of strong ligaments depicted as spring elements. Pedicle screws with 5.5mm diameters were modeled as titanium cylinders. A load was applied at the T10 superior endplate to simulate gravimetric loading in a standing position. Overall spine stiffness was increased by 200% for "solid" fused spines and by 100% for pseudoarthrosis.
Results: Upper body weight acting on the long instrumented spine created bending moments of 17.3Nm and 20.7Nm in the non-failure and major-failure groups, respectively. Loading resulted in 14mm translation and 4.9 degree rotation for the major-failure group - 18% and 14% higher than the non-failure group. Solid posterior fusion dramatically increased the stiffness of the spine, decreasing translation by 42% and 41% and rotation by 35% and 31% in the non-failure and major-failure groups, respectively. Higher stress was observed in the rod at the L1-L2 and L4-L5 levels in both groups. Rod stress was 20% higher in the major failure group than in the non-failure group (see figure). Solid posterior fusion alleviated rod stress in the lower lumber region. Conversely, simulated pseudoarthrosis produced stress patterns similar to those observed in the construct-only spine, regardless of spinopelvic parameters.
Conclusion: The spinopelvic parameters of the major-failure group produced increased gravity load, resulting in increased motion, stress, and strain compared to the non-failure group. Simulated "solid" fusion in the lumbar region further increased lumbar rigidity, decreasing range of motion and stress/strain, while shifting stress concentrations to the proximal treated levels. In total, the results emphasize both the importance of sagittal alignment and the necessity of biological fusion to reduce mechanical demand on screw and rod fixation in lumbopelvic fixation.