Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P7. Gravity line and sagittal alignment are both risk factors for increased rod stresses in long lumbopelvic fixation: A biomechanics study

Abstract

BACKGROUND CONTEXT Posterior lumbopelvic fixation with iliac screws is a method that is commonly used for reconstructing the spine. However, clinically significant failure of lumbopelvic fixation (11.9%) and other complications such as pseudoarthrosis (observed in up to 50% of patients with rod failure) are possible, requiring revision surgery. With high rates of implant failure, questions remain regarding mechanical risk factors, or if there is any relationship between implant type, spinopelvic parameters, and failure to achieve fusion. PURPOSE The purpose of this study is to identify if—and to what extent—spinal-pelvic parameters play a role in construct failure using an in silico model. STUDY DESIGN/SETTING Finite element analysis. OUTCOME MEASURES Range of motion, rod stress or strain. 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 reported in the literature, major-failure (defined as pseudoarthrosis or rod fracture above S1) and nonfailure groups. In both groups, vertebral segments were modeled as three-dimensional solid elements. Intervertebral discs 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 5.5 mm diameters were modeled as titanium cylinders (yield strength=795 MPa). A moment load was applied at the T10 superior endplate to simulate gravimetric loading in a standing position. Both nonfailure and major-failure spines had a similar gravity line offset from the heels, as provided by the literature. A model validation was carried out to compare the range of motion of the intact spine unit with the experiments under 10Nm moment in flexion and extension. RESULTS Despite a fixed gravity line position relative to the heels, differences in spinopelvic parameters resulted in a neutral sagittal alignment in the nonfailure spine model, but produced a “sagittal forward” alignment of the major-failure spine model. In order for the latter to maintain sagittal balance, pelvic retroversion was reported and the major-failure spine was translated toward the heel by 10 mm to simulate that. As a result, the bending moment was approximately 17.3 Nm in the nonfailure group and 20.7 Nm in the major-failure group. Differences in loads produced 14 mm translation and 4.9° rotation for the major-failure group—18% and 14% higher, respectively, than in the nonfailure group (11.9 mm translation/4.3° rotation). Rod stresses were highest at L1–L2 and L4–L5. In the major-failure group, maximum stress (138.3 MPa) was observed at the left rod between L4 and L5. In the nonfailure group, maximum stress (115.4 MPa) was at the left rod surface between L1 and L2. High stress (141.0 MPa) was also observed in the right S1 screws in the major-failure group; it was 42% higher than the stress observed in the nonfailure group. CONCLUSIONS Due to compensatory differences in alignment of spinopelvic parameters between normal and failed spines, in the presence of a fixed gravity line, the major failure cohort in this study observed a 20% higher load and 18% greater instability. The higher load and instability further increased loading and mechanical demand on the posterior rods in the lower lumbar spine, further emphasizing the importance of proper sagittal alignment.