Lightning Podiums: Spinal Potpourri - 803B

Presented by: W. Cho

Author(s):

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

Abstract

Introduction: 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. 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.

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 reported in the literature [1], major-failure (defined as pseudoarthrosis or rod fracture above S1) and non-failure 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 5.5mm diameters were modeled as titanium cylinders (yield strength = 795MPa). A moment load was applied at the T10 superior endplate to simulate gravimetric loading in a standing position.

Results: Despite a fixed gravity line position relative to the heels, differences in spinopelvic parameters resulted in a neutral sagittal alignment in the non-failure spine model, but produced a "sagittal forward" alignment of the major-failure spine model [3]. In order for the latter to maintain sagittal balance, pelvic retroversion was reported [3] and the major-failure spine was translated toward the heel by 10mm to simulate that. As a result, the bending moment was approximately 17.3Nm in the non-failure group and 20.7Nm in the major-failure group. Differences in loads produced 14mm translation and 4.9 degree rotation for the major-failure group - 18% and 14% higher than in the non-failure group (11.9mm translation/4.3 degree rotation) (Figure). Rod stresses were highest at L1-L2 and L4-L5. In the major-failure group, the maximum stress (138.3 MPa) was observed at the left rod between L4 and L5. In the non-failure group, the maximum stress (115.4 MPa) was at the left rod surface between L1 and L2. High stress (141.0 MPa) was also observed in right S1 screws in the major-failure group; it was 42% higher than the stress observed in the non-failure group.

Conclusion: 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.

Figure 1