Poster Presentation Sydney Spinal Symposium 2022

Quantifying the biomechanical effects of posterior fixation in XLIF (#114)

Vivek A.S. Ramakrishna 1 2 3 , Uphar Chamoli 4 5 , Alessandro G. Larosa 5 , Subhas C. Mukhopadhyay 3 , Gangadhara Prusty 1 , Ashish D Diwan 2 6
  1. School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW, Australia
  2. Spine Labs, St. George & Sutherland Clinical School, University of New South Wales, Sydney, NSW, Australia
  3. School of Engineering, Faculty of Science and Engineering, Macquarie University, Sydney, NSW, Australia
  4. St. George and Sutherland Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW, Australia
  5. School of Biomedical Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney, Ultimo, NSW, Australia
  6. Spine Service, Department of Orthopaedic Surgery, St. George Hospital Campus, Kogarah, NSW, Australia

Aims

Extreme Lateral Interbody Fusion (XLIF) may be performed with a standalone interbody cage, or with the addition of unilateral or bilateral pedicle screws. Whilst decisions regarding supplemental fixation are predominantly based on clinical indicators, the aim of this study was to quantify the biomechanical impacts of posterior supplemental fixation by examining its effect on facet micromotions, cage loads, and load-patterns at adjacent levels in a L4-L5 XLIF. Further, these were to be evaluated at partial and full fusion stages.

Methods

Computed tomography data from an asymptomatic subject were anatomically segmented and digitally stitched into a surface mesh of the lumbosacral spine (L1-S1). The interbody cage and posterior instrumentation were inserted at L4-L5. The volumetric mesh was imported into finite element software for pre-processing, running nonlinear static solves, and post-processing. XLIF with no fixation (NF), unilateral fixation (UL), and bilateral fixation (BL) were studied.

Results

Loads and micromotions at index-level facets reduced commensurately with extent of posterior fixation. Load-pattern changes observed at adjacent facets, however, were potentially anatomically dependent. In flexion at partial fusion, compressive strain on the cage-graft construct reduced by 48% and 64% in UL and BL models respectively; in extension the reductions were 51% and 69%. A similar pattern was observed at full fusion. There was a notable reduction in cage stress-risers with BL compared to UL and NF. No changes were found at adjacent discs. Posterior ligaments were more strained in flexion in adjacent regions (12-21%) with posterior fixation compared to NF. No biomechanical differences were found between UL and BL models at adjacent facets, discs, or ligaments.

Conclusion

Posterior supplemental fixation with XLIF alters biomechanics at index and adjacent levels. Results suggest that single-sided fixation provides sufficient biomechanical stability to the joint, which warrants consideration alongside clinical information (such as subsidence risk) during pre-surgical planning.