Stress distribution over lumbosacral vertebrae and axial transsacral rod after axial lumbar interbody fusion (AxiaLIF): finite element analysis

Objective: To evaluate stress distribution over lumbosacral vertebrae and axial transsacral rod by means of finite element analysis. Methods: The intact finite element model was established along L4-S1 lumbosacral vertebrae and validated. To build the standalone AxiaLIF model, an axial transsacral rod was inserted into L5-S1 segment of the validated intact model in accordance with the AxiaLIF technique. For patients with spondylolysis or unilateral direct decompression who received AxiaLIF, corresponding models were established. Comparison of Von Mises stress was made to evaluate the stress distribution endured by bony components or transsacral rod with preload imposed on the models. Results: A higher concentration of stress was registered at the bone-rod interface. The stress distribution over vertebrae was witnessed in the anterior and anterolateral region of S1 vertebra during flexion and axial rotation, while such stress concentration moved to the posterior and lateral region of L5 vertebra in extension and lateral bending respectively. The highest stress turned out to be localizing somewhere one or two thread pitches away from the middle part of the rod. There was no obvious differences of stress distribution among the surgery-simulated models. Conclusion: Standalone AxiaLIF meets the basic need to provide ample stress resistance for body’s normal activities. For patients suffering from spondylolysis, or demanding unilateral direct decompression, of no significance was the influence that was exerted on load transfer after AxiaLIF.