Short segment posterior fusion (SSPF) vs. long segment posterior fusion (LSPF) for a burst fracture and the biomechanical consequences on adjacent IVDs

Introduction: A burst fracture is a common thoracolumbar spine fracture. The primary treatment goal is to achieve optimal biomechanical stability while reducing the number of instrumented vertebrae. Studies have shown that the lower the spine is instrumented, the higher the risk of intervertebral disc degeneration. However, there are currently no studies that analyze intervertebral compression forces in a moving, living organism. This study aims to gain insights into spinal biomechanics and to compare the biomechanical effects of SSPF and LSPF constructs in a daily activity.

Methods: A multi-body simulation (MBS) approach was used to evaluate spinal biomechanics during daily activities, considering the anatomical characteristics of muscles and ligaments. Simulations were performed for both lifting and walking tasks, comparing forces in a healthy, SSPF and LSPF spine. (Fig. 1) Three different BMI cases (24, 28 and 32 kg/m²) were analyzed. In the LSPF, the spine was fused from L1 to L5, whereas in the SSPF from L2 to L4. The L5/S1 compression force was evaluated and analyzed for the different scenarios.

Fig. 1: Simulated daily lifting task and gait

Fig. 2: L5/S1 compression forces plotted for three different BMIs

Results: Across the different simulation scenarios, the highest compression forces occurred during the lifting task for the long segment fused spine, with forces increasing with BMI. (Fig. 2) In detail, the maximum recorded compression force was 5314 N for a BMI of 24, 5830 N for a BMI of 28 and 6345 N for a BMI of 32. During gait, peak compression forces were 3439 N for BMI 24, 3687 N for BMI 28 and 3934 N for BMI 32.

Conclusion: SSPF and LSPF don´t lead to a large increase in compression force during gait. During lifting, the compression forces are up to 20 % higher in the LSPF and up to 6 % higher in the SSPF spine compared to a healthy one.

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