A Biomechanical Comparison of 2 Different Topping-off Devices and Their Influence on the Sacroiliac Joint Following Lumbosacral Fusion Surgery

Objective Interspinous spacer (ISS)-based and pedicle screw-rod dynamic fixator (PDF)-based topping-off devices have been applied in lumbar/lumbosacral fusion surgeries for preventing the development of proximal adjacent segment degeneration. However, little attention has been paid to sacroiliac joint (SIJ), which belongs to the adjacent joints. Accordingly, the objective of this study was to compare how these 2 topping-off devices affect the SIJ biomechanics. Methods A validated, normal finite-element lumbopelvic model (L3–pelvis) was initially adjusted to simulate interbody fusion with rigid fixation at the L5–S1 level, and then the DIAM or BioFlex system was instrumented at the L4–5 level to establish the ISS-based or PDF-based topping-off model, respectively. All the developed models were loaded with moments of 4 physiological motions using hybrid loading protocol. Results Compared with the rigid fusion model (without topping-off devices), range of motion and von-Mises stress at the SIJs were increased by 23.1%–64.1% and 23.6%–62.8%, respectively, for the ISS-based model and by 51.2%–126.7% and 50.4%–108.7%, respectively, for the PDF-based model. Conclusion The obtained results suggest that the PDF-based topping-off device leads to higher increments in SIJ motion and stress than ISS-based topping-off device following lumbosacral fusion, implying topping-off technique could be linked to an increased risk of SIJ degeneration, especially when using PDF-based device.