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Biomechanical Investigation of Lumbar Interbody Fusion Supplemented with Topping-off Instrumentation Using Different Dynamic Stabilization Devices

A normal finite element model of human lumbosacral spine was modified to simulate interbody fusion with topping-off instrumentation using pedicle-based dynamic stabilization system (PBDSS) or interspinous process spacer (ISP). Biomechanical effects of the PBDSS-based and ISP-based topping-off devices on the transition segment and its supra-adjacent segment were compared. Study Design. A biomechanical comparison study using finite element method. Objective. The aim of this study was to investigate effects of different dynamic stabilization devices, including pedicle-based dynamic stabilization system (PBDSS) and interspinous process spacer (ISP), used for topping-off implants on biomechanical responses of human spine after lumbar interbody fusion. Summary of Background Data. Topping-off stabilization technique has been proposed to prevent adjacent segment degeneration following lumbar spine fusion. PBDSS and ISP are the most used dynamic stabilizers for topping-off instrumentation. However, biomechanical differences between them still remain unclear. Methods. A validated, normal FE model of human lumbosacral spine was employed. Based on this model, rigid fusion at L4–L5 and moderately disc degeneration at L3–L4 were simulated and used as a comparison baseline. Subsequently, Bioflex and DIAM systems were instrumented at L3–L4 segment to construct PBDSS-based and ISP-based topping-off models. Biomechanical responses of the models to bending moments and vertical vibrational excitation were computed using FE static and random response analyses, respectively. Results. Results from static analysis showed that at L3–L4, the response parameters including annulus stress and range of motion were decreased by 41.6% to 85.2% for PBDSS-based model and by 6.3% to 67% for ISP-based model compared with rigid fusion model. At L2–L3, these parameters were lower in ISP-based model than in PBDSS-based model. Results from random response analysis showed that topping-off instrumentation increased resonant frequency of spine system but decreased dynamic response of annulus stress at L3–L4. PBDSS-based model generated lower dynamic stress than ISP-based model at L3–L4, but the dynamic stress was higher at L2–L3 for PBDSSbased model. Conclusion. Under static and vibration loadings, the PBDSSbased topping-off device (Bioflex) provided a better protection for transition segment, and likelihood of degeneration of supraadjacent segment might be relatively lower when using the ISPbased topping-off device (DIAM). Level of Evidence: 5