Study design:

A biomechanical comparison study using finite element (FE) method.

Objective:

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 41.6-85.2% for PBDSS-based model and by 6.3-67% 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 PBDSS-based model.

Conclusions:

Under static and vibration loadings, the PBDSS-based topping-off device (Bioflex) provided a better protection for transition segment, and likelihood of degeneration of supra-adjacent segment might be relatively lower when using the ISP-based topping-off device (DIAM).Level of Evidence: 5.