Introduction

Interspinous fixation devices (IFDs) serve as less invasive alternatives to pedicle screw fixation for treating spinal stenosis and facilitating stabilization. Over time, their design has evolved from basic extension blocks to sophisticated systems incorporating set‐screw locking mechanisms and plate geometries. However, comparative data evaluating how differing design features influence fixation strength have been lacking. This study aims to fill that gap by biomechanically testing two IFD designs: the symmetrical, dual-locking InSpan (InSpan LLC), and the asymmetrical, single-locking Aspen (ZimVie) Journal of Spine Surgery.

Methods

The study employed bench tests under ambient conditions using an INSTRON 8874 servohydraulic testing system. Two testing modalities were used:

1. Static Pullout Testing: Each device was installed into both stainless steel and 40 pcf polyurethane foam substrates, then subjected to pullout loads. Measurements recorded included maximum pullout force and displacement.

2. Static Disassembly Testing: Devices—both “pristine” and “used” (post-pullout)—were tested for resistance to mechanical disassembly through set manipulations of the plates Journal of Spine Surgery.

The metrics focused on load-to-failure data (peak force before failure) and failure modes.

Results

• Pullout Resistance: The InSpan device exhibited approximately 94.8% greater resistance to pullout relative to the Aspen device (statistically significant, P < 0.05). This was ascribed in part to InSpan’s significantly larger footprint (69.8% larger) Journal of Spine Surgery.

• Static Disassembly:

• Pristine Devices: InSpan required 60.7% more force to disassemble than the Aspen.

• Used Devices: The difference was even more dramatic—InSpan demanded 401.3% more force to disassemble compared to the Aspen Journal of Spine Surgery.

• Failure Mechanics: Both devices failed primarily at the foam block–implant interface. Observed failure patterns involved gradual plate distraction and material removal at the set-screw contact points, underscoring the critical roles of plate surface area and tooth design in preventing dislodgment Journal of Spine Surgery.

Discussion and Conclusion

The biomechanical findings clearly favor the dual-locking symmetrical InSpan design in terms of both pullout and disassembly strength. These results suggest that increased contact footprint and symmetry—with dual set-screw engagement—enhance fixation stability significantly compared to an asymmetrical, single-locking model like Aspen.

Implication: Device architecture matters. For improved long-term biomechanical performance in spinal fixation—especially in dynamic loading scenarios—designs like InSpan may offer enhanced durability and reduced risk of loosening or dislodgement.

Next Steps: These results pave the way for clinical investigation to verify whether these biomechanical advantages translate to better patient outcomes in vivo, such as reduced migration, lower complication rates, or improved fusion stability.