New percutaneously inserted spinal fixation system

We describe a new percutaneous minimally invasive spinal fixation system based on pedicle screws and inflatable rods. The rods are inserted in a flexible state and harden following deployment. We test this system in terms of biocompatibility, ferromagnetism, magnetic resonance artifact production, bench top mechanical testing, ease of insertion within cadavers, potential thermal damage to paraspinous muscles in pigs, and long-term device tolerability in sheep.

To determine the safety and utility of this system before its use in human subjects.

Composite materials and epoxy compounds have been used safely in a variety of implanted medical devices for years with no signs of systemic toxicity or significant device failures.

Long-term biocompatibility test of system components was conducted according to International Standards Organization 10993 and Food and Drug Administration Blue Book Memorandum #G95-1 standards. Device components were assessed for magnetic deflection and torque and imaged in a 1.5 Tesla magnetic resonance unit. Full constructs of the system were tested for compression strength, torque, and fatigue per American Society for Testing and Materials F1717 standards. The system was deployed using C-arm fluoroscopic guidance in 11 cadavers and 2 live sheep. Further, the inflatable rods were tested for exothermic damage to paraspinous musculature in 2 pigs.

All system components were found to be biocompatible, nonferromagnetic, and produce little magnetic resonance artifact. Compression and torque results for the new system were found to be comparable to standard metallic pedicle screw and rod fixation systems. However, the new system displayed a superior modulus of elasticity relative to standard surgical systems. The new system endured 5 million cycles of repetitive compressions without breakage or significant wear. All cadaver and sheep insertions were performed successfully. Sheep suffered no complications, and minimal blood loss occurred during device insertions. One of the animals killed at 6 months demonstrated no internal organ damage. The self-curing version of polymer used to inflate the flexible rods cured to approximately 53% of its final strength in 90 minutes with maximum external rod temperature of 40.5 C. and no adjacent thermal damage within porcine paraspinous musculature.

The new spinal fixation system is biocompatible, uses a nontoxic polymer, is magnetic resonance compatible, displays favorable biomechanical characteristics, can be easily deployed percutaneously using simple fluoroscopic guidance, is well tolerated in living sheep, caused no muscular thermal damage, and could be used in humans within a reasonable operative time frame. The new system demonstrates the feasibility of percutaneously constructing composite structures in situ within the body.