Implantable Spinal Fusion Stimulator

The implantable spinal fusion stimulators by Biomet consist of a direct current generator (pink arrow) in a titanium shell with a focal coating of platinum (serves as the anode) connected to insulated nonmagnetic steel leads (white arrows) that in turn connect to bare wire cathodes (blue arrows). The cathodes get embedded in pieces of bone graft by the surgeon and are thought to stimulate fusion of the allograft. The SpF-XL IIb Spinal Fusion Stimulator model is shown in the image above.

The generator is powered by a lithium iodine battery that is capable of delivering 20-μA of current for about 6 months. After that, the device is non-functional.

The device can be safely used in magnets with static field strengths of 1.5 T or less. The cathodes should ideally be placed at least 1 cm from nerve roots (to reduce the possibility of nerve excitation during MR imaging). It would also be nice if the stimulator is placed as far as possible from the spinal canal and bone graft to decrease the effect of artifact.

In addition,

• Radiographs should be obtained prior to MRI to verify that the leads are intact. Broken leads can lead to excessive heating during imaging. If radiographs cannot make this determination, the risk of heating should be discussed with the patient prior to the study.
• MRI should be performed in magnets with static fields of 1.5 T or less.
• Spin-echo, fast spin-echo, and gradient echo pulse sequences are permissible.
• Echo planar techniques, conditions that produce exposures to high levels of RF energy (exceeding a whole-body averaged specific absorption rate of 1.0 W/kg), exposure to gradient fields that exceed 20 T/sec, or unconventional MR sequences should be avoided.
• Patients should be continuously observed during the study and instructed to report any unusual sensations including any feelings of warming, burning, or neuromuscular excitation or stimulation.

References

• Biomet. MRI Safety Information - SpF®-XL IIb.
• Schwardt JD, Jankowski GB. Preformed extendable mesh cathode for implantable bone growth stimulator. US Patent 6,112,122
• Shellock FG, Hatfield M, Simon BJ, Block S, Wamboldt J, Starewicz PM, Punchard WF. Implantable spinal fusion stimulator: assessment of MR safety and artifacts. J Magn Reson Imaging. 2000 Aug;12(2):214-23.
• Tejano NA, Puno R, Ignacio JM. The use of implantable direct current stimulation in multilevel spinal fusion without instrumentation. A prospective clinical and radiographic evaluation with long-term follow-up. Spine (Phila Pa 1976). 1996 Aug 15;21(16):1904-8.