Bone Fusion (Spinal) Fusion Stimulator

Bone Fusion Stimulator/Spinal Fusion Stimulator
The implantable bone fusion or spinal fusion stimulator (EBI, LLC., Biomet, Inc.) is designed for use as an adjunct therapy to a spinal fusion procedure. The implantable spinal fusion stimulator consists of a direct current generator with a lithium iodine battery and solid-state electronics encased in a titanium shell, partially-coated with platinum that acts as an anode. The generator is implanted beneath the skin and muscle near the vertebral column and provides the full-rated current for approximately 24 to 26 weeks. The use of this electronic implant provides a faster consolidation of the bone grafts, leading to higher fusion rates and improved surgical outcomes, along with a reduced need for orthopedic instrumentation.

Studies using excessively-high electromagnetic fields under highly-specific experimental conditions and modeling scenarios for the lumbar/torso area (i.e., 1.5-Tesla MR system, excessive exposures to RF fields, excessive exposures to gradient magnetic fields, etc.) have demonstrated that the implantable spinal fusion stimulator will not present a hazard to a patient undergoing MR imaging with respect to movement, heating, or induced electrical fields during the use of conventional MR techniques.

Additionally, there was no evidence of malfunction of the implantable spinal fusion stimulator based on in vitro and in vivo experimental findings. These studies addressed the use of conventional pulse sequences and parameters with an acknowledgement that echo planar techniques or imaging parameters that require excessive RF power will have different implications and consequences for the patient with an implantable spinal fusion stimulator.

To date, MR examinations have been performed in many patients (conceivably, using MR imaging conditions that involved a wide-variety of imaging parameters and conditions) with implantable spinal fusion stimulators with no reports of substantial adverse events (based on a review of data obtained through the Freedom of Information Act and Unpublished Observations, Simon BJ, EBI, LLC, Parsippany, NJ).

In an in vivo study, there were no reports of immediate or delayed (minimum of one month follow up) adverse events from patients with implantable spinal fusion stimulators who underwent MR imaging at 1.5-Tesla. Each patient was visually inspected following the MRI study and there was no evidence of excessive heating (i.e., change in skin color or other similar response).

One patient indicated a sensation of “warming” felt at the site of the stimulator, however, this feeling was described as minor and the MR examination was completed without further indication of unusual sensations or problems. Of further note is that there were no reports of excessive heating or neuromuscular stimulation in association with the presence of the implantable spinal fusion stimulators in patients that underwent MR procedures.

Chou et al. conducted a thorough investigation of the effect of heating of the implantable spinal fusion stimulator associated with MR imaging. This work was performed using a human phantom during MR procedures involving a relatively high exposure to RF energy (i.e., at whole body averaged specific absorption rates of approximately 1.0-W/kg). Fluoroptic thermometry probes were placed at various positions on and near the cathodes, leads, and the stimulator for each experiment to record temperature changes.

The phantom used by Chou et al. did not include the effects of blood flow, which obviously would help dissipate heating that may occur during MR imaging and, therefore, it further represents an excessive RF exposure condition. With the implantable spinal fusion stimulator in place and the leads intact, the maximum temperature rise after 25 minutes of scanning occurred at the center of the stimulator and was less than 2 degrees C.

The temperature rise at the cathodes was less than 1 degree C. When the stimulator was removed, the maximum temperature rise was less than 1.5 degrees C, recorded at the tip of the electrode with insignificant temperature changes occurring at the cathode. These temperature changes are within physiologically acceptable ranges for the tissues where the implantable spinal fusion stimulator is implanted, especially considering that the temperatures for muscle and subcutaneous tissues are at levels that are known to be several degrees below the normal core temperature of 37 degrees C.

Chou et al. also investigated heating of the tips of broken leads of the implantable spinal fusion stimulator (this device was the same as that which underwent testing in the present study). Temperature changes occurred in localized regions that were within a few millimeters of the cut ends of the leads, with maximum temperature increases that ranged from 11 to 14 degrees C.

If these levels of temperatures occurred during MR imaging, the amount of possible tissue damage would be comparable in characteristics and clinical significance to a small electrosurgical lesion and would likely occur in the scar tissue that typically forms around the implanted leads. Additionally, the potential for tissue damage is only theoretical and a brief temperature elevation around a broken lead, over an approximated volume of 2- to 3-mm radius may not be clinically worse than the scar tissue that forms over the leads during implantation. Fortunately, broken leads are rare, occurring in approximately 10 out of the 70,000 devices implanted over the last ten years (Personal Communication, Simon BJ, EBI, LLC, Parsippany, NJ).

Based on findings from the various investigations that have been conducted, RF energy-induced heating during MR imaging does not appear to present a major problem for a patient with the implantable spinal fusion stimulator, as long as there is no broken lead. Accordingly, the integrity of the leads should be assessed using a radiograph prior to the MR procedure.

MRI Safety Information - SpF-XL IIb
SpF-XL IIb Model
Safety information for the use of Magnetic Resonance Imaging (MRI) procedures (i.e., imaging, angiography, functional imaging, spectroscopy, etc.) pertains to shielded MRI systems with static magnetic fields of 1.5 Tesla or less (maximum spatial gradient 450 gauss/cm), gradient magnetic fields of 20 Tesla/second or less, and a maximum whole body averaged Specific Absorption Rate (SAR) of 1.1 W/kg for 25 minutes of imaging. The effects of MRI procedures using MR systems and conditions above these levels have not been determined.

The SpF devices have been determined to be MR safe, thereby (when used in the MRI environment) there is no additional risk to the patient, but may affect image quality. MRI procedures must only be performed according to the following guidelines:

-Plain films (radiographs) must be obtained to assess the site of the implanted SpF prior to the MRI examination to verify that there are no broken leads present.
-If this cannot be reliably determined, then the potential risks and benefits to the patient requiring the MRI examination must be carefully assessed in consideration of the possibility of excessive heating to develop in the leads.
-The patient must be continuously observed during the MRI procedure and instructed to report any unusual sensations including any feelings of warming, burning, or neuromuscular excitation or stimulation.
-If these occur, the MRI procedure must be discontinued.

Static Magnetic Field of MR Systems
A patient with a SpF may safely undergo an MRI procedure using a shielded MR system with a static magnetic field of 1.5 Tesla or less (maximum spatial gradient magnetic field of 450 gauss/cm)

Gradient Magnetic Fields of MR Systems
Pulse sequences (e.g., echo planar imaging techniques or other rapid imaging pulse sequences), gradient coils or other techniques, and procedures that exceed a gradient magnetic field of 20 Tesla/second must not be used for MRI procedures. The use of unconventional or non-standard MRI techniques must be avoided. Standard or conventional pulse sequences (e.g., spin echo, fast spin echo, gradient echo, etc.) may be used for MRI examinations.

Radio Frequency (RF) Fields of MR Systems
MRI procedures must not exceed exposures to RF fields greater than a whole body averaged specific absorption rate (SAR) of 1.1 W/kg for 25 minutes of imaging. The use of unconventional or non-standard MRI techniques must be avoided.

MRI Artifacts
Artifacts for the SpF have been characterized using a 1.5 Tesla MR system (maximum spatial gradient 450 gauss/cm) and various pulse sequences. Based on this information, implantation of the SpF (i.e., with reference to the center of the device) a distance of at least 5-8 cm from the imaging area of interest is likely to maintain the diagnostic quality of the MRI examination. Artifact size is dependent on the type of pulse sequence used for imaging (e.g., larger for gradient echo pulse sequences and smaller for fast spin echo pulse sequences), the direction of the frequency encoding direction (larger if the frequency encoding direction is perpendicular to the device and smaller if it is parallel to the device), and the size of the field of view. Positional errors and artifacts on MR images may be larger for MR systems with static magnetic field strengths greater than 1.5 Tesla or smaller for MR systems with lower static magnetic fields strengths using the same imaging parameters.

Implant the SpF as far as possible from the spinal canal and bone graft is desirable since this will decrease the possibility that artifacts will affect this area of interest on MRI examinations. The use of fast spin echo pulse sequences will minimize the amount of artifact associated with the presence of the SpF compared to the use of other imaging techniques.

Nerve Excitation
The cathodes of the implantable spinal fusion stimulator must be positioned a minimum of 1 cm from nerve roots to reduce the possibility of nerve excitation during a MRI procedure.

Torque
To minimize the possibility of magnetically induced torque during MR imaging, the stimulator should be oriented with its broad face (36 mm x 23 mm plane) parallel to the body and to the static field lines inside the bore.

MRI Safety Information - SpF PLUS-Mini
SpF PLUS-Mini 60 microA/W and 60 microA/M
Safety information for the use of Magnetic Resonance Imaging (MRI) procedures (i.e., imaging, angiography, functional imaging, spectroscopy, etc.) pertains to shielded MRI systems with static magnetic fields of 1.5 Tesla or less (maximum spatial gradient 250 gauss/cm), gradient magnetic fields of 20 Tesla/second or less, and a maximum whole body averaged Specific Absorption Rate (SAR) of 1.1 W/kg for 25 minutes of imaging. The effects of MRI procedures using MR systems and conditions above these levels have not been determined.

The SpF devices have been determined to be MR safe, thereby (when used in the MRI environment) there is no additional risk to the patient, but may affect image quality. MRI procedures must only be performed according to the following guidelines:

-Plain films (radiographs) must be obtained to assess the site of the implanted SpF prior to the MRI examination to verify that there are no broken leads present.
-If this cannot be reliably determined, then the potential risks and benefits to the patient requiring the MRI examination must be carefully assessed in consideration of the possibility of excessive heating to develop in the leads.
-The patient must be continuously observed during the MRI procedure and instructed to report any unusual sensations including any feelings of warming, burning, or neuromuscular excitation or stimulation.
-If these occur, the MRI procedure must be discontinued.

Static Magnetic Field of MR Systems
A patient with a SpF may safely undergo an MRI procedure using a shielded MR system with a static magnetic field of 1.5 Tesla or less (maximum spatial gradient magnetic field of 250 gauss/cm)

Gradient Magnetic Fields of MR Systems
Pulse sequences (e.g., echo planar imaging techniques or other rapid imaging pulse sequences), gradient coils or other techniques, and procedures that exceed a gradient magnetic field of 20 Tesla/second must not be used for MRI procedures. The use of unconventional or non-standard MRI techniques must be avoided. Standard or conventional pulse sequences (e.g., spin echo, fast spin echo, gradient echo, etc.) may be used for MRI examinations.

Radio Frequency (RF) Fields of MR Systems
MRI procedures must not exceed exposures to RF fields greater than a whole body averaged specific absorption rate (SAR) of 1.1 W/kg for 25 minutes of imaging. The use of unconventional or non-standard MRI techniques must be avoided.

MRI Artifacts
Artifacts for the SpF have been characterized using a 1.5 Tesla MR system (maximum spatial gradient 250 gauss/cm) and various pulse sequences. Based on this information, implantation of the SpF (i.e., with reference to the center of the device) a distance of at least 5-8 cm from the imaging area of interest is likely to maintain the diagnostic quality of the MRI examination. Artifact size is dependent on the type of pulse sequence used for imaging (e.g., larger for gradient echo pulse sequences and smaller for fast spin echo pulse sequences), the direction of the frequency encoding direction (larger if the frequency encoding direction is perpendicular to the device and smaller if it is parallel to the device), and the size of the field of view. Positional errors and artifacts on MR images may be larger for MR systems with static magnetic field strengths greater than 1.5 Tesla or smaller for MR systems with lower static magnetic fields strengths using the same imaging parameters.

Implant the SpF as far as possible from the spinal canal and bone graft is desirable since this will decrease the possibility that artifacts will affect this area of interest on MRI examinations. The use of fast spin echo pulse sequences will minimize the amount of artifact associated with the presence of the SpF compared to the use of other imaging techniques.

Nerve Excitation
The cathodes of the implantable spinal fusion stimulator must be positioned a minimum of 1 cm from nerve roots to reduce the possibility of nerve excitation during a MRI procedure.

Torque
To minimize the possibility of magnetically induced torque during MR imaging, the stimulator should be oriented with its broad face (36 mm x 23 mm plane) parallel to the body and to the static field lines inside the bore.

[MR healthcare professionals are advised to contact the respective manufacturer in order to obtain the latest safety information to ensure patient safety relative to the use of an MR procedure.]

REFERENCES
http://www.biomet.com/spine/mriInfo.cfm?pdid=3

Chou C-K, McDougall JA, Chan KW. RF heating of implanted spinal fusion stimulator during magnetic resonance imaging. IEEE Trans Biomed Engineering 1997;44:357-373.

Shellock FG. Magnetic Resonance Procedures: Health Effects and Safety. CRC Press, LLC, Boca Raton, FL, 2001.

Shellock FG, Hatfield M, Simon BJ, Block S, Wamboldt J, Starewicz PM, Punchard WFB. Implantable spinal fusion stimulator: assessment of MRI safety. J Magn Reson Imaging 2000;12:214-223.