Surgery at the lumbar levels of the spine carries serious risks which the monitor can help reduce. Follow the table above as a guide to electrode placement.
Use 1699-00 electrodes inserted when the patient is in the final position on the operating table. Four sets of electrodes should be inserted into each leg with an assistant holding the leg if necessary. Tape all electrodes so they are flat. Connect to preamplifier as the electrodes are inserted.
Two 1705-00 reference electrodes should be inserted into the buttock or biceps femoris on each leg and taped down. A further single electrode will be required for the stimulator. This should be inserted into the shoulder.
Attach the pre-amplifier pod to the side rail of the operating table. Switch the main unit on and connect the pre-amplifier, stimulating pod and the mute sensor. Apply the Pedicle Screw setting.
Observe the bargraph or waveform when the channels are switched on. If there are no segments lit, or only the bottom one or two are flickering, then impedance is good. If in waveform mode, ensure the screen is not updating. If several segments are permanently lit or the screen is continuously updating, impedance is poor and the electrodes should be re-inserted.
The monopolar pedicle screw probe 3603-00 is recommended.
If necessary, adjust the stimulator output to 7mA. The stimulator probe should be connected when required rather than at the start of the surgery.
Ideally, levels above and below the immediate area of surgery should be monitored – patients tend to be individual, with the degree of innervation from the different levels varying from person to person.
Use the pedicle screw probe to stimulate. If the nerve root is directly stimulated, use a lower current level (<2mA); in order to stimulate through the pedicle wall, set the current level to 7mA.
Once the hole for the screw has been made, the stimulating probe can be run down it. The probe should be positioned at the thinnest part of the pedicle, about ⅓ from the top; this is the point of stimulation. If there is no response at 7mA, or the response is below 100µV as shown on the bargraph display, the screw may be inserted. Listen carefully for any indication that the pedicle wall is being breached. The principle is that bone acts as an insulator, so if no current flows to the nerve root then the bone is intact; if current flows, it implies that the bone is very thin or that there is a crack in the bone.
If there is significant response at 7mA, maintain the probe in position and reduce the stimulating current. Note the level at which the response drops below 100µV. If this level is reached above 5mA, continue and insert the screw with caution, listening for any activity.
If there is significant response between 3mA and 5mA, consider repositioning the hole.
If there is significant response at 2mA, the pedicle wall is almost certainly breached.
Products & consumables able to be used for this procedure include: