Pedicle instrumentation is often used to facilitate spinal fusion. Pedicle screws extend through the pedicles of vertebrae and into the body of the vertebrae. The screws are connected by rods or plates to eliminate motion between the vertebrae that are fused together.
Misplaced pedicle screws can injury the nerves and blood vessels that surround the vertebrae. Numerous techniques are used to help surgeons guide screws into the pedicles of the vertebrae. For example, surgeons often use x-rays including fluoroscopy to confirm the position of pedicle screws.
Nerve compression by pedicle screws can also be determined through electrical stimulation of the pedicle screws. Prior-art techniques involve recording electrical impulses in the legs or arms after electrical stimulation of the pedicles. High conductivity of the electrical impulses suggests the pedicle screws are too close to spinal nerves. High conductivity is determined by recording electrical impulses in the legs or arms of a patient after applying electrical impulses of relatively low amplitude to the pedicle screws.
Prior art “neurophysiology” techniques have several deficiencies. First, existing systems rely on the conductivity through a patient's body from the pedicle screw to electrodes in extremities or electrodes on the skin of the extremities. False negative values, low conductivity, can occur if the nerves or the skin do not conduct electricity well. Damaged nerves can be relatively poor conductors of electricity. Second, electrical impulses of relatively high magnitudes must be used to overcome the resistance of the skin, muscles, and nerves. Stimulation by electrical impulses of large amplitude can damage nerves. Third, the variable resistance of patient's bodies leads to a relatively wide range of “normal” values recorded from the extremities. The wide range of normal values decreases the sensitivity and the specificity of the prior art technologies.
NuVasive, Inc. of San Diego, Calif. offers a product that uses “screw test” technology to determine if a screw or similar device is being positioned close to a nerve during a surgical procedure. Surgeons typically use NuVasive's system to stimulate screws, guidewires, and taps placed into the pedicles of vertebrae. Recording surface electrodes are placed over the legs of the patient. Nerves conduct electricity very efficiently, such that electrical stimulation of the metal objects placed into the vertebrae can be recorded in the legs.
Using the NuVasive system, an electrical charge is sent through the screw, and a circle lights up on a computer screen giving a simple number to indicate the amount of charge reaching sensors placed on the patient's leg muscles. A high number, such as a 20, suggests the screw is clear of the nerve. A lower reading, like a 3, indicates the nerve is being stimulated and the surgeon needs to consider moving the screw. Thus, the lower the amplitude needed to record activity in the legs, the closer the metal objects are to the spinal nerves.
Research has shown that if the surface electrodes record electrical activity with stimulation of less than 8 milliamps, the metal objects are too close to the spinal nerves. The system can also be used in the cervical spine. The surface electrodes are placed on the arms for recording stimulation of devices placed into the cervical spine.
The NuVasive system has a several shortcomings. For one, the system does not yield real-time data. Nor does the system allow for efficient, repeated stimulation of instruments that are turned. This is due to the fact that the NuVasive system uses a ball-tipped stimulating probe, and the ball of the probe slips off the circular shaft of the instruments. In addition, while the system helps surgeons identify holes in the pedicle, it does not identify the location of the hole in the pedicle. Also, the instruments and screws that are placed into the spine cannot touch the skin, muscles, and subcutaneous tissues surrounding the spine during electrical stimulation. If the metal instruments touch the surrounding tissues during stimulation, the electricity can be shunted from the vertebrae. Shunting of electricity can lead to false recordings in the legs or arms (during stimulation in the cervical spine). Furthermore, the existing NuVasive system requires two different probes; one to stimulate screws and a second probe to stimulate wires.