An emerging trend in spinal surgery is to perform surgery in a minimally invasive or minimal access fashion to avoid the trauma of so-called open or “direct access” procedures. A specific area of interest is in the placement of pedicle screws, which are typically employed to effect posterior fixation in spinal fusion procedures. While great strides are being made in this area, a risk exists (as it does in open procedures) that the pedicle may become breached, cracked, or otherwise compromised during the procedure. If the pedicle (or more specifically, the cortex of the medial wall, lateral wall, superior wall and/or inferior wall) is breached, cracked, or otherwise compromised, the patient may experience pain or neurologic deficit due to unwanted contact between the pedicle screw and exiting nerve roots. This often necessitates revision surgery, which can be painful and costly, both in terms of recovery time and hospitalization.
Some attempts to minimize the risk of a pedicle breach involve capitalizing on the insulating characteristics of bone and the conductivity of the exiting nerve roots themselves to perform pedicle integrity assessments. That is, if the wall of the pedicle is breached, a stimulation signal applied to the pedicle screw and/or the pilot hole (prior to screw introduction) will cause the various muscle groups coupled to the exiting nerve roots to contract. If the pedicle wall has not been breached, the insulating nature of the pedicle will prevent the stimulation signal from innervating the given nerve roots such that the associated muscle groups will not twitch. Traditional EMG monitoring systems may be employed to augment the ability to detect such innervation.
One period during a pedicle screw procedure in which the risk of a pedicle breach is prevalent is during the initial access of the pedicle. Typically, initial access to a pedicle may be achieved by inserting a needle to the target site and driving the needle point into the pedicle, creating a pilot hole. Due to the size and shape of the typical needle, however, manipulation and maneuvering of the needle may be awkward or difficult, increasing the risk of complication. Additionally, the pedicle may be breached and nerve damage done during the initial drive of the needle into the pedicle, before a pedicle integrity test assessment may be performed.
A problem that may arise when various medical instruments are electrified and used with traditional EMG monitoring systems is that different instruments may produce different EMG stimulation thresholds. For example, an electrified needle may exhibit a threshold stimulation of approximately 5-6 mA, while a bone screw placed in the same location may exhibit a threshold stimulation of approximately 16-20 mA. This can be problematic in that an electrified needle may tend to indicate a breach in the pedicle wall when in fact the pedicle wall is intact.
The present invention is directed at eliminating, or at least improving upon, the shortcomings of the prior art.