Handheld Nerve Monitoring with the CHECKPOINT® Stimulator/Locator
Before the birth of Checkpoint Surgical in late 2009, our team began working with a small group of surgeons who had identified a need for a better way to locate and protect nerves intraoperatively in a significant set of nerve exploration/repair and complex reconstructive surgeries. These surgeons believed that too often in these procedures, nerves in the surgical field are at undue risk of intraoperative injury due to the prevalence of scar tissue or otherwise altered anatomy. These surgeons also had experience using available nerve stimulation and nerve monitoring technologies but found them inadequate and/or impractical. What they wanted was a “surgeon controlled” device that would be safe and effective, while also easy to use with a low technical burden.
Our team had substantial expertise in the development of neurostimulation technology, primarily in the arena of implantable devices. The challenge in developing ffthe desired device was to convert sophisticated implantable neurostimulation technology into a low cost, handheld device that would still be safe, reliable and accurate, while meeting the specific functional needs identified by the surgeons.
Out of this collaboration was born the CHECKPOINT® Stimulator/Locator, cleared by the FDA in late 2009 and introduced to market in 2010. The Checkpoint quickly proved an effective tool for locating and mapping nerves in the surgical field and verifying nerve integrity. Checkpoint also proved to be a helpful decision-making tool, aiding the surgeon in the assessment of neuromuscular function and determining the most appropriate course of intervention. The Checkpoint is now being used in a variety of surgical specialties in hundreds of hospitals across the United States and in four foreign countries.
The CHECKPOINT® offers the surgeon total independence from outside technical support in a simple to use, yet versatile tool for a variety of surgical procedures The Checkpoint circuitry is located on a single circuit board assembly with 77 components and proprietary embedded software. This micro-controller based circuit constantly monitors the internal circuitry of the Checkpoint with integrated self-testing operations to ensure the Checkpoint consistently produces safe, precise and reliable stimulus. The Checkpoint has three amplitude setting options (.5mA, 2mA and 20mA) and an adjustable pulse width control with a range of zero to 200 microseconds. The widely variable and reproducible stimulus intensity of the Checkpoint is useful whenever problems are encountered identifying and protecting the critical neurovascular structures in the surgical field. This may be as a result of altered anatomy due to previous surgery, trauma, or other conditions such as tumors or infection. The stimulator can be used to identify a suspect area that may contain a critical motor nerve and can then be used to identify precisely the nerve at risk and to confirm its identity by stimulating it and observing the distal motor response.
The Checkpoint circuitry is designed to be resistant to electrical impedance, which helps to avoid a false negative response. This component of the system generates a constant nerve-depolarizing field regardless of the type of impedance that is presented within the surgical environment. Therefore, the spatial field of excitation does not vary with the impedance of the tissue directly under the probe tip nor does it vary with the area of the probe tip in contact with tissue. This can be of significant value when working on nerves through surrounding tissue and at varying depths following trauma or previous surgery.
Checkpoint employs a biphasic waveform, which allows for repeated and continuous stimulation to an area of interest or directly on a nerve, without concern for diminished response or damage to the tissue area. The microcontroller-based circuit generates a controlled current, biphasic, capacitively coupled waveform. This precisely controlled biphasic stimulus, in combination with the stimulus frequency employed in the Checkpoint, elicits a tetanic muscle contraction that provides the surgeon unambiguous data. Other disposable stimulators employ direct current stimulation that elicits only a single muscle twitch and limits nerve contact to no longer than one second without risk of nerve injury. Furthermore, locating or activating nerves through surrounding tissue with these other stimulators is impractical.
The Checkpoint also may be used to evaluate the functional condition of the nerve using its variable pulse width setting. After the nerve is identified and initial dissection is performed, individual fascicle identification can also be done to further identify specific area of injury or for performing nerve transfers. Once the area of concern is identified, the nerve may be stimulated and the threshold parameter, which is the lowest stimulus intensity at which a muscle response, is noted. Without this variable pulse width control, establishing the threshold of nerve activation is challenging. Following a neurolysis, the nerve can be rechecked to see if the threshold stimulus intensity has dropped to the normal level confirming that nerve function has improved and eliminating the need to perform a graft which would require much longer recovery time. This type of use also may be helpful in a neuroma case to determine whether or not there is any functional axonal continuity or the neuroma must be resected and grafted or a nerve transfer procedure performed. Some response should be seen in the 2 mA range. Even a weak distal response indicates at least some functioning nerve fascicles and the capacity for further recovery, indicating that resection of the neuroma-in-continuity and grafting may not be necessary.
The 20 mA setting, in addition to regional stimulation for nerve location, can be used to stimulate entire muscles and thus evaluate their viability or to test them during muscle and tendon transfer procedures allowing precise evaluation of both tendon excursion and setting the tension in tendon transfers. At 20 mA, there is significant stimulus spill into adjacent tissues. 20 mA is not used to directly stimulate nerves, although it may be used to stimulate a region of tissue surrounding a nerve to try to determine if a nerve is present within the tissue mass.
Checkpoint Surgical is headquartered in Cleveland, Ohio, a leading center for neurodevice research and development. The CHECKPOINT® Stimulator/Locator is the flagship product of Checkpoint Surgical, an emerging leader in the intraoperative nerve protection and repair market.