The spinal column is a highly complex system of bones and connective tissues that provide support for the body and protect the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked one atop the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord is located behind the vertebral bodies.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), excess kyphosis (abnormal forward curvature of the spine), excess lordosis (abnormal backward curvature of the spine), spondylothesis (forward displacement of one vertebra over another), and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function.
A noteworthy trend in the medical community is the move away from performing surgery via traditional “open” techniques in favor of minimally invasive or minimal access techniques. Open surgical techniques are generally undesirable in that they typically require large incisions and high amounts of tissue displacement to gain access to the surgical target site, which produces concomitantly high amounts of pain, lengthened hospitalization (increasing health care costs), and high morbidity in the patient population. Less-invasive surgical techniques (including so-called “minimal access” and “minimally invasive” techniques are gaining favor due to the fact that they involve accessing the surgical target site via incisions of substantially smaller size with greatly reduced tissue displacement requirements. This, in turn, reduces the pain, morbidity, and cost associated with such procedures. One such minimally invasive approach, a lateral trans-psoas approach to the spine, developed by NuVasive, Inc., San Diego, Calif. (XLIF®) has demonstrated great success in reducing patient morbidity, shortening the duration of hospitalization, and speeding recovery time if it is employed.
One example of a surgical retraction system and methods of use in a lateral trans-psoas approach to the spine is described in commonly owned U.S. Pat. No. 7,905,840, issued on Mar. 15, 2011 and entitled “Surgical Access System and Related Methods,” the entire contents of which are incorporated by reference into this disclosure as if set forth fully herein. To create the lateral access corridor to the lumbar spine, the patient is positioned on his or her side and a surgical access system is advanced through an incision, into the retroperitoneal space, and then through the psoas muscle until the target spinal site (for example, a disc space between a pair of adjacent vertebral bodies) is reached. The surgical access system may include a sequential dilation assembly of increasing diameter and a tissue retraction assembly. The sequential dilation assembly is advanced to the target site first and the retractor assembly is then advanced to the target site over the sequential dilation system. One or more stimulating electrodes may be provided on the distal tip of each component of the surgical access system. Nerve monitoring may be performed while advancing each of the dilation and retraction assemblies to the target site to detect the presence of, and thereby avoid, nerves lying in the trans-psoas path to the target site.
Once the retractor assembly has been docked at a target site however, a nerve situated near any location along the length of a retractor blade (for example, a center (posterior) blade) might come into inadvertent contact with the blade which could cause the nerve to become compressed over the course of the surgical procedure. As such, information regarding the proximity, health, and status of nearby nerves (e.g., the ipsilateral femoral nerve, which is a “mixed” nerve) during maintenance of a lateral access corridor is desirable. Such information may include neurogenic responses (e.g. compound nerve action potential (CNAP) responses) obtained in response to stimulation of the sensory branches of the nerve and myogenic responses (e.g. compound muscle action potential (CMAP) responses or neuromuscular responses) obtained in response to stimulation of the motor branches of the nerve. Changes in these sensory and motor responses from baseline (i.e., prior to positioning of the retractor) could indicate that the nerve is potentially compromised at the location of the surgical site. A quick, reliable means of obtaining these sensory and motor responses could provide an advanced neuromonitoring capability that may further reduce the risk of neural compromise during spine surgery.