Neural locating method

A method of locating a nerve within an intracorporeal space includes advancing a distal end portion of a stimulator toward an anatomical target within the intracorporeal space, and periodically applying a first electrical stimulus from a first electrode disposed on a central axis of the stimulator. If a muscular response to the first electrical stimulus is detected, a locating electrical stimulus is then applied from a plurality of locations offset from the central axis of the stimulator, and a magnitude of the response of the muscle is monitored. A distance between the nerve and each of the plurality of locations is determined from the magnitude of the muscular response, and from a magnitude of the locating electrical stimulus provided at each of the plurality of locations. The location of the nerve is then triangulated from the determined distance at each of the plurality of locations.

TECHNICAL FIELD

The present invention relates generally to a surgical diagnostic system for detecting the presence of one or more nerves.

BACKGROUND

Traditional surgical practices emphasize the importance of recognizing or verifying the location of nerves to avoid injuring them. Advances in surgical techniques include development of techniques with ever smaller exposures, such as minimally invasive surgical procedures, and the insertion of ever more complex medical devices. With these advances in surgical techniques, there is a corresponding need for improvements in methods of detecting and/or avoiding nerves during surgery.

SUMMARY

In an embodiment, a method of locating a nerve within an intracorporeal space includes advancing a distal end portion of a stimulator toward an anatomical target within the intracorporeal space, and periodically applying a first electrical stimulus from a first electrode disposed on a central axis of the stimulator while the stimulator is advancing toward the anatomical target. If a response of a muscle innervated by a nerve within the intracorporeal space to the first electrical stimulus is detected, a locating electrical stimulus may then be applied from a plurality of locations offset from the central axis of the stimulator. The method may continue by monitoring a magnitude of the response of the muscle to the locating electrical stimulus at each of the plurality of locations, and determining a distance between the nerve and each of the plurality of locations. Using the determined distance, the location of the nerve from the determined distance at each of the plurality of locations.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numerals are used to identify like or identical components in the various views,FIG. 1schematically illustrates a neural monitoring system10that may be used to identify the presence of one or more nerves within an intracorporeal treatment area12of a subject14. As will be described in greater detail below, in one configuration, the system10may monitor one or more muscles of the subject14for a mechanical motion, and may be capable of determining whether the mechanical motion is an artificially-induced mechanical response of a muscle to a provided stimulus (also referred to as an “artificially-induced mechanical muscle response”) or a motion cause by another factor (e.g., a subject-intended muscle contraction/relaxation and/or an environmentally caused movement). If an artificially-induced mechanical muscle response is detected during the procedure, the system10may provide an indication to a user, such as via a display or perform another appropriate action.

As used herein, an artificially-induced mechanical muscle response refers to a contraction or relaxation of a muscle in response to a stimulus that is not received through natural sensory means (e.g., sight, sound, taste, smell, and touch). Instead, it is a contraction/relaxation of a muscle that is induced by the application of a stimulus directly to a nerve that innervates the muscle. Examples of stimuli that may cause an “artificially-induced” muscle response may include an electrical current applied directly to the nerve or to intracorporeal tissue or fluid immediately surrounding the nerve. In this example, if the applied electrical current is sufficiently strong and/or sufficiently close to the nerve, it may artificially cause the nerve to depolarize (resulting in a corresponding contraction of the muscle innervated by that nerve). Other examples of such “artificial stimuli” may involve mechanically-induced depolarization (e.g., physically stretching or compressing a nerve, such as with a tissue retractor), thermally-induced depolarization (e.g., through ultrasonic cautery), or chemically-induced depolarization (e.g., through the application of a chemical agent to the tissue surrounding the nerve).

During an artificially-induced mechanical muscle response, a muscle innervated by the artificially depolarized nerve may physically contract or relax (i.e., a mechanical response). Such a mechanical reaction may primarily occur along a longitudinal direction of the muscle (i.e., a direction aligned with the constituent fibers of the muscle), though may further result in a respective swelling/relaxing of the muscle in a lateral direction (which may be substantially normal to the skin for most skeletal muscles). This local movement of the muscle during an artificially-induced mechanical muscle response may be measured relative to the position of the muscle when in a non-stimulated state, and is distinguished from other global translations of the muscle.

The neural monitoring system10may include a processor20that is in communication with at least one mechanical sensor22. The mechanical sensor22may include, for example, a strain gauge, a force transducer, a position encoder, an accelerometer, a piezoelectric material, or any other transducer or combination of transducers that may convert a physical motion into a variable electrical signal.

Each mechanical sensor22may specially be configured to monitor a local mechanical movement of a muscle of the subject14. For example, each sensor22may include a fastening means, such as an adhesive material/patch, that allows the sensor22to be adhered, bandaged, or otherwise affixed to the skin of the subject14(i.e. affixed on an external skin surface). Other examples of suitable fastening means may include bandages, sleeves, or other elastic fastening devices that may hold the sensor22in physical contact with the subject14. Alternatively, the mechanical sensor22(and/or coupled device) may be configured to monitor a local mechanical movement of a muscle by virtue of its physical design. For example, the sensors/coupled devices may include catheters, balloons, bite guards, orifice plugs or endotracheal tubes that may be positioned within a lumen or natural opening of the subject to monitor a response of the lumen or orifice, or of a muscle that is directly adjacent to and/or connected with the lumen or orifice. In a preferred embodiment, the mechanical sensor is a non-invasive device, whereby the term “non-invasive” is intended to mean that the sensor is not surgically placed within the body of the subject (i.e., via cutting of tissue to effectuate the placement). For the purposes of this disclosure, non-invasive sensors may include sensors that are placed within naturally occurring body lumens that are accessible without the need for an incision.

In one configuration, the sensor22may include a contact detection device, that may provide an indication if the sensor22is in physical contact with the skin of the subject14. The contact detection device may, for example, include a pair of electrodes that are configured to contact the skin of the subject14when the sensor22is properly positioned. The sensor22and/or contact detection device may then monitor an impedance between the electrodes to determine whether the electrodes are in contact with the skin. Other examples of suitable contact detection devices may include capacitive touch sensors or buttons that protrude slightly beyond the surface of the sensor.

The system10may further include one or more elongate medical instruments30that are capable of selectively providing a stimulus within the intracorporeal treatment area12of the subject14(i.e., also referred to as a stimulator30). For example, in one configuration, the elongate medical instrument30may include a probe32(e.g., a ball-tip probe, k-wire, or needle) that has one or more electrodes34disposed on a distal end portion36. The electrode(s)34may be selectively electrified, at either the request of a user/physician, or at the command of the processor20, to provide an electrical stimulus38to intracorporeal tissue of the subject14. For some procedures, the elongate medical instrument30may include a dialator, retractor, clip, cautery probe, pedicle screw, or any other medical instrument that may be used in an invasive medical procedure. Regardless of the instrument, if the intended artificial stimulus is an electrical current, the instrument30may include one or more selectively electrifiable electrodes34disposed at a portion of the instrument that is intended to contact tissue within the intracorporeal treatment area12during a procedure.

During a surgical procedure, the user/surgeon may selectively administer the stimulus to intracorporeal tissue within the treatment area12to identify the presence of one or more nerve bundles or fibers. For an electrical stimulus38, the user/surgeon may administer the stimulus, for example, upon depressing a button or foot pedal that is in communication with the system10, and more specifically in communication with the stimulator30. The electrical stimulus38may, for example, be a discrete pulse (e.g., a step pulse) having a pulse width within the range of about 30 μs to about 500 μs. In other examples, the discrete pulse may have a pulse width within the range of about 50 μs to about 200 μs, or within the range of about 75 μs to about 125 μs. The discrete pulse may be periodically applied at a frequency of, for example, between about 1 Hz and about 10 Hz.

If a nerve extends within a predetermined distance of the electrode34, the electrical stimulus38may cause the nerve to depolarize, resulting in a mechanical twitch of a muscle that is innervated by the nerve (i.e., an artificially-induced mechanical muscle response). In general, the magnitude of the response/twitch may be directly correlated to the distance between the electrode and the nerve, and the magnitude of the stimulus current.FIG. 2illustrates a graph50of these relationships where the magnitude52of the sensed response is shown as a function of the distance54between the stimulator and the nerve, and the magnitude56of the applied electrical current stimulus. In one configuration, the relationships illustrated inFIG. 2(or variants thereof) may be stored in a lookup table associated with the processor20. The lookup table may then be employed by the processor20to provide an approximate distance54between the electrode34and the nerve, given a known stimulus magnitude56and a measured mechanical muscle response magnitude52.

Referring again toFIG. 1, prior to beginning a surgical procedure, the one or more mechanical sensors22may be placed in mechanical communication with one or more muscles of the subject14. In the present context, a sensor22may be in mechanical communication with the muscle if it can physically detect a movement, velocity, acceleration, strain or other physical response of the muscle, either via direct contact with the muscle, or via a mechanical relationship through one or more intermediate materials and/or tissues (e.g., skin and/or subcutaneous tissue).

FIG. 3illustrates an example of the placement of a plurality of mechanical sensors22for a surgical procedure that may occur proximate the L2, L3, and/or L4 vertebrae of the lumbar spine (shown schematically inFIG. 4). The nerves60,62and64exiting the L2, L3 and L4 foramen66,68,70may therefore either lie within the treatment area12(i.e., the area surrounding the L2, L3, and/or L4 vertebrae), or may be immediately proximate to this area. Using common anatomical knowledge, the surgeon may understand that damage to these nerves60,62,64may affect the functioning of the vastus medialis muscles and the tibialis anterior muscles. As such, the surgeon may place mechanical sensors22a-22don or near the vastus medialis muscles and the tibialis anterior muscles to guard against inadvertent manipulation of the nerves during the procedure. For example, mechanical sensors22aand22bare placed on the vastus medialis muscles, which are innervated by the nerves60,62exiting the L2 and L3 foramen66,68, and sensors22cand22dare placed on the tibialis anterior muscles, which are innervated by the nerves64exiting the L4 foramen70.In general, each mechanical sensor22may generate a mechanomyography (MMG) output signal (schematically shown inFIG. 1at72) that corresponds to a sensed mechanical movement/response of the adjacent muscle. The MMG output signal72may be either a digital or analog signal, and may typically be provided to the processor20through either wired or wireless communication means (e.g., through a physical wire, or using radio frequency communication protocols, such as according to IEEE 802.11 or another protocol such as a BLUETOOTH protocol). As a specific signal, the MMG output signal72is intended to be separate and distinct from any electrical potentials of the muscle or skin (often referred to as electromyography (EMG) signals). While electrical (EMG) and mechanical (MMG) muscle responses may be related, their relationship is complex, and not easily described (e.g., electrical potentials are very location specific, with a potentially variable electrical potential across the volume of the muscle of interest).

Referring again toFIG. 1, the processor20may be in communication with the stimulator30and the mechanical sensor22, and may be configured to receive the MMG output signal72from the mechanical sensor22. The processor20may be embodied as one or multiple digital computers, data processing devices, and/or digital signal processors (DSPs), which may have one or more microcontrollers or central processing units (CPUs), read only memory (ROM), random access memory (RAM), electrically-erasable programmable read only memory (EEPROM), a high-speed clock, analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, input/output (I/O) circuitry, and/or signal conditioning and buffering electronics.

The processor20may be configured to automatically perform one or more signal processing algorithms80or methods to determine whether a sensed mechanical movement (i.e., via the MMG output signal72) is representative of an artificially-induced mechanical muscle response or if it is merely a subject-intended muscle movement and/or an environmentally caused movement. These processing algorithms80may be embodied as software or firmware, and may either be stored locally on the processor20, or may be readily assessable by the processor20.

During an invasive procedure, as discussed above, the processor20may determine the distance between an electrically stimulating electrode34and a nerve by providing an electrical stimulus38to the electrode34at a known or measurable current magnitude, and by measuring the magnitude of the mechanical muscle response. In one configuration, a surgeon may be able to surmise the relative location of the nerve by dithering the stimulator30, and monitoring the changes in the magnitude of the response (i.e., moving the stimulator30closer to the nerve would yield a greater response). In another embodiment, the system10may be configured to automatically determine the position of the nerve relative to the stimulator30without the need for mechanical dithering. For example, the stimulator30may be provided with a plurality of electrodes that may collectively be used to triangulate the position of the nerve.

It is preferable that any electrodes disposed on the stimulator30are configured to make leading contact with intracorporeal tissue as the probe is being advanced in a longitudinal direction. This maximizes the likelihood that each electrode will remain in contact with the tissue. Examples of designs that place the electrodes on a leading surface include, for example, positioning an electrode on a tip of the probe, positioning an electrode on a sloped or conical advancing face, and/or extending/protruding the electrode radially outward from a perimeter surface.

While the above-described technology is useful in providing a real-time directional reference to a user, in a further extension, the processor20may be configured to create and maintain a three-dimensional nerve model100, such as shown inFIG. 5.

FIG. 6schematically illustrates a data flow diagram for creating and utilizing a 3D nerve model. As shown, the processor20may utilize a determined distance54between a stimulator electrode34and a nerve, together with a known position104of the electrode34as determined by a position sensor, locating device, or kinematic algorithm (generally at106), to build a 3D nerve probability model at108. As discussed above, the determined distance54may be a function of the stimulus intensity56and the amplitude of the MMG response52, and may be determined at110using established relationships, such as shown inFIG. 2. In one configuration the processor20may use these relationships to directly solve for distance54given a variable stim current56and a variable MMG response52. In another configuration, the processor20may establish a threshold MMG response level that is indicative of a “response” and then may determine the minimum stim current56that is required to induce this threshold response. From this minimum current, distance may be easily computed and/or inferred.

Once created, the nerve model100may be output via a display at112, provided to a robotic controller at114, or passed to an approach planning module116(which, in turn, may output to the display112and/or robotic controller114). The displayed model may be, for example, either viewed as a stand-alone model, or merged with other imagery. If passed to the robotic controller114directly, the model100may, for example, inform feed-forward robotic control techniques to provide more accurate and dynamic tool control in the proximity of nerves. The approach planning module116may be software-implemented and may merge the nerve model100with a separate anatomical model to optimize an ideal tool path/surgical approach toward an anatomical target. Finally, in some configurations, determined nerve distance54may also be displayed to a user at112as an informational metric.

In one configuration, the processor20may build the three-dimensional nerve model100(at108) by sampling/stimulating at differing locations within the intraoperative space to triangulate nerve position and progressively construct and/or refine the nerve model100. In one configuration, this triangulation may be performed using geometric equations (i.e., where a triangle can be constructed between the nerve and two known stimulation locations, and the location of the nerve can be resolved by knowing the distances between all three vertices of the triangle). In another embodiment, a model-based triangulation approach may be performed, such as schematically illustrated inFIGS. 7, 9A-9D, and 10A-10B. This model-based triangulation approach may prove less computationally intensive for model building than the geometric equation-based alternative.

The method120schematically illustrated inFIG. 7generally operates on the assumption that a stimulator electrode34transmits current to the surrounding tissue in a generally uniform, omnidirectional manner. As such, if a nerve is stimulated, and a distance determined (in the manner described above), the nerve may exist at some location on a shell that surrounds the electrode at the determined distance. Conversely, if no threshold muscle response is detected, it can be assumed that no nerve exists within a radius of the electrode that is defined by the stimulus magnitude. By aggregating shells constructed from a plurality of electrode locations, the system may define areas that are more likely to represent nerves based on the density of recorded shells. In effect, this is a model-based triangulation method.

The method120may begin by segmenting a virtual 3D workspace150(shown inFIG. 5) into a plurality of voxels (at122). As is well understood in computer graphics and modeling, a “voxel” is a three-dimensional, volumetric element that is a defined subdivision of a larger volume (i.e., much in the same way that a 2D pixel is a defined subdivision of a larger 2D area or image). Each voxel may have a spatial location relative to the surrounding voxels, and may be capable of holding an assigned value. The virtual 3D workspace150may be registered with a corresponding physical 3D workspace152(illustrated inFIG. 1) that includes the intraoperative site12. Registration generally involves establishing a relationship between the coordinate spaces of the two workspaces such that there is direct correspondence. In the case of a robotic surgical procedure, the virtual workspace150may be coincident with, for example, the physical workspace152that is accessible by the robot.

Once the virtual workspace150is created and registered to the physical workspace152, one or more stimulation electrodes34within the physical workspace may then be registered/located within the virtual workspace150(at124). In one configuration, electrode position within the physical 3D workspace152may be determined by a locating device106and/or kinematic algorithm.

In one embodiment, a locating device106used to detect the location of the one or more electrodes34within the physical workspace152may include a multi-axial, spatial input device154that may be affixed to the stimulator30, and may monitor the position of the stimulator throughout the modeling procedure. An embodiment of a spatial input device154is generally shown inFIG. 8. In this design, the spatial input device154may include a plurality of instrumented rotatable joints156, which may monitor the physical location of the stimulator30in three spatial dimensions (x, y, and z), as well as in three orientations (roll, pitch, and yaw). In this manner, the position of the distal end portion may be reconciled and provided to the processor20. Commercially available examples of a spatial input device of this nature include the Touch Haptic Input Device or the Phantom Haptic Input Device, both made by Geomagic Solutions. In a similar manner, the location of the one or more electrodes34may be determined through the joint motion/kinematics of a surgical robot that actively controls the position and posture of the stimulator30throughout the procedure.

In still other embodiments, the one or more electrodes34may be located within the physical 3D workspace152by monitoring the position of the distal end portion of the stimulator30using a non-contact position locating device. Examples of non-contact position locating devices may use ultrasound, electrical fields, magnetic fields, fluoroscopy, or optical recognition to locate the stimulator (i.e., the distal end portion of the stimulator) within three-dimensional space.

Referring again toFIG. 7, once the relative position of the one or more electrodes34have been registered within the virtual workspace (at124), and assuming that the distal tip of the stimulator30has been advanced to an intracorporeal location, an electric current38may be applied via the electrode34to the surrounding intracorporeal tissue (at126). If a threshold MMG response is detected (at128) the processor20may determine a distance between the electrode34and the nerve (at130) and update the nerve model (at132). Alternatively, if a threshold MMG response is not detected (at128) the processor20may skip directly to updating the nerve model (at132). As discussed above, in one configuration, determining the distance (at130) may include directly determining the distance from a variable stim magnitude and a variable MMG response magnitude. In another configuration, determining the distance (at130) may include determining a minimum stim current that is required to induce the threshold response at the stim location, and using that minimum inducing current to determine the distance. In such an embodiment, determining the minimum stim current may include providing one or more additional “locating” electrical stimuli of progressively decreasing intensity until the threshold response is no longer induced.

In general, the process of updating the model (at132) involves progressively refining the voxel model to distinguish between areas that are “safe” and areas that may potentially be a nerve.FIGS. 9A-9Dschematically illustrate this process. As shown, each voxel160within the workspace100may have one of three states: no-information; no-nerve (i.e., “safe”); or nerve. In one configuration, no-information may be represented by an empty or “null” value, no-nerve may be represented by a zero, and “nerve” may be represented by an integer value that is greater than or equal to one.

Initially, all voxels160may be initialized to “no-information.” If a stimulus is delivered (at126), and no threshold muscle response is measured/detected (at128), the processor20may conclude that no nerve lies within a predefined radius of the electrode. As such, it may change all voxels within this area162to “no-nerve,” such as shown inFIG. 9Aand represented at134inFIG. 7. Once a voxel160is painted as “safe” (i.e., no-nerve) it should remain in that state indefinitely. If a stimulus is delivered and a nerve164is detected at a certain distance, such as shown inFIG. 9B, then a shell166may be constructed at a radius equal to the determined distance, and all voxels160coincident with the shell166would be changed to a “one” (represented at136inFIG. 7) (i.e., with the exception of voxels already identified as safe). Additionally, because the determined distance generally represents the minimum distance to the nerve164, all voxels160interior to the shell may be deemed “safe” and changed to “no-nerve” (represented at138inFIG. 7).

With reference toFIG. 7, after each test, if further model-refining detail is required (at140) the electrodes34may be repositioned within the physical 3D workspace/intracorporeal area (at142), and another location may be tested via the MMG stimulation routine. By sampling at many different locations, such as shown inFIG. 9C, subsequent shells166may be constructed, and voxels with more than one shell may be incremented to a value that reflects the number of shells at that point. Once a sufficient number of points have been tested, the voxel model may be smoothed and filtered (at144) such that only voxels with a value above a certain threshold remain (generally represented inFIG. 9D). This remaining structure168provides a 3D probabilistic nerve model100that is representative of the actual nerve anatomy.

In one configuration, these same techniques can be used to map a plurality of different nerves concurrently. This concept of multiple nerve models would rely on a plurality of mechanical sensors distributed about the body in communication with various muscle groups, such as generally shown inFIG. 3. Each sensor may be considered its own channel that can be used to determine distances to the nerve that primarily innervates that respective group. As nerves are identified and located by the processor20, they may be each stored in a different respective “layer” of the model100. Upon completion of the mapping, the various distinct layers may be merged together to form the final nerve map100.

FIGS. 10A-10Bschematically illustrate the multi-nerve technique being used to identify two different nerve bundles170,172. As shown, the system may construct a first plurality of shells174by identifying the mechanical response of a muscle to the simulation of the first nerve bundle170(shown inFIG. 10A). Either sequentially or concurrently with the creation of the first plurality of shells174, the system may construct a second plurality of shells176by identifying the mechanical response of a muscle to the simulation of the second nerve bundle176. In one configuration, each of these nerves may be separately maintained within the model to avoid having the readings of the first nerve bundle170erroneously affect the readings of the second nerve bundle172(or vice versa).

Referring again toFIG. 6, once the three dimensional nerve model100is created, it may be output to a display device at112either as a stand alone model, or by merging it with other imagery/anatomical models. For example, in one configuration, the nerve model100may be aligned and merged with three-dimensional CT or MRI models using anatomical landmarks/reference points, known coordinate transforms, or other such methods of merging different anatomical imaging modalities. In another configuration, a 2D view of the model may be overlaid on, for example, a 2D fluoroscope or ultrasound image. To provide enhanced clarity/visibility, the graphical overlay of the nerve model100may be provided in color, which would contrast against the traditionally black and white 2D image.

In another embodiment, the nerve model100may be output to a robotic controller114for the purpose of real-time guidance. More specifically, the nerve model100may inform the robotic controller114about the intracorporeal surroundings and/or may define one or more boundaries or restricted areas that constrain the motion of the end effector. In this sense, the model100may aid in any feedforward control (whereas any real-time MMG sensing may serve as feedback control).

In still another embodiment, the nerve model100may be used, together with an anatomical model, to calculate an optimal surgical approach toward an anatomical target. More specifically, the nerve model100may be merged with an anatomical model that is representative of the intracorporeal treatment area and a portion of the anatomical model may be identified as the “anatomical target.” This target may be the ultimate destination for a surgical instrument and/or procedure, and may either generally identify a structure, such as a vertebral disk, or may more narrowly identify the specific site of the procedure.

Once an anatomical target is identified, the processor20may use one or more optimization routines to determine an optimal approach from an outer surface of the anatomical model to the anatomical target that minimizes the potential for contact with a nerve represented within the nerve model100. Additionally, the optimization may account for, and minimize contact with at least one constraining/obstructing anatomical structure that lies between the outer surface of the model and the anatomical target. Examples of obstructing structures may include organs, such as the intestines, kidneys, liver, or bones, such as the ribs or pelvis/iliac crest. In a specific sense, the “optimal” approach may be the shortest linear (or non-linear) approach that reaches the anatomical target while minimizing the potential for contact with nerves or other constraining physical anatomy. Such path planning capabilities may be particularly useful when attempting to pass through areas with uncertain and/or complexly defined nerve paths, such as within the psoas muscle, when attempting to access very specific locations adjacent to critical nerves, such as docking in Kambin's triangle (a small access window to the vertebral disk that is, in part, defined by the nerve root), or when approaching locations that may be traditionally difficult to access due to obstructing anatomy, such as the L5-S1 vertebral joint.

Once an optimal path is defined, it may be displayed either alone, or overlayed/merged with an anatomical model to guide a surgeon in performing an access procedure. For example, an image of the probe (e.g., from fluoro or computer imagery) may be represented in a first manner if the probe is on the optimal path (e.g., colored green), and may be represented in a second manner if the probe is off of the optimal path (e.g., colored red). The optimal path may also (or alternatively) be passed to a robotic controller, which may constrain the motion of a robotically controlled tool and/or end effector within a predefined tolerance of the path, or may provide a fully automated approach along the path. In one configuration, the robotic controller114may be operative to simply constrain the motion of a tool that is under the primary (manual) control of the surgeon.

In general, the goal of the present modeling routine is to facilitate faster and safer surgical access than is possible with traditional procedures. To further this goal, in one configuration, the system may utilize an adaptive stimulation technique that can alternate between a lower resolution “searching current” and a higher resolution “locating current” to classify the intracorporeal tissue as rapidly as possible. In general, the searching current may be a higher current stimulus can more quickly characterize a large area, whereas the locating current may be a lower current stimulus that can more accurately hone in on a specific location of a nerve. These adaptive stimulation techniques are much like painting a wall with a variety of different sized brushes. While it is certainly possible to paint the entire wall with the smallest, finest brush to ensure control, it would be more efficient to paint the center of large areas with a larger brush (or roller), and switch to finer brushes only where more control and precision is required.

FIGS. 11-13schematically illustrate three embodiments of adaptive stimulation techniques that attempt to balance the detection speed provided by larger stim currents with the precision afforded by smaller stim currents. Specifically,FIG. 11illustrates a first embodiment of an evidence-based adaptive stimulation method180,FIG. 12illustrates a second embodiment of an evidence-based adaptive stimulation method181, andFIG. 13illustrates a model-based adaptive stimulation method182.

Referring toFIG. 11, the first embodiment of the evidence-based adaptive stimulation method180generally operates by stimulating with a larger, searching current during the approach until an MMG response is detected. In this manner, the processor20may paint the largest area possible as “safe” with the fewest number of stimulations/samples. Once a response is detected, the processor20may downshift to a smaller, locating current to more precisely locate the nerve (i.e., at the expense of speed).

As shown inFIG. 11, the method180begins at184by administering a searching stim current upon entry into the intracorporeal treatment area12. If no MMG response is detected (at186), then the processor20may update the model (at132) as discussed above with respect toFIG. 7. The electrodes may be repositioned (at142), and the processor20may re-administer the searching stimulus (at184). If, at some point, an MMG response is detected in response to an applied searching stimulus, the processor20may then administer the lesser locating stim current (at188) in an effort to more accurately determine a distance to the nerve (at130). Once a distance is determined in response to the locating stimulus, the processor may update the model (at132), reposition the electrodes (at142), and then re-stim using only the lesser locating current (at188).

In the embodiment illustrated inFIG. 12, the purpose of the “searching stimulus” is to identify safe areas, whereas the purpose of the “locating stimulus” is to identify the location of a nerve. In general, this technique contemplates that the precise location of the nerve is only relevant if it is close to the stimulator, otherwise, it is more important to know where the nerve is not located. The method181is similar to the method180shown inFIG. 11, with the exception that if an MMG response is detected (at186) following the application of a searching stimulus, the stimulus level is reduced (at190) and then compared to a threshold (at192). If the reduced searching current level is still greater than the threshold (i.e., it is still sufficiently large), the reduced searching current may be reapplied (at184) to see if the nerve response remains. If the reduced searching current level falls below the threshold (at192) then the method181may downshift into a locating mode and apply the higher resolution locating stimulus in an effort to locate the nerve. In general the threshold (at192) is a current level that defines the difference between the two modes and between a “searching stimulus” and a “locating stimulus.” In one configuration, the threshold may be set by a user based on a perceived comfort with the procedure. In another configuration, the threshold may be within a range of from about 6 mA to about 12 mA. It should also be appreciated that the order of reducing the stimulus (at190) and comparing to the threshold (at192) is not significantly material, and can be reversed.

The model-based adaptive stimulation method182shown inFIG. 13is much like the method181ofFIG. 12, except that the processor20uses an indication of the location of the electrode34, together with a general understanding of human anatomy to select whether to apply a searching stimulus or a locating stimulus given the likelihood of a proximate nerve. For example, in an area where nerves are not expected, the processor20may try to identify safe areas using the larger searching current. Conversely, as the stimulator/electrode approaches an area where nerves are expected (based on the anatomical model), the processor20may reduce the current to begin painting the area with finer resolution and/or with a better ability to accurately locate the nerve.

As shown inFIG. 13, the method182begins by comparing electrode location104with a model of neurological structure194(at196) to estimate a distance between the electrode34and a nerve. In this embodiment, the model of neurological structure194may be either a previously acquired model of the actual patient (e.g., from CT or MRI), or may be a more generalized model of an “average” person.

Once the nerve distance is estimated (at196), the estimate is then compared to a threshold (at198) to determine whether to apply a searching current (at200) or a locating current (at202). The threshold may be, for example, a similar threshold as used at192inFIG. 12(and/or a distance threshold that corresponds to the stimulus threshold ofFIG. 12).

If the distance estimate is greater than the threshold (at198) and the searching current is applied (at200), the processor20then examines whether an MMG response was induced by the stimulus (at204). If no MMG response is detected, then the method182proceeds to update the model (at132), as no nerve is within the searching radius. If, however, a response is detected, then the processor20may applying a locating stimulus (at198) to determine a distance to the unexpectedly present nerve.

If the distance estimate is less than the threshold (at198) and the locating current is applied (at202), the processor20then examines whether an MMG response was induced by the locating stimulus (at206). If no response is detected, the processor20may elect to apply a searching stimulus (at200) to further explore the area. If a response is detected to the locating stimulus, however, the processor20may determine a distance to the nerve (at130) and update the model (at132). Following the update of the model, the electrodes may be repositioned (at142), and the process may repeat. If the processor20fails to sense an expected MMG response at204or206, then the processor20may attempt to adjust the anatomical model194to account for the newly acquired nerve response information (at208). For example, the processor20may attempt to stretch, skew, scale, or rotate portions of the anatomical model to better match the nerve model100.

It should be appreciated that any of the adaptive stimulation techniques described with respect toFIGS. 11-13(or variations thereof) may be used in the nerve modeling techniques described above. In particular, the nerve model100may operatively model the presence of one or more nerves and/or may model the absence of any nerves.

As generally illustrated inFIGS. 9C, 10A, and 10B, to properly construct a nerve model, the electrode(s) must be capable of stimulating at multiple locations throughout the intracorporeal treatment area12. Simulation only along a single insertion axis may not provide an ability to accurately triangulate nerve position. Therefore, as illustrated inFIGS. 14A and 14B, in a first embodiment, stimulation throughout the intracorporeal treatment area12may be performed utilizing a plurality of thin stimulator probes210(e.g. K-wire style probes), each having an electrode disposed on its distal tip.

As shown inFIG. 14A, in a first configuration, each probe may be inserted through the skin212of the patient at a different respective location. Despite the varying entry, the probes210may each extend along a respective trajectory that converges toward a target location/area214. As a slight variation on this concept,FIG. 14Billustrates the plurality of thin stimulator probes210extending in a closely spaced, probe array216, where each probe210is parallel to the other probes. This probe array216may be collectively inserted through a singular incision in the skin to minimize total access points.

In either configuration illustrated inFIG. 14AorFIG. 14B, each probe210may be capable of independent movement and independent stimulation. In this manner, if one probe's trajectory is found to potentially intersect a nerve, its longitudinal progress may be halted while the remaining probes may be advanced.

In another embodiment, one or more muti-electrode stimulators may be used to perform the mapping.FIGS. 15A-15B and 16A-16Billustrate two embodiments of a muti-electrode stimulator220,222that may be used in the present mapping process. Each embodiment220,222generally illustrates a leading electrode224disposed on the distal tip226, together with one or more electrodes228that are offset from the stimulator's longitudinal axis230. More specifically, the stimulator220shown inFIGS. 15A-15Bis illustrated with a single offset electrode228, while the stimulator222shown inFIGS. 16A-16Bis illustrated with multiple offset electrodes228.

In either embodiment, each electrode224,228may be selectively and independently energized at the direction of a processor20and may be configured to provide an electrical stimulus38to tissue of the subject14. Likewise, it is preferable for the electrodes224,228to be disposed on the stimulator in a manner such that they make leading contact with intracorporeal tissue as the probe is being advanced in a longitudinal direction. This maximizes the likelihood that each electrode will remain in contact with the tissue.

During a mapping procedure, in one configuration, the stimulator220illustrated inFIGS. 15A-15Bmay create a 3D stimulation array by rotating the stimulator as it is longitudinally advanced. In this manner, the leading electrode224and offset electrode228may traverse a helical pattern. In another embodiment, which may provide a faster mapping procedure in some circumstances, the leading electrode224may be regarded as a nerve “searching electrode,” while the offset electrode228may be a “triangulation electrode.” In this embodiment, the leading/searching electrode224may be the sole electrode used while advancing the stimulator (i.e., the sole electrode providing the searching current). Once a nerve is detected, however, the stimulator220may be rotated with the offset/triangulation electrode228providing a locating stimulus at multiple points throughout the rotation. Such a use of the offset/triangulation electrode is generally illustrated inFIGS. 10A-10B(i.e., where multiple stimulation sites traverse an arc around a central electrode).

In another embodiment, the need to rotate the stimulator may be reduced or eliminated by including multiple offset electrodes228, such as shown inFIGS. 16A-16B.

One embodiment of a method for utilizing the stimulators ofFIGS. 15A-15B and 16A-16Binvolves advancing the distal end portion36of a stimulator32toward an anatomical target within the intracorporeal space. A first (searching) electrical stimulus may be applied from a leading electrode224disposed on a central axis230of the stimulator32while the stimulator32is advancing toward the anatomical target. If the processor20detects a response of a muscle innervated by a nerve within the intracorporeal space to the searching electrical stimulus, it may then apply a locating electrical stimulus from a plurality of locations offset from the central axis230of the stimulator32. The processor20may then monitor a magnitude of the response of the muscle to the locating electrical stimulus at each of the plurality of locations, determine a distance between the nerve and each of the plurality of locations from the magnitude of the response and a magnitude of the locating electrical stimulus provided at each of the plurality of locations, and triangulate the location of the nerve from the determined distance at each of the plurality of locations. If the stimulator220ofFIGS. 15A-15Bis used in this method, the processor20may stimulate at the plurality of offset locations using the single offset electrode228by stimulating between successive rotations of the stimulator220.

FIG. 17schematically illustrates an embodiment of a robotic surgical system250that may use the present nerve detection/modeling techniques. Such a system is further described in U.S. patent application Ser. No. 13/428,693, filed 23 Mar. 2012, entitled “ROBOTIC SURGICAL SYSTEM WITH MECHANOMYOGRAPHY FEEDBACK,” which is incorporated by reference in its entirety and for all of the disclosure setforth therein.

As illustrated, the displayed embodiment of the robotic surgical system250includes a nerve detection processor20and a robotic controller114. The robotic controller114is configured to control the motion of an elongate surgical instrument252that includes a proximal end portion254and a distal end portion256.

During a surgical procedure, the surgical instrument252may extend through an opening258in the body of the subject14, with the distal end portion256disposed within the intracorporeal treatment area12, and the proximal end portion254disposed outside of the subject14. In one configuration, the surgical instrument252may generally be defined by a rigid elongate body260, such that movement of the proximal end portion254of the instrument252may result in a predictable movement of the distal end portion256. In another configuration, the surgical instrument252may be defined by a controllably flexible body, such as an endoscope.

The surgical instrument252may further include an end effector262disposed at the distal end portion256. The end effector262may be responsible for performing one or more cutting, grasping, cauterizing, or ablating functions, and may be selectively actuatable in at least one degree of freedom (i.e. a movable degree of freedom, such as rotation, or an electrical degree of freedom, such as selectively delivering ablative energy). Additionally, the end effector262may be configured to selectively rotate and/or articulate about the distal end portion256of the surgical instrument252to enable a greater range of motion/dexterity during a procedure. The end effector262and/or distal end portion256of the instrument252may include a plurality of electrodes (as generally discussed above, that may each be configured to provide a respective electrical stimulus38to tissue within the treatment area12.

In one embodiment, such as generally illustrated inFIG. 17, the end effector262may be configured to resemble forceps, and may have one or more controllably movable jaws adapted to articulate about a hinged joint. The selective articulation of the one or more jaws may be enabled, for example, by cables or pull wires extending to the robotic controller through the rigid elongate body260of the instrument252.

The robotic controller114may be responsible for controllably performing a minimally invasive surgical procedure within the body of the subject14by controllably manipulating the proximal end254of the surgical instrument252in a manner that results in a controlled motion of the distal end portion256. As generally illustrated inFIG. 18, in one configuration, the robotic controller114may include a motion controller270, a location detection module272and a supervisory processor274. The motion controller270may include a plurality of motors, linear actuators, or other such components that may be required to manipulate the proximal end254of the surgical instrument252in six or more degrees of freedom. (e.g., three degrees of translation, three degrees of rotation, and/or one or more degrees of actuation). Additionally, the motion controller270may include one or more processors or digital computers and/or power electronics that may be required to convert a received motion command into a physical actuation of a motor or actuator.

The location detection module272may include one or more digital computers or processing devices that may be configured to determine the position/motion of the distal end portion256of the surgical instrument252, such as relative to one or more external reference frames. In one configuration, the location detection module272may monitor the behavior of the motion controller270to determine the motion of the distal end portion256using kinematic relationships of the surgical instrument252. In another configuration, the location detection module272may receive a location signal276from an external, locating device106, which may resolve the position of the distal end portion256of the surgical instrument252using, for example, encoded joints/linkages, ultrasound energy, magnetic energy, or electromagnetic energy that may be propagated through the subject14.

The supervisory processor274may be embodied as one or more digital computers or data processing devices, each having one or more microprocessors or central processing units (CPU), read only memory (ROM), random access memory (RAM), electrically-erasable programmable read only memory (EEPROM), a high-speed clock, analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, input/output (I/O) circuitry, power electronics/transformers, and/or signal conditioning and buffering electronics. The individual control routines/systems resident in the supervisory processor274or readily accessible thereby may be stored in ROM or other suitable tangible memory location and/or memory device, and automatically executed by associated hardware components of the processor274to provide the respective control functionality. In one embodiment, the supervisory processor274may provide the motion controller270with actuation commands in a closed loop manner using the positional feedback provided by the location detection module272. The supervisory processor274may perform any combination of feedforward, feedback, and/or predictive control schemes to accurately control the motion and/or actuation of the distal end portion256of the surgical instrument252.

Additionally, the robotic controller114may be in communication with a master station280that includes a user input device282and a user feedback device such as a display284(e.g., which may be similar to display112provided inFIG. 6). The user input device282may receive an input286from a user that corresponds to an intended movement of the distal end portion256of the surgical instrument252. The master station280may then provide a motion command to the robotic controller114that corresponds to the received input286. Similarly, the master station280may receive visual information288from the robotic controller and convey it to the user via the display284.

WhileFIG. 18provides one embodiment of a robotic controller114, other embodiments, configurations, and or control schemes may similarly be used to manipulate the surgical instrument252in a manner that results in a controlled and intended motion of the distal end portion256. While the robotic controller114and surgical instrument12described above are generally of the kind used for robotic laparoscopy, such description is made for illustrative purposes and should not be limiting. Other minimally invasive surgical systems that employ a robotic controller114to control the motion of the distal end of an elongate surgical instrument may include, for example, robotic catheter systems and/or robotic endoscopic systems.

Referring again toFIG. 17, the robotic surgical system250includes (and/or may be in communication with) a neural monitoring system10that may digitally communicate with the robotic controller114. As described above, the neural monitoring system10may include at least one mechanical sensor22and a nerve monitoring processor20in communication with the mechanical sensor22. The neural monitoring system10may provide the robotic controller114with an awareness of nerves that may be adjacent to the distal end portion256of the surgical instrument252. In this manner, the robotic system250may avoid manipulating tissue (either through translational motion or actuation of an end effector262) that may jeopardize neural integrity.

If the nerve monitoring processor20detects the presence of a nerve proximate to the elongate instrument252(i.e., via the mechanical sensor22), it may then provide a control signal290to the robotic controller114. The control signal290may include an indication of the relative position/direction of the nerve, and may further include an indication of proximity between the distal end portion256of the surgical instrument252and the nerve.

Upon receipt of a control signal290, the robotic controller114may artificially constrain the motion of the distal end portion256of the surgical instrument252to avoid inadvertent contact with a proximate nerve. For example, in one configuration, the robotic controller114may be configured to prevent all motion of the distal end portion256of the surgical instrument252in response to the received control signal290. As such, if the distal end portion256was in motion, the received control signal290may cause the controller114to halt such motion and await a further command from the user. Additionally, the robotic controller114may be configured to limit or prevent actuation of an end effector262upon receipt of the control signal290. Conversely, in certain therapeutic procedures, the robotic controller114may be configured to actuate the end effector262upon receipt of the control signal290(e.g., selectively deliver ablative energy to tissue proximate to the nerve).

In another configuration, such as schematically illustrated inFIG. 19, upon receipt of the control signal290, the robotic controller may limit the instrument's ability to move in a direction toward the nerve292. In still another configuration, the robotic controller114may construct a virtual barrier294about the nerve292which may prevent the instrument252from moving within a prescribed distance of the nerve292. The virtual barrier294may be maintained in an associated memory of the robotic controller114and/or may be associated with the 3D nerve model100that may be maintained by the nerve monitoring processor20. In general, the virtual barrier294may limit the allowed range of motion of the surgical instrument252, such that the surgical instrument252is artificially restricted from crossing the virtual barrier294. As generally illustrated inFIG. 20, as the surgical instrument252moves and acquires additional nerve directionality information, the virtual barrier294may be refined.

In still another configuration, once a nerve is detected, the robotic controller114may be configured to vary the permitted speed of the distal end portion256of the surgical instrument252as a function of the indicated proximity between the real-time location of the instrument252and the estimated relative position of the nerve. As such, the instrument252may be allowed to move more quickly and/or at a higher rate of speed when it is farther from the nerve. In this manner, the precision of the movements may be enhanced as one or more nerves become more proximate.

If the presence of a proximate nerve is detected, and/or if an action is performed by the robotic controller114to adjust or limit the allowed motion of the surgical instrument252, the robotic controller114may likewise transmit an alert (i.e., a visual alert or an auditory alert) to the user via the master station280.

While the above-described technology is primarily focused on determining the position of a nerve relative to a stimulator30and creating a nerve probability model, the nerve monitoring processor20may further include one or more filtering algorithms that may allow the system10to distinguish an artificially-induced mechanical muscle response from a patient-intended response and/or a global translation of a portion of the patient. Suitable filtering algorithms may include analog filtering algorithms, such as those described in U.S. Pat. No. 8,343,079, which is incorporated by reference in its entirety, and/or digital filtering algorithms, such as those described in U.S. Patent Application No. US2015/0051506, filed on 13 Aug. 2013 and entitled “Neural Event Detection,” which is incorporated by reference in its entirety. These filtering algorithms may look at time correlations between an applied stimulus and a detected response, the rise time/slope of a monitored response, and/or frequency characteristics of the monitored response to discern whether a detected mechanical muscle movement is attributable to a provided stimulus. In one configuration, such filtering may precede any proximity detection and/or position triangulation.

While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not as limiting.