U.S. patent application number 13/427760 was filed with the patent office on 2012-09-20 for system and methods for determining nerve direction to a surgical instrument.
This patent application is currently assigned to NUVASIVE, INC.. Invention is credited to Jeffrey Blewett, Allen Farquhar, James Gharib, Norbert Kaula, Goretti Medeiros.
Application Number | 20120238893 13/427760 |
Document ID | / |
Family ID | 37662554 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120238893 |
Kind Code |
A1 |
Farquhar; Allen ; et
al. |
September 20, 2012 |
System and Methods for Determining Nerve Direction to a Surgical
Instrument
Abstract
System (20) and related methods for performing surgical
procedures and assessments, including the use of
neurophysiology-based monitoring to determine nerve proximity and
nerve direction to surgical instruments (30) employed in accessing
a surgical target site.
Inventors: |
Farquhar; Allen; (Portland,
OR) ; Gharib; James; (San Diego, CA) ; Kaula;
Norbert; (Arvada, CO) ; Blewett; Jeffrey; (San
Diego, CA) ; Medeiros; Goretti; (Plantsville,
CT) |
Assignee: |
NUVASIVE, INC.
San Diego
CA
|
Family ID: |
37662554 |
Appl. No.: |
13/427760 |
Filed: |
March 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11182545 |
Jul 15, 2005 |
8147421 |
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13427760 |
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PCT/US03/02056 |
Jan 15, 2003 |
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11182545 |
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60382318 |
May 22, 2002 |
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Current U.S.
Class: |
600/546 ;
600/554 |
Current CPC
Class: |
A61B 90/04 20160201;
A61B 5/4893 20130101; A61B 5/0488 20130101; A61B 5/05 20130101;
A61B 5/0492 20130101 |
Class at
Publication: |
600/546 ;
600/554 |
International
Class: |
A61B 5/05 20060101
A61B005/05; A61B 5/0488 20060101 A61B005/0488 |
Claims
1. A system comprising: a surgical accessory having at least one
stimulation electrode; and a control unit capable of electrically
stimulating said at least one stimulation electrode on said
surgical accessory, sensing a response of a nerve depolarized by
said stimulation, determining a direction from the surgical
accessory to the nerve based upon the sensed response, and
communicating said direction to a user.
2. The system of claim 1, further comprising an electrode
configured to sense a neuromuscular response of a muscle coupled to
said depolarized nerve, the electrode being operable to send the
response to the control unit.
3. The system of claim 1, wherein said surgical accessory comprises
a system for establishing an operative corridor to a surgical
target site.
4. The system of claim 3, wherein said system for establishing an
operative corridor to a surgical target site comprises a series of
sequential dilation cannulae, at least one cannula having said at
least one stimulation electrode near a distal end.
5. The system of claim 4, wherein said system for establishing an
operative corridor to a surgical target site further comprises a
K-wire.
6. The system of claim 5, wherein said K-wire has a first
stimulation electrode at a distal tip of the K-wire.
7. The system of claim 6, wherein said K-wire has a second
stimulation electrode away from the distal tip.
8. The system of claim 6, wherein said K-wire is slidably received
in the surgical accessory, the surgical accessory having a
plurality of electrodes.
9. The system of claim 3, wherein said system for establishing an
operative corridor to a surgical target site is configured to
access a spinal target site.
10. The system of claim 9, wherein said system for establishing an
operative corridor to a surgical target site is configured to
establish said operative corridor via a lateral, trans-psoas
approach.
11. The system of claim 1, further comprising a handle coupled to
the surgical accessory, the handle having at least one button for
initiating the electrical stimulation from said control unit to
said at least one stimulation electrode on said surgical
accessory.
12. The system of claim 1, wherein the control unit comprises a
display operable to display an electromyographic (EMG) response of
the muscle.
13. The system of claim 1, wherein the control unit comprises a
touch-screen display operable to receive commands from a user.
14. The system of claim 1, wherein the surgical accessory comprises
a plurality of stimulation electrodes.
15. The system of claim 14, wherein the stimulation electrodes are
positioned near a distal end of the surgical accessory.
16. The system of claim 14, wherein the stimulation electrodes are
positioned in a two-dimensional plane.
17. The system of claim 14, wherein the stimulation electrodes are
positioned orthogonally to form a cross.
18. The system of claim 17, the control unit derives x and y
Cartesian coordinates of a nerve direction with respect to the
surgical accessory by using x=i.sub.w.sup.2-i.sub.e.sup.2 and
y=i.sub.s.sup.2-i.sub.n.sup.2 where i.sub.e, i.sub.w, i.sub.n, and
i.sub.s represent stimulation current thresholds for east, west,
north, and south electrodes.
19. The system of claim 14, wherein the stimulation electrodes
comprise a first set of electrodes in a first two-dimensional plane
and a second set of at least one electrode in another plane that is
parallel to the first plane.
20. The system of claim 19, wherein the stimulation electrodes form
a tetrahedron.
21. The system of claim 19, wherein the control unit is configured
to determine a three-dimensional vector from a reference point on
the surgical accessory to a nerve.
22. The system of claim 21, wherein the control unit is configured
to determine a three-dimensional vector from a reference point on
the surgical accessory to a nerve by using: x = 1 4 R ( d w 2 - d e
2 ) ##EQU00015## y = 1 4 R ( d s 2 - d n 2 ) ##EQU00015.2## and
##EQU00015.3## z = 1 4 D ( d o 2 - d k 2 ) . ##EQU00015.4##
23. The system of claim 21, wherein the control unit is configured
to determine a three-dimensional vector from a reference point on
the surgical accessory to a nerve by using: x = 1 4 RK ( i w - i e
) ##EQU00016## y = 1 4 RK ( i s - i n ) ##EQU00016.2## and
##EQU00016.3## z = 1 4 DK ( i c - i k ) ##EQU00016.4## where
i.sub.x is a stimulation current threshold of a corresponding
stimulation electrode (west, east, south or north), i.sub.k is the
stimulation current threshold of a k-wire electrode, i.sub.c is
calculated from: i c + KR 2 = 1 4 ( i w + i e + i s + i n ) .
##EQU00017##
24. The system of claim 21, wherein the control unit is further
configured to display the three-dimensional vector to a user.
25. The system of claim 14, wherein the stimulation electrodes
comprise two pairs of electrodes.
26. The system of claim 14, wherein the stimulation electrodes
comprise a first electrode at a first longitudinal level of the
surgical accessory and a second electrode at a second longitudinal
level of the surgical accessory.
27. The system of claim 14, wherein the control unit is configured
to electrically stimulate a first stimulation electrode with a
first current signal, determine whether a first stimulation current
threshold has been bracketed, stimulate a second stimulation
electrode with a second current signal, and determine whether a
second stimulation current threshold has been bracketed.
28. The system of claim 27, wherein first and second current
signals are equal.
29. The system of claim 27, wherein the control unit is further
configured to determine a first range for the first stimulation
current threshold, and determine a second range for the second
stimulation current threshold, each range having a maximum
stimulation current threshold value and a minimum stimulation
current threshold value.
30. The system of claim 29, wherein the control unit is configured
to process the first and second ranges by using
x.sub.min=i.sub.e,min.sup.2-i.sub.e,max.sup.2;
x.sub.max=i.sub.w,max.sup.2-i.sub.e,min.sup.2;
y.sub.min=i.sub.s,min.sup.2-i.sub.n,max.sup.2;
y.sub.max=i.sub.s,max.sup.2-i.sub.n,min.sup.2, where i.sub.e,
i.sub.w, i.sub.n, and i.sub.s represent stimulation current
thresholds for east, west, north, and south electrodes.
31. The system of claim 29, wherein the control unit is configured
to process the first and second ranges by using x min = 1 4 R ( d w
, min 2 - d e , max 2 ) ##EQU00018## x max = 1 4 R ( d w , max 2 -
d e , min 2 ) ##EQU00018.2## y min = 1 4 R ( d s , min 2 - d n ,
max 2 ) ##EQU00018.3## y max = 1 4 R ( d s , max 2 - d n , min 2 )
##EQU00018.4## where d is a distance from a nerve to east, west,
north, and south electrodes.
32. The system of claim 29, wherein the control unit is configured
to process the first and second ranges and display an arc
indicating a general direction of a nerve from the surgical
accessory.
33. The system of claim 27, wherein the control unit is further
configured to electrically stimulate the first stimulation
electrode with a third current signal, determine whether the first
stimulation current threshold has been bracketed, stimulate the
second stimulation electrode with a fourth current signal, and
determine whether the second stimulation current threshold has been
bracketed.
34. The system of claim 33, wherein the control unit is configured
to electrically stimulate each electrode until a stimulation
current threshold has been bracketed.
35. The system of claim 34, wherein the control unit is configured
to display an arc indicating a general direction of a nerve from
the surgical accessory and narrow the arc as stimulation current
thresholds are bracketed.
36. The system of claim 34, wherein the control unit is further
configured to electrically stimulate the first and second
stimulation electrodes to bisect each bracket until a first
stimulation current threshold has been found for the first
stimulation electrode and a second stimulation current threshold
has been found for the second stimulation electrode within a
predetermined range of accuracy.
37. The system of claim 36, wherein the control unit is configured
to display an arc indicating a general direction of a nerve from
the surgical accessory and narrow the arc as stimulation current
threshold brackets are bisected.
38. The system of claim 1, wherein the control unit is configured
to emit a sound when the control unit determines a distance between
the surgical accessory and the nerve has reached a predetermined
level.
39. The system of claim 1, wherein the control unit is configured
to emit a sound that indicates a distance between the surgical
accessory and the nerve.
40. The system of claim 1, wherein the surgical accessory is
dimensioned to be inserted percutaneously through a hole to a
surgical site.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/182,545, filed on Jul. 15, 2005 (now issued
as U.S. Pat. No. 8,147,421), which is a continuation of PCT
Application Serial No. PCT/US03/02056, filed Jan. 15, 2003
(published as PCT Pub. No. WO/04/06434), which claims priority to
U.S. Provisional Patent application Ser. No. 60/382,318, filed on
May 21, 2002, the entire contents of which are hereby expressly
incorporated by reference as if set forth fully herein.
BACKGROUND
[0002] I. Field
[0003] The present application relates to a system and methods
generally aimed at surgery. More particularly, the present
application relates to a system and related methods for determining
the direction of a surgical instrument to a nerve during surgical
access procedures.
[0004] II. Description of Related Art
[0005] A variety of surgeries involve establishing a working
channel to gain access to a surgical target site. Oftentimes, based
on the anatomical location of the surgical target site (as well as
the approach thereto), the instruments required to form or create
or maintain the working channel may have to pass near or close to
nerve structures which, if contacted or disturbed, may be
problematic to the patient. Examples of such "nerve sensitive"
procedures may include, but are not necessarily limited to, spine
surgery and prostrate or urology-related surgery.
[0006] Systems and methods exist for monitoring nerves and nerve
muscles. One such system determines when a needle is approaching a
nerve. The system applies a current to the needle to evoke a
muscular response. The muscular response is visually monitored,
typically as a shake or "twitch." When such a muscular response is
observed by the user, the needle is considered to be near the nerve
coupled to the responsive muscle. These systems require the user to
observe the muscular response (to determine that the needle has
approached the nerve). This may be difficult depending on the
competing tasks of the user. In addition, when general anesthesia
is used during a procedure, muscular response may be suppressed,
limiting the ability of a user to detect the response.
[0007] While generally effective (although crude) in determining
nerve proximity, such existing systems are incapable of determining
the direction of the nerve to the needle or instrument passing
through tissue or passing by the nerves. While the surgeon may
appreciate that a nerve is in the general proximity of the
instrument, the inability to determine the direction of the nerve
relative to the instrument can lead to guess work by the surgeon in
advancing the instrument, which raises the specter of inadvertent
contact with, and possible damage to, the nerve.
SUMMARY
[0008] The present application may be directed to at least reduce
the effects of the above-described problems with the prior art. The
present application includes a system and related methods for
determining the direction of a surgical instrument to a nerve
during surgical procedures. According to one aspect of the system,
this involves the use of neurophysiology-based monitoring to
determine nerve direction to surgical instruments employed in
accessing a surgical target site. The system may do so in an
automated, easy to use, and easy to interpret fashion so as to
provide a surgeon-driven system.
[0009] The system may combine neurophysiology monitoring with any
of a variety of instruments used in or in accessing a surgical
target site (referred to herein as "surgical access instruments").
By way of example only, such surgical access instruments may
include, but are not necessarily limited to, any number of devices
or components for creating an operative corridor to a surgical
target site, such as K-wires, sequentially dilating cannula
systems, distractor systems, and/or retractor systems. Although
described herein largely in terms of use in spinal surgery, it is
to be readily appreciated that the teachings of the methods and
systems may be suitable for use in any number of additional
surgical procedures where tissue having significant neural
structures must be passed through (or near) in order to establish
an operative corridor to a surgical target site.
[0010] A general method according to the present application may
include: (a) providing multiple (e.g., four orthogonally-disposed)
electrodes around the periphery of the surgical access instrument;
(b) stimulating the electrodes to identify the current threshold
(I.sub.Thresh) necessary to innervate the muscle myotome coupled to
the nerve near the surgical access instrument; (c) determining the
direction of the nerve relative to the surgical access instrument
via successive approximation; and (d) communicating this successive
approximation direction information to the surgeon in an
easy-to-interpret fashion.
[0011] The act of providing multiple (e.g., four
orthogonally-disposed) electrodes around the periphery of the
surgical access instrument may be accomplished in any number of
suitable fashions depending upon the surgical access instrument in
question. For example, the electrodes may be disposed orthogonally
on any or all components of a sequential dilation system (including
an initial dilator, dilating cannulae, and working cannula), as
well as speculum-type and/or retractor-based access systems. The
act of stimulating may be accomplished by applying any of a variety
of suitable stimulation signals to the electrode(s) on the surgical
accessory, including voltage and/or current pulses of varying
magnitude and/or frequency. The stimulating act may be performed
during and/or after the process of creating an operative corridor
to the surgical target site.
[0012] The act of determining the direction of the surgical access
instrument relative to the nerve via successive approximation is
preferably performed by monitoring or measuring the EMG responses
of muscle groups associated with a particular nerve and innervated
by the nerve(s) stimulated during the process of gaining surgical
access to a desired surgical target site.
[0013] The act of communicating this successive approximation
information to the surgeon in an easy-to-interpret fashion may be
accomplished in any number of suitable fashions, including but not
limited to the use of visual indicia (such as alpha-numeric
characters, light-emitting elements, and/or graphics) and audio
communications (such as a speaker element). By way of example only,
this may include providing an arc or other graphical representation
that indicates the general direction to the nerve. The direction
indicator may quickly start off relatively wide, become
successively more narrow (based on improved accuracy over time),
and may conclude with a single arrow designating the relative
direction to the nerve.
[0014] Communicating this successive approximation information may
be an important feature. By providing such direction information, a
user will be kept informed as to whether a nerve is too close to a
given surgical accessory element during and/or after the operative
corridor is established to the surgical target site. This is
particularly advantageous during the process of accessing the
surgical target site in that it allows the user to actively avoid
nerves and redirect the surgical access components to successfully
create the operative corridor without impinging or otherwise
compromising the nerves.
[0015] Based on this nerve direction feature, an instrument is
capable of passing through virtually any tissue with minimal (if
any) risk of impinging or otherwise damaging associated neural
structures within the tissue, thereby making the system suitable
for a wide variety of surgical applications.
[0016] A direction-finding algorithm that finds a stimulation
threshold current one electrode at a time for a plurality of
electrodes (e.g., four electrodes) may require 40 to 80
stimulations in order to conclude with a direction vector. At a
stimulation rate of 10 Hz, this method may take four to eight
seconds before any direction information is available to a surgeon.
A surgeon may grow impatient with the system. An "arc" method
described herein may improve the direction-finding algorithm and
provide nerve direction information to the surgeon sooner. The
system may display direction to the nerve during a sequence of
stimulations as an "arc" (or wedge), which represents a zone
containing the nerve. Computation of the direction arc (wedge) may
be based on stimulation current threshold ranges, instead of
precise, finally-calculated stimulation current threshold levels.
Display of the direction arc (wedge) is possible at any time that
the stimulation current thresholds are known to fall within a range
of values.
[0017] One aspect relates to a system comprising: a surgical
accessory having at least one stimulation electrode; and a control
unit capable of electrically stimulating at least one stimulation
electrode on said surgical accessory, sensing a response of a nerve
depolarized by said stimulation, determining a direction from the
surgical accessory to the nerve based upon the sensed response, and
communicating said direction to a user. The system may further
comprise an electrode configured to sense a neuromuscular response
of a muscle coupled to said depolarized nerve. The electrode may be
operable to send the response to the control unit.
[0018] The surgical accessory may comprise a system for
establishing an operative corridor to a surgical target site. The
system for establishing an operative corridor may comprise a series
of sequential dilation cannulae, where at least one cannula has
said at least one stimulation electrode near a distal end. The
system for establishing an operative corridor may further comprise
a K-wire. The K-wire may have a first stimulation electrode at a
distal tip. The K-wire may have a second stimulation electrode away
from the distal tip. The K-wire may be slidably received in the
surgical accessory, where the surgical accessory has a plurality of
electrodes. The system for establishing an operative corridor may
be configured to access a spinal target site. The system for
establishing an operative corridor may be configured to establish
an operative corridor via a lateral, trans-psoas approach.
[0019] The system may further comprise a handle coupled to the
surgical accessory. The handle may have at least one button for
initiating the electrical stimulation from the control unit to at
least one stimulation electrode on the surgical accessory.
[0020] The control unit may comprise a display operable to display
an electromyographic (EMG) response of the muscle. The control unit
may comprise a touch-screen display operable to receive commands
from a user.
[0021] The surgical accessory may comprise a plurality of
stimulation electrodes. The stimulation electrodes may be
positioned near a distal end of the surgical accessory. The
stimulation electrodes may be positioned in a two-dimensional
plane. The stimulation electrodes may be positioned orthogonally to
form a cross. The control unit may derive x and y Cartesian
coordinates of a nerve direction with respect to the surgical
accessory by using x=i.sub.w.sup.2-i.sub.e.sup.2 and
y=i.sub.s.sup.2-i.sub.n.sup.2 where i.sub.e, i.sub.w, i.sub.n, and
i.sub.s represent stimulation current thresholds for east, west,
north, and south electrodes. The stimulation electrodes may
comprise a first set of electrodes in a first two-dimensional plane
and a second set of at least one electrode in another plane that is
parallel to the first plane. The stimulation electrodes may form a
tetrahedron. The control unit may be configured to determine a
three-dimensional vector from a reference point on the surgical
accessory to a nerve.
[0022] The control unit may determine a three-dimensional vector
from a reference point on the surgical accessory to a nerve by
using:
x = 1 4 R ( d w 2 - d e 2 ) ##EQU00001## y = 1 4 R ( d s 2 - d n 2
) ##EQU00001.2## and ##EQU00001.3## z = 1 4 D ( d o 2 - d k 2 ) .
##EQU00001.4##
The control unit may be configured to determine a three-dimensional
vector from a reference point on the surgical accessory to a nerve
by using:
x = 1 4 RK ( i w - i e ) ##EQU00002## y = 1 4 RK ( i s - i n )
##EQU00002.2## and ##EQU00002.3## z = 1 4 DK ( i c - i k )
##EQU00002.4##
where i.sub.x is a stimulation current threshold of a corresponding
stimulation electrode (west, east, south or north), i.sub.k is the
stimulation current threshold of a k-wire electrode, i.sub.c is
calculated from:
i c + KR 2 = 1 4 ( i w + i e + i s + i n ) . ##EQU00003##
[0023] The control unit may be further configured to display the
three-dimensional vector to a user.
[0024] The stimulation electrodes may comprise two pairs of
electrodes. The stimulation electrodes may comprise a first
electrode at a first longitudinal level of the surgical accessory
and a second electrode at a second longitudinal level of the
surgical accessory.
[0025] The control unit may be configured to electrically stimulate
a first stimulation electrode with a first current signal,
determine whether a first stimulation current threshold has been
bracketed, stimulate a second stimulation electrode with a second
current signal, and determine whether a second stimulation current
threshold has been bracketed. The first and second current signals
may be equal. The control unit may be further configured to
determine a first range for the first stimulation current
threshold, and determine a second range for the second stimulation
current threshold. Each range may have a maximum stimulation
current threshold value and a minimum stimulation current threshold
value. The control unit may be configured to process the first and
second ranges by using
x.sub.min=i.sub.w,min.sup.2-i.sub.e,max.sup.2;
x.sub.max=i.sub.w,max.sup.2-i.sub.e,min.sup.2;
y.sub.min=i.sub.s,min.sup.2--i.sub.n,max.sup.2;
y.sub.max=i.sub.s,max.sup.2-i.sub.n,min.sup.2, where i.sub.e,
i.sub.w, i.sub.n, and i.sub.s represent stimulation current
thresholds for east, west, north, and south electrodes. The control
unit may be configured to process the first and second ranges by
using
x min = 1 4 R ( d w , min 2 - d e , max 2 ) , x max = 1 4 R ( d w ,
max 2 - d e , min 2 ) ##EQU00004## y min = 1 4 R ( d s , min 2 - d
n , max 2 ) , y max = 1 4 R ( d s , max 2 - d n , min 2 )
##EQU00004.2##
where d is a distance from a nerve to east, west, north, and south
electrodes.
[0026] The control unit may be configured to process the first and
second ranges and display an arc indicating a general direction of
a nerve from the surgical accessory. The control unit may be
further configured to electrically stimulate the first stimulation
electrode with a third current signal, determine whether the first
stimulation current threshold has been bracketed, stimulate the
second stimulation electrode with a fourth current signal, and
determine whether the second stimulation current threshold has been
bracketed.
[0027] The control unit may be configured to electrically stimulate
each electrode until a stimulation current threshold has been
bracketed. The control unit may be configured to display an arc
indicating a general direction of a nerve from the surgical
accessory and narrow the arc as stimulation current thresholds are
bracketed. The control unit may be further configured to
electrically stimulate the first and second stimulation electrodes
to bisect each bracket until a first stimulation current threshold
has been found for the first stimulation electrode and a second
stimulation current threshold has been found for the second
stimulation electrode within a predetermined range of accuracy. The
control unit may be configured to display an arc indicating a
general direction of a nerve from the surgical accessory and narrow
the arc as stimulation current threshold brackets are bisected.
[0028] The control unit may be configured to emit a sound when the
control unit determines a distance between the surgical accessory
and the nerve has reached a predetermined level. The control unit
may be configured to emit a sound that indicates a distance between
the surgical accessory and the nerve. The surgical accessory may be
dimensioned to be inserted percutaneously through a hole to a
surgical site.
[0029] Another aspect relates to a surgical instrument comprising
an elongated body and a plurality of electrodes on the elongated
body. Each electrode is configured to produce electrical current
pulses at a plurality of current levels. At least one current level
being sufficient to depolarize a nerve when the elongated body is
near the nerve. The elongated body may comprise a K-wire. The
elongated body may comprise a cannula. The electrodes may comprise
four orthogonal electrodes in a two-dimensional plane. The
electrodes may comprise a first set of electrodes in a first
two-dimensional plane and a second set of at least one electrode in
another plane that is parallel to the first plane. The electrodes
may be configured to produce electrical current pulses round-robin
at a first current level, then produce electrical current pulses
round-robin at a second current level. The elongated body may
comprise a sequential dilation system.
[0030] Another aspect relates to a processing unit operable to
determine ranges of nerve-stimulation threshold current levels for
a plurality of electrodes on a surgical instrument inserted into a
body.
[0031] Another aspect relates to a method comprising providing a
system operable to determine a direction of a nerve from a surgical
instrument inserted in a body; and installing software in the
system. The method may further comprise configuring a minimum
threshold peak-to-peak voltage level of a neuromuscular
response.
[0032] Another aspect relates to a method of finding a direction of
a nerve from a surgical instrument. The method comprises:
electrically stimulating a first stimulation electrode on a
surgical instrument inserted in a body with a first current signal;
determining whether a first stimulation current threshold has been
bracketed by the first stimulation current signal; electrically
stimulating a second stimulation electrode on the surgical
instrument with a second current signal; and determining whether a
second stimulation current threshold has been bracketed by the
second stimulation current signal.
[0033] The first and second current signals may be equal. The
method may further comprise determining a first range for the first
stimulation current threshold, and determining a second range for
the second stimulation current threshold, each range having a
maximum stimulation current threshold value and a minimum
stimulation current threshold value. The method may further
comprise processing the first and second ranges and displaying an
arc indicating a general direction of a nerve from the surgical
accessory.
[0034] The method may further comprise electrically stimulating the
first stimulation electrode with a third current signal;
determining whether the first stimulation current threshold has
been bracketed; stimulating the second stimulation electrode with a
fourth current signal; and determining whether the second
stimulation current threshold has been bracketed. The method may
further comprise electrically stimulating each electrode until a
stimulation current threshold has been bracketed. The method may
further comprise displaying an arc indicating a general direction
of a nerve from the surgical accessory and narrowing the arc as
stimulation current thresholds are bracketed.
[0035] The method may further comprise electrically stimulating the
first and second stimulation electrodes to bisect each bracket
until a first stimulation current threshold has been found for the
first stimulation electrode and a second stimulation current
threshold has been found for the second stimulation electrode
within a predetermined range of accuracy. The method may further
comprise displaying an arc indicating a general direction of a
nerve from the surgical accessory and narrow the arc as stimulation
current threshold brackets are bisected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a flow chart illustrating fundamental functions of
a neurophysiology-based surgical system according to one embodiment
of the present application;
[0037] FIG. 2 is a perspective view of an exemplary surgical system
capable of performing the functions in FIG. 1 and determining nerve
direction to surgical instruments employed in accessing a surgical
target site;
[0038] FIG. 3 is a block diagram of the surgical system shown in
FIG. 2;
[0039] FIG. 4 is a graph illustrating a plot of a stimulation
current pulse capable of producing a neuromuscular response (EMG)
of the type shown in FIG. 5;
[0040] FIG. 5 is a graph illustrating a plot of the neuromuscular
response (EMG) of a given myotome over time based on a current
stimulation pulse (such as shown in FIG. 4) applied to a nerve
bundle coupled to the given myotome;
[0041] FIG. 6 is a graph illustrating a plot of EMG response
peak-to-peak voltage (Vpp) for each given stimulation current level
(I.sub.Stim) forming a stimulation current pulse (otherwise known
as a "recruitment curve") for the system of FIG. 2;
[0042] FIG. 7A-7E are graphs illustrating a current
threshold-hunting algorithm that may be used by the system of FIG.
2;
[0043] FIG. 8 illustrates four orthogonal electrodes near a distal
end of a surgical instrument, such as a cannula, modeled as north,
south, east and west points in a two-dimensional X-Y plane for the
system of FIG. 2;
[0044] FIG. 9 illustrates a nerve point (x, y) bounded by maximum
and minimum x- and y-values, which forms a rectangle;
[0045] FIG. 10 illustrates a vector from an origin (center axis of
an instrument with electrodes) to a nerve point (x, y) and an arc
containing that vector found by the system of FIG. 2;
[0046] FIG. 11 illustrates a stimulation site and multiple EMG
response sensing sites for the system of FIG. 2;
[0047] FIG. 12 is a graph illustrating a plot of a neuromuscular
response (EMG) over time in response to a stimulus current pulse,
where the plot shows voltage extrema at times T1 and T2;
[0048] FIG. 13 is a graph illustrating a method of determining the
direction of a nerve (denoted as a "hexagon") relative to an
instrument having four (4) orthogonally disposed stimulation
electrodes (denoted by the "circles") for the system of FIG. 2;
[0049] FIG. 14A is a side view and FIG. 14B is a front view of a
distal end of a surgical instrument, such as a cannula in FIG. 2,
with four orthogonal electrodes and a fifth electrode;
[0050] FIG. 15A-15C are displays of a surgical instrument in FIG. 2
and a nerve direction arc that may become progressively smaller
until it becomes an arrow as stimulation threshold levels are
bracketed, bisected and found for a plurality of electrodes in
FIGS. 14A-14B;
[0051] FIGS. 16-19 illustrate a sequential dilation access system
of FIG. 2 in use creating an operative corridor to an
intervertebral disk;
[0052] FIGS. 20-21 are exemplary screen displays illustrating one
embodiment of the nerve direction feature of the surgical access
system of FIG. 2;
[0053] FIG. 22 illustrates a generalized one-dimensional,
two-electrode, direction-finding model;
[0054] FIG. 23 illustrates an electrode positioned along the x=y=0
z-axis, which is in a different x-y plane than a plurality of other
electrodes;
[0055] FIG. 24 illustrates a first pair of electrodes in one plane,
a second pair of electrodes in another plane and a nerve activation
site;
[0056] FIG. 25 illustrates a device with four electrodes in a
tetrahedron configuration;
[0057] FIG. 26 illustrates a device and a K-wire slidably received
in the device with electrodes.
DETAILED DESCRIPTION
[0058] Illustrative embodiments of the application are described
below. In the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions must be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure. The systems disclosed herein boast a variety of
inventive features and components that warrant patent protection,
both individually and in combination.
[0059] FIG. 1 illustrates general functions according to one
embodiment of the present application, namely: (a) providing
multiple (e.g., four orthogonally-disposed) electrodes around the
periphery of the surgical access instrument; (b) stimulating the
electrodes to identify the current threshold (I.sub.Thresh)
necessary to innervate the muscle myotome coupled to the nerve near
the surgical access instrument; (c) determining the direction of
the nerve relative to the surgical access instrument via successive
approximation; and (d) communicating this successive approximation
direction information to the surgeon in an easy-to-interpret
fashion.
[0060] The act of providing multiple (e.g., four
orthogonally-disposed) electrodes around the periphery of the
surgical access instrument may be accomplished in any number of
suitable fashions depending upon the surgical access instrument in
question. For example, the electrodes may be disposed orthogonally
on any or all components of a sequential dilation system (including
an initial dilator, dilating cannulae, and working cannula), as
well as speculum-type and/or retractor-based access systems, as
disclosed in the co-pending, co-assigned May, 2002 U.S. Provisional
application incorporated above. The act of stimulating may be
accomplished by applying any of a variety of suitable stimulation
signals to the electrode(s) on the surgical accessory, including
voltage and/or current pulses of varying magnitude and/or
frequency. The stimulating act may be performed during and/or after
the process of creating an operative corridor to the surgical
target site.
[0061] The act of determining the direction of the surgical access
instrument relative to the nerve via successive approximation is
preferably performed by monitoring or measuring the EMG responses
of muscle groups associated with a particular nerve and innervated
by the nerve(s) stimulated during the process of gaining surgical
access to a desired surgical target site.
[0062] The act of communicating this successive approximation
information to the surgeon in an easy-to-interpret fashion may be
accomplished in any number of suitable fashions, including but not
limited to the use of visual indicia (such as alpha-numeric
characters, light-emitting elements, and/or graphics) and audio
communications (such as a speaker element). By way of example only,
this may include providing an arc or other graphical representation
that indicates the general direction to the nerve. The direction
indicator may quickly start off relatively wide, become
successively more narrow (based on improved accuracy over time),
and may conclude with a single arrow designating the relative
direction to the nerve.
[0063] The direction indicator may be an important feature. By
providing such direction information, a user will be kept informed
as to whether a nerve is too close to a given surgical accessory
element during and/or after the operative corridor is established
to the surgical target site. This is particularly advantageous
during the process of accessing the surgical target site in that it
allows the user to actively avoid nerves and redirect the surgical
access components to successfully create the operative corridor
without impinging or otherwise compromising the nerves.
[0064] Based on this nerve direction feature, then, an instrument
is capable of passing through virtually any tissue with minimal (if
any) risk of impinging or otherwise damaging associated neural
structures within the tissue, thereby making the system suitable
for a wide variety of surgical applications.
[0065] FIGS. 2-3 illustrate, by way of example only, a surgical
system 20 provided in accordance with a broad aspect of the present
application. The surgical system 20 includes a control unit 22, a
patient module 24, an EMG harness 26 and return electrode 28
coupled to the patient module 24, and (by way of example only) a
sequential dilation surgical access system 34 capable of being
coupled to the patient module 24 via cable 32. The sequential
dilation access system 34 comprises, by way of example only, a
K-wire 46, one or more dilating cannula 48, and a working cannula
50.
[0066] The control unit 22 includes a touch screen display 40 and a
base 42, which collectively contain the essential processing
capabilities (software and/or hardware) for controlling the
surgical system 20. The control unit 22 may include an audio unit
18 that emits sounds according to a location of a surgical element
with respect to a nerve, as described herein.
[0067] The patient module 24 is connected to the control unit 22
via a data cable 44, which establishes the electrical connections
and communications (digital and/or analog) between the control unit
22 and patient module 24. The main functions of the control unit 22
include receiving user commands via the touch screen display 40,
activating stimulation electrodes on the surgical access
instruments 30, processing signal data according to defined
algorithms (described below), displaying received parameters and
processed data, and monitoring system status and report fault
conditions. The touch screen display 40 is preferably equipped with
a graphical user interface (GUI) capable of communicating
information to the user and receiving instructions from the user.
The display 40 and/or base 42 may contain patient module interface
circuitry (hardware and/or software) that commands the stimulation
sources, receives digitized signals and other information from the
patient module 24, processes the EMG responses to extract
characteristic information for each muscle group, and displays the
processed data to the operator via the display 40.
[0068] In one embodiment, the surgical system 20 is capable of
determining nerve direction relative to each K-wire 46, dilation
cannula 48 and/or the working cannula 50 during and/or following
the creation of an operative corridor to a surgical target site.
Surgical system 20 accomplishes this by having the control unit 22
and patient module 24 cooperate to send electrical stimulation
signals to each of the orthogonally-disposed stimulation electrodes
1402A-1402D (FIGS. 14A-14B) on the various surgical access
instruments 46-50 (e.g., electrodes on the distal ends of the
instruments 46-50). Depending upon the location of the surgical
access instruments 46-50 within a patient, the stimulation signals
may cause nerves adjacent to or in the general proximity of the
surgical instruments 46-50 to depolarize. This causes muscle groups
to innervate and generate EMG responses, which can be sensed via
the EMG harness 26. The nerve direction feature of the system 20 is
based on assessing the evoked response of the various muscle
myotomes monitored by the surgical system 20 via the EMG harness
26.
[0069] The sequential dilation surgical access system 34 is
designed to bluntly dissect the tissue between the patient's skin
and the surgical target site. Each K-wire 46, dilating cannula 48
and/or working cannula 50 may be equipped with multiple (e.g., four
orthogonally-disposed) stimulation electrodes to detect the
location of nerves in between the skin of the patient and the
surgical target site. To facilitate this, a surgical hand-piece 52
is provided for electrically coupling the surgical accessories
46-50 to the patient module 24 (via cable 32). In a preferred
embodiment, the surgical hand piece 52 includes one or more buttons
for selectively initiating the stimulation signal (preferably, a
current signal) from the control unit 22 to a particular surgical
access instrument 46-50. Stimulating the electrode(s) on these
surgical access instruments 46-50 during passage through tissue in
forming the operative corridor will cause nerves that come into
close or relative proximity to the surgical access instruments
46-50 to depolarize, producing a response in the innervated
myotome.
[0070] By monitoring the myotomes associated with the nerves (via
the EMG harness 26 and recording electrode 27) and assessing the
resulting EMG responses (via the control unit 22), the sequential
dilation access system 34 is capable of detecting the direction to
such nerves. Direction determination provides the ability to
actively negotiate around or past such nerves to safely and
reproducibly form the operative corridor to a particular surgical
target site. In one embodiment, by way of example only, the
sequential dilation access system 34 is particularly suited for
establishing an operative corridor to an intervertebral target site
in a postero-lateral, trans-psoas fashion so as to avoid the bony
posterior elements of the spinal column.
[0071] A discussion of the algorithms and principles behind the
neurophysiology for accomplishing these functions will now be
undertaken, followed by a detailed description of the various
implementations of these principles.
[0072] FIGS. 4 and 5 illustrate a fundamental aspect of the present
application: a stimulation signal (FIG. 4) and a resulting evoked
response (FIG. 5). By way of example only, the stimulation signal
is preferably a stimulation current signal (I.sub.Stim) having
rectangular monophasic pulses with a frequency and amplitude
adjustable by system software. In one embodiment, the stimulation
current (I.sub.Stim) may be coupled in any suitable fashion (i.e.,
AC or DC) and comprises rectangular monophasic pulses of 200
microsecond duration. The amplitude of the current pulses may be
fixed, but may preferably sweep from current amplitudes of any
suitable range, such as from 2 to 100 mA. For each nerve and
myotome there is a characteristic delay from the stimulation
current pulse to the EMG response (typically between 5 to 20 ms).
To account for this, the frequency of the current pulses may be set
at a suitable level, such as, in a preferred embodiment, 4 Hz to 10
Hz (and most preferably 4.5 Hz), so as to prevent stimulating the
nerve before it has a chance to recover from depolarization.
[0073] The EMG response shown in FIG. 5 can be characterized by a
peak-to-peak voltage of V.sub.pp=V.sub.max-V.sub.min.
[0074] A basic premise behind the neurophysiology employed by the
system 20 is that each nerve has a characteristic threshold current
level (I.sub.Thresh) at which it will depolarize. Below this
threshold, current stimulation will not evoke a significant EMG
response (V.sub.pp). Once the stimulation threshold (I.sub.Thresh)
is reached, the evoked response is reproducible and increases with
increasing stimulation until saturation is reached. This
relationship between stimulation current and EMG response may be
represented graphically via a so-called "recruitment curve," such
as shown in FIG. 6, which includes an onset region, a linear
region, and a saturation region. By way of example only, the system
20 may define a significant EMG response to have a Vpp of
approximately 100 uV. In a preferred embodiment, the lowest
stimulation current that evokes this threshold voltage
(V.sub.Thresh) is called a stimulation current threshold or
"I.sub.Thresh."
[0075] In order to obtain this useful information, the system 20
should first identify the peak-to-peak voltage (Vpp) of each EMG
response that corresponds to a given stimulation current
(I.sub.Stim). The existence of stimulation and/or noise artifacts,
however, can conspire to create an erroneous Vpp measurement of the
electrically evoked EMG response. To overcome this challenge, the
surgical system 20 may employ any number of suitable artifact
rejection techniques. Having measured each Vpp EMG response (as
facilitated by the stimulation and/or noise artifact rejection
techniques), this Vpp information is then analyzed relative to the
stimulation current in order to determine a relationship between
the nerve and the given electrode on the surgical access instrument
46-50 transmitting the stimulation current. More specifically, the
system 20 determines these relationships (between nerve and
surgical accessory) by identifying the minimum stimulation current
(I.sub.Thresh) capable of producing a predetermined Vpp EMG
response.
[0076] I.sub.Threshmay be determined for each of the four
orthogonal electrodes 1402A-1402D (FIGS. 14A-14B) in an effort to
determine the direction between the surgical access instrument 34,
36 and the nerve. This may be accomplished by employing a two-part
threshold-hunting algorithm, including a bracketing process and a
bi-section (or bisecting) process, which may proceed step-wise for
each stimulation electrode to provide successive directional
information to the user.
[0077] Arc Method
[0078] In one embodiment, successive directional information may
take the form of an arc or wedge (or range) representing a zone
that contains the nerve, according to an "arc" method described
below. This successive directional information is based on
stimulation current threshold "ranges," and may be displayed (FIGS.
15A-15C) or otherwise communicated to the surgeon any time the
stimulation current thresholds are known to fall within a range of
values.
[0079] In the bracketing process, an electrical stimulus is
provided at each of the four orthogonal electrodes 1402A-1402D
(FIGS. 14A-14B), beginning with a small current level (e.g. 0.2 mA)
and ramping up. In the "arc" method, each of the four electrodes
1402A-1402D may be stimulated at the same current level, in
sequence, before proceeding to the next higher current level (as
opposed to another method that completes the bracketing for one
electrode 1402 before advancing to another electrode 1402). The
goal is to identify a bracket around the stimulation current for
each of the four stimulation electrodes 1402A-1402D. If a
stimulation current threshold has been bracketed for an electrode
1402, the bracketing act is complete for that electrode 1402, and
stimulation proceeds for the remaining electrodes until the
stimulation current threshold has been bracketed for each
electrode. As the bracketing process proceeds, each new stimulation
provides information about the "range" of the current threshold for
that electrode 1402. This information may bracket the stimulation
current threshold (e.g., between 1.6 and 3.2 mA), or it may only
provide a lower bound for the current threshold (e.g., threshold is
greater than 5.0 mA). In either event, the "arc" bracketing process
proceeds for each of the stimulation electrodes 1402A-1402D to
provide, in succession, more accurate information regarding the
direction of the nerve relative to the surgical access instrument
46-50.
[0080] As shown in FIG. 10, an arc (wedge) containing the final
direction vector is computed from the range information for the
stimulation current thresholds corresponding to the four
stimulation electrodes 1402A-1402D. This can be done as often as
desired as the bracketing method proceeds. The arc (wedge) may then
be used to display directional information to the operator, as in
FIGS. 15A-15C.
[0081] This successive approximation information may be
communicated to the surgeon in a number of easy-to-interpret
fashions, including but not limited to the use of visual indicia
(such as alpha-numeric characters, light-emitting elements, and/or
graphics, as in FIGS. 15A-15C and 20-21) and audio communications
(such as a speaker element 18 in FIG. 3). By way of example only,
this successive directional information may include providing an
"arc" 1502 (hence the name "arc" method) or other graphical
representation that indicates the general direction to the nerve,
which may start off relatively wide, become successively more
narrow (based on improved accuracy over time), and may conclude
with a single arrow designating the relative direction to the
nerve. FIGS. 15A-15C illustrate screenshots of a cross-section of
an instrument 1500 and a wide direction arc 1502A, a narrower
direction arc 1502B and an arrow 1502C as more stimulation current
pulses are generated and EMG responses are analyzed during the
bracketing and bisecting processes.
[0082] There are a number of possibilities for displaying the arc
information. An arc or wedge might be displayed. Alternatively, an
arrow might point to the midpoint of the arc. Another indicator
might be used to illustrate the width of the arc (i.e. the
uncertainty remaining in the result).
[0083] Upon completion of the bracketing process, a bisection
process may determine more precisely the stimulation current
thresholds. As with the bracketing process, current stimulations
may be "rotated" among the stimulation electrodes so that the
thresholds are refined substantially in parallel, according to the
"arc" method. As with the bracketing method, the arc (wedge) 1502
containing the final direction vector may be computed and displayed
(FIGS. 15A-15C) frequently during the process. Upon completion of
the bisection method for all electrodes 1402A-1402D, the
stimulation current thresholds are identified precisely. At that
time, the final direction vector 1502C (FIG. 15C and FIGS. 20-21)
may be displayed.
[0084] The above-identified two-part hunting-algorithm may be
further explained with reference to FIGS. 7A-7E. According to the
arc method, each electrode 1402 is stimulated at the same
stimulation current level before passing to the next stimulation
current level. In this fashion, successive directional information
can be obtained as described above. Threshold current
(I.sub.Thresh) is the minimum stimulation current (I.sub.Stim)
(FIG. 6) that produces a Vpp (FIG. 5) greater than a known
threshold voltage (V.sub.Thresh). The value of I.sub.Stim may be
adjusted by a bracketing method as follows. The first bracket may
be 0.2 mA and 0.3 mA. If the Vpp corresponding to both of these
stimulation currents is lower than Vthresh, then the bracket size
may be doubled to 0.2 mA and 0.4 mA. This doubling of the bracket
size continues until the upper end of the bracket results in a Vpp
that is above V.sub.Thresh.
[0085] The size of the brackets may then be reduced by a bisection
method. A current stimulation value at the midpoint of the bracket
is used, and if this results in a Vpp that is above Vthresh, then
the lower half becomes the new bracket. Likewise, if the midpoint
Vpp is below Vthresh, then the upper half becomes the new bracket.
This bisection method is used until the bracket size has been
reduced to I.sub.Thresh mA. I.sub.Thresh may be selected as a value
falling within the bracket, but is preferably defined as the
midpoint of the bracket.
[0086] The threshold-hunting algorithm of this embodiment may
support three states: bracketing, bisection, and monitoring. A
"stimulation current bracket" is a range of stimulation currents
that bracket the stimulation current threshold I.sub.Thresh. The
width of a bracket is the upper boundary value minus the lower
boundary value. If the stimulation current threshold I.sub.Thresh
of a channel exceeds the maximum stimulation current, that
threshold is considered out-of-range. During the bracketing state,
threshold hunting will employ the method described herein to select
stimulation currents and identify stimulation current brackets for
each EMG channel in range.
[0087] The initial bracketing range may be provided in any number
of suitable ranges. In one embodiment, the initial bracketing range
is 0.2 to 0.3 mA. If the upper stimulation current does not evoke a
response, the upper end of the range should be increased. For
example, the range scale factor may be 2. The stimulation current
should preferably not be increased by more than 10 mA in one
iteration. The stimulation current should preferably never exceed a
programmed maximum stimulation current (to prevent nerve damage,
injury or other undesirable effects). For each stimulation, the
algorithm will examine the response of each active channel to
determine whether the stimulation current falls within that
bracket. Once the stimulation current threshold of each channel has
been bracketed, the algorithm transitions to the bisection
state.
[0088] During the bisection state (FIGS. 7C and 7D), threshold
hunting may select stimulation currents and narrow the bracket to a
selected width (for example, 0.1 mA) for each EMG channel with an
in-range threshold. After the minimum stimulation current has been
bracketed (FIG. 7B), the range containing the root is refined until
the root is known with a specified accuracy. The bisection method
is used to refine the range containing the root. In one embodiment,
the root should be found to a precision of 0.1 mA. During the
bisection method, the stimulation current at the midpoint of the
bracket is used. If the stimulation evokes a response, the bracket
shrinks to the lower half of the previous range. If the stimulation
fails to evoke a response, the bracket shrinks to the upper half of
the previous range. The nerve proximity/direction detection
algorithm is locked on the electrode position when the response
threshold is bracketed by stimulation currents separated by the
selected width (i.e., 0.1 mA). The process is repeated for each of
the active channels until all thresholds are precisely known. At
that time, the algorithm may enter the monitoring state.
[0089] During the monitoring state (FIG. 7E), threshold hunting may
employ the method described below to select stimulation currents
and identify whether stimulation current thresholds are changing.
In the monitoring state, the stimulation current level may be
decremented or incremented by 0.1 mA, depending on the response of
a specific channel. If the threshold has not changed, then the
lower end of the bracket should not evoke a response, while the
upper end of the bracket should. If either of these conditions
fail, the bracket is adjusted accordingly. The process is repeated
for each of the active channels to continue to assure that each
threshold is bracketed. If stimulations fail to evoke the expected
response three times in a row, then the algorithm may transition
back to the bracketing state in order to reestablish the
bracket.
[0090] A method for computing the successive arc/wedge directional
information from stimulation current threshold range information is
described. The stimulation current threshold is presumed to be
proportional to a distance to the nerve. The nerve may be modeled
as a single point. Since stimulation current electrodes are in an
orthogonal array, calculation of the X- and Y-dimension components
of the direction vector may proceed independently. With reference
to FIG. 8, the North and South electrodes 800A, 800C contribute to
the Y-dimension component, while the East and West electrodes
800B-800D contribute to the X-dimension component. The direction
vector <x, y> to a nerve may be defined as:
x=i.sub.w.sup.2-i.sub.e.sup.2
y=i.sub.s.sup.2-i.sub.n.sup.2 (1)
where i.sub.e, i.sub.w, i.sub.n, and i.sub.s represent the
stimulation current thresholds for the east, west, north, and south
electrodes 802B, 802D, 802A, 802C, respectively. (The equations may
be normalized to an arbitrary scale for convenience.)
[0091] As the threshold hunting method begins, the stimulation
current thresholds are known only within a range of values.
Therefore, the X- and Y-dimension components are known only within
a range. This method provides an extension to the previous
definitions, as follows:
x.sub.min=i.sub.w,min.sup.2-i.sub.e,max.sup.2
x.sub.max=i.sub.w,max.sup.2-i.sub.e,min.sup.2
y.sub.min=i.sub.s,min.sup.2-i.sub.n,max.sup.2
y.sub.max=i.sub.s,max.sup.2-i.sub.n,min.sup.2 (2)
Just as i.sub.e,min and i.sub.e,max bracket i.sub.e, x and y are
bracketed by [x.sub.min, x.sub.max] and [y.sub.min, y.sub.max].
Stated another way, the point (x,y) lies within a rectangle 900
described by these boundaries, as shown in FIG. 9. As shown in FIG.
10, just as the point (x,y) represents a vector from the origin to
a nerve modeled as a point, the bounding rectangle 900 represents
an arc (wedge) containing that vector.
[0092] The arc method may have several advantages. First, the arc
is capable of narrowing in a relatively quick fashion as more
stimulations and responses are analyzed. This method provides
general directional information much faster than if the current
threshold for each electrode 1402 was determined before moving on
the next electrode 1402. With general directional information, it
may be possible to terminate the stimulation before having the
ultimate precision of the stimulation current vectors. This will
result in a faster response, in many instances. The arc method may
provide a real-time view of the data-analysis. This helps
illustrate the value of additional stimulations to a user. This
educates and empowers the user. The user can observe the progress
of this method, which aids in the understanding of the time the
system 20 takes to converge on the final direction vector. More
frequent display updates help time "go faster" for the user. This
method avoids a long pause that might seem even longer. Disclosure
of the intermediate acts (narrowing arcs) in the process of finding
the direction vector invites mutual trust between the user and the
access system 30. An arc may provide a more intuitive visualization
for neural tissue than a direction vector.
[0093] The arc method may also be useful in tracking direction as
the instrument and stimulation electrodes move relative to the
nerve. For example, if the uncertainty in the stimulation current
threshold increases, this can be reflected in an increasing arc
size.
[0094] The sequential dilation access system 34 (FIG. 2) of the
system 20 is capable of accomplishing safe and reproducible access
to a surgical target site. It does so by detecting the existence of
and direction to neural structures before, during, and after the
establishment of an operative corridor through (or near) any of a
variety of tissues having such neural structures. If neural
structures are contacted or impinged, this may result in neural
impairment for the patient.
[0095] In one embodiment, the surgical system 20 accomplishes this
through the use of the surgical hand-piece 52, which may be
electrically coupled to the K-wire 46 via a first cable connector
51a, 51b and to either the dilating cannula 48 or the working
cannula 50 via a second cable connector 53a, 53b. For the K-wire 46
and working cannula 50, cables are directly connected between these
accessories and the respective cable connectors 51a, 53a for
establishing electrical connection to the stimulation electrode(s).
In one embodiment, a pincher or clamp-type device 57 is provided to
selectively establish electrical communication between the surgical
hand-piece 52 and the stimulation electrode(s) on the distal end of
the cannula 48. This is accomplished by providing electrical
contacts on the inner surface of the opposing arms forming the
clamp-type device 57, wherein the contacts are dimensioned to be
engaged with electrical contacts (preferably in a male-female
engagement scenario) provided on the dilating cannula 48 and
working cannula 50. The surgical hand-piece 52 includes one or more
buttons such that a user may selectively direct a stimulation
current signal from the control unit 22 to the electrode(s) on the
distal ends of the surgical access components 46-50. In an
important aspect, each surgical access component 46-50 is insulated
along its entire length, with the exception of the electrode(s) at
their distal end. In the case of the dilating cannula 48 and
working cannula 50, the electrical contacts at their proximal ends
for engagement with the clamp 57 are not insulated. The EMG
responses corresponding to such stimulation may be monitored and
assessed in order to provide nerve proximity and/or nerve direction
information to the user.
[0096] When employed in spinal procedures, for example, such EMG
monitoring would preferably be accomplished by connecting the EMG
harness 26 to the myotomes in the patient's legs corresponding to
the exiting nerve roots associated with the particular spinal
operation level (see FIGS. 11 and 20-21). In a preferred
embodiment, this is accomplished via 8 pairs of EMG electrodes 27
(FIG. 2) placed on the skin over the major muscle groups on the
legs (four per side), an anode electrode 29 providing a return path
for the stimulation current, and a common electrode 31 providing a
ground reference to pre-amplifiers in the patient module 24.
Although not shown, it will be appreciated that any of a variety of
electrodes can be employed, including but not limited to needle
electrodes. The EMG responses measured via the EMG harness 26
provide a quantitative measure of the nerve depolarization caused
by the electrical stimulus. By way of example, the placement of EMG
electrodes 27 may be undertaken according to the manner shown in
Table 1 below for spinal surgery:
TABLE-US-00001 TABLE 1 Color Channel ID Myotome Spinal Level Blue
Right 1 Right Vastus Medialis L2, L3, L4 Violet Right 2 Right
Tibialis Anterior L4, L5 Grey Right 3 Right Biceps Femoris L5, S1,
S2 White Right 4 Right Gastroc. Medial S1, S2 Red Left 1 Left
Vastus Medialis L2, L3, L4 Orange Left 2 Left Tibialis Anterior L4,
L5 Yellow Left 3 Left Biceps Femoris L5, S1, S2 Green Left 4 Left
Gastroc. Medial S1, S2
[0097] FIGS. 16-19 illustrate the sequential dilation access system
34 in FIG. 2 in use creating an operative corridor to an
intervertebral disk. As shown in FIG. 16, an initial dilating
cannula 48A is advanced towards the target site with the K-wire 46
disposed within an inner lumen within the dilating cannula 48. This
may be facilitated by first aligning the K-wire 46 and initial
dilating cannula 48A using any number of commercially available
surgical guide frames. In one embodiment, as best shown in the
expanded insets A and B, the K-wire 46 and initial dilating cannula
48A are each equipped with a single stimulation electrode 70 to
detect the presence and/or location of nerves in between the skin
of the patient and the surgical target site. More specifically,
each electrode 70 may be positioned at an angle relative to the
longitudinal axis of the K-wire 46 and dilator 48 (and working
cannula 50). In one embodiment, this angle may range from 5 to 85
degrees from the longitudinal axis of these surgical access
components 46-50. By providing each stimulation electrode 70 in
this fashion, the stimulation current will be directed angularly
from the distal tip of the respective accessory 46, 48. This
electrode configuration is advantageous in determining proximity,
as well as direction, according to the present application in that
a user may simply rotate the K-wire 46 and/or dilating cannula 48
while stimulating the electrode 70. This may be done continuously
or step-wise, and preferably while in a fixed axial position. In
either case, the user will be able to determine the location of
nerves by viewing the proximity information on the display screen
40 and observing changes as the electrode 70 is rotated. This may
be facilitated by placing a reference mark 72 on the K-wire 46
and/or dilator 48 (or a control element coupled thereto),
indicating the orientation of the electrode 70 to the user.
[0098] In another embodiment, the K-wire 46 and dilating cannula 48
in FIG. 2 may each have multiple electrodes, as described above and
shown in FIGS. 14A-14B.
[0099] In the embodiment shown, the trajectory of the K-wire 46 and
initial dilator 48A is such that they progress towards an
intervertebral target site in a postero-lateral, trans-psoas
fashion so as to avoid the bony posterior elements of the spinal
column. Once the K-wire 46 is docked against the annulus of the
particular intervertebral disk, cannulae of increasing diameter
48B-48D may then be guided over the previously installed cannula
48A (sequential dilation) until a desired lumen diameter is
installed, as shown in FIG. 17. By way of example only, the
dilating cannulae 48A-48D may range in diameter from 6 mm to 30 mm,
with length generally decreasing with increasing diameter size.
Depth indicia 72 may be optionally provided along the length of
each dilating cannula 48 to aid the user in gauging the depth
between the skin of the patient and the surgical target site. As
shown in FIG. 18, the working cannula 50 may be slideably advanced
over the last dilating cannula 48D after a desired level of tissue
dilation has been achieved. As shown in FIG. 19, the last dilating
cannula 48D and then all the dilating cannulae 48 may then be
removed from inside the inner lumen of the working cannula 50 to
establish the operative corridor therethrough.
[0100] Once a nerve is detected using the K-wire 46, dilating
cannula 48, or the working cannula 50, the surgeon may select the
DIRECTION function to determine the angular direction to the nerve
relative to a reference mark on the access components 46-50, as
shown in FIG. 21. In one embodiment, a directional arrow 90 is
provided, by way of example only, disposed around the cannula
graphic 87 for the purpose of graphically indicating to the user
which direction the nerve is relative to the access components
46-50. This information helps the surgeon avoid the nerve as he or
she advances the K-wire 46 and cannulae 48, 50. In one embodiment,
this directional capability is accomplished by equipping the K-wire
46, dilators 48 and working cannula 50 with four (4) stimulation
electrodes disposed orthogonally on their distal tip (FIGS.
14A-14B). These electrodes are preferably scanned in a monopolar
configuration (that is, using each of the 4 electrodes as the
stimulation source). The threshold current (I.sub.thresh) is found
for each of the electrodes by measuring the muscle evoked potential
response Vpp and comparing it to a known threshold Vthresh. From
this information, the direction from a stimulation electrode (or
device 46-50) to a nerve may be determined according to the
algorithm and technique described herein and with reference to
FIGS. 8-10, 13, 14A-14B and 15A-15C. In FIGS. 8 and 13, the four
(4) electrodes 800A-800D are placed on the x and y axes of a two
dimensional coordinate system at a radius R from the origin. A
vector is drawn from the origin along the axis corresponding to
each electrode. Each vector has a length equal to I.sub.Thresh for
that electrode. Thus, with four electrodes 800A-800D, four vectors
are drawn from the origin along the four axi corresponding to the
four electrodes. The vector from the origin to a direction pointing
toward the nerve is then computed. Using the geometry shown, the
(x,y) coordinates of the nerve, taken as a single point, can be
determined as a function of the distance from the nerve to each of
four electrodes 800A-800D. This can be expressly mathematically as
follows: [0101] Where the "circles" in FIG. 13 denote the position
of the electrode respective to the origin or center of the cannula,
the "hexagon" denotes the position of a nerve, and d.sub.1,
d.sub.2, d.sub.3, and d.sub.4 denote the distance between the nerve
point and stimulation electrodes 1-4 (north, east, south and west)
respectively, it can be shown that:
[0101] y = d 1 2 - d 3 2 - 4 R ##EQU00005## and ##EQU00005.2## x =
d 2 2 - d 4 2 - 4 R ##EQU00005.3## [0102] Where R is the cannula
radius, standardized to 1, since angles and not absolute values are
measured.
[0103] After conversion from Cartesian coordinates (x,y) to polar
coordinates (r,.theta.), then .theta. is the angular direction to
the nerve. This angular direction may then be displayed to the
user, by way of example only, as the arrow 90 shown in FIG. 21
pointing towards the nerve. In this fashion, the surgeon can
actively avoid the nerve, thereby increasing patient safety while
accessing the surgical target site. The surgeon may select any one
of the 4 channels available to perform the Direction Function,
although the channel with the lowest stimulation current threshold
(that indicates a nerve closest to the instrument) should probably
be used. The surgeon should preferably not move or rotate the
instrument while using the Direction Function, but rather should
return to the Detection Function to continue advancing the
instrument.
[0104] After establishing an operative corridor to a surgical
target site via the surgical access system 34, any number of
suitable instruments and/or implants may be introduced into the
surgical target site depending upon the particular type of surgery
and surgical need. By way of example only, in spinal applications,
any number of implants and/or instruments may be introduced through
the working cannula 50, including but not limited to spinal fusion
constructs (such as allograft implants, ceramic implants, cages,
mesh, etc.), fixation devices (such as pedicle and/or facet screws
and related tension bands or rod systems), and any number of
motion-preserving devices (including but not limited to total disc
replacement systems).
[0105] Segregating the Geometric and Electrical Direction Algorithm
Models
[0106] There are other relationships resulting from the symmetry of
the four electrodes described above:
d w 2 + d e 2 = d s 2 + d n 2 and ( 1 ) d 0 2 + R 2 = 1 4 ( d w 2 +
d e 2 + d s 2 + d n 2 ) ( 2 ) ##EQU00006##
where d.sub.0 is the distance between the nerve activation site and
the midpoint between the electrodes (i.e., the origin (0, 0) or
"virtual center"). These results are based purely on geometry and
apply independent of an electrical model.
[0107] As described above under the "arc" method, the geometric
model can be extended to define a region of uncertainty based on
the uncertainty in the distance to the nerve:
x min = 1 4 R ( d w , min 2 - d e , max 2 ) x max = 1 4 R ( d w ,
max 2 - d e , min 2 ) y min = 1 4 R ( d s , min 2 - d n , max 2 ) y
max = 1 4 R ( d s , max 2 - d n , min 2 ) ( 3 ) ##EQU00007##
[0108] In FIGS. 8 and 13, the two-dimensional x-y model assumes
that the nerve activation site lies in the same plane as the
stimulation electrodes or the entire z-axis space may be considered
to be projected onto the x-y plane. It has been found that the
z-dimension has no effect on direction "in the plane." The distance
equations presented in the previous sections also apply when the
nerve activation site is out of the plane of the stimulation
electrodes.
Generalized 1-D Model
[0109] FIG. 22 illustrates two electrodes 2200A, 2200B. Given any
two electrodes 2200A, 2200B, the absolute position of the nerve
activation site 2202 in one dimension can be computed from the
distances from those two electrodes:
d = 1 4 D ( d 1 2 - d 2 2 ) ( 4 ) ##EQU00008##
[0110] The 1-D model can be extended to two or three dimensions by
the addition of electrodes.
3-D Geometric Model
[0111] Using the four co-planar electrodes (FIGS. 8 and 13), it is
not particularly easy to identify direction to the nerve along the
z-axis. This may be easily rectified by the addition of one or more
stimulation electrodes 2300 (FIG. 23) out of the original x-y
electrode plane. For example, FIG. 23 shows a k-wire electrode
2300, positioned along the x=y=0 z-axis. D is half the distance
between the K-wire electrode 2300 and the plane 2302 of the other
four electrodes.
[0112] 3-D direction to the nerve is possible by comparing the
distance to the nerve activation site from the k-wire electrode
2300 to that of the other electrodes.
z = 1 4 D ( d o 2 - d k 2 ) ( 5 ) ##EQU00009##
where d.sub.o (as noted above) is the distance between the nerve
activation site and the midpoint between the electrodes (i.e., the
origin (0, 0) or "virtual center"), and d.sub.k is the distance
between the nerve activation site and the k-wire electrode 2300.
3-D direction is possible by converting from Cartesian (x, y, z) to
spherical (.rho., .theta., .phi.) coordinates. The arc method
described above may also be extended to three dimensions. Other 3-D
geometric models may be constructed. One possibility is to retain
the four planar electrodes 1402A-1402D and add a fifth electrode
1404 along the side of the cannula 1400, as shown in FIG. 14A.
[0113] Another possibility is to replace the four planar electrodes
2502A-2502D with two pairs (e.g., vertices of a tetrahedron), as
shown in FIG. 25. FIG. 25 illustrates a device 2500 with four
electrodes 2502A-2502D in a tetrahedron configuration, which may be
used with the system 20. Four electrodes may be a minimum for
spanning a 3-D space, and may be the most efficient in terms of
number of stimulations required to find the stimulation current
thresholds.
Electric Model
[0114] The direction algorithm described above assumes direct
proportionality between distance and the stimulation current
threshold, as in equation (2).
i.sub.th=Kd (6)
where i.sub.th is the threshold current, K is a proportionality
constant denoting a relationship between current and distance, and
d is the distance between an electrode and a nerve.
[0115] An alternative model expects the stimulation current
threshold to increase with the square of distance:
i.sub.th=i.sub.o+Kd.sup.2 (7)
Using the distance-squared model, the Cartesian coordinates for the
nerve activation site can be derived from equations (5), (7) and
the following:
x = 1 4 R ( d w 2 - d e 2 ) y = 1 4 R ( d s 2 - d n 2 ) x = 1 4 RK
( i w - i e ) y = 1 4 RK ( i s - i n ) z = 1 4 DK ( i c - i k ) ( 8
) ##EQU00010##
where i.sub.s is the stimulation current threshold of the
corresponding stimulation electrode (west, east, south or north),
i.sub.k is the stimulation current threshold of the k-wire
electrode 2602A (FIG. 26), and i.sub.c is calculated from:
i c + KR 2 = 1 4 ( i w + i e + i s + i n ) ( 9 ) ##EQU00011##
FIG. 26 illustrates a device 2600, such as a cannula 48A in FIG.
16, and a K-wire 46 slidably received in the device 2600. Both the
K-wire 46 and the device 2600 have electrodes 2602A-2602F.
[0116] Other sets of equations may be similarly derived for
alternative electrode geometries. Note that in each case, i.sub.0
is eliminated from the calculations. This suggests that the
absolute position of the nerve activation site relative to the
stimulation electrodes may be calculated knowing only K. As noted
above, K is a proportionality constant denoting the relationship
between current and distance.
[0117] Distance or position of the neural tissue may be determined
independent of nerve status or pathology (i.e., elevated i.sub.0),
so long as stimulation current thresholds can be found for each
electrode.
Measuring Nerve Pathology
[0118] If the distance to the nerve is known (perhaps through the
methods described above), then it is possible to solve equation (8)
for i.sub.0. This would permit detection of nerves with elevated
stimulation thresholds, which may provide useful clinical (nerve
pathology) information.
i.sub.o=i.sub.th-Kd.sup.2
[0119] Removing Dependence on K
[0120] The preceding descriptions assume that the value for K is
known. It is also possible to measure distance to a nerve
activation site without knowing K, by performing the same
measurement from two different electrode sets. FIG. 24 illustrates
two pairs of electrodes 2400A, 2400B, 2402A, 2402B and a nerve
activation site 2404. The top two electrodes 2400A, 2400B form one
pair, and the bottom two electrodes 2402A, 2402B form a second
pair. Using the electrodes 2400A, 2400B, 2402A, 2402B and distances
defined in FIG. 24, the geometric model from equation (4)
becomes:
d a d b = D b D a ( d 3 2 - d 4 2 d 1 2 - d 2 2 ) = d 0 + d b d b =
d 0 d b + 1 ( 11 ) ##EQU00012##
Adding the electrical model from equation (7), the dependence on K
is removed. Solve equation (11) for d.sub.b to get the distance in
one dimension:
D b D a ( i 3 - i 4 i 1 - i 2 ) = d 0 d b + 1 ( 12 )
##EQU00013##
Finally, it is possible to solve for the value of K itself:
K = i 1 - i 2 4 D b d b ##EQU00014##
[0121] Although the configuration in FIG. 14 shows four electrodes,
the technique may also work with three collinear electrodes.
Electrode Redundancy
[0122] Whichever electrical model is used, the relationship
expressed in equation (1) means that the current at any of the
electrodes at the four compass points can be "predicted" from the
current values of the other three electrodes. Using the electrical
model of equation (7) yields:
i.sub.w.sup.2+i.sub.e.sup.2=i.sub.s.sup.2+i.sub.n.sup.2
Using the electrical model of equation (6) yields:
i.sub.w+i.sub.e=i.sub.s+i.sub.n
This provides a simple means to validate either electrical
model.
[0123] Applying the tools of geometric and electrical modeling may
help to create more efficient, accurate measurements of the nerve
location.
[0124] While certain embodiments have been described, it will be
appreciated by those skilled in the art that variations may be
accomplished in view of these teachings without deviating from the
spirit or scope of the present application. For example, the system
22 may be implemented using any combination of computer programming
software, firmware or hardware. As a preparatory act to practicing
the system 20 or constructing an apparatus according to the
application, the computer programming code (whether software or
firmware) according to the application will typically be stored in
one or more machine readable storage mediums such as fixed (hard)
drives, diskettes, optical disks, magnetic tape, semiconductor
memories such as ROMs, PROMs, etc., thereby making an article of
manufacture in accordance with the application. The article of
manufacture containing the computer programming code may be used by
either executing the code directly from the storage device, by
copying the code from the storage device into another storage
device such as a hard disk, RAM, etc. or by transmitting the code
on a network for remote execution. As can be envisioned by one of
skill in the art, many different combinations of the above may be
used and accordingly the present application is not limited by the
scope of the appended claims.
* * * * *