Medical devices and methods for penetrating an anatomical structure based on sensed electrical characteristics
A medical device for penetrating a bone structure including a processing unit having a transfer function that associates an electrical conductivity value S with a depth value d, wherein the processing unit is configured to detect a threshold selected from amongst an absolute threshold, a relative threshold and a critical gradient, and to emit a warning signal and/or control signal responsive to detection of the threshold.
1 . A medical device for penetrating an anatomical structure having multiple anatomical media with a range of electrical characteristics, the medical device comprising:
a body having a tip disposed at its distal end configured to penetrate into the anatomical structure, a first electrode having a first contact surface disposed at the distal end, a second electrode having a second contact surface disposed at the distal end and spaced apart from the first contact surface; and
a processing unit operably coupled to the first electrode and the second electrode, the processing unit configured to:
store a set of criteria representative of an electrical characteristic of an anatomical medium as a function of depth in the anatomical medium;
measure an electrical characteristic of tissue disposed between the first contact surface and the second contact surface;
determine a critical gradient as at least one slope p(d) of an evolution of electrical characteristic values as a function of depth, representative of a change in a first anatomical medium towards a second anatomical medium;
detect, using the set of criteria, an electrical characteristic threshold from amongst an absolute threshold, a relative threshold, and the critical gradient; and
emit a warning signal responsive to detection of the electrical characteristic threshold indicative of a predetermined transition between anatomical media.
2 . The medical device of claim 1 , wherein the anatomical structure includes bone and soft tissue, and the predetermined transition indicates actual or impending penetration of the tip into soft tissue corresponding to nervous tissue.
3 . The medical device of claim 1 , further comprising a robotic arm and end effector, and the processing unit further is configured to issue a control signal to the robotic arm or end effector responsive to detection of the electrical characteristic threshold indicative of a predetermined transition between anatomical media.
4 . The medical device of claim 1 , further comprising a robotic arm, end effector, and a force sensing unit, wherein the processing unit further is configured to issue a control signal to the force sensing unit to vary a force applied by the robotic arm or end effector responsive to detection of the electrical characteristic threshold indicative of a predetermined transition between anatomical media.
5 . The medical device of claim 4 , further comprising a depth detection unit, wherein the processing unit further is configured to receive a depth signal from the depth detection unit responsive to movement associated with respiration, and to issue a control signal to the force sensing unit responsive to the depth signal to control displacement of the end effector responsive to the movement associated with respiration.
6 . The medical device of claim 1 , further comprising a robotic arm, end effector, and a depth detection unit, wherein the processing unit further is configured to receive a depth signal from the depth detection unit, and to issue a control signal to the robotic arm or end effector responsive to detection of the electrical characteristic threshold indicative of a predetermined transition between anatomical media.
7 . The medical device of claim 1 , further comprising a robotic arm, end effector, and a depth detection unit, wherein the processing unit further is configured to receive a depth signal, and to control operation of the robotic arm or end effector in penetrating the anatomical structure responsive to the depth signal.
8 . The medical device of claim 1 , wherein the electrical characteristic of the anatomical medium is selected from amongst electrical conductivity, impedance, conductance, voltage, electrical intensity or a combination thereof.
9 . The medical device of claim 1 , wherein the processing unit is configured to determine the absolute threshold as N a ×Ds, where Ds is the range of electrical characteristic values of the multiple anatomical media and N a is a real number between 0 and 1.
10 . The medical device of claim 1 , wherein the processing unit is configured to determine the relative threshold as N r ×MA(d) where MA is an average of the electrical characteristic values between an initial depth value and a current depth value d, and N r is a real number between 0 and 5.
11 . The medical device of claim 1 , wherein the at least one slope p(d) of the evolution of electrical characteristic values is computed as:
p
(
d
)
=
T
[
A
(
d
)
]
-
T
[
A
(
d
-
k
x
E
c
)
]
k
x
E
c
where
T[A(d)] is an electrical conductivity associated by a transfer function T based on a value of an electrical characteristic A at depth d,
T[A(d−k×E c )] is an electrical conductivity associated by the transfer function T based on a value of electrical characteristic A at depth d−k×E c located at a distance k×E c of the depth d,
E c is a thickness of an anatomical medium in millimeters, and
k is a positive real number between 0 and 5.
12 . The medical device of claim 11 , wherein the processing unit is configured to verify the critical gradient based upon occurrence of either:
(a) an increase in conductivity if the second electrical conductivity is greater than the first electrical conductivity:
p
(
d
)
>
C
×
Ds
1
mm
where
Ds is the range of electrical characteristic values for at least one of the anatomical media and C is a real number between 0 and 10, or
(b) a decrease in conductivity if the second electrical conductivity is lower than the first electrical conductivity:
p
(
d
)
<
C
′
×
Ds
1
mm
where Ds is the range of electrical characteristic values for at least one of the anatomical media and C′ is a real number between −10 and 0.
13 . The medical device of claim 11 , further configured to penetrate an anatomical structure comprising a third anatomical medium constituting an interface between a first anatomical medium having a first conductivity and a second anatomical medium having a second conductivity, the third anatomical medium having a third electrical conductivity, wherein the processing unit is configured to verify the critical gradient based upon occurrence of:
(a) if the third conductivity is lower than the first and second electrical conductivities
p
(
d
1
)
<
C
1
×
Ds
1
mm
and
p
(
d
2
)
>
C
2
×
Ds
1
mm
where
p(d 1 ) is a slope at a depth value d 1 ,
C 1 is a real number between −10 and 0,
p(d 2 ) is a slope at a depth value d 2 greater than d 1 ,
C 2 is a real number between 0 and 10, or
(b) if the third conductivity is greater than the first and second electrical conductivities:
p
(
d
1
)
>
C
3
×
Ds
1
mm
and
p
(
d
2
)
<
C
4
×
Ds
1
mm
where
C 3 is a real number between 0 and 10,
C 4 is a real number between −10 and 0.
14 . The medical device of claim 13 , wherein the processing unit is configured to check for a change in electrical characteristic if the depth values d 1 and d 2 have a maximum difference of e such that e=m×E c , where m is a positive real number between 0 and 80.
15 . The medical device of claim 13 , wherein the body is configured for use in penetrating an anatomical structure including a bone structure and soft tissue, wherein the bone structure has a layer of trabecular bone constituting the first anatomical medium and a layer of cortical bone constituting the third anatomical medium, and soft tissues constitute the second anatomical medium, the second electrical conductivity being greater than the first electrical conductivity and the third electrical conductivity being less than the first and second electrical conductivities.
16 . The medical device of claim 15 , wherein the body is configured to drill into the bone structure.
17 . The medical device of claim 1 , wherein the processing unit is configured to define a plurality of critical gradients based on average values of electrical characteristics of the multiple anatomical media.
18 . The medical device of claim 1 , wherein the body comprises a drill, a threaded tool, a screw, an implant, a needle, a cutting blade, a nail, an osteotome, a burr, a spindle, a probe, a square tip, a spatula, a curette and or a tap.
19 . The medical device of claim 1 , further comprising a base and an end effector, the body being mounted on the end effector, wherein the processing unit includes a force measurement unit configured to determine a force exerted on the body, and the processing unit is configured to control the displacement of the end effector relative to the base responsive to a setpoint force.
20 . The medical device of claim 19 , wherein the processing unit is configured to modify the setpoint force responsive to the warning signal.
21 . The medical device of claim 1 , wherein the medical device is embodied in a hand-held tool.
22 . A method of penetrating an anatomical structure having multiple anatomical media with a medical device, the multiple anatomical media having a range of electrical characteristics, the medical device including a body having a tip disposed at its distal end configured to penetrate into the anatomical structure, a first electrode having a first contact surface disposed at the distal end, a second electrode having a second contact surface disposed at the distal end and spaced apart from the first contact surface, and a processing unit operably coupled to the first electrode and the second electrode, the method comprising:
storing in the processing unit a set of criteria representative of an electrical characteristic of an anatomical medium as a function of depth in the anatomical medium;
contacting the first contact surface and the second contact surface to the anatomical structure;
measuring an electrical characteristic of tissue disposed between the first contact surface and the second contact surface;
determining a critical gradient as at least one slope p(d) of an evolution of electrical characteristic values as a function of depth, representative of a change in a first anatomical medium towards a second anatomical medium;
detecting, using the set of criteria processed by the processing unit, a threshold amongst an absolute threshold, a relative threshold, and the critical gradient; and
emitting from the processing unit a warning signal responsive to detection of the threshold indicative of a predetermined transition between anatomical media.
23 . The method of claim 22 , wherein determining the critical gradient as at least one slope p(d) of the evolution of electrical characteristic values as a function of depth comprises computing the at least one slope p(d) of the evolution of electrical characteristic values as:
p
(
d
)
=
T
[
A
(
d
)
]
-
T
[
A
(
d
-
k
×
E
c
)
]
k
×
E
c
where
T[A(d)] is an electrical conductivity associated by a transfer function T based on a value of an electrical characteristic A at depth d,
T[A(d−k×E c )] is an electrical conductivity associated by the transfer function T based on a value of electrical characteristic A at depth d−k×E c located at a distance k×E c of the depth d,
E c is a thickness of an anatomical medium in millimeters, and
k is a positive real number between 0 and 5.
24 . The method of claim 22 , wherein the anatomical structure includes bone and soft tissue, and the detecting a threshold corresponds to detecting a predetermined transition indicative of actual or impending penetration of the tip into soft tissue corresponding to nervous tissue.
25 . The method of claim 22 , wherein the medical device further comprises a robotic arm and end effector, the method further comprising issuing a control signal, by the processing unit, to the robotic arm or end effector responsive to detection of the threshold indicative of a predetermined transition between anatomical media.
26 . The method of claim 22 , wherein the medical device further comprises a robotic arm, end effector, and a force sensing unit, the method further comprising issuing a control signal, by the processing unit, to the force sensing unit to vary a force applied by the robotic arm or end effector responsive to detection of the threshold indicative of a predetermined transition between anatomical media.
27 . The method of claim 26 , wherein the medical device further comprises a depth detection unit, the method further comprising receiving a depth signal from the depth detection unit responsive to movement associated with respiration, and issuing a control signal, by the processing unit, to the force sensing unit to control displacement of the end effector responsive to the depth signal.
28 . The method of claim 22 , wherein the medical device further comprises a robotic arm, end effector, and a depth detection unit, the method further comprising receiving a depth signal from the depth detection unit, and issuing, by the processing unit, a control signal to the robotic arm or end effector responsive to detection of the threshold indicative of a predetermined transition between anatomical media.
29 . The method of claim 22 , wherein the medical device further comprises a robotic arm, end effector, and a depth detection unit, the method further comprising receiving a depth signal, and controlling operation, by the processing unit, of the robotic arm or end effector to penetrate the anatomical structure responsive to the depth signal.