Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2002-05-07
2004-01-06
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S067000
Reexamination Certificate
active
06675046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and method for electrically stimulating neural tissue including, but not limited to, a spinal cord. More specifically, this invention relates to an apparatus and method for applying a precursor electrical pulse to neural tissue prior to a stimulation pulse with the first pulse “conditioning” the tissue for the application of the stimulation pulse.
2. Description of the Prior Art
Nerve cells in the brain and the spinal cord have a variety of shapes and sizes. A typical nerve cell has the shape shown in
FIG. 1
generally labeled
1
. The classical parts of nerve cell
1
are the cell body
2
, the dendritic tree
3
and the axon
4
(including its terminal branches). Nerve cells convey information to other cells at junctions called synapses.
An important property of the nerve cell is the electrical potential that exists across the cell's outer membrane
5
. Normally, when cell
1
is at rest, the inside
6
of cell
1
is 70-80 mV negative with respect to the outside
7
of cell
1
. As shown in
FIG. 2
, cell
1
has chemical pumps
8
imbedded in the cell membrane
5
. Pumps
8
consume energy to move sodium ions
9
outside and potassium ions
10
into the cell
1
to maintain the concentration gradients and therefore the electrical potential difference across membrane
5
.
The membrane
5
of the axon
4
has specific dynamic properties related to its function to transmit information. In man, like in other mammals, it contains sodium channels
11
and leakage channels
12
. Membrane
5
has a voltage and time dependent sodium conductivity that is related to the number of open sodium channels. Channels
11
open and close in response to changes in the potential across the membrane
5
of the cell
1
. When the membrane
5
is in its resting state (70-80 mV negative at the inside), only few sodium channels
11
are open. However, when the electrical potential across membrane
5
is reduced (membrane depolarization) to a value called the excitation threshold, the sodium channels
11
open up allowing sodium ions
9
to rush in (excitation). As a result, the electrical potential across membrane
5
changes by almost 100 mV, so that the inside
6
of the axon
4
gets positive with respect to the outside
7
.
After a short time the sodium channels
11
close again and the resting value of the membrane potential is restored by the flow of ions through the leakage channels
12
. This transient double reversal of the potential across the membrane
5
is named “action potential”. The action potential, which is initiated at a restricted part of membrane
5
, also depolarizes adjacent portions of the membrane
5
up to their excitation threshold. Channels
11
in these portions begin to open, resulting in an action potential at that portion of the membrane
5
which then affects the next section of membrane
5
and so on and so on. In this way the action potential is propagated as a wave of electrical depolarization along the length of the axon
4
(FIG.
3
).
After an action potential has been generated, there is a refractory period during which nerve cell
1
cannot generate another action potential. The sodium channels
11
do not open again when the membrane
5
is depolarized shortly after its excitation. The effect of the refractory period is that action potentials are discrete signals. Trains of propagating action potentials transmit information within the nervous system, e.g. from sense organs in the skin to the spinal cord and the brain.
There are two categories of nerve fibers that carry sensory information from remote sites to the spinal cord, small diameter afferent nerve fibers
13
and large diameter afferent nerve fibers
14
. Generally speaking, the small diameter afferent nerve fibers
13
carry pain and temperature information to the spinal cord while the large diameter afferent nerve fibers
14
carry other sensory information such as information about touch, skin pressure, joint position and vibration to the spinal cord. As shown in
FIG. 4
, both the small and large diameter afferent nerve fibers
13
,
14
enter the spinal cord
16
at the dorsal roots
17
. Only large diameter nerve fibers
14
contribute branches to the dorsal columns
15
.
Melzack and Wall published a theory of pain which they called the “gate control theory.” (R. Melzack, P. D. Wall, Pain Mechanisms: A new theory.
Science
1965, 150:971-979) They reviewed past theories and data on pain and stated that there seems to be a method to block pain at the spinal level. Within the dorsal horn of gray matter of the spinal cord, there is an interaction of small and large diameter afferent nerve fibers
13
,
14
through a proposed interneuron. When action potentials are transmitted in the large diameter afferent nerve fibers
14
, action potentials arriving along small diameter nerve fibers
13
(pain information) are blocked and pain signals are not sent to the brain. Therefore, it is possible to stop pain signals of some origins by initiating action potentials in the large diameter fibers. The type of pain that can be blocked by such activity is called neuropathic pain. Chronic neuropathic pain often results from damage done to neurons in the past.
Spinal Cord Stimulation (SCS) is one method to preferentially induce action potentials in large diameter afferent nerve fibers
14
. These fibers
14
bifurcate at their entry in the dorsal columns
15
into an ascending and a descending branch (dorsal column fiber), each having many ramifications into the spinal gray matter to affect motor reflexes, pain message transmission or other functions. Only 20% of the ascending branches reach the brain (for conscious sensations).
Action potentials in the large diameter nerve fibers
14
are usually generated at lower stimulation voltages than action potentials in small diameter nerve fibers
13
. While the dorsal roots
17
could be stimulated to cause action potentials in the large diameter afferent nerve fibers
14
, stimulation there can easily cause motor effects like muscle cramps or even uncomfortable sensations. A preferred method is to place electrodes near the midline of spinal cord
16
to limit stimulation of the nerve fibers in dorsal root
17
.
Today, SCS systems use cylindrical leads or paddle-type leads to place multiple electrodes in the epidural space over the dorsal columns
15
. Often the surgeon will spend an hour or more to position the leads exactly, both to maximize pain relief and to minimize side effects. One of the current problems with SCS is the preferential stimulation of nerve fibers in the dorsal roots (dorsal root nerve fibers) instead of nerve fibers in the dorsal columns (dorsal column fibers) especially at mid-thoracic and low-thoracic vertebral levels. This is in part because the largest dorsal root fibers
14
have larger diameters than the largest nearby dorsal column fibers. Other factors contributing to the smaller stimulus needed to excite dorsal root fibers are the curved shape of the dorsal root fibers and the stepwise change in electrical conductivity of the surrounding medium at the entrance of a dorsal root into the spinal cord (J. J. Struijk et al.,
IEEE Trans Biomed Eng
1993, 40:632-639). Stimulation of fibers in one or more dorsal roots results in a restricted area of paresthesia. That is, paresthesia is felt in only a few dermatomes (body zones innervated by a given nerve). In contrast, dorsal column stimulation results in paresthesia in a large number of dermatomes.
One approach to suppress the activation of dorsal root fibers and thereby favor dorsal column stimulation has been the application of an electric field to the tissue where the shape of the electric field is changeable and, as a result, where the location of the electric field in the tissue is steerable. This technique has been described in U.S. Pat. No. 5,501,703 entitled Multichannel Apparatus For Epidural Spinal Cord Stimulation that issued Mar. 26, 1996 with Jan Holsheimer and Johannes J. Struijk a
Bauer Stephan W.
Evanisko George R.
Kinghorn Curtis D.
Medtronic Inc.
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