|
|
|
|
Electrodes
To evaluate individual motor
units within a muscle, an electrode must be placed into
the muscle itself. Needle electrodes used for this purpose
will be described below. As with all work involving electrical
equipment, a ground electrode must be used and should
be placed on the same extremity that is being investigated.
For grounding, a plate electrode is the most useful and
best tolerated by the patient, though uninsulated needles
have been used.
The electromyographer chooses the
type of needle electrode to use, and it is preferable
that he or she uses one type consistently to become
well acquainted with its characteristics.
Monopolar
Needle
Made of stainless steel, the
monopolar needle electrode has a very finely sharpened
point and is covered with Teflon or other insulating material
over its entire length, except for a 0.5 mm exposure at
the tip. The needle serves as the active electrode, and
a surface electrode placed on the skin close to it serves
as a reference.
 The
main advantage of monopolar needle electrodes is that
patients accept them better because they are of small
diameter and Teflon covering allows them to slide in
and out of the muscle easily. Moving the needle causes
less discomfort.
The major disadvantages of this
needle is that, with repeated use, the size of the bare
tip changes, thereby limiting the number of examinations
for which that needle can be used. The Teflon peels
back, exposing a larger area that then changes the recorded
characteristics of the motor unit potentials. Because
the active electrode tip and the surface electrode are
separated by some distance, the background noise becomes
much greater as remote muscle contractions may be picked
up easier.
Concentric Needle
With the concentric needle
consists of a cannula with an insulated wire (or wires)
down the middle. The active electrode is the small tip
of the center wire, and the reference electrode is the
outside cannula. Concentric needles may have two central
wires (bipolar), in which case the active and reference
electrodes are at the tip and the outside cannula acts
as the ground.
 Because
the active and reference electrodes are closer together,
using the concentric electrode minimizes background
noise. The electrode picks up motor units from only
a very small distance. Another advantage of this electrodes
is that no (reference) surface electrode is needed.
The main disadvantage of the concentric
electrode is that, by comparison with other needles,
its larger diameter can cause more pain, and moving
the electrode around is uncomfortable. When one tries
to use a small-gauge concentric needle, bending becomes
a problem when the needle is dulled by repeated use.
This needle can be resharpened with a fine honing stone.
Single-Fiber
Needle
 Used
for special studies, this needle consists of a 0.5-0.6
mm stainless steel cannula with a 25 µm fine platinum
wire inside its hollow shaft. In a side port 3 mm behind
its tip, the cut end of the platinum wire is exposed.
Surface
Electrode
Because they can pick up gross
motor unit activities, the evaluation of single motor
unit potentials with surface electrodes is difficult.
They are best used as reference electrodes when monopolar
needles are used. They can also be used however, in kinesiology
to obtain gross indications about muscle activity and
in gait analysis.
|
|
home
about
us 
patient
forums
doctors
forums
online
store 
contact
us 
