The
mode of vocal cord vibration in normal and pathological conditions has been
investigated in several studies. We believe that the vocal cord should be
regarded as a layer-structured vibrator in order to explain the features of the
vibratory mode in various conditions. This conception is very important for
diagnostic aspects and also for simulation studies of vocal cord
vibration. This is a frontal section of the vocal cord. As it is the portion at
and around the free edge which is moved most markedly during phonation, we will
concentrate our attention on this area. The surface of the free edge is covered
with thin stratified squamous epithelium about 50 micrometers in thickness which
extends an average of 1.8 millimeters lateral to the edge and an average of
2.3 millimeters inferior to the edge. From a mechanical point of view, this
may be regarded as a capsule whose purpose is to maintain the vocal cords
shape. Underneath the epithelium there is the lamina propria of the mucosa which
can be divided into three layers according to the distribution of the
fibers components, that is the elastic and collagenous fibers. The superficial
layer is rather poor in the fibers components and appears loose and pliant.
This layer is known as Reynke's space. From a mechanical point of view, this
layer may be regarded as somewhat like a mass of soft gelatin. The intermediate
layer is primarily composed of elastic fibers whereas the deep layer is dense
with mostly collagenous fibers. The entire structure consisting of these two
layers is known as the vocal ligament. From a mechanical point of view, the
elastic fibers are like rubber bands and the collagenous fibers are more like
cotton thread. Underneath the mucosa lies the vocalis muscle, the main body of the
vocal cord. The vocalis muscle is like a bundle of rather stiff rubber bands. Thus
quite clearly the vocal cord consists of multiple layers of tissue, all having
different mechanical properties. This shows the layered structure
schematically. From a mechanical point of view, we can differentiate the layers
into three sections. The cover consisting of the epithelium and the
superficial layer of the lamina propria, the transition consisting of the
intermediate and deep layers of the lamina propria, and the body consisting
of the vocalis muscle. The average thickness of the cover is 0.3
millimeters and that of the transition is 0.8 millimeters. This shows the load
strain curves of each section. The strain for the cover is much greater than that
for the body. The strain for the transition is in between those for the
cover and the body, but is closer to that of the cover. On the basis of the
measurements described so far, and those on laryngograms, we determined the
average vocal cord position during phonation as shown in this figure. The
three layers are simplified into two, the cover and the body. The thickness of the
cover in this simplified model is assumed to be 0.8 millimeters. The ratio
of the stiffness of the cover to that of the body can be assumed to be one to
three. Now let us observe the vocal cord vibrations in various conditions. The
subject of the first film is a bass singer in a non-professional choir.
Almost symmetrical vibrations are repeatedly regularly. You can clearly
see wavy movements on the mucosa traveling from below to above. The glottis
is closed completely.
This shows the results of frame-by-frame analysis over one period of the
vibrations. Lateral excursion of three points, that is the upper lip, the lower
lip and the tip of the wave, is drawn as a function of time. On the basis of this
figure and the nature of the layer structured vibrator mentioned earlier, we
made schemata of the frontal section of the vocal cord model at the time shown
with the numbered thin vertical blue lines. This shows the behavior of the
layered structure schematically.
Let's look at the actual vibrations again.
Next is falsetto of a tenor singer. In falsetto, the vocalis muscle is almost
relaxed. The entire structure of the vocal cord is stretched by the
cricothyroid muscle, resulting in an increase in stiffness of the cover. The
amplitude of lateral excursion is smaller than that in heavy voice and
little wave is seen on the mucosa. The glottis is not completely closed. The
behavior of the layered structure is shown schematically.
Let's look at the actual vibrations again. In recurrent laryngeal nerve
paralysis, mass and stiffness of the body are decreased and therefore the
difference in stiffness between the cover and the body is decreased. The left
paretic vocal cord is seen on the left side unlike a normal face-to-face
examination. It moves like a flag flapping in the wind, presenting a
missing motion. Very small waves are seen on the paretic cord. The glottis remains
open throughout. This shows the behavior of the layered structure schematically.
Let's look at the actual vibrations again. In sulcus vocalis, the mass of the
cover is decreased, whereas stiffness of the cover is increased at the sulcus or
the furrow. The sulcus on the right vocal cord is not visualized because the angle
of the light during the filming was slightly inclined. The amplitude of
lateral excursion of the vocal cords is small. No wave is seen at and below the
sulcus. The glottis is not completely closed. In the schemata, the right vocal
cord is drawn symmetrical to the left.
The actual vibrations are shown again.
In polyp, the mass of the cover is increased. Stiffness seems decreased in
this particular case because the polyp is edematous. Although there is a small
polyp on the upper surface of the left vocal cord, it does not affect the
vibration significantly. The polyp on the right moves slightly later than the
vocal cord proper, showing a coupled vibration. The glottis anterior to the
polyp is not completely closed. The small polyp on the left side is neglected in
schemata. Let's look at the actual vibrations again.
In polypoid vocal cord, or Reimke's edema, the mass of the cover is increased
and the stiffness of the cover is decreased. The movements of both the
vocal cords are asymmetrical. The bilateral polypoid swellings touch each
other for a rather long time, affecting the movements of the opposite side
significantly. Mucosal waves are clearly seen, especially on the right side.
Deformation of the polypoid swellings, as well as the behavior of each layer, is
shown schematically.
The actual vibrations are shown again. In amyloid tumor, mass and stiffness of
the cover are increased. The tumor always remains touching the
contralateral vocal cord, interfering with its movement. The glottis anterior
and posterior to the tumor is not completely closed.
The behavior of the layered structure is shown schematically.
Let's look at the actual vibrations again. In epithelial hyperplasia, mass and
stiffness of the cover are increased. The vocal cords present small asymmetrical
movements. No wave is seen on the hyperplastic epithelium. The movements of
the lesions are like that of a boat rolling on the waves. The glottis is not
completely closed. Now let us look at the schematical representations.
Let's look at the actual vibrations again. In carcinoma, mass and stiffness of
both the cover and the body are increased. The right vocal cord, which has a
carcinomatous lesion, shows little movement. No wave is seen on the lesion,
either. The movement of the unaffected cord is small and irregular. The glottis
is not completely closed.
The behavior is shown schematically.
The actual vibrations are shown again.
Thank you for watching.