Prognathism
—method of
determining.
Objections
to Flower’s
index.
to the horizontal in which the skull is oriented. Since the Frankfort-Munich plane,
i. e. that passing through the upper part of the external auditory meatus and the
infra-orbital margin, has no direct association with any of the angles of the aforesaid
triangle, it becomes a matter of extreme difficulty to orient the triangle in relation to
this plane, more particularly since the Frankfort-Munich plane is based on points which
lie lateral to the mesial plane, and has no constant relation to points which fall within
the mesial plane.
The alveolo-condylic plane of Broca, whilst referable to points in the mesial plane,
i. e. the alveolar point, is open to the grave objection that the whole orientation of the
skull depends on the development of the upper facial area; in other words, when the
face is proportionately long, the whole skull when oriented in this plane acquires an
upward tilt and the disposition of the calvaria is correspondingly altered, which is not
necessarily the case, as may be readily proved. These difficulties, however, are in great
part overcome if the skull is so oriented that the basi-nasal line is disposed at an angle
of 2 70 with the horizontal; the nasion then becomes the point below and in front of which
the face is developed, whilst it serves
as the point from which the curve and
disposition of the cranial vault may be
estimated above and behind. If such
an orientation be adopted, it will then
be possible to determine the position
of the alveolar point below the nasion,
whilst by dropping a perpendicular from
the nasion it will also be possible to
estimate the projection of the alveolar
line in advance of a vertical line passing
through the nasion. This may be seen
in the accompanying figure (Fig. 6),
where the line n b corresponding to the
basi-nasal line is prolonged to meet the
horizontal h h at an angle of 270 at c. n v is a vertical dropped from n the nasion; the
projection of the alveolar points a , a ' in the two outlines can easily be measured by taking
their distance in front of the vertical n v .
These are advantages which none of the more generally accepted systems possess,
as will be at once apparent when some of the more usually adopted methods of
estimating prognathism are criticized. The plan suggested by Flower, and since largely
employed for the estimation of prognathism, is open to serious objection. The basi-
alveolar length is expressed in terms of 100 by reference to the basi-nasal length.
Such a procedure, however, does not take into consideration any variation in the length
of the third side of the triangle, i. e. the nasi-alveolar length ; consequently it sometimes
happens, and that more frequently than might at first appear likely, that the results
obtained by Flowers method are misleading and in some cases suggest a degree of
projection where none such exists.
This may perhaps be best explained by a diagram (Fig. 7). Here a b the basi-
nasal line = 100. b c and b d are each basi-alveolar lengths measuring 100, but in the
case of b c the point c falls well in front of a e , the vertical dropped from a the nasion,
whilst in the case of b d the point d lies closer to that perpendicular. Yet in each
instance the index of prognathism (10 0) is the same, though, as has been shown, the point c Objections
projects considerably in advance of d . The error, of course, is due to the fact that S
Flower s method does not take any account of the variations in the nasi-alveolar length,
as is represented by the differences in length between the points a and c, and a and d .
That such objection to his system is not
hypercritical is borne out by the fact
that in testing the value of Flower’s index
as an estimate of prognathism in a series
of 161 skulls, it became apparent that
indices which were identical even to the
superfluous place of decimals were proved
1
to be absolutely misleading as regards
the relative projection of the jaw. This
view may be confirmed by a reference
to the Appendix (inset, 1-32), where our
1
tables of prognathic projection may be
DT .—^
compared with Flower’s indices.
27"''-..
F ig . 7.
Gnathic
Angle.
In regard to the gnathic angle, we
may point out that this merely expresses
the angle formed by the convergence of the nasi-alveolar and basi-alveolar lines, and that
this angle may remain the same despite variations in the length of these lines and
simultaneous differences in the length of the basi-nasal line. But if we regard the position
of the cranial base, i. e. the basi-nasal line, as constant in its relation to the cranial vault
and the facial skeleton, these variations in length of the sides of the triangle, i. e. the
basi-alveolar and the nasi-alveolar, whilst not altering the included angle will certainly
give rise to an alteration in the disposition of that angle relative to the base. In
Fig. 8 the two triangles a c b and a d b
have the same base a b (the basi-nasal
line), and they have also similar angles
at c and d , yet they are so disposed that
the angle c lies farther away from the
vertical a e than does the angle d. From
this it will be evident that the mere record
of the gnathic angle is no criterion of the
projection of the face beyond the vertical,
however this vertical may be determined
to fall. In other words, it fails to give us
any accurate record of prognathism, whatever
the system of orientation employed
may be. I n short, prognathism cannot be
determined without also taking into consideration
27'
F ig . 8.
the absolute or relative measures of some two of the three sides of the triangle.
The weakness of Flower’s gnathic or alveolar index, on the other hand, is that it only
expresses the proportionate length of the basi-alveolar line to the basi-nasal. It fails,
because it leaves out of account the nasi-alveolar length, to express correctly the precise
position of the alveolar point, which can only be determined when Flower’s index is
supplemented by the gnathic angle.
I i
Il II