12
3
Myrin Inclinometer;
4
Spinal Pantopraph;
5
Flexicurve
7.5
SETUP AND USE
The arcometer is a manual device made of a ruled bar
bearing three perpendicular arms on it: the first one fixed
at one end, the intermediate one is mobile on two axes
and the third one mobile on one axis only.
The ends of the three arms identify three points through
which a single
circumference can be drawn. Using the arcometer we can
measure the chord and the rise. These two values allow us
to calculate the radius of the curve and Cobb angle by
using a two entry table.
NB:
the arcometer works according to the trigonometric
assioma that through three points only a single
circumference can be drawn. For this pourpose the
segment of the spine is arbitrary approximated to a part
(arch) of a circumference.
Arcometer schema with its working way: note that, in order
to make calculations easier, the intermediate arm has to
be placed into the median point between the lateral arms.
The validation of the insturment was made studying two
aspects:
Concordance X-rays
–
arcometer: the mean difference
between two series of measurements was 2.9° Cobb; the
concordance between the two methods of measurement
was good, without sistematic errors.
The lenght of the chord can have an influence on the
results since a shorter chord bears a risk of underestimating
the angle.
Concordance
intra-observator
:
the
concordance
between two series of measurements was good, without
sistematic errors: The mean of the differences risulted 0.1°
Cobb (90% of the cases difference within ±10° Cobb; 82%
of the cases within ±5° Cobb; the difference shows a small
increase for kyphosis values between 44° and 63.5°).
Bibliography: D’Osualdo F., Schierano S., Iannis M. Validation of
clinical measurement of kyphosis with a simple intrument, the
arcometer.
Spine 1997; 22:408-13.
7.6
PROCEDURE OF MEASUREMENT
To measure the spine on the sagittal plane (Kyphosis and
lordosis) the arcometer is placed with the external bars at
the end of the segment: the length is the chord.
The central bar is then fixed on the middle point of the
length, then gently approached to the back: on the same
bar one can read the value of the rise.
Cobb’s angle along with the radius value can be easily
read on a two entry table.
