Full text of DOI:10.1016/S0889-5406(00)70234-4

ORIGINAL ARTICLE
Otic axis locator: Closing the accuracy gap in cephalometrics
and cast mounting
Raymond C. Thurow, DDS
Verona, Wisc
An overview is presented of a new technique to improve the accuracy and reproducibility of earpost-
dependent orientation in procedures such as cephalometric radiography and cast mounting, using a
custom-molded ear canal insert to provide a stable fitted socket for more exact earpost fit. This is especially
valuable in follow-up treatment and serial studies because the insert can be filed for future use on the same
patient to provide identical positioning in successive registrations. Cephalometric radiography is further
enhanced by the addition of a radiopaque x-ray marker deep in the canal, otherwise inaccessible, close to
the bony ear canal (anatomical porion) and the mandibular condyle. This marker is also visible in all 3
cephalometric views—lateral, frontal (posteroanterior) and coronal (submental-vertex)—providing an exact
common horizontal axis (Otic axis) for a three-dimensional Cartesian coordinate system of measurement.
(Am J Orthod Dentofacial Orthop 2000;117:298-302)
B. Holly Broadbent’s vision of the future
holder was a modified craniostat positioned with one x-
ray generator exactly aligned with the earpost axis and
another perpendicular to that axis. Channels to hold film
cassettes were mounted on the craniostat perpendicular
to each x-ray beam to complete the major modifications.
His description of patient orientation is simple:
of cephalometrics, when he introduced it to the profes-
sion in 1930, did not include its current widespread
1
use in orthodontic diagnosis. He saw it only as a
research tool for pooled serial growth studies encom-
passing three dimensions, using the “frontal” (pos-
teroanterior) view in combination with the lateral view.
The x-ray measurement system was patterned after
craniometrics, an anthropometric technique for mea-
suring dried skulls by using an instrument called a
craniostat to orient, hold, and measure the skull.
Like the relation of the skull to the cephalostat, the
head rests on the uppermost sides of the rods that are
inserted into the ear holes....After the head has been cen-
tered, the chair is lowered or the child instructed to settle
down so that the under surfaces of the superior border of
the ear holes are resting on the upper sides of the ear
rods. Then the head is rotated on this Porion axis by lift-
ing or lowering the face until the lowest point on the infe-
rior bony border of the left orbit is at the level of the top
of the ear supports as indicated by the Orbital marker.
The first step in orienting a skull is to suspend it by
the most obvious bilateral structures, the ear holes
external auditory meati). These openings appear
(
ready-made for that function. Gently inserting the mov-
able earpost tips of the instrument as far as possible
gives a very accurate, reproducible position. The con-
tact point where the earpost engages the superior sur-
face of the opening has been termed porion, and the
axis connecting the right and the left porions is the
baseline for most orientation and measurement.
Rotation around the earpost axis is then adjusted
to place the inferior margin of the left orbit (orbitale)
on the same level as the porions to establish the
Frankfort horizontal plane, which is the generally
accepted horizontal reference plane.
He goes on to describe the final step of “Placing this
(Nasion) rest against the root of the nose clamps the
head firmly in the instrument.”
The top of the soft tissue ear hole is called porion
and is treated the same as the underlying bony struc-
ture, despite the obvious differences imposed by the
interposition of mobile soft tissue. The loose-fitting
movable serpentine cartilaginous ear canal displaces
the earpost laterally about 2 cm and introduces wide
parasagittal variability that leaves the porion wander-
ing from film to film.
Broadbent adopted this same orientation for the
cephalometric radiography of living patients. His head
That relocation and resulting variability of the
cephalometric porion is well known but generally
ignored in clinical application, being viewed as an
unavoidable variable that can be only moderately
US Patent application serial no. 09/286, 847 4/6/99
Reprint requests to: Raymond C. Thurow, DDS, 8058 Coray Ln,
Verona, WI 53593-9072; e-mail, www.thurowrc@chorus.net
Copyright © 2000 by the American Association of Orthodontists.
2
-4
controlled.
This is inconsequential in typical
0
889-5406/2000/$12.00 + 0 8/1/103776
doi:10.1067/mod.2000.103776
cephalometric diagnosis based on lateral cephalomet-
298

American Journal of Orthodontics and Dentofacial Orthopedics
Thurow 299
Volume 117, Number 3
ric films, because significant decisions do not turn on
the several millimeters or degrees of variability
involved, but the reliability of the porion can be crit-
ical in our evaluating individual serial changes or
precise functional occlusal orientation.
5
Ricketts has repeatedly called attention to this
problem and recommends use of the x-ray image of the
bony aperture in the lateral view, instead of the top of
the earpost. This is an accurate landmark, which makes
use of the original porion of craniometrics, but the dif-
ficulty of locating it consistently and identifying right
and left on typical clinical films has limited its useful-
ness and wider adoption. Like most landmarks, it is
also identifiable on only one x-ray view, further limit-
ing its applicability in three-dimensional studies.
Fig 1. Superior (S) and posterior (P) views of left ear-
mold impression showing canal form, earpost socket
liner, and x-ray marker.
Earpost Orientation of the Living Head
Broadbent does not indicate how far he inserted the
earposts, merely describing the objective of centering
the head by using scales on the earposts before further
stabilizing with the nasion rest. The ear canals are
highly variable among individuals, irregular in shape
and both flexible and sensitive. Those anatomic char-
acteristics make it impossible to engage a canal with
the consistent accuracy required for precise orientation,
measurement, or reproducible positioning. Broadbent
tacitly recognizes this when he notes little change in
some cranial base areas and recommends superimpos-
ing their images for serial evaluations.
Earposts for ear canal engagement have been fabri-
cated in endless variations of cylindrical, conical, and
bulbous forms. The technique of their insertion has also
been varied in many ways in attempts to improve con-
sistency and accuracy, but significant improvement
continues to elude us.
At one end of the earpost spectrum are the bulbous
forms, which are often used on face-bows for cast
mounting. This is usually not intimidating to the patient
because the mechanism is not very threatening and the
earposts engage the external auditory meatus at the least
sensitive outer opening. That is also the largest and most
mobile part of the canal, so the technique must be aimed
at minimizing tissue distortion as the face-bow is
adjusted and finally locked into position. The operative
word in the statement above is “minimizing,” because
even a highly skilled operator must contend with the
inevitable variability of final earpost position.
At the other end of the earpost spectrum are the
small, sometimes tapered shapes intended for the
deeper insertion needed to optimize accuracy in
cephalometrics. The cephalostat mechanism that closes
the 2 earposts toward each other facilitates deeper
insertion, and tissue distortion becomes an essential
objective to enable insertion into the smaller, deeper
part of the canal. This coordinated insertion of both
earposts is a trying procedure for both operator and
patient. While the operator is trying to insert the ear-
posts uniformly to maximum depth with the head per-
fectly oriented, the patient is responding predictably
with unpredictable avoidance moves. The lining of the
canal is progressively thinner and more sensitive as the
earposts move inward. When the operator feels that the
earposts are inserted as far as possible, a final instruc-
tion to close the teeth together and hold still can cause
yet another shift as the condyle intrudes on the already
tight space just before the x-ray exposure.
Predictably, exact alignment of both sides with any
of these techniques is even more rare than a hole in one.
Taking another film or making another face-bow regis-
tration with the same orientation is virtually impossible.
Even successive films exposed without removing the
earposts can be expected to show different positions.
This is easily seen in the placement of the images of
bilateral structures on individual lateral cephalometric
films, as well as by superimposing successive films
over each other. Face-bow registrations can be com-
pared by mounting a pair of casts and then remounting
only the upper cast while using a new registration.
Ear Canal Anatomy
A close look at the anatomy of the ear canal will
show why the many variations in earpost form and man-
agement have failed to solve the competing problems of
accuracy and comfort. Fig 1 shows the irregular shape of
a left ear canal in superior and posterior views of a full-
size impression of the outer portion of the canal. The
most movable part of the sound conduction system is the
concha of the external ear which collects sound and

300 Thurow
American Journal of Orthodontics and Dentofacial Orthopedics
March 2000
The wide individual variation in the deviant course
and changing cross-section of the canal make accurate
engagement with any earpost design an unattainable
objective. Off-the-shelf earposts can be expected to fit
no better than off-the-shelf dentures. Clinicians have
always been aware of these problems, which are
responsible for countless adjustments of dental devices
fitted to articulators and continue to limit the utility of
cephalometric measurement to applications that can
tolerate the large variations.
Anatomical X-ray Landmarks in Cephalometrics
Orienting the head is just the first of the two func-
tions of ear canal engagement in cephalometrics. The
other is to provide a bilateral pair of anatomical ref-
erence points at or near the temporomandibular joint
to establish a transverse axis for orientation and mea-
surement of x-ray images. This is the primary axis
for the same Cartesian coordinate system of three-
dimensional orientation and measurement long used
in craniometry. If we had the same access to the bony
canal as the physical anthropologist does in measur-
ing dried skulls, this whole discussion would be irrel-
evant, but no such solid engagement is available on a
living head. Nor are there any anatomical points that
we can consistently identify for a transverse axis on
any x-ray view.
Radiopaque markers are the only alternative.
Implants in bone are out of the question in human sub-
jects today, so we must rely on removable radiopaque
markers. A marker attached to the earpost has always
served this purpose in cephalometrics, but for reasons
already discussed, they bear such a loose relationship
to underlying anatomical structures that they are use-
less for accurate measurement. They also lie so far out-
side of the underlying bone structures that they are
sometimes lost outside the edges of the image area of
the film on frontal and coronal views.
Fig 2. Earmold impression (A) with socket insert (flange
B] and socket [C]).
[
directs it toward the funnel-shaped aperture of the canal.
The tragus partially hides and protects the opening, and
the canal continues directly inward from there. Below
the overlying skin and cartilage, it makes a sharp poste-
rior, and slightly upward, turn. That first turn is as far as
we can see in most cases without instrumentation, and it
is also as far as a bulbous earpost can penetrate. The
canal extends inward a long way from there, and as soon
as there is room for another turn, it turns more inward
and slightly upward as is passes behind the mandibular
condyle. It then turns more inward after another short
run and becomes smaller and more round as it enters the
temporal bone and continues on toward the eardrum in a
posteromedial direction. The turn behind the condyle is
as far as a conventional earpost can be expected to pen-
etrate because the canal is coursing at a significant angle,
while the earpost is pointed medially.
A NEW SOLUTION
The proximity of underlying cartilage and bone
progressively decrease mobility, while the sensitivity of
the lining of the canal increases as it courses inward
toward and through the bony entrance into the skull.
The external third of the canal is surrounded by carti-
lage and other soft tissue, but beyond the osteo-carti-
laginous junction behind the medial part of the
condyle, its character changes. The lining within the
temporal bone is thinner, devoid of glands and hairs,
The objective of the otic axis locator described here
is to significantly reduce the long-accepted level of
variability by enhancing the reliability of ear canal
engagement and, thus, raise the accuracy of dependent
procedures to a new level. A molded ear canal insert
flexible enough to allow for easy removal and reinser-
tion reshapes the entry to the large irregular canal with
an integral socket designed to accurately receive the
earpost of the cephalostat or face-bow. A pair of these
ear canal inserts provide a degree of accuracy in posi-
tioning and reproducibility unachievable with conven-
tional methods, while also improving patient comfort.
Both the exact positioning of the earpost and the added
6
and much more sensitive . Fig 1 shows how the cross-
section of the canal progressively diminishes from a
large flattened ovoid to a much smaller round shape as
it progresses into the bone.

American Journal of Orthodontics and Dentofacial Orthopedics
Thurow 301
Volume 117, Number 3
stability provided by the close fit throughout the ear
canal contribute to the accuracy of positioning.
1. One method is to attach it to the retrieval thread used on the
impression blocker described below with the impression
technique. Both thread and marker are inserted into a small
section of heat-shrinkable tubing, placed against or inside of
the blocker, and then secured by the application of heat to
lock the tubing onto the blocker and thread. This provides a
very secure attachment for the assemblage, which will
become embedded in the impression. This has the advan-
tage of single-stage fabrication, with the disadvantage that
the exact location in the impression cannot be controlled.
2. For more exact positioning of the marker, it is inserted
directly into the finished impression after it has been
removed from the canal. A hole is made at the desired
location, and the marker is then inserted and secured to
prevent accidental dislodgement.
Forming the socket directly in the earmold insert is
adequate for face-bow orientation, but incorporating a
rigid radiolucent flanged socket liner further improves
stability and facilitates earpost insertion for cephalomet-
rics, where the ear canal must control the entire head.
Cephalometrics also requires a radiopaque marker
embedded in the insert to provide an x-ray reference
point close to the condyle and the more stable bony
canal. This places the marker close to the anatomical
porion of craniometrics. To avoid compounding the
confusion by adding yet another porion to the points
already given that name, these markers are identified as
otic markers. The axis connecting the right and the left
otic markers is the otic axis. These markers provide the
only stable landmarks that can be clearly identified in
all 3 x-ray views, so the same otic axis can be reliably
identified in all films for reliable and three-dimensional
measurements and analyses.
Impression Technique
The impression technique is a modification of the
technique typically used in the fabrication of hearing
7
aids that fit inside the canal, with use of impression
materials made for that purpose. These are typically
silicone-based materials with an incorporated lubricant
that facilitates removal from the dry lining of the canal.
Preparation for the impression is brief. First, the
ear canal is examined with an otoscope to verify that
there are no blockages, inflammation, or other abnor-
malities that might interfere with the impression.
(These are uncommon and in most cases would also
require modification or postponement of conven-
tional earpost insertion.)
Next, the procedure is rehearsed so the patient knows
what to expect and how to respond to instructions. The
socket liner is positioned and the flange is trimmed if
necessary. If a face-bow is being used to hold the socket,
it is then placed on the bow and positioned with the bow.
The patient is instructed to hold the teeth in occlusion as
will be done later while the impression sets; dentists are
already familiar with the intrusion of the lateral pole of
the condyle on the canal in many individuals, as demon-
strated by palpation, and the closing of the jaw ensures
that the earmold will be formed to accommodate this
movement. Finally, the liner assemblage is removed, and
the patient is ready for the impression.
Placing an Earpost Socket Liner
The primary alignment for accurate repro-
ducible placement of earposts is provided by the
socket that receives the earpost. Embedding a rigid
radiolucent flanged earpost socket liner in the
external face of the earmold impression adds rigid-
ity and facilitates earpost insertion.
The socket liner is located to minimize distortion of
the surrounding ear tissues. This may place it in a
somewhat different position than the typical earpost
that is forced through bends in the canal. The place-
ment should be checked before mixing the impression
material, so the flange can be trimmed as may be
required to fit behind the tragus and accommodate any
unusual anatomical variations. Final placement in the
soft impression is facilitated by holding it with a light-
weight bow similar to those used to hold paired
earplugs or audio earpieces. The bow is stabilized by a
simple earplug on the other side as it holds the insert in
position while the impression material sets.
Placing a Radiopaque X-ray Marker (Otic Marker)
For cephalometric applications, a radiopaque x-ray
landmark is inserted near the most stable inner end of
the insert, close to the temporomandibular joint and the
original anatomical porion. A metal ball about 2 mm in
diameter is an appropriate marker for embedment or
other secure attachment near the inner end of the
impression. Right and left can be distinguished by plac-
ing double markers on the left or by using unique sizes
or shapes on opposite sides. Two methods may be used
for placing these markers:
The first step in the impression procedure is placing
a soft canal blocker, attached to a strong thread, into
the ear canal at the desired depth, leaving the end of the
thread hanging outside of the canal to aid in retrieval of
the impression. This limits penetration of the impres-
sion material and can facilitate retrieval of the finished
impression. Blockers are typically fabricated from cot-
ton pellets or soft foam.
The impression material is then mixed, placed in a
syringe, and immediately injected into the canal,

302 Thurow
American Journal of Orthodontics and Dentofacial Orthopedics
March 2000
beginning with full insertion of the syringe tip and then
withdrawing it as the canal is filled. The area surround-
ing the opening of the canal is also filled, and the
socket liner is immediately positioned in the soft
impression material. The patient is then instructed to
hold the teeth in occlusion while the material sets.
After the material has set, the impression is
removed with the aid of the protruding thread. Setting
times are, depending on the choice of impression mate-
rial, around 10 minutes.
The impression on the opposite side can be made
as soon as the first impression has set. These impres-
sions are ready to use as soon as the material sets but
must be removed for post-impression placement of
an x-ray marker or to confirm appropriate positioning
of a marker embedded in the impression at the time
of molding.
SUMMARY
An otic axis locator consists of an earpost socket in
a removable/reinsertable insert molded to fill the outer
portion of the external auditory canal. This provides for
greatly improved stability and accuracy in earpost place-
ment, enabling a more accurate and consistent horizon-
tal axis in the condyle area for mounting dental casts in
articulators and for head positioning for cephalometric
radiography. It also eliminates most of the pain, discom-
fort, and associated patient movement that typically
occurs in positioning the head for cephalometrics.
Placement of a radiopaque marker near the inner end
of the insert adds several important advantages for
cephalometric imaging: (1) Locating the marker close to
the bony ear canal provides maximum stability. (2)
Locating the marker deep in the canal places it very close
to anatomical (true) porion. (3) The same point can be
identified in all three cephalometric views. (4) Its posi-
tion close to the mandibular condyle facilitates the accu-
rate relating of the temporomandibular joint to the rest of
the facial skeleton. (5) The more medial location close to
skeletal structures helps keep it from being lost off the
edge of the image area in frontal and vertical views.
Using the Otic Axis Locator
Use of these otic axis locators is simple, requiring
little change in technique from current practice. Nor-
mal procedures are used after the inserts are fully re-
seated in both ear canals. For cephalometrics, there is a
slight change in earpost placement because they should
be tightened only enough to fully seat them; excessive
tightening may distort the insert and cause unnecessary
patient discomfort. The whole procedure after the
inserts are placed is actually faster and more comfort-
able than conventional methods.
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1
2
.
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Broadbent BH. A new x-ray technique and its application to orthodontia. Angle
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Krogman WM, Sassouni V. Syllabus in Roentgenographic Cephalometry. Philadel-
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3. Riolo ML, Moyers RE, McNamara JA, Hunter WS. An Atlas of Craniofacial Growth.
Ann Arbor: Center for Human Growth; 1974. p. 20.
These inserts should be filed after use so that they
are available later for follow-up films or other proce-
dures. They can be re-inserted at any time after verify-
ing that the canals are clear. The only reason for fabri-
cating new inserts is alteration of the canals by growth,
injury, or surgery.
4
.
Thurow RC. Atlas of Orthodontic Principles, ed. 2. St. Louis: Mosby; 1977. p. 282-3,
293.
Ricketts RM. Perspectives in clinical application of cephalometrics. Angle Orthod
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980;51:115-50.
6. Martin FN. Introduction to Audiology. Englewood Cliffs (NJ): Prentice-Hall; 1975.
p. 211.
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Mueller HG, Hall JW. Audiologists’ Desk Reference. San Diego: Singular; 1998. p.
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