Dentistry – Apparatus – Having gauge or guide
Reexamination Certificate
1999-04-06
2001-12-11
Wilson, John J. (Department: 3732)
Dentistry
Apparatus
Having gauge or guide
C181S130000
Reexamination Certificate
active
06328564
ABSTRACT:
FIELD OF THE INVENTION
A device fitted to the external auditory canal for the purpose of reproducibly engaging the head for orientation and stabilization for dental or radiographic (commonly known as ‘cephalometric’) recording and for placing a radiographic marker in the canal.
BACKGROUND OF THE INVENTION
The external ear canal has been used as a basis for dental orientation and measurement since the nineteenth century. Artificial dentures and other dental prostheses are typically fabricated in a laboratory on casts of the jaws mounted on a mechanical articulator which approximates the functional relationships of the jaws. How well the teeth on the articulator imitate natural function depends in a large part on relating them correctly to the joints that control jaw movement.
Positioning the casts of the teeth in the articulator is typically accomplished by a ‘face bow transfer’ which uses an adjustable rigid bow that engages the upper teeth in front and the two ear canals in back. This relies on earposts in the ear canals as the basis for estimating the actual position of the joints, and it is subject to significant error because of the anatomical characteristics of the external ear canal that is engaged by the facebow. Efforts continue today to improve the accuracy and reproducibility of these procedures.
When cephalometric radiography was developed by Broadbent in the 1920's, it was also based on ear canal orientation of the head. The external mechanism is aligned with the x-ray source through movable right and left earposts that engage the outer ear canals. Cephalometric radiographs have become a standard component of orthodontic diagnosis, and the axis connecting the ear canals or related proxy landmarks are still the underlying foundation for measurements derived from these films in all three planes of space.
The orientation of the head on the ear canal axis is controlled by engaging the outer ear canals with a rigidly mounted mechanism usually called a ‘cephalostat’ or ‘cephalometer,’ and this continues to be a major source of error in this technique. Limits of precision exist for every method of measurement, and over time they often become accepted as an integral part of the technique and either factored into measurements or ignored. That is the current state in cephalometrics. The orienting mechanism can be made to any degree of precision, but the anatomy of the engaged ear structures still provides a very poor and inconsistent engagement. Alternative methods proposed up to this time have failed to offer significant improvement.
The external auditory meatus area is irregular in shape, highly variable, mobile, and very sensitive. Those anatomical characteristics make it impossible to engage it with the consistency required for reproducible positioning and precision measurement, but it is all that we have had. The ear canals are the only bilateral structures that can be readily engaged mechanically. Earposts for engaging the canals have been fabricated in various cylindrical, conical or bulbous forms, but all suffer from the same basic problems that prevent predictable or reproducible orientations with the accuracy required for precision applications.
SHORTCOMINGS OF EARPOST ORIENTATION OF THE HEAD
The classical cephalometric engagement of the head by coordinated insertion of machine-mounted earposts into the external auditory canals is a trying one for both operator and patient. While the operator is trying to insert the two earposts uniformly to maximum depth, keeping the head perfectly oriented with the earpost axis intersecting the sagittal plane at a 90 deg. angle, the patient is responding predictably to the resulting discomfort or pain. Shifting and squirming are typical in the search for a more comfortable position for both sides. When the operator feels that the earposts are inserted as far and as uniformly as possible the instruction to, “hold still” is issued while retreating behind a barrier to make the x-ray exposure. Predictably, the two sides are rarely aligned exactly, and taking another film with the identical orientation is impossible. This becomes especially critical when the mechanism and patient must be rotated to one of more companion views oriented in different Cartesian planes.
A close look at the anatomy of the ear canal will quickly show why the many variations in earpost form and size have failed to solve the problems of accuracy and comfort.
FIG. 1
shows the relevant anatomy of the external ear canal
10
and the position and general course of the canal upward and posteriorly as it progresses inward to the ear drum. These top and rear views of a full-size impression of the outer portion of the canal clearly show the irregular shape of the canal that defies consistent engagement by any standardized earpost. The outside view in
FIG. 2
completes the picture. The concha
12
of the external ear collects sound and directs it toward the funnel-shaped aperture of the external acoustic meatus
16
immediately behind the tragus
18
. The canal initially continues directly inward from there, and that is as far as we can usually see. It is also as far as a cephalostat earpost can initially penetrate, but the canal extends inward a long way from there. As soon as there is room for a turn, it turns sharply in a posterior
14
and slightly upward
20
direction, and then turns more inward
22
after another short run. The canal becomes smaller and more round as it continues on at about a 45 degree angle beyond the reach of the impression shown at
26
to the eardrum.
Individual canals vary widely in size and shape as they progress inward, with even wider variations among individuals. As can be seen in
FIG. 1
, the crosssection even in a single canal varies from nearly round
28
to flattened ovoid
30
, and from large to small. The deviant course and changing cross-section of the external canal makes accurate engagement for orientation of the head with any one earpost shape an unattainable objective.
The problem is further complicated by the proximity of underlying cartilage and bone, combined with the sensitivity of the lining of the canal. The outer portions are supported only by very mobile cartilage and other soft tissues, gradually approaching rigid bone as the canal progresses inward. This increases both stability and sensitivity as we progress inward, causing patients to shift unpredicably as they search for a more comfortable position.
The above problems have always been obvious to clinicians, and they have limited the utility of cephalometric measurement to structures and applications that can tolerate the inevitable variability.
IDENTIFYING ANATOMICAL POSITION
Orienting the head is just the first of two functions of the engagement of the ear canal. The other is to provide a pair of bilateral anatomic reference points to establish a line that can serve as the basis for measurement in the horizontal, vertical and transverse Cartesian planes of space. There are no bilateral anatomical points that can be reliably identified on x-ray images in more than one plane of space, so it is necessary to rely on secondary markers. Common practice uses a radiopaque marker attached to the earpost, so the same variability described above also affects these anatomic markers.
REFERENCES:
patent: 3097059 (1963-07-01), Hoffman
patent: 4616998 (1986-10-01), Wong
patent: 5781637 (1998-07-01), Heide et al.
Bumgarner Melba
Cummings Henry W.
Wilson John J.
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