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(Fig. 2). The proper height found, the patient's head is laid upon the plate holder, in such a position that the area to be examined is placed at point where the focal-ray would strike. The areas for upper

and lower maxilla are attained by shifting the focal ray either forward or backward one inch. The area for molars, bicuspids and cuspid are further assured by the position of the nose. For molars the nose is slightly raised from the plate. For bicuspids and cuspid, the nose is flattened on the plate. For these exposures, it is advisible to retract the body in order to prevent the vertebrae from being projected into the area under examination. This method brings the root, crown and surrounding tissues into closer contact to the plate of any position so far advocated. Cieszynski has accurately measured the distortion resulting from these positions and claims that when roperly applied, the distortion is less than ten per cent of the length of the tooth. This position and technique accomplishes several things. First, each radiograph presents the same field with landmarks always in similar positions. The thickness of the object and distance always the same, the time of exposure is the same. Immobility is always accomplished. Given the same distance, the current, in a tube, of known and equal penetration and intensity, a developer with known factor, and the correct time of exposure maintained, the results will always be the same, i. e., the degree of density is that desired for this kind of work. Hence, one will readily recognize normal areas in a negative produced by exactly similar technique.

The title of this paper calls principally for the Interpretation of Dental Radiography.

The field which I will consider, takes into consideration only detailed interpretations, and can better be demonstrated by limiting myself almost exclusively as these are apparent on plates. By plates

is meant an exposure covering the area, including all the teeth from the cuspids back, both upper and lower maxilla, and the antrum.

We have been called upon to clear up certain foci of infection, which according to present knowledge, are responsi. ble for certain systemic disturbances. That we may not fail in our duty, it is as necessary to determine the extent of a sinus, as it is to locate the tooth causing it.

The one elementary factor of interpretation is, that a Roentgenogram shows only one thing, namely, difference in tissue density. In this, physical laws alone prevail, nothing being left to the imagination.

Air shows the least density, because no rays are absorbed, and therefore reach the plate with full intensity, blackening it deeply, resulting in the so-called high lights.

Water and soft tissue, skin, muscle, etc., have the same density, that is, absorbability for the Roentgen Rays. These allow a lesser amount of rays to penetrate them, and act chemically upon the plate, hence, are accordingly lighter.

Bone tissue has greater density on account of the quantity of mineral, (calcium) salts in its composition. Therefore, decalcified bone shows less density, and eburnated or ebonized bone greater density. The first will be found in rhacitis and tuberculosis, in arterio-sclerosis, etc., the second in tertiary stages of syphilis, or late osteomyelitis. Enamel shows

greater density than the last mentioned alteration, and metal like gold fillings the greatest.

Pathologic conditions may only be assumed, never asserted. We may say that an area of increased density signifies an abscess, but we ought never positively identify it as such from radiographic findings alone. The extent of infection cannot be determined by a radiogram beyond the line of demarcation, which indicate the destroyed tissue. A round cell

even

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infiltration, however, may cause an area of decreased density due to an inflammatory condition, causing an absorption of calcium salts. An apical abscess is more or less distinct according to the amount of bone tissue destroyed. There is, however, a vast difference in an abscess, be it acute, post-acute or chronic. So called blind abscesses, with but little destruction of hard tissue are often difficult to diagnose. Such abscesses will show but faintly and in many cases will be entirely overlooked. It is in such cases that I enlist the attention, study and discussion of men in this section to determine and give expression to views and knowledge to guide us in recognizing detail in the reading of Roentgenograms.

Distinguishing normal from abnormal is here a question, and I ask the question—"What indicates an abnormal condition where bone tissue is not actually destroyed ?"

We see an area surrounding each tooth which we assume to be the “lamina dura" to which Dr. Hopewell-Smith has given the radiographic designation "linea dura." By what sign may we detect an abnormal condition? Is it its thickness, or absence that indicates anything? Assuredly without a clinical examination nothing could be assumed, but even so, it is sometimes impossible to even surmise the underlying condition.

In many cases of blind abscess, so called, it is a matter of study to detect any change in tissue density, and one thing more than anything else is apparent, namely, that absorption from putrescent canals, with attendent absorption of toxins seems to show least evidence upon the negative. In cases of arthritis, the absorption of toxins seems to come mostly from this class of abscess.

The radiological picture may be so questionable as to be overlooked entirely, and in many cases undoubtedly is,

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abscess the following plate (Fig. 4) is shown, interpreted at the time as a bone Cyst.

When operated upon subsequently by Dr. G. V. I. Brown its nature was definitely established, but not its etiology. You will note quite a difference in this and the foregoing, in that the degree of density within the cavity, and the margin denoting its extent, are very different. Upon clinical examination it also gives an entirely different picture from an api

advanced to a degree where loss of tissue may be determined in the roots' length, we may easily overlook the difference in density when destruction has taken place on the roots lateral surfaces.

Interpretation here depends upon assumption as to the length of its chronic pathological state and a practised eye, ready to detect the almost microscopical indentation manifested in the radiograph, so slight that the degree of density on the plate of necrotic tissue appears almost

like the cancellated one of the alveolus. The case now on the screen (Fig. 5) gives a fair example of this class.

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Fig. 4.

While necrosis was here assumed to be present, still its existence, and further its extent on the plate was not apparent until the tooth was extracted.

When, however, the tooth and the radiograph were compared, it left no question but that the exact outline of the necrotic area was plainly visible on the film.

cal abscess, showing a hard, immovable, non-inflammatory mass. It's extent and its relationship to the surrounding teeth could not have been determined by a film.

DETECTION OF NECROSIS. Necrosis of a tooth root presents two radiographic manifestations; that of acute destruction of tissue, and that of rarefaction of the calcium salts. The presence of actual necrosis with a destruction of tooth tissue is sometimes difficult to detect because a tooth root presents no definite destruction in anatomical outline. Unless the necrosis has

THE NECESSITY OF BEING ABLE TO NOTE THE RELATION OF TOOTH ROOTS TO THE INFERIOR

DENTAL NERVE.

In many cases of neuralgia, it is imperative to note the relation of teeth to the mandibular nerve. Its contact, or its relationship to the tooth roots must be determined thru its entire course,

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