Radiographic diagnosis of canine oral pathology (Proceedings)

Radiographs should be made whenever clinical examination indicates that there might be subgingival pathology that could be identified or better characterized by evaluating the hidden hard tissues. A partial list of some observed abnormalities that should be further investigated by making oral radiographs includes:

     • Fractured tooth

     • Discolored tooth

     • Tooth resorption

     • Before extractions

     • Fractured jaw

     • Dental caries

     • Abrasion wear

     • Missing teeth

     • Periodontal disease

     • Developmental anomalies

     • Tumors

There is some evidence that routine full-mouth radiographs can identify pathology even when the oral examination is normal. For this reason, some practitioners recommend a full-mouth series of radiographs at a patient's first dental visit and periodically after that. The question of the value of identifying those abnormal findings (how many are causing a problem that requires intervention) has not been made clear.

When deciding the benefit-to-cost ratio for making a dental radiograph, the benefit to the patient (diagnosing problems, verifying quality of treatment, evaluating response to treatment) is weighed against the cost to the patient and staff (radiation exposure, time under anesthesia). The International Commission on Radiological Protection recommends that radiographs should only be made when they are justified (they should not be done unless a positive net benefit can be expected), optimized (the least amount of radiation required to produce the needed result should be used), and limited (the dose equivalence limits allowed for employees needs to be monitored and followed).

While the main goal of making a dental radiograph is to provide the practitioner with additional information to assist in making clinical decisions, it is also a powerful tool to help show a pet owner the extent of, or existence of, a problem. A client can actually see the amount of bone damage from periodontal or endodontic disease. This can be a strong motivating force to help them understand the importance and urgency of the recommended treatment.

Interpreting radiographs

When x-rays (electromagnetic energy waves) pass through tissues, their travel is interrupted differentially depending on degree of penetration or deflection of those tissues. X-rays that are able to pass through tissues interact with the film or sensor, while those that are absorbed or deflected by radiopaque tissues do not reach the recording device resulting in white shadows. Radiographs are images of these shadows that consist of black areas where x-rays were unimpeded, white areas where they did not penetrate at all, and a gray scale of shades between the two.

All of the shadows made by all the structures that interfere with the x-rays along the path of the beam are collapsed into a two-dimensional image. Imagine making multiple CT slices along the beam axis and projecting them all superimposed on top of each other, then trying to understand the resulting jumble. You must mentally re-expand the collapsed third dimension of the imaged structure into the most likely 3-D arrangement.

"Summation" can produce either addition or subtraction, resulting in increased or decreased radiopacity of structures due to superimposed adjacent anatomy. A superimposed radiopaque object adds to the radiopacity of a superimposed structure in the area of overlap while a superimposed radiolucent object subtracts from it. Summation can also occur within a single structure due to the "tangential effect"; when a structure is perpendicular to the x-ray beam its effect (whether increased or decreased radiopacity) is diminished and when it is parallel to the x-ray beam its effect is enhanced.

A dental radiograph is read similar to the way that other radiographs are read. Evaluate the entire film in a consistent manner. Use a good source of illumination and magnification. Start by scanning the entire film for the overall anatomy. Look for things that are present but should not be, and for things that are not present but should be. Then examine each part of each tooth individually, followed by the peridental and interdental structures.

It is important to be familiar with normal and abnormal radiographic anatomy. Most pathological lesions have typical pathognomonic radiographic appearances, as well as deviations from them. For these deviations, an understanding of the various pathological processes combined with logical evaluation will help the evaluator to reach an accurate diagnosis.

Periodontal disease

The typical radiographic lesion of periodontal disease is loss of marginal alveolar bone. This can be horizontal loss along the margin of the teeth, or vertical loss along the tooth root. Other related radiographic findings include: extension of disease from the periodontal ligament to infect the pulp either around the root tip or through a lateral or furcation canal, secondary mandibular fracture caused by bone loss that weakened the mandible, and extension to develop osteomyelitis.

Endodontic disease

The archetypal radiographic lesion of endodontic disease is a periapical radiolucency. This is evidence of apical periodontitis caused by inflammatory bone loss at the root apex that is the largest area of communication to the pulp. Similar lesions can also occur at the site of lateral canals or in the furcation area where communicating canals can also occur. Other signs of endodontic disease include: increased radiopacity at these same sites indicating condensing osteitis, arrested pulp maturation (wide pulp cavity) from a necrotic pulp, accelerated pulp maturation (narrow pulp cavity) from generalized pulpitis of a vital pulp, irregular pulp space contour, and radiolucencies on the root indicating external or internal root resorption.

Tooth resorption

Idiopathic tooth resorption is one of the most common lesions found in cats. It is also seen, although much less frequently, in dogs, humans and many other species. Radiographically it appears as areas of the tooth with decreased radiopacity. It can be found focally on external root surfaces in areas of inflammation where it appears as a radiolucent area (type 1 resorption), or it can be seen as a generalized slight decrease in root radiopacity that approximates the density of the surrounding bone (type 2 resorption). In dogs or cats that have type 2 resorption, full mouth radiographs are recommended since multiple teeth are often involved.

Oral swelling

Causes of oral swellings include infection including osteomyelitis and abscesses, inflammatory lesions such as fibromas, cysts such as dentigerous cysts, benign tumors (the most common is the peripheral odontogenic fibroma, previously classified as a fibromatous or ossifying epulis), locally invasive tumors (the most common is the acanthomatous ameloblastoma, previously classified as an acanthomaouts epulis), neoplasia (the most common being squamous cell carcinoma in cats, which is also common in dogs along with fibrosarcoma and malignant melanoma), metabolic abnormalities and developmental problems.

Developmental abnormalities

Teeth can suffer from multiple developmental problems. Some may not cause clinical problems, and others can be very problematic. Supernumerary teeth and missing teeth are common. Incomplete twinning or fusion of tooth buds can result in large abnormally shaped teeth. Trauma to a developing tooth bud can cause dilacerations, or a drastic bend in the root or crown that complicates normal eruption. Roots or crowns can fail to develop or properly mature, and infected deciduous teeth can interfere with the developing subjacent permanent tooth causing various abnormalities.

Trauma

Radiographic signs of trauma are generally straightforward. Common injuries that require radiographic evaluation include mandibular or maxillary fractures, separation of the symphysis, fractured, discolored, or avulsed teeth, severe tooth abrasional wear, temporomandibular joint dislocation, etc.

Caries

Caries ("cavity") is an infection of the tooth that is relatively uncommon but does occur in dogs. It has not been shown to occur in cats. Clinically it appears as a brown area on the occlusal or interproximal surface of molar teeth. It is differentiated from the clinically unimportant dentin staining that can look similar to it by pushing a sharp dental explorer into the discolored area. If the explorer can penetrate it, then this is caries. A stained area with the same appearance but without caries will feel hard and smooth to the explorer and it will not be able to penetrate it. Radiographs of teeth affected by caries are important because we often find these lesions late in their course after the pulp has already become infected. A radiograph will show the typical signs of endodontic disease if the pulp is infected. Saving the tooth would then require root canal treatment. If the caries has been discovered before endodontic involvement, then it can be "drilled-and-filled" with mechanical removal of the infected dentin and placement of a bonded composite resin restoration.

Summary

Interpreting films requires familiarity with normal radiographic appearance and anatomy as well as an understanding of the pathological processes that occur and the changes from normal that can be appreciated on radiographs. Radiographs can sometimes be challenging to interpret, and can even be misleading at times. However, like any other undertaking, the more radiographs that you read the easier it becomes. Having films of both normal and abnormal patients available for comparison to those of your patient can be helpful. For this purpose, keeping a veterinary dental radiography text available can prove valuable.

While the main goal of making a dental radiograph is to provide the practitioner with additional information to assist in making clinical decisions, it is also a powerful tool to help show a pet owner the extent of, or existence of, a problem.