Obtaining a diagnostic radiograph is necessary to interpret it. Correct projections and exposing techniques are necessary to get the good-quality radiograph. The correct way of reading dental radiographs will be presented with presentation of most common radiopaque and radiolucent lesions.
Evaluation of dental radiographs starts with appropriate orientation of the image according to established standards.
The key to properly identifying the imaged teeth on standard (analog films) radiographs is the embossed dot, which is near one corner of the film. When exposing a radiograph on standard radiographic films, the convex surface points toward the radiographic tube head when the film is properly positioned. It is not possible to obtain a diagnostic radiograph with the film in backward, because of the lead sheet on the back side of the film. Therefore, when exposing the film, the embossed dot must be facing out of the mouth.
Interpreting dental radiographs starts with the appropriate orientation. First, place the convex side of the dot toward you. This means you are looking at the teeth as if your eyes are the x-ray beam. This step is done for you on most digital systems. The dot should always be located in such a way that it is not superimposed on structures being imaged. When chemical development is performed, place the clip to hold the film adjacent to the dot. This will provide an area of interest free of interfering artifacts. Next, rotate the film so that the roots are in their natural position (pointing up on maxillary views and down on mandibular). When this is done, it is necessary to determine if it is the left or right side of the patient. For lateral oblique projections (canine, premolar, and maxillary molar teeth) or parallel projections (mandibular molar teeth), the side of the film where the more mesial teeth are located indicates the side that was imaged. In other words, if the mesial teeth are on the right side of the film, it is an image of the right side of the patient. With other projections, such as dorsoventral (DV) or ventrodorsal (VD) images (i.e., incisors or canines), the right side of the mouth is on the left side of the film and vice versa for the left side of the mouth. This is similar to a VD image of the abdomen.
To distinguish between mandibular and maxillary images, certain landmarks should be evaluated.
For mandible, the presence of the mandibular canal, mental foramina, mandibular symphysis and ventral mandibular margin (cortex). The most rostral mental foramen is located in the second incisor area, the middle is at the level of apex of the second premolar, and the caudal is at the level of the third premolar. In dogs, the mandibular second, third, and fourth premolars and the first and second molars should have two roots. In cats there are normally only three teeth caudal to the canine. There are obviously exceptions to these rules (e.g., third root in a molar, fused roots or the presence of the second premolar in cats, and supernumerary teeth).
In maxilla, the presence of palatine fissures, incisive canal; the conchal crest rostrally and pterygopalatine fossa caudally. The radiopaque line running across the canine root and just dorsally to the roots of the premolars and molars is the nasal surface of the alveolar process of the maxilla. Nasal structures are visible above the conceal crest with symmetric turbinate details. Typical structures for the nasal cavity are the palatine fissures and incisive foramen. In dogs, the fourth premolar as well as two maxillary molars normally have three roots; however, the second molar often has fused roots. In cats, the zygomatic arch is typically superimposed on the maxillary cheek teeth.
Normal radiographic anatomy. There are numerous structures within the oral cavity that mimic pathologic states, depending on the projection. Knowledge of normal radiographic anatomy will help avoid over-interpretation.
Normal alveolar bone will appear gray and relatively uniform throughout the arcade. It is slightly more radiopaque, “darker” than tooth roots. In addition, it appears slightly but regularly mottled. Alveolar bone should completely fill the area between the roots (furcation) and end at the cementoenamel junction (CEJ). The root canals should all be the same width, allowing for differences in the diameters of the root. There should be no radiolucent areas in teeth or bone. A regular thin dark line (periodontal ligament) should be visualized around the roots.
Periodontal disease. Periodontal bone loss results from the combination of bacteria-induced inflammation and host response creating osteoclastic resorption of bone. This resorption will result in crestal bone loss to a level below the cementoenamel junction. This decrease in bone height may also create furcation exposure. Horizontal bone loss is the most common pattern in veterinary patients. This appears as generalized bone loss of a similar level across all or part of an arcade. The other pattern is angular (vertical) bone loss. The radiographic appearance of angular bone loss is one area of recession below the surrounding bone. The surrounding bone may be normal or be undergoing horizontal bone loss. Therefore, it is common to have a combination of the two types in the same arcade. Bone loss does not become radiographically evident until 30–50% of the mineralization is lost. Therefore, radiographic findings will always underestimate bone loss. In addition, bone loss on only one surface (i.e., lingual, palatal, or facial) may be hidden by superimposition of bone or tooth. This may result in a nondiagnosed bony pocket. Always interpret radiographs in light of the complete oral examination findings.
Endodontic disease. Endodontic disease may be demonstrated radiographically in several ways. An individual tooth may have one, some, or all of the different changes listed below. However, only one need be present to establish a presumptive diagnosis of endodontic disease. Radiographic changes can be broken into two major classifications: 1) changes in the surrounding bone, or 2) changes within the tooth itself.
Tooth resorption. Physiologically, tooth resorption occurs during changing of dentition from deciduous to permanent teeth. The erupting permanent tooth causes resorption of the deciduous tooth root. Persistent deciduous dentition teeth very often undergo resorption even without permanent tooth eruption, and therefore the lifespan and time of functionality of these teeth is often very limited.
The radiographic appearance of different types of resorption does not always relate to the type of disease; however, replacement resorption has some typical features. In addition, localization of the lesion also could be linked to the specific type. For example, PIRR is often located at the cervical area of the tooth as the consequence of damaged cervical root surface and therefore was previously called a “neck lesion”.
The importance of dental radiography in TR cases cannot be overstated. Type 1 lesions typically retain a viable root canal system and will result in pain and endodontic infection if the roots are not completely extracted. However, the concurrent presence of a normal periodontal ligament makes these extractions routine. With type 2 lesions, there are areas lacking a normal periodontal ligament (ankylosis) which also demonstrate varying degrees of root resorption, which makes extraction by conventional elevation difficult to impossible. The continued resorption in type 2 teeth is the basis for crown amputation therapy. It is this author’s opinion that teeth with an identifiable root canal on dental radiographs must be extracted completely, while teeth with no discernable root canal may be treated with crown amputation. If there is any question, always err on the side of complete extraction.
Neoplasia. Neoplasia is defined as the abnormal growth of cells that is not responsive to normal growth control. Neoplasms can be further classified by their biologic behavior as benign or malignant.
Benign masses: Most benign neoplastic growths will have no boney involvement on dental radiographs. If bone involvement does occur with a benign growth, it will be expansive, resulting in the bone “pulling away” from the advancing tumor, leaving a decalcified soft tissue-filled space in the tumor site. Bony margins are usually distinct. Finally, this expansive growth will typically result in tooth movement.
Malignant neoplasia: Malignant oral neoplasms typically invade bone early in the course of disease, resulting in irregular, ragged bone destruction. Initially, the bone will have a mottled “moth-eaten” appearance, but radiographs late in the disease course will reveal a complete loss of bone (the teeth will appear to float in space). If the cortex is involved, an irregular periosteal reaction will be seen.