Intraoral radiography in the cat
Equipment and techniques
Dental radiography requires a dental x-ray generator/dental radiographic unit (e.g., wall-mounted, cart-mounted, hand-held) and intraoral receptors/detection system (e.g., intraoral conventional dental films, “direct” digital radiography (DR) systems, or computed radiography (CR) systems). All equipment must be regularly checked and the staff working with it properly trained to assure radiation safety.
To obtain dental radiographs, veterinary patients should be under general anaesthesia to avoid any trauma to the patient and damage to the equipment. Also, obtaining dental radiographs is usually just a part of the dental procedure and is followed by other diagnostic and therapeutic procedures. There are three techniques to obtain dental radiographs in cats: parallel technique (for caudal mandibles), extra-oral near-parallel technique (for caudal maxillae), and bisecting angle technique for all other areas of the jaws. For the parallel technique, place the film intra-orally at the teeth to be radiographed, so that the film is parallel to the long axis of those teeth. Position the x-ray cone so that the central x-ray beam is perpendicular to the film, and bring the x-ray cone as close as possible to the object (jaw). Check if the teeth of interest are within the circumference of the cone. Bisecting angle technique is used when the film cannot be placed parallel to the tooth and perpendicular to the x-ray beam due to anatomy of the oral cavity and teeth. Once the film is placed in the mouth, visualize an angle between the long axis of (the root of) the tooth to be radiographed and the plane of the film. Then bisect this angle with an imaginary line and position the x-ray cone so that the central x-ray beam is perpendicular to the imaginary bisecting line. Bring the x-ray cone as close as possible to the object and check if the teeth of interest are within the circumference of the cone. Extra-oral near-parallel technique is used to radiograph maxillary premolar and molar teeth in cats to avoid image overlap of the zygomatic arch and the teeth of the caudal maxilla if using intraoral bisecting angle technique. Position the cat in lateral recumbency and place a film on the table under the cat’s head/maxilla (the side to be radiographed is the side closer to the table/film). Open cat’s mouth with a gag (only use a gag for short period of time – e.g., just to obtain the radiograph – to avoid possible complications associated with mouth gag use). Then, one option is to position the cat’s head slightly obliquely (to avoid superimposition of the contralateral maxillary teeth) and position the x-ray cone so that the central x-ray beam is perpendicular to the film. To use maximum amount of the film (e.g., if using #2 size film), position the head so, that the tips of the cusps of the teeth to be radiographed are lined up along the edge of the film. The other option is to place the cat’s head parallel with the film and position the x-ray cone slightly obliquely to avoid the teeth of the jaw that lies away from the film.
Standard views for the cat include 1) occlusal view of the maxillary incisor and canine teeth (bisecting angle technique), 2) lateral view of the maxillary canine teeth (bisecting angle technique), 3) extra-oral (near-parallel) view of the maxillae (P2-M1; near-parallel technique), 4) occlusal view of the mandibular incisor and canine teeth (bisecting angle technique), 5) lateral view of the mandibular canine teeth (bisecting angle technique), 6) caudal mandibles (P3-M1; parallel technique).
Technical quality and orientation of dental radiographs
Obtained radiographs should be first examined for their technical quality – check, if 1) the area of interest is on the image, 2) there is any elongation/foreshortening of the teeth radiographed, 3) the quality of exposure is appropriate, 4) there are any processing errors.
Radiographs should then be properly oriented using “labial mounting” – 1) if using conventional dental films assure that the embossed dot/orientation mark faces up for all radiographs, where intra-oral technique was used, 2) by knowing anatomical features, determine, what are maxillary and what mandibular views, 3) crowns of the maxillary teeth are to point down and crowns of the mandibular teeth up, 4) occlusal views are in the center, with first incisor teeth at the midline, 5) last molar teeth are on the periphery. This orientation results in the radiographs of the teeth from the patient’s left side to be on the right side and vice-versa (note positioning of the extra-oral views).
Interpretation of dental radiographs and common dental pathology
Diagnostic-quality radiographs are then systematically examined. Radiographs can be evaluated on a tooth-by-tooth basis and findings directly compared to those found on the detailed dental examination (dental charting). Closely examine the crown, root (and apex), dentin, enamel, pulp cavity, alveolar margin, periodontal ligament space (lamina lucida), alveolar bone (with the cortical bone of the alveolus - lamina dura, and trabecular bone of the alveolus) and bone forming the jaw. Interpretation of dental radiographs requires knowledge of normal dental/oral radiographic anatomy in order to be able to diagnose any anatomical/developmental abnormalities, periodontal pathologies, endodontal pathologies and other abnormalities. It is also important to remember, that bone loss is only radiographically evident once 30-50% of mineralized component is lost, hence bone loss will be underestimated on the radiographs. Also, radiographs will only give a 2-dimensional view of a 3-dimensional structure, therefore sometimes several views may help to better visualize a specific structure.
Periodontal disease is the most common oral inflammatory disease. The first radiographic signs of the disease are seen as rounding of the alveolar margin. Further, widening of the periodontal ligament space and the loss of integrity of the lamina dura are seen. Alveolar bone osteolysis can be further interpreted as horizontal bone loss (most common in cats), vertical bone loss or combined bone loss. Periodontitis may also present as alveolar bone expansion. Severe cases with total loss of attachment may lead to “perio-endo” type lesions – when primary advanced periodontal disease causes endodontic infection. Vacated alveoli may be noted if periodontitis was so advanced that the tooth exfoliated. Pathologic jaw fractures are possible in chronic advanced periodontitis cases.
Tooth resorption also commonly affects domestic cats. It may or may not be associated with periodontitis. While clinical examination is of great importance to diagnose the stage of the disease, radiographs are a must as in majority of the cats they reveal additional information on extent of the lesions and the type of the lesions, as well as reveal lesions limited to the root. Type 1 describes lack of any dental hard tissue replacement by bone (lamina lucida and lamina dura can be distinguished), while in cases, where type 2 tooth resorption is diagnosed, dental hard tissues are morphed with bone and lamina lucida and lamina dura cannot be distinguished. The radiodensity of the roots in the latter case is not homogenous.
When looking for signs of endodontic disease, evaluate the integrity of the crown and/or root, pulp cavity width and shape (especially in comparison with the contralateral tooth), width of the periodontal ligament space (especially apically) and integrity of the periapical bone (presence of periapical lesion) and of the apex (inflammatory root resorption). Ill-defined moth-eaten bone resorption and periosteal reaction in chronic cases may be suggestive of osteomyelitis. “Endo-perio” lesions may develop, when primary endodontic infection spreads into the periodontal ligament and causes total loss of attachment. Sometimes there are no radiographic changes associated with a fractured tooth with exposed pulp as some time is needed before radiographic changes are detected after pulp exposure, infection, inflammation and necrosis; such tooth still requires treatment.
Although selected dental radiographs only can be obtained (either based on clinical examination or with a “screening” approach), full-mouth dental radiographs are still the golden standard, especially if the animal is presented for the first time, or if the clinical condition has changed significantly since the previous visit. In cats, clinical examination without supporting dental radiographs could miss incidental and clinically important findings in 46.5% of the animals. Dental radiographs are of an utmost importance also during treatment. For example, endodontic treatment and evaluation of the treatment outcome cannot be accurately performed without dental radiographs. The same is true for crown amputation with intentional root retention procedure to treat type 2 tooth resorption lesions in cats. Dental radiographs are also of an extreme help when dealing with complications of treatments (e.g., root fractures during dental extractions). However, the limitations of intraoral radiography must be considered, especially when dealing with e.g., palatal defects, maxillofacial trauma, TMJ disease or oral neoplasia, when advanced 3-dimensional imaging techniques (usually computed tomography or cone-beam computed tomography) are recommended.
If you have noted any problems with your animal, please consult your veterinarian.