Feline Oral–Dental Radiographic Examination and Interpretation

FELINE DENTISTRY

0195-5616/92 $0.00 + .20

FELINE ORAL-DENTAL RADIOGRAPHIC EXAMINATION AND INTERPRETATION Colin E. Harvey, BVSc, FRCVS, and Bonnie M. Flax, RDH, BS

RADIATION SAFETY

Radiation used to produce radiographs is biologically damaging. The amount of radiation exposure involved in a dental radiograph is very small and is dependent on the film speed, technique used, the kilovolts used, and the x-ray source and type of collimation used. Patient Protection

Fast film should be used to reduce patient exposure. Group D and Group E are the two film speeds used for intraoral radiography. Because low-energy electrons are absorbed by the patient's tissues, the x-ray unit should be operated using at least 60 kVp (preferably 70 kVp or above), therefore increasing the speed of electrons striking the target. The x-ray beam collimation (restriction of the diameter of the beam) should not expose the patient to any more than a circle 7 em (2 %inches) in diameter for a dental radiographic unit. This can be achieved by using a lead-lined, open-ended cylinder or the newer rectangular collimating devices. Pointed plastic cones, which are still found on many older model dental x-ray units, should be discarded because they tend to scatter xrays in a wide pattern. Retakes should be minimized, so be sure that film placement and tubehead positioning are correct the first time. Operator Protection

The operator can minimize radiation exposure by standing behind a lead barrier while exposing films. In the absence of a barrier, there should be at From the Department of Clinical Studies-Philadelphia, University of Pennsylvania School of Veterinary Medicine, Philadelphia, Pennsylvania

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE \'OLUME 22 • NUMBER 6 • NOVEMBER 1992

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least 6 feet between the patient and the operator, who should be standing at right angles to the beam. Never stand in the direct line of the open end of the cylinder or hold the film in the animal's mouth with an ungloved hand. The film can be secured in its proper position by using folded or rolled gauze squares or rubber bands. In situations in which anesthesia is not appropriate, the person holding the film in the mouth must wear lead-lined gloves, a thyroid collar, and a chest/lap apron having a lead equivalent of 0.5 mm. The room containing the x-ray unit should be constructed of cinderblock or at least 2lh inches of plaster or drywall. Wood panelling alone is not sufficient enough to provide a safe environment for staff or patients in surrounding areas.

INTRAORAL RADIOGRAPHS

Radiographs are images of three-dimensional objects represented on film in two dimensions, length and width. Dental radiographs are most commonly intraoral and are made by directing the x-ray beam from a source outside the face, through the anatomic structures, and onto the film that has been placed in the mouth. Two types of intraoral radiographs, periapical and occlusal, are most commonly used in veterinary practice. A periapical radiograph is an image of a group of teeth, along with their supporting structures (bone and periodontal ligaments) located in one specific area of the dental arch. Because the periapical film is used to interpret normal anatomy and pathology of the crown, root, and surrounding structures, it is necessary to include in the image the entire length of the teeth plus 3 to 4 mm beyond the root apex. The film sizes used for periapical radiographs vary depending on the size of the oral cavity (Table 1). The smallest films, sizes 0 and 1, are useful for periapical radiographs of cats, fitting easily into the sublingual space when a mandibular view is taken. The film size 2, slightly larger than size 1, is especially useful for periapical views of the longer rooted canines, maxillary premolar and molar teeth, and incisors. Films are rectangular in shape; thus Table 1. DENTAL X-RAY FILM ULTRASPEED Type (1 Film/ Packet)

Size

OF 54

0

Description

22 x 35 mm Periapical 24 x 40 mm Periapical

OF 56

OF 58

2

31 x 41 mm Periapical

OF 50

4

57 x 76 mm Occlusal

Desired Image Mandibular premolars, molars Mandibular or maxillary premolars, molars Canine, incisors, maxillary premolars, molars Entire arch, nasal cavity

Type (2 Films/ Packet) OF 53

OF 55

OF 57

OF 49

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periapical radiographs of the incisor and canine areas are usually placed with the longest dimension vertically (parallel to the long axis of the tooth), and films of the premolar and molar areas are placed with the longest dimension horizontally (perpendicular to the long axis of the tooth). The occlusal radiograph uses larger film (size 4) to record images of an entire arch. The film is placed over the incisal and occlusal surfaces of the teeth as if the patient were biting it, with either the long or short dimension across the mouth. If a more caudal view is desired to include the nasal cavity, the film may be placed diagonally with one corner extending over the soft palate. The occlusal image is useful to determine the location of objects in all three dimensions, to locate retained root fragments or unerupted or supernumerary teeth, to visualize outlines of the nasal cavity, to determine the nature of fractures of the maxilla and mandible, and to determine the presence or extent of cysts and tumors. Dental x-ray film is available in two speeds, Ultraspeed and Ecktaspeed. Ecktaspeed is the fastest, reducing radiation exposure by 50%. Owing to the increased speed, adjustments must be made in settings to avoid overexposure, and care must be taken during developing because the fast film is more sensitive to fogging as a result of light exposure. Use of Ultraspeed film is preferable for intraoral radiographs because it has higher contr<)st than Ecktaspeed film. It is less grainy; therefore fine details are more clearly seen. Because radiographic film is affected by background radiation, humidity, and heat, it should be stored in an area shielded from x-rays and,preferably in a refrigerator (50° to 70° F with 30% to 50% humidity). TECHNIQUES OF INTRAORAL RADIOGRAPHS

Two techniques for achieving diagnostic, undistorted images are employed in dental radiography. The choice of technique is dependent on the location

within the oral cavity of the tissue to be imaged and the film type being used (periapical or occlusal). Film placement and the angulation of the x-ray beam vary, depending on which technique is being used. Variations in applying the techniques will also occur as a result of the need to adapt to the anatomy of the mouth of the cat. Periapical and occlusal radiographs require the use of either the parallel technique or the bisecting angle technique (Figs. 1 through 4). Parallel Technique

The parallel periapical technique uses a geometric principle that minimizes distortion of the desired image. The film is placed along the lingual surfaces of the teeth to be radiographed, parallel to the long axes of the teeth. The central x-ray beam is then directed perpendicular to the teeth and the film. For mandibular x-rays, the film will slide down between the tongue and the teeth; it is usually necessary to secure it low in the sublingual furrow to ensure that the apex of the tooth and surrounding 3 to 4 mm of bone is visualized on the film. Folded gauze squares placed between the top edge of the film and the occlusal surfaces of the maxillary teeth will push the film low in the floor of the mouth and secure it in place. The parallel technique is best used for mandibular periapical radiographs, where the film can be placed parallel to the long axis of the tooth. The shallow palatal vault of the maxilla dictates the use of an alternate technique.

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b

d

~

__a

A

Figure 1. A-C, Bisecting angle technique for occlusal film of maxillary incisors and canines. a = line of plane of radiograph; b = line of long axis of tooth ; c = line bisecting angle between lines a and b; and d = central beam from x-ray source.

Bisecting Angle Technique

The bisecting angle technique is based on the geometric principle of equally dividing a triangle. The central ray is directed perpendicular to an imaginary line that bisects the angle formed by the plane of the film and the long axis of the tooth. The film is placed in the mouth touching the teeth at the incisal edge of the incisor or canine teeth or at the lingual-occlusal edge of the premolarmolar teeth. The apical portion of the film is away from the tooth apex owing to the anatomy of the palate. The film meets the teeth at an angle; therefore parallel position is not possible. The bisecting angle technique is used for premolar-molar periapical and incisor-canine occlusal films of the maxilla and incisor-canine occlusal films of the mandible. Owing to the anatomy of the maxillary arch of cats, modifications in the bisecting angle and in the horizontal placement of the tube are necessary. The zygomatic arch interferes with the ability to visualize the roots of the maxillary premolars and molars; therefore adjustments must be made in the vertical angulation. The beam is aimed more laterally to the zygomatic arch. The horizontal angulation, w hich n ormally should be at a true perpendicular angle to the film, will need to be modified to visualize the separation of the three roots of the maxillary fourth premolar tooth. Because a true perpen-

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b

c ._,.d

A

Figure 2. A-C, Bisecting angle technique for periapical film of maxillary premolars and molars. a = line of plane of radiograph ; b = line of long axis of tooth; c = line bisecting angle between lines a and b; and d = central beam from x-ray source.

dicular angle will cause the buccal and palatal roots to be superimposed, the horizontal angle of the tube should be moved so the beam is directed from a more rostral position. Owing to the position of the zygomatic arch and the roots of the maxillary caudal teeth, it may not be possible to have on one film a diagnostic visualization of all of the roots and the crowns of the teeth. Separate radiograph s using different angulations are necessary, depending on your purpose for taking the radiograph. For example, to have clear visualization of the cementoenamel junction (neck) area of the tooth to detect resorptive lesions, the angulation may be different from a radiograph taken to determine an apical infection caused by a pulp exposure. Perpendicular position (x-ray perpendicular to the long axis of the teeth, with the central beam aimed directly along the long axis of the teeth) will give the best view of the lamina durae of the maxillary premolar teeth (Fig. 5); this view is p articularly useful for identifying remaining fragments. Specific vertical and horizontal angulations are difficult to give for learning purposes becau se modifications will be necessary depending on the size and facial structure of each cat. A thorough understanding of the basic theories of parallel and bisecting angulation will enable you to make necessary angulation adjustments. X-RAY BEAM ANGULATION

Obtaining a diagnostic radiograph requires the adjustment of the x-ray tube in two directional positions for each exposure. The up and down move-

C,

Figure 3. A-C, Bisecting angle technique for occlusal film of mandibular canines and incisors. a = line of plane of radiograph; b = line of long axis of tooth; c = line bisecting angle between lines a and b; and d = central beam from x-ray source.

a b

-- - --

-~-d

A

Figure 4. A-C, Parallel technique for periapical film of mandibular premolars and molars. a = line of plane of rad iograph ; b = line of long axis of tooth ; and d = central beam from x-ray source.

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Figure 5. Radiograph of the maxillary premolar teeth taken in perpendicular position, showing the lamina durae surrounding the roots of the teeth.

ment of the tubehead is the vertical angulation. Most x-ray units have dials on the arms that indicate the vertical angulation by degree. For example, in the paralleling technique, the vertical angle of the x-ray beam must be perpendicular to the film and the long axes of the teeth. Incorrect vertical angulation will cause images on the film to appear either elongated or foreshortened (Fig. 6). The horizontal angulation is the position of the x-ray tube as it rotates around the patient's head in the same direction as the horizon. The horizontal angulation of the x-ray beam must be perpendicular to the horizontal plane of the film. Incorrect horizontal angulation will result in overlapped images (Fig. 7). Standard veterinary radiography machines can be used to obtain oral radiographs; however, there will be some loss of detail owing to the greater focal spot-film distance (FFD) (75 em). The end of the collimator on a dental xray machine should touch the skull of the cat, creating a short FFD of 25 to 30 em. Standard x-ray units with fixed heads are less adaptable to altering the direction of the beam and require more movement of the animal to obtain correct head positioning.

FILM PROCESSING

Radiation (energy) reacts with chemicals (emulsion) on the film to produce the final image visible by transmitted light. To understand the process, it is necessary to study the composition of the film and the chemicals used in the process of development and fixation.

Film Composition

The outermost wrapping of the dental film packet is covered with a white plastic or paper cover having an opening tab on one side. This tab must always be on the surface of the film that is not in contact with the teeth. It may be helpful at this point to examine a film packet and open it when reading this section. The semiflexible plastic film is surrounded by a black wrapping. A sheet of thin lead foil is placed at the back of the film packet to protect the tissues that lie in the path of the x-ray beam beyond the film. The film is a thin plastic, coated on both sides with a mixture of gelatin and silver halide crystals. The clear gelatin holds the silver halide grains in suspension. The silver halide grains store the energy from radiation of the x-

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B

Figure 6. A, Roots elongated owing to insufficient vertical angulation. B,C, Roots foreshortened owing to excessive vertical angulation (maxillary teeth), film not parallel to long axis of roots (mandibular teeth, or not perpendicular to central beam) .

ray beam. As the beam passes through the tissue, denser tissue absorbs more energy, and thus less energy reaches the film directly beyond the denser tissue.

Processing Processing begins with the chemical reactions when the film is placed in the developer. The gelatin softens, permitting the silver halide crystals that

Figure 7. Oblique positioning of up-

per premolar teeth in cat. The root structure and lamina dura is obstructed owing to inaccurate film positioning in ventral and horizontal angulation-zygomatic arch is overlying the teeth.

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were struck by the x-rays to react with the chemicals in the developer. The silver halide separates into bromide and metallic silver. The black metallic silver is deposited onto the film, and the crystals that have not been energized will be washed away. The next step of film processing is fixation. The fixer removes undeveloped and unexposed crystals from the film (which creates the white or clear areas) and preserves the developed, energized crystals on the film. A film will be dark if it is overdeveloped or overexposed. It will be light if it is overfixed, underdeveloped, or underexposed. Manual Processing

Processing may be done in a darkroom or in a portable developer, which can be located close to the operating table. The developing solutions fill the left tank, the fixing solutions fill the right tank, and a water bath fills the center tank. Prepare the solutions according to the manufacturer's directions. Important conditions to consider when developing films are the length of time the film is in the solutions and the temperature of the developer. The higher the temperature of the solutions, the shorter the developing time. Use of "rapid" processing solutions will also reduce developing times. An accurate timer and thermometer are necessary. Follow the manufacturer's chart for temperature and time relations. The solutions in the portable developer must be replaced daily because they weaken owing to number of times used, contamination with water or the opposite chemical, and exposure to air. In a darkroom, or with the hands through the portals in a chair-side darkroom (developer box), remove the exposed film from the packet, handling the film by the edges to avoid finger marks. Clip the film onto a film hanger and run your finger along the film edge to see that it is tightly clipped. Quickly immerse the films into the developer solution, setting the timer or checking a clock as you do so. Agitate the film hanger for 5 seconds to break up air bubbles. After the correct processing time, remove the hanger from the developer and immerse the film into the water tank for 20 seconds while agitating the film hanger up and down. Immerse the film into the fixer tank for 10 minutes. The films may be viewed after 30 seconds in the fixer; however, they must then be returned to the fixer for the full 10 minutes. Wash films in clean running water for 10 minutes and allow to dry in a dust-free area. A single-step manual processing system is also available, using an injectable developer-fixer system and films that are specifically packaged for the system. A needle is used to inject the fluid into the film packet, and the fluid is then spread by massaging the film packet for 15 seconds. The film can be viewed wet; however, further fixing is necessary. Automatic Processing

Manual processing is gradually being replaced by the automatic developers, which shorten the total processing time to as little as 4 or 5 minutes. They produce consistently good results by eliminating operator error; however, there is no option for quick interpretation of the wet film. Like the portable manual processor, the automatic processor is small enough to be placed close to the surgical table and has daylight loading capabilities, which eliminates the need for a darkroom. Automatic dental film processors are significantly more expensive than hand developing systems.

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TROUBLESHOOTING ERRORS

The clinician's goal of achieving a diagnostically useful radiograph may be prevented owing to mistakes made in either the exposure technique or the processing technique, resulting in excess exposure to radiation for the cat when retakes are necessary. Exposure Technique Errors

Several errors caused by the clinician in the exposure phase of intraoral radiographs can occur. Incorrect film positioning is the most common cause for retakes, either because the root ap ex is missed or because of superimposition of roots. The tooth or area of interest should be centered on the film and secured in place to avoid slipping. Choose the correct film size to cover adequately the areas to be radiographed. Selecting a film size that is too small may result in missing the region of interest. Cone cutting is a term used when the teeth are correctly centered on the film, but the open-ended tube is directed incorrectly toward the film, resulting in images appearing on only a portion of the film (Fig. 8). Errors occurring from incorrect exposure will result in films either too light or too dark for diagnostic use. Because most dental x-ray machines have a fixed kVp (70 to 90) and a fixed milliamperage (10 to 15), overexposed and underexposed films are a result of incorrect time selection. Adjustments in time selection need to be made depending on the size of the patient and the thickness of the tissues. For example, the exposure time will be longer when filming a maxillary tooth in comparison with teeth on the mandibular arch. The setting of exposure factors is dependent on the preset kilovoltage and the milliamperage of each individual x-ray machine, the specific tooth that is being x-rayed, and the density of the surrounding bone; therefore settings for exposure times need to be individualized in each office. Exposure times should be tested to determine the proper settings for each tooth, which will result in obtaining a radiograph that has good contrast and detail. A list stating the exposure time for each specific tooth should be placed next to the machine following test exposures and calibration. Processing Errors

Stained films may result from water drops left on the film long enough to dissolve the emulsion. Inadequate washing after the film has been fixed will result in yellow or brown discoloring of the film .

Figure 8. Cone cutting-image appears only partially on film owing to incorrect alignment of tube end with film.

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Images in a fogged film appear normal; however, the film is gray overall with little black/white contrast. Most fogged films are uninterpretable and are caused by either light leaks during developing or out of date film. Dark or black films are caused by stray light also, an error easily eliminated by not removing your hands from the portals of the developer box until the film is well into the development process. Dark films also are a result of an overlong development time (easily done during hand processing) or if the developing solution is too strong. If the fixing solution is too weak to remove the excess developer from the film, the image will be too dark. Light films may be caused by the developing time being too short, a weak developing solution, or excessive fixation. Automatic developers may cause films to stick to each other if the operator feeds them into the processor too soon after each other. These films may be peeled apart; however, the areas that were touching will not be developed. INTERPRETATION

Accurate interpretation of dental films requires correctly positioned, correctly exposed, and correctly developed radiographs; a good viewing system (radiographic viewbox and magnifying glass or magnifying viewbox); and a thorough knowledge of normal and abnormal radiographic features. Interpretation of dental radiographs is made simpler because of the differences in radiodensity of dental and adjoining tissue. Tooth substance is principally dentin, which is evenly radiodense. Dentin surrounds the pulp chamber and root canal (endodontic system), which are noncalcified. On the crown, the dentin is covered by enamel, the most densely mineralized tissue in the body. On the root is a thin (nonradiographically discernible) layer of cementum. The tooth is held in place by fibrous tissue (periodontal ligament) that extends from the alveolar bone to the cementum. It is the periodontal ligament that provides the most distinctive and useful feature in dental radiographs-a radiolucent line between the dentin/cementum of the root and the cortical bone (lamina dura) of the jaw. In most parts of the jaw, the lamina dura is seen as a distinct dense line because it is surrounded by less dense trabecular bone. These normal anatomic features are best seen as distinct entities in the mandibular premolar and molar teeth radiographed in parallel position (Fig. 9). For other teeth, the inability to obtain true parallel position interferes with the full appreciation of these features (see Fig. 7). In addition, adjacent anatomic features may prevent visualization of the expected features. For example, the midline radiolucent symphyseal line separating the two halves of the mandible prevents clear identification of the lamina dura of the first incisor teeth (see Fig. 3). The lateral alveolar plate of the mandibular canine often is continuous with the lateral cortex of the mandible. The upper jaw incisor and canine teeth and alveolar bone are seen superimposed on the nasal and maxillary bones and nasal conchae (see Fig. 1). Other features that may cause misinterpretation are the mental foramina in the mandibles. If there is any doubt about the location of a suspected abnormal radiolucency or radiodensity relative to a tooth, take an additional radiograph at a different angle; if the suspected abnormality remains "attached" to the tooth, it is real, whereas an adjacent, previously superimposed, anatomic feature will now be located away from the tooth. Radiographic Abnormalities in Tooth Substance in Cats 1. Even band of radiolucency in the crowns of the teeth, most often noticeable

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--------PL

RC---

A----

MC LD

CB

Figure 9. Radiographic structures visible in a normal jaw. A = apex of root; CB = cortical bone of mandible; F = crestal bone filling the furcation; LD = lamina dura; MC = mandibular canal; PC = pulp chamber; PL = periodontal ligament; RC = root canal.

in premolar and lower first molar teeth (Fig. 10). This is of unknown significance, although poor mineralization of feline teeth is associated with the presence of periodontitis and resorptive lesions. 5 2, Traumatic fracture of part or all of a crown, seen most often affecting the canine teeth in cats. The rest of the tooth may be normal, there may be injury of adjacent bone, or there may be periapical disease if the fracture is long-standing and involves the pulp (Fig. 11). 3. Resorptive lesions. These can occur in many locations and can affect any tooth. Lesions that are clinically evident generally are visible as radiolucent areas, most often at the cementoenamel junction (Fig. 12). Lesions on the buccal or lingual surface of a tooth may not be visible radiograph~ ically in a film made in parallel position, whereas lesions at the mesial or distal end of the tooth often are startlingly clear (Fig. 12). Resorptive lesions are considered to be either internal or external. The typical clinical neck lesion in cats is an external resorption. Radiographs are useful for determining the depth of the lesion-whether the lesion has penetrated the pulp chamber is significant in determining the treatment required if the tooth is to be considered for restoration. Radiographs are useful for demonstrating lesions in the furcation area (Fig. 12), which inay be difficult to examine thoroughly clinically. External resorptive lesions may lead to fracture and loss of the crown, with retained root fragments (Fig. 13). Internal resorptive lesions can be seen only by radiographic examination (unless the lesion has penetrated through full thickness of dentin and enamel). Internal resorptive lesions are identified most often in canine teeth as irregular radiolucent areas in dentin or in more severe cases as disruption of all organized root structure (Fig. 14). 4. Apical changes. In young cats, the apex is incomplete and the root canal is wide (Fig. 15). The apex closes at 12 to 18 months of age, and the root canal continues to narrow throughout life.

Fig Lire 10. Horizontal band of radiolucency affecting the lower premolar and molar teeth. There is a resorptive lesion on the distal surface of the lower first molar tooth (arrow).

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Figure 11. A, Fractured canine tooth, with widening of the periapical periodontal ligament space (compare with the intact canine tooth shown in B); the pulp chamber and root canal of the fractured tooth are wider than in the intact tooth, indicating pulpal death some months or years previously. Some incisor teeth are missing or present only as root fragments. C, Very long-standing periapical abscessation secondary to crown fracture, with fistulation and enlargement of the rostral end of the mandible. The apical end of the affected root is undergoing resorption. Note difference in size of root canal in affected tooth compared to normal canine tooth.

Figure 12. Resorptive lesions, visible as radiolucent areas most often at cementoenamel junction at the mesial or distal end of the tooth (arrows) (A,B), or in the furcation area (B,C,D, arrows ).

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Figure 13. Retained root fragment (distal root of the third premolar tooth-large arrow). There are also resorptive lesions present on the fourth premolar and first molar teeth (small · arrows).

Figure 14. Internal resorptive lesions, with severe disruption of all organized root structure in both lower canine teeth.

Figure 15. Young cat, with open apexes of the lower canine teeth.

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Radiographic Abnormalities in Bone Structure Changes in Lamina Dura

The thin cortical plate of the alveolar bone is most often attacked at its most coronal or most apical extent. Coronal. Bone loss associated with periodontal disease is first evident at the crestal margin, initially as loss of definition and later as loss of height of the bone. Radiographic bone loss most commonly is horizootal-adjacent root surfaces are approximately equally affected (Fig. 16A). Vertical bone loss (Fig. 16B) is less common in cats than in some other species because the teeth of cats are smaller and closer together; bony structures within about 2 to 3 mm of each other are lil}ely to be affected by demineralization or resorptive processes to a similar extent. The extent of periodontal bone loss in cats is correlated statistically with the extent of resorptive tooth destruction. Oblique bone loss forming a radiolucent bowl around one or both roots of the lower first molar tooth is common (see Fig. 18C). Apical. Apical bone loss is less common than coronal bone loss and is usually caused by endodontic periapical disease. When the contents of the endodontic system (pulp chamber and root canal) die, the contents become necrotic and tissue breakdown fluids seep through the apical delta into the periodontal space surrounding the root apex (periapical space). These chemicals stimulate an inflammatory response locally, the first visible effect being widening of the periodontal ligament space and loss of distinction of the lamina dura periapically (see Fig. llA). If endodontic disease is allowed to continue, the area of radiolucency becomes more extensive. A periapical abscess may develop from an infected root canal, causing extensive tissue destruction and fistula formation (see Fig. llC). A second type of change seen periapically is increased density, most typically in older cats. Sometimes referred to as hypercementosis, the change is often a combination of thickening of the radiodense root structure and of the periapical alveolar bone. In many cases, the periodontal ligament is not clearly discernible as a radiolucent line, and thus distinction between thickening of tooth substance and thickening of alveolar bone cannot be made. Thickening of the root-tip area, without development of periapical radiolucent spaces and when the root itself shows no radiolucent resorptive areas, is regarded as a benign aging change, even though the changes are irregular in thickness. A normal anatomic feature may appear as a periapical radiolucency owing to superimposition. The most clinically significant is the middle mental foramen, which may appear as a large radiolucency over the apex of the lower canine or possibly the third premolar tooth. Entire Circumference

Loss of radiodensity of the entire circumference of the lamina dura is seen rarely in cats. Severe periodontal disease or combined endodontic-periodontal lesions are the most likely cause. Another possible cause is renal or nutritional secondary hyperparathyroidism. Irregular loss of bone around the teeth and including loss of cortical bone of the jaw is most likely to be due to neoplastic invasion or secondary to trauma (Fig. 17). The most common tumor is squamous cell carcinoma; the jaw squamous tumors are bone invasive, the tongue-based lesions are generally confined to soft tissues. Other bone-invasive tumors seen are fibrosarcoma and aggressive epulides.

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Figure 16. A, Horizontal bone loss affecting the lower premolar and molar teeth. 8 , Vertical bone loss, mesial root of lower first molar-a resorptive lesion is present. C, Periodontal bone loss forming a radiolucent 'bowl' around the roots of the lower first molar tooth.

Loss of normal cortical pattern away from teeth is most likely due to neoplasia. Irregularities in density are commonly seen in the mandibular symphysis in aging cats (Fig. 18), often exacerbated by severe periodontal disease of incisor or canine teeth. This may be confused with symphyseal separation or fracture because this is the most common area for clinically evident jaw trauma in cats. Loss of continuity of the cortical line of the mandible or maxilla is most likely to be due to trauma, although mandibular body fractures are less common in cats than in dogs.

Figure 17. Irregular loss of mandibular bone due to neoplastic invasion by a fibrosarcoma.

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Figure 18. Irregularities in bone density of the mandibular symphysis in an aging cat with extensive canine tooth periodontitis. ·

Radiographs of the Temporomandibular Joint

One other oral area where radiographs are of particular value is the temporomandibular joint, the second most common site for jaw fractures in cats.• To be interpreted correctly, temporomandibular joint radiographs must be made in excellent position, either absolutely symmetric ventrodorsal (or dorsoventral view) or both right and left oblique radiographs taken at the same angle (positioning under fluoroscopic guidance is particularly useful). The major abnormalities seen are luxation (most typically rostrodorsal), fracture, fracture-luxation, and osteoarthritis secondary to trauma. References l. Kodak Dental Products Catalog and Reference Guide. Rochester, New York, Eastman

Kodak Company, 1991 " Emily P, Penman S: Handbook of Small Animal Dentistry. Oxford, Pergamon Press, 1990 3. Miles DA, VanDis ML, Jensen CW, et al: Radiographic Imaging for Dental Auxiliaries. Philadelphia, WB Saunders, 1989 ... Ticer JW, Spencer CP: Injury cif the feline temporo-mandibular joint: Radiographic signs. Vet Rad 19:146- 156, 1978 _. Zetner K: Neck lesions bei der Katze: Diagnostisch-atiologische Untersuchungen uber Zusammenhagne. Waltham Report 30:15- 23, 1990

Address reprint requests to Colin E. Harvey, BVSc, FRCVS Department of Clinical Studies---:-Philadelphia University of Pennsylvania School of Veterinary Medicine 3850 Spruce Street Philadelphia, PA 19104-6010