Introduction

Radiography is indispensable to human medicine and dentistry. It is an established part of general veterinary medicine, but unlike human dentists, vets seem reluctant to use this essential tool routinely for dental assessment. Clients expect a high level of service from our profession. This requires that suitable investigation such as radiography is undertaken prior to therapy. As tooth roots, the largest part of most teeth, and their associated support structures are hidden within the jaws we cannot complete a dental examination without radiography, so why is it not even offered by many practices as part of their dental service?

Concern relating to monetary cost is a possible reason, but this is for the client to decide, not the vet. Another reason for failure to perform dental radiography and radiology is the assumption that it is difficult. Well possibly it is, but so are radiography and radiology of the general skeleton, yet it is rare for orthopaedic surgery to be undertaken without radiographic control.

Table 1. Indications for oral radiography.

a. Identification of normal features:

 Anatomical variation

 Deciduous tooth root resorption

 Differentiation between

 Presence of permanent dentition pre eruption

 Tooth root development

b. Recognition of dental pathology:

 Avulsion or luxation

 Caries or resorptive cavities

 Crown fractures

 Delayed tooth eruption

 Developmental abnormalities

 Endodontic assessment

 Monitoring treatment

 Periodontal abscessation

 Periodontal bone loss

 Periapical pathology

 Retention of deciduous teeth

 Root deformity

 Root fracture

 Root resorption

 Supernumerary teeth

c. Recognition of other oral pathology:

 Abscesses

 Bone atrophy

 Craniomandibular osteopathy

 Foreign body

 Metabolic bone disease

 Orthodontic assessment

 Osteomyelitis

 Pathological fracture

 Traumatic injury

 Tumours

Not knowing all the facts about a case can put the patient at risk! Advanced periodontal disease is common and leads to loss of alveolar bone. The only practical way to assess this in general practice is by radiography. Failure to identify cases with advanced bone loss commonly leads to iatrogenic jaw fracture due to improper handling during examination and treatment. The timing of radiography during examinations can be varied. In cases with indications of advanced periodontal lesions the best time is immediately on induction of anaesthesia, the latter being done with extremely gentle handling and avoidance of applying pressure to the mandibles. In cases with minimal periodontal involvement it may be appropriate to wait until after scaling and polishing of the teeth so that the oral cavity is clean and a more pleasant area to work in. In many cases further radiographs are required at different times during a procedure to complete a diagnosis, investigate new findings or monitor treatment progress.

Oral Radiography

Obtaining diagnostic radiographs of oral structures is sometimes complicated by their anatomical arrangement, but by applying the basic principles used with radiography in other areas, the crowns, roots and supporting alveolar bone of almost all teeth can be demonstrated clearly in dogs and cats. It is a little more difficult in herbivores due to their longer oral cavities, longer teeth and limited oral opening.

At least two views are needed in order to adequately assess three dimensional anatomical structures. In the mandible there is limited space for hidden abnormalities medial and lateral to the teeth so a single view will demonstrate the majority of lesions. Even in the maxilla, with use of intra-oral film placement to minimise superimposition of other structures on the image, a single view is often all that is required to confirm anatomical relationships and locate pathology. Additional films are readily obtained using a differing view for the identification and specific localisation of suspect lesions once their presence is known. The occlusal view (with the x-ray beam directed along the length of tooth roots) is particularly useful for locating retained root tips.

In order to obtain a realistic image of the subject, it should generally be positioned close to and as near parallel to the film as possible, with the central x-ray beam perpendicular to the film, i.e., using "parallel technique. With long bone radiography this is usually straightforward, but is much less so for most teeth. In the carnivores, only the caudal mandibular premolars and molars can be imaged in this manner. The bisecting angle method is an alternative for use when parallel positioning is not practical. In this technique the x-ray beam is directed perpendicular to a line which bisects the angle between the long axis of the object being imaged and the film plane. The bisecting angle technique makes use of the geometry of the isosceles triangle, the two halves of which have the same dimensions giving the image the same length as the object being imaged. The image will show increasing magnification of those parts further from the film so the image is not a perfect representation of the teeth.

Whilst it is possible to use a conventional x-ray machine for dental radiography, it is much easier if a dental x-ray machine is used. When dental radiography is integrated into dental procedures (it should be considered essential not optional) the frequent use of the machine will rapidly pay for its purchase and maintenance allowing it to become a profit centre for the veterinary practice.

Obtaining Good Radiographs

Radiologists have individual preferences regarding x-ray images, some preferring light films, others dark ones. As there is a reasonable range of image densities which provide sufficient contrast and definition for film assessment the actual image density is unimportant providing the film is of diagnostic quality. Guidelines for producing radiographs of this standard are given in Table 2.

Table 2. Guidance on obtaining diagnostic dental radiographs.

 Immobilise the patient as even a small movements will lead to blurring of the image. Dental radiography, as with thorough intra-oral examination, requires general anaesthesia so the two can be performed at the same time.

 Position the patient, film and x-ray machine to demonstrate the features of interest. As mentioned above, a dental x-ray machine simplifies positioning.

 Use a film or film/screen combination to provide optimum definition whilst minimising exposure.

 Use the appropriate x-ray exposure for the area under investigation. Exposures can be varied a little to satisfy the radiologist's density preferences.

 Process the film correctly and ensure that subsequent handling and storage are meticulous to maximise final image quality and longevity.

Suitable viewing conditions are almost as important as correct exposure and processing. The best method of viewing radiographs is to work with subdued room lighting. Avoid looking at bright light sources for several minutes prior to examining radiographs. The film is placed on a viewing box and the area around the film collimated to cut out unwanted light prior to switching on the illumination. A bright area adjacent to the film will tend to dazzle the viewer significantly reducing ability to resolve fine detail and subtle changes. The light intensity should be varied according to the film density in the area of interest, particularly dark areas on a film being viewed with bright spot illumination, just the same as in general radiology. Ones eyes need to dark adapt to the level of light transmitted through the film so a quick glance will not suffice. In dentistry the assessment of fine detail such as suspected root resorption in cats requires the additional use of magnification.

Dental Radiographic Film

Dental radiographic film is non-screen film cut to appropriate sizes for use in the human oral cavity. There are different "speeds" of dental film. The slower Ultraspeed films have the highest exposure latitude and finest grain. Ektaspeed film requires only half the exposure (having a thicker emulsion) but has a lower contrast range and larger silver grain size and is less tolerant of exposure and processing errors. Recently even faster films have been introduced with the aim of reducing patient exposure, but these also require very precise exposure and processing to get the best results. Digital dental radiography systems have been introduced. These require a lower exposure than film systems but most of those in general use have a much lower resolution. A big advantage is the ability to manipulate image contrast, brightness and magnification for viewing. With modern digital cameras the same effects can be achieved by viewing photographs of conventional films on a computer.

Three sizes of dental film are in common use in veterinary dentistry: pediatric periapical, e.g., Kodak DF54 (22 x 35 mm), standard periapical, e.g., Kodak DF58 (31 x 44 mm), and occlusal, e.g., Kodak DF 50 (57 x 75 mm). Human dentists use several other sizes so have a look in catalogues and see whether any of these may be of use to you. The majority of dental films have a small indentation in one corner as a positioning marker. The raised dot is on the side facing the patient/tooth/x-ray machine.

When processing the film it is necessary to separate it from the packaging. Processing film plus packaging or just a layer of backing card doe not work ;-) Unfortunately the small size of dental films and their thick emulsion preclude their development in ordinary automatic processors. Whilst special automatic processors are available, manually processing is quick and easy using either conventional chemicals or rapid processing solutions (e.g., Kodak Rapid Access Developer and Fixer). There are even films supplied with their own processing chemicals within the film sachet. Processing is simplified in this manner, but image contrast and keeping quality tends to be poor. Whatever processing method is used it is important that the films are thoroughly fixed and washed. Re-fixing and thorough washing under running water is advisable after initial viewing of hand processed films to ensure keeping quality once dried.

In cats a full mouth dental survey, using a combination of parallel and bisecting angle views, can be obtained using six standard periapical film. In dogs a larger number of films are usually needed in a combination of sizes. When one has had a little practice the whole series can be obtained in a matter of minutes. It then takes a while to examine the films properly, but it is always time well spent. An additional benefit of routine dental radiography is that there is something to show clients at the end of the procedure and correctly processed films combined with the dental chart will become part of the permanent case record for future reference.

References

1.  Colmery B, DeForge D. Atlas of Veterinary Dental Radiology. Iowa State University Press 2000

2.  Crossley and Penman, BSAVA Manual of Small Animal Dentistry, 2nd edition. BSAVA, Cheltenham 1995.

3.  Harvey and Emily, Small Animal Dentistry, Mosby, 1993

4.  Verstraete FJM, Kaas PH, Terpak CH. Diagnostic value of full-mouth radiography in dogs. American Journal of Veterinary Research 1998, 59, 686-691

5.  Verstraete FJM, Kaas PH, Terpak CH. Diagnostic value of full-mouth radiography in cats. American Journal of Veterinary Research 1998, 59, 692-695

6.  Wiggs, Lobprise. Veterinary Dentistry, Principles and Practice. Lippincott - Raven, Philadelphia 1997,

7.  Zontine EJ. Dental Radiographic technique and Interpretation. Vet.Clinics N. America, 1974, 4: 4, 741-750

8.  Zontine WJ. Canine dental radiology; radiographic technic, development, and anatomy of the teeth. Journal of the American Veterinary Radiology Society 1975, 16: 3, 75-83