TMJ’s, Extra Oral Radiographs and Film Interpretation
Norman Johnston United Kingdom
Radiography of the mouth and associated structures can occasionally be frustrating, but once the techniques are mastered, quality diagnostic radiographs should be well within the reach of the practitioner. Oral/Dental radiography is a seriously under-used technique in many practices. In many procedures, diagnostic radiographs are essential to the treatment plan, and in others, treatment may be contraindicated without them. The oral cavity requires both intra-oral and extra-oral techniques. Although most radiographs of teeth are performed using intra-oral techniques (when the radiograph is located inside the mouth), extra-oral techniques (where the film is located outside the mouth) are often used for certain teeth (e.g., caudal maxillary teeth or mandibular premolars 1 and 2) and for large areas or lesions that may not fit onto small dental films. For neophyte veterinary dentists who may not stock special dental films, it may allow a degree of diagnostic ability for the oral cavity structures.
EXTRA-ORAL STANDARD VIEWS
Left-Right Lateral or Right-Left Lateral. Patient is in lateral recumbency and the rostral aspect of the head is raised to create a parallel relationship with the film. Often used for routine surveys but open-mouth view prevents overlay of the coronoid processes of the mandibles. Diagnostic ability is limited by superimposition of structures.
Ventro-dorsal Skull. The patient is in dorsal recumbency with the hard palate parallel to the film. If possible, the endotracheal tube should be removed before exposure. Diagnostic ability is limited by superimposition of structures.
Dorso-ventral Skull. As for VD with patient in sternal recumbency.
EXTRA-ORAL SUPPLEMENTAL VIEWS
Various oblique views are helpful to reduce superimposition
Open Mouth 45° Lateral Oblique for both mandible and maxilla. The patient is in lateral recumbency and the mouth fully opened with a non radio-opaque gag (foam wedge or syringe case). The target area is nearest the film and the sagittal plane is rotated 45° with a wedge.
Sagittal Oblique for the temperomandibular joints: see below.
Extra-oral Near Parallel Technique. For dental radiographs, this technique is an alternative to bisecting angle for the maxillary cheek teeth. It is of particular use in cats where the zygomatic arch superimposes over standard intra-oral bisecting angle views. The patient is in lateral recumbency with the target teeth nearest the table. The long axis of the target teeth is as near parallel to the film as possible and the beam is angled at 90° to the film and the target. The mouth is opened with a prop to direct the beam onto the film at 90° without superimposing the top cheek teeth on the bottom cheek teeth. Accuracy is dependent on ability to keep teeth as near parallel to the film as possible.
Imaging of the temperomandibular joints can be difficult. The main problems are those of positioning for imaging and also reproducing images repeatedly. In addition, the resultant radiographs can be hard to interpret.
Indications for TMJ Imaging
Pain/reluctance to close mouth.
Malocclusion of teeth.
Reluctance /Inability to masticate.
Assessment of TMJ involvement in mandibular fracture and other regional trauma.
Views for TMJ
Sagittal Oblique for the temperomandibular joint space. The patient is in lateral recumbency with the joint to be examined nearest the table. The rostral aspect of the head is raised so that the sagittal plane is raised rostro-caudally by 25° for brachycephalic breeds, 15° for mesocephalics and 10° for dolicocephalics. The mouth is opened with a foam block. The central beam is angled onto the joint to be examined to align the long axis of the mandibular condyle perpendicular to the film for better visualisation of the joint space.
Lateral Oblique Views
Dog: Dorso 45° lateral oblique. Patient lies in lateral recumbency with target joint down. Sagittal plane is rotated 45° from lateral with foam wedge and beam is directed through lower TMJ.
Cat: Ventro 20° lateral-dorsolateral oblique
Due to the prominence of the zygomatic arch in the cat, a slightly different view is employed. The patient is placed in lateral recumbency with the target joint away from the table. The head is tipped up 20° from the lateral and the beam is directed perpendicularly through the upper TMJ.
Summary of TMJ Imaging
Oblique views are hard to position.
Oblique views are hard to reproduce.
Oblique views are hard to interpret.
INTERPRETATION OF DENTAL RADIOGRAPHS
The interpretation of any radiograph requires time, equipment and a logical approach to prevent missing lesions.
Magnifying glass or block is very useful, particularly for small lesions.
Film quality depends on many variables: exposure, film speed, development quality, positioning, etc. It may help to cut a “keyhole” in a card and view the radiograph through the keyhole with the background room lights off.
A hot light within the viewer can help, as good contrast exists between the dense tissues of the oral cavity and air.
Evaluate the image quality: image too light or dark? Contrast? Processed properly? Image distorted or superimposed?
Specifically identify the species, location and structures.
Examine whole radiograph from left to right.
Teeth: check each tooth for:
Changes in contour and/or density of dentine.
Changes in bone level around roots (particularly furcation and interproximal).
Changes in pulp chamber or periodontal space.
Changes in bone density around root and integrity of lamina dura.
Jaw: examine lesion in jaw:
Site: location, extent, solitary, multi-focal or generalized.
Size and shape: measure and describe. May require one or more views.
Symmetry: examine contralateral site. Bilateral symmetry suggestive of normal variant.
Border: sclerosis, resorption, lack of continuity.
Contents: lucent or opaque. Homogenous or varying density.
Association with other structures. Teeth displaced or resorbing.
Attempt a diagnosis or assess need for further tests.
COMMON INDICATIONS and FINDINGS FOR ORAL RADIOGRAPHS
Trauma and Exodontia
Diagnosis and treatment planning of fractured teeth or bones and surrounding tissues.
Crown fractures: complicated (if pulp exposed) or uncomplicated.
Root fractures require radiographs to be taken with the primary beam parallel to the angle of fracture for true representation and visualisation of beginning and end of fracture line.
Pre extraction radiographs ensure procedure can be properly planned and that no developmental abnormalities or resorptive lesions and/or ankylosis will surprise operator. Post extraction radiographs ensure that all root fragments are removed and that no collateral damage has been caused.
Loss of attachment is crucial in the treatment planning of periodontal disease. Significant features to look for are:
Receding bone height relative to cemento-enamel junction (CEJ).
Loss of bone at interproximal space or at furcation.
Widening periodontal space is highly significant.
Loss of integrity of lamina dura. The lamina dura is a thin white line around the root and represents dense cortical bone, NOT a structure in its own right. A complete lamina dura is suggestive of good periodontal health. The lamina dura is separated from the tooth by the periodontal ligament, which is relatively radiolucent. The jawbone is trabecular in pattern and varies in density with age and location. A lack of visible space in the area of the PL may indicate ankylosis of the root.
Apical rarefaction or halo. Suggestive of periodontal pathology leading to endodontic pathology (class 1 combined lesion), endodontic pathology leading to periodontal pathology (class 2) or true combined perio/endo lesions (class 3).
Defects and Variations in Tooth Density
Caries usually affect the molar teeth of dogs. Loss of normal contour and density will only be visible on a radiograph if pathology is advanced. Assess whether pulp canal affected by loss of dentine.
Feline Odontoclastic Resorptive Lesions. Treatment planning of these lesions must involve radiography.
Internal or external root resorption. Often secondary to periodontal or endodontic disease.
Developmental Defects and Anomalies
Detection of missing permanent teeth.
Teeth with developmental problems.
Teeth having eruption problems: impacted or delayed.
Swellings, Cysts and Neoplasms
Cysts present as well demarcated and expansive lytic lesions.
Neoplasms may present as increased or decreased densities. They are often irregular and poorly demarcated with lysis of bone. Close examination of the periphery of the lesion will help, as this is often the most active zone in a pathological process.
Craniomandibular osteopathy (CMO) is usually a lesion of the mandibular body, occasionally the base of the cranium or TMJ’s, and produces a proliferative periosteal reaction.
Osteomyelitis will often present with a proliferative reaction at the periphery with decreased density at the centre of the lesion.
Diseases that affect calcium metabolism, such as hyperparathyroidism, present as reduced bone density. Teeth are often quoted as “floating” when advanced.
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