4
mentioned under signs that may be associated with dental disease; however, the underlying aetiology could be ocular disease. Ultrasonography is also a very useful diagnostic technique for investigation of disease of the eye and the bony orbit (see Chapter 8).
Neurological signs for which skull radiography is indicated include seizures, head tilt, vestibular signs, personality changes, nystagmus, ataxia, deficient palpebral closure, drooling, facial asymmetry, one ear lying down, drooping or retracted lip, incoordin-ation and any other cranial nerve deficits identified, for example deviation of the tongue.
Cases of otitis externa where there is suspicion of otitis media or interna should be investigated radio -graphically. If a bulging tympanic membrane is found, this also warrants radiographic investigation.
A history of trauma, abnormal temporomandib-ular joint movement, facial pain, any palpable swelling, deformity, asymmetry of the skull and the investigation of any non-specific clinical signs may also be indications for skull radiography in the rabbit.
Radiographic anatomy and interpretation
Dental anatomy
Normal dental anatomy is rarely seen in rabbits pre-sented at veterinary surgeries. Most rabbits, except the very young, have some radiographic signs of dental disease. It is important to be familiar with the normal, however, in order to be able to fully evaluate the abnormal.
The examination should start rostrally, at the inci-sors, noting the shape of their occlusal surfaces and how they have worn. The layer of enamel on the labial surface of the primary incisors results in a 45-degree chisel shape. The tips of the mandibular incis rs s ld e et een t e 1st and 2nd a il-lary incisors when at rest. The 2nd maxilil-lary incisors should not be chisel-shaped. These are missing in some rabbits. The occlusal surface of the mandi-bular incisors should line up roughly with a line that follows the curvature of the mandible (Figure 1 l n ati n and increased r decreased c r a-ture of the crown, can also be gauged using this imag inary line. The primary maxillary incisors are
more curved than the mandibular incisors (Figure 4.2). The enamel of the incisors should be seen to be smooth, especially on the labial surfaces. The pulp cavity can be used as a measure of eruption.
Normally, as the tooth grows, dentine slowly fills in the pulp cavity. If the growth rate is slower than normal, or even arrested, the pulp cavity will be red ced r e en a sent see i re 1 r t e normal anatomy; Figure 4.7, later, shows the pulp cavity when tooth growth has been arrested). A more rapid growth rate than normal will result in the pulp cavity being close to or even beyond the gingi-val margin, as in cases where the incisors are not occluding and there is no negative pressure on growth (see Figure 4.6). This is clinically significant if reduction of the height of the clinical crown is planned. The apices of the mandibular incisors are
st r stral t t e irst c ee teet see i re 1 e apices t e 1st a illar incis rs are at t e junction between the incisive and maxillary bones of the diastema. Each should be at least the diameter of its apex away from the diastema. The lamina dura is visualized as a more radiopaque line of denser bone demarcating the alveolus. At the area of the apex there should be room between the tooth and the lamina dura for the germinal tissue. The perio-dontal space is the more radiolucent line between the tooth and the lamina dura.
The cheek teeth should be evaluated next. With the mandible in the resting position, i.e. with the inci-sors in normal occlusion on a lateral view, the occlusal plane of the cheek teeth should be seen as a i a line as in i re 1 e r str ca dal view can also be used to evaluate the occlusal plane of the cheek teeth (Figure 4.3). The apices of the cheek teeth are more difficult to visualize because of the superimposition of the contralateral side. Lateral oblique views allow a clearer view of the apices. The tympanic bullae are also sep arated on this view (Figure 4.4). Both a right and a left oblique lateral view are required to visualize all the apical areas. As with the incisors, the lamina dura should be seen as a smooth thin curved radiopaque line; the space between the lamina dura and the tooth represents the periodontal ligaments and, at the end of the tooth, the germinal tissues.
Detail of the mandibular incisor. The solid white arrow marks the more radiodense line of the lamina dura, the wall of the dental alveolus. The line of the occlusal surface of the mandibular incisors is a continuation of the line of the diastema. Note the normal position of the apex in relation to the 1st mandibular cheek tooth and the normal distance between the apical end of the tooth and the lamina dura in this radiograph (open arrow). The black arrow indicates the normal coronal extent of the pulp cavity, extending for approximately two-thirds the length of the tooth.
(© Aidan Raftery) Palatal 4.1
surface
Rostral margin of the pulp cavity Occlusal
surface
Lamina dura Clinical crown
labial surface
Periodontal
space Apex
Perforated part of the maxillary
bone
Rostral orbital margin
*
Zygomatic process of
temporal bone Condylar process of the
mandible
Tympanic bulla Diastema
Lateral views of the skull and mandible showing important anatomical landmarks. Solid arrows mark the temporal processes of the zygomatic bones (right and left).
The dotted line marks the rostral border of the mandibular ramus, the coronoid process. The asterisk marks the orbital foramen. Note: this radiograph is not a true lateral view, which is why there are two lines around the tympanic bullae and two zygomatic processes of the temporal bone. (Radiograph
© Aidan Raftery; skull model image courtesy of Frances Harcourt-Brown)
4.2 Rostral orbital
margin Nasal bone
Incisive bone
Diastema
Rostral border of the mandibular ramus
Right and left
mandibular rami Tympanic bullae Apex of the
4th maxillary cheek tooth (1st molar)
Frontal bone Zygomatic processesof the temporal bones Parietal
bone Condylar processes of the mandible
* The mandibular cheek teeth should show at least the same distance between the tooth and the lamina dura as between the lamina dura and the line of the ventral cortex of the mandible. Evaluating the maxillary teeth for apical elongation is more difficult because there is no similar reference point.
Various guidelines have been proposed, although all have their limitations. Elongation of teeth at the apices will result in loss of the normal appearance in this area and loss of the line of the lamina dura.
Many clinicians evaluate the height of the clini-cal crowns of the cheek teeth by projecting the line of the maxillary diastema and the mandibular dia-stema rostrally. This line should be converging to
eet in r nt t e ra it’s n se t ese lines are parallel or diverging, the most common reason is elongation of the clinical crowns of the cheek teeth (Figure 4.5). There are, however, normal skulls where this guideline does not work.
On the right and left lateral oblique views an attempt should be made to identify each of the
cheek teeth separately. There are five mandibular cheek teeth and six maxillary cheek teeth on each side. The 5th mandibular cheek tooth is signifi-cantly smaller, while the 2nd and 3rd are less c r ed t an t e t ers i re e 1st a il-lary cheek tooth is smaller and shorter than the others, and the 6th is shorter still. The apex of the 3rd maxillary cheek tooth usually extends beyond those of its neighbours (see Figure 4.5). A longitudinal ridge, which is an enamel infolding, should be visible from the apex to the occlusal surface of each tooth. This should be a smooth unbroken line, where not obscured by superimpo-sition. The extent of the pulp cavity is not easily seen in the cheek teeth. In the oblique views, where rotation frees the distal part of the tooth from superimposition by the contralateral side, the pulp cavity can be more easily evaluated just prox-imal to the apices. The degree of curvature of the teeth and the size of the interproximal spaces should also be noted.
Supraorbital margin of the frontal bone
Zygomatic arch
Facial tuberosity Masseteric fossa
Temporomandibular joint
Rostrocaudal views of the skull showing the occlusal plane of the cheek teeth (arrows). The maxillary and mandibular cheek teeth are outlined in red. The maxillary and mandibular incisors are outlined in blue. The
temporomandibular joints are also seen on this view (yellow line). (Radiograph
© Aidan Raftery; skull model image courtesy of Frances Harcourt-Brown)
4.3
Tympanic bulla
Angular process of the mandible Masseteric
fossa
Zygomatic arch Temporomandibular
joint Temporal fossa Parietal bone
Rostral orbital margin
Zygomatic process of the temporal bone
Right condular process of mandible
Left tympanic bulla
Left angular process of mandible Apices of the left
mandibular cheek teeth Secondary maxillary
incisor Nasal bone
Apex of the 1st right maxillary cheek tooth
Lateral oblique views of the skull. This radiographic view allows the mandibular cheek teeth apices on one side and the maxillary cheek teeth apices on the contralateral side to be projected free from
superimposition by the opposite sides. (Radiograph courtesy of Matthew Baraclough)
(continues) 4.4
Nasal bone
Rostral orbital
margin Zygomatic process of
the temporal bone
Condylar process of mandible
Tympanic bulla
Angular process of mandible Secondary maxillary
incisor
Ventral cortex of mandible
(continued) Lateral oblique views of the skull. (© Aidan Raftery)
4.4
Converging diastema as a measure of normal clinical crown height. Elongation of the mandibular incisors is shown by the closed arrow.
Elongation of the mandibular incisor apices occurs behind the 1st mandibular cheek teeth. There is an abnormal angle to the occlusal surface of the mandibular incisors (dotted arrow). Although still converging, the lines of the diastema (dashed lines) are moving closer to parallel, indicating elongation of the clinical crowns of the cheek teeth. (© Aidan Raftery)
4.5
Secondary maxillary incisor
Apex of the 3rd maxillary cheek tooth
Rostral margins of the coronoid processes of
the mandible
4.6 A fracture of the mandible can be seen just caudal to the 5th cheek tooth (dotted arrow) in this radiograph of a 3-year-old neutered female Dwarf Lop that had been attacked by a dog. The ramus of the mandible is displaced ventrally (solid arrowheads), which has resulted in incisor malocclusion.
The upper and lower sides of the diastema (dotted lines) should be convergent or parallel and not diverging as here. The 2nd maxillary incisors are missing.
This is common in dwarf rabbits.
Each cheek tooth should be identified and evaluated separately. The 3rd maxillary cheek tooth is often the longest.
There is elongation of the clinical crowns of all teeth, together with malocclusion of the incisors. The pulp cavity is more proximal in the incisors than normal, indicating a more rapid growth rate. (Courtesy of Frances Harcourt-Brown) Ethmoid
turbinates 3rd maxillary cheek tooth Nasal
turbinates
Upper respiratory tract
Upper respiratory tract disease is a common indica-tion for radiography in rabbits. The lateral and DV views are the most useful. The area of the nasal and ethmoid turbinates and the areas of the nasal cavity where air provides good negative contrast should be evaluated (see Figure 4.6). The surround-ing structures should also be evaluated for patho-logical processes that may be affecting the nasal cavity. The nasal turbinates are in the rostral area of the nasal cavity and have a linear pattern caused by the delicate bony scrolls and their surrounding air. Loss of the linear pattern is seen when the air spaces between the turbinate bones fill up with accumulated exudate, reducing the contrast between bone and surrounding material. There is often destruction of the nasal turbinates, which is seen as an area of decreased radiopacity. In some cases of chronic upper respiratory tract disease there may be mixed areas of increased and decreased radiopacity. Localized chronic disease may sometimes show radiographically as focal radiodense lesions where there has been minerali-zation of chronic inflammatory or neoplastic tissue.
Comparison of a lesion on different views will be necessary to localize it. These lesions can be fur-ther investigated by rhinoscopy and/or computed t rap see apters and 1 e ethmoid turbinates are seen as a more irregular pattern of thin radiopaque lines caudal to the nasal turbinates (see Figure 4.6). The radiographic appearance of pathological changes is broadly sim-ilar to that described for the nasal turbinates.
Paranasal sinuses
Sinusitis is a common sequel to chronic upper res-piratory tract disease. Diagnosis is difficult and the condition is often chronically undiagnosed. The clini-cal signs are similar to those of chronic rhinitis, and the findings on rhinoscopy will raise the suspicion of sinusitis. Skull radiology will identify clinically signifi-cant chronic sinusitis, but CT scans are better at identifying mild sinusitis. However, because of cost
considerations, radiology currently plays an impor-tant part in the diagnosis and management of sinus-itis for most patients. The exception is sinussinus-itis of the sphenoidal sinus, which is much more difficult to identify by conventional radiography.
Owing to the anatomy of the dorsal conchal sinus and the maxillary sinus, inflammatory exu-dates tend to accumulate in the ventral recess of the maxillary sinus, it being the most ventral sinus com-partment. It has no gravitational drainage. Drainage relies on the normal functioning of the ciliary system and occurs against gravity. On lateral radiographs the ventral recess of the maxillary sinus can be visu-alized, if it is filled with inflammatory exudates, as a thick linear area of increased radiopacity projecting from the area of the apex of the primary maxillary incisors to the angle between the ethmoid turbinates and t e ape t e 1st a illar c ee t t (Figure 4.7). In less severe cases, when the recess is not filled with accumulated exudate, it may be visualized as a semi-opaque line. DV views are not as useful, but an increased soft tissue density on one side will help to localize the lesion. Pathology within the nasal cavities may obscure the contrast, and changes in the sinuses may be more difficult to recognize. Sinus puncture with the collection of diagnostic samples for culture and sensitivity is the gold standard for the diagnosis of sinus diseases
see apter 1 The ear
The DV view is the best view for evaluating the tym-panic bullae, although they are superimposed on the temporal petrous bone. The other advantage of the DV or VD views is that both bullae are visible and directly comparable with the same exposure deve-lopment factors (Figure 4.8).
Lateral views result in superimposition of the tympanic bullae and are therefore of limited value for evaluating them; also each bulla cannot be com-pared easily with the contralateral one. The lateral oblique views separate most of the structure of the bullae from superimposition. However, the tympanic The maxillary sinus is visible because it is filled with inflammatory e udate (open arrowheads). This rabbit has more advanced dental disease, with mineralization of areas of chronic inflammation (dotted arrow). There is
remodelling of the ventral cortex of the mandible as the apices of the cheek teeth elongate.
Malocclusion of the incisors is present, and the pulp cavity is reduced in size, indicating slow or arrested growth. The maxillary sinus is also visible in Figure 4.6. (© Aidan Raftery)
4.7
Detail of the DV view of chronically diseased tympanic bullae with extensive remodelling of the lateral and rostral walls of the tympanic bulla, most severe on the right side (open arrowheads). The spheno-occipital fi ssure is marked by a closed arrowhead. Aidan aftery
4.8
bullae are not completely free from superimposition until 30 degrees of rotation from the lateral is obtained along the long axis. The upper tympanic bulla still overlies the cranium at this point (King et al., 2002a). The rostrocaudal view with the mouth open is used in the dog and cat to evaluate
Tympanic bullae Foramen magnum
Spheno-occipital fissure Palatine
bone
Facial tuberosity
Mandible
Mandible
Zygomatic prominence of temporal bone First maxillary cheek tooth
Zygomatic prominence of maxilla Zygomatic arch
(a) This rabbit had minerali ation of chronic infl ammatory tissue associated with an apical abscess e tending from the right 1st and 2nd maxillary cheek teeth. The zygomatic process of the temporal bone is marked with a solid arrowhead. The temporal process of the zygomatic bone is marked with a dotted arrow. The 2nd maxillary cheek tooth has an elongated apex and is marked with a solid arrow. The bulla is normal. (b) Skull model image of the same view. (Radiograph © Aidan Raftery; skull model image courtesy of Frances Harcourt-Brown)
4.9
(b) Primary maxillary
incisor
Secondary maxillary
incisor
Mandible
Mandible Palatine
bone
Spheno-occipital fissure Tympanic bullae
Mastoid process
Transverse process
of the atlas Foramen magnum
External acoustic meatus Basisphenoidal
bone Temporomandibular
joint
1st maxillary cheek tooth
Facial tuberosity Nasal cavity and paranasal sinuses Nasal cavity Incisive bone
Nasal bone
(a)
the tymphanic bullae; however, the rabbit jaw will only open 20–25 degrees, which is not sufficient to prevent superimposition of the jaws on the bullae (King et al., 2002b).
On the DV view the bones of the external acous-tic meatus can be seen extending laterally from the tympanic bulla. The rostral border of the tympanic bulla should be seen as a thin curved arc of bone, which should not extend past the spheno-occipital iss re see i re 1 1 e lateral e tent t e bulla should not be superimposed on the angular process of the mandible. With disease this bone may become irregular, thicker in some areas and thinner in others. There may be remodelling, with extra bone laid down extending rostrally, with the bulla appearing irregular and larger than that on the opposite side. It often also extends to overlap the angular process of the mandible (Figure 4.9).
Occasionally it can extend rostrally past the spheno-occipital fissure. The air density within the tympanic bulla may be partially or completely replaced with a soft tissue density, and occasionally the bulla may be partly or completely filled with bone density. This may represent neoplasia or mineraliza-tion resulting from a chronic inflammatory process.
If there is disease of the external ear canal, the bony external acoustic meatus may not be visible as a consequence of its destruction or the external ear canal being filled with soft tissue density. The dia-meter of the external ear canal at that level is often increased (see Figure 4.8).
When otitis media is diagnosed the radiographs should also be evaluated for concurrent upper res-piratory tract disease. Otitis media can be second-ary to upper respiratory tract infections with access gained up the auditory tube. Disease of the inner ear is rarely appreciated radiographically unless it is an extension of otitis media.
Canalography is a contrast technique used to eval - uate the patency of the tympanic membrane (see
apter 1
The eye and associated structures
Radiographic evaluation of ocular disease is limited to the evaluation of other structures that are affect-ing the eye. Apical abscessation affectaffect-ing the 5th or 6th maxillary cheek tooth is a common cause of exophthalmia. Caudal extension of an infection in the maxillary sinus is also seen as a cause of exophthalmia. Space-occupying lesions in the thor-acic cavity can increase blood pressure, causing bilateral exophthalmia due to congestion of the orbital venous sinus. In the case of exophthalmia caused by increased blood pressure, skull radio-graphs will be non-diagnostic.
On the lateral view the rostral margin of the bony orbit is formed by the zygomatic prominence of the maxilla and can usually be seen as a curved line of increased density starting at the apex of the 2nd maxillary cheek tooth. The caudal border is not delineated but is roughly dorsal to the zygomatic process of the temporal bone. The orbital foramen should be clearly visible (see Figure 4.2). These structures may appear as double lines, depending on positioning. When an apical abscess is causing
exophthalmia it may be visible on the radiograph as a delineated area of increased soft tissue density extending caudally and dorsally, often partially obscuring the optic foramen.
On the DV view the eye can be seen as a soft tissue density in the area bounded by the zygo-matic arch. The lateral margin of the orbit is not normally visible beyond the soft tissues on the zygomatic arch.
The nasolacrimal duct is often partially or com-pletely blocked at some point by secondary dis-ease processes, most commonly by pathology at the apices of the maxillary teeth. At the level of the apex of the primary maxillary incisors the naso-lacrimal duct narrows and makes a right angled bend. This is the most common area affected by dental disease. It also runs in close association with the apical areas of the rostral cheek teeth, and any pathology causing bone remodelling at this level can also affect the duct. Upper respiratory tract infections where thick exudates are produced may also interfere with the duct as will any con-dition causing scarring just inside the nose.
Exudates in the nasal cavities will obscure the detail of the nasal turbinates.
Nasolacrimal duct contrast studies will help to identi t e site str cti n i re 1 Occasionally the nasolacrimal duct will rupture through its medial wall into the maxillary sinus. This is more common when there are attempts to cannu-late t e d ct it s t re aterial see apter 1 The temporomandibular joint
Visualization of the temporomandibular joint radio-graphically is difficult in the rabbit. It can be seen in the DV view but the vertical ramus of the mandible is s peri p sed i re 11 str ca dal ie s with the mouth closed give the most clinically use-ful views of the temporomandibular joint. Both sides are visible on the same view, so they can be
Nasolacrimal duct contrast study. The dotted arrow marks the end of the first narrow section. After this the duct is approximately 2 mm in diameter until the area of the apex of the primary maxillary incisor. At this point, which is marked by a solid arrowhead, the duct narrows to approximately 1 mm in diameter, makes a 90-degree bend medially and traverses to exit several millimetres caudal to the mucocutaneous junction at the alar fold. Note the normal occlusal plane of the cheek teeth.
(© Aidan Raftery) 4.10