Types of teeth

There are two basic types of teeth:

The brachyodont tooth has a short crown : root ratio, with a true root. Once the tooth has matured, the root apex closes and the potential for further tooth growth ceases. Humans, dogs, cats and ferrets have a brachyodont dentition.

The hypsodont tooth is a tooth with a long anatomic crown, and a comparatively short root. The subgingival part of the crown is called the reserve crown. Hypsodont teeth are either radicular or aradicular. The radicular hypsodont tooth eventually forms a true root. The tooth grows for most of the life of the animal, but late in life, the root apex closes and tooth growth ceases. Horses and cows have radicular hypsodont teeth. The aradicular hypsodont tooth never forms a true root with an apex and the tooth grows continuously throughout the animal’s life. Rabbits, guinea pigs, chinchillas and degus have aradicular hypsodont teeth. The incisors of all rodents are aradicular hypsodont, while the cheek teeth are either aradicular hypsodont or brachyodont depending on the species.

If eruption of continuously growing teeth is hindered, e.g. mechanical obstruction due to a malocclusion resulting in abnormal occlusal forces, the continued growth of the tooth will result in destruction of the alveolar bone and apparent ‘apical growth’ of the tooth. This may result in perforation of the cortical bone of the jawbones. In advanced stages, the tooth itself may show severe deformation in the more apical portion.

Lagomorphs

The Order of Lagomorphs includes rabbits, hares, cottontails and pikas. All teeth in lagomorphs are aradicular hypsodont. They have four incisor teeth in the upper jaw. This clearly differentiates them from rodents who only have two incisors in the upper jaw. The lagomorphs do not have canine teeth (Box 14.1).

The four incisor teeth in the upper jaw are placed in two rows with the two large incisors located labially and the two smaller rudimentary incisors (peg teeth) located palatally. In occlusion, the crown tips of the mandibular incisor teeth rest between the 1st and 2nd row of upper jaw incisors. At rest, the incisors are held in occlusion and the cheek teeth are held out of occlusion (Crossley 1995a). A relatively normal rabbit skull is depicted in Figure 14.1.

Rabbits do not gnaw like rodents, unless there is some cheek tooth problem interfering with normal mastication (Crossley 1995a). The incisors are mainly used in a lateral slicing motion, so they more or less cut their food into smaller apprehensible pieces. The large upper incisors grow at an average rate of 2.0 mm per week and the lower incisors at a rate of 2.4 mm per week (Wiggs and Lobprise 1995). A rabbit with normal incisor occlusion, eating a normally abrasive diet such as hay, grass and fresh greens, will wear down the teeth at a similar rate. The incisor teeth have thick white enamel on the labial surface and almost no enamel on the palatal/lingual surface. Normal tooth wear thus results in a chisel-shaped tooth as the softer dentine wears down faster than the thick enamel.

A large diastema separates the incisor teeth from the cheek teeth (premolar and molar teeth).

The upper jaw is wider than the mandible (anisognathic) and when there are no cheek tooth problems, and no other interference such as overgrown incisors, the rabbit chews its foods using a wide lateral (side to side) motion.

Rodents

Rodentia is the largest Mammal Order, with weights ranging from 4 g to over 50 kg. All rodents are ‘gnawers’, with a wide rostrocaudal movement range in the temporomandibular joint and chisel-shaped continuously growing incisor teeth designed for this dorsoventral motion. They are anisognathic, but, in contrast to the lagomorphs, the mandible is wider than the maxilla.

While the incisors are aradicular hypsodont, the cheek teeth are either aradicular hypsodont or brachyodont depending on species. The strict herbivores eating a highly abrasive diet have aradicular hypsodont cheek teeth, e.g. guinea pigs, chinchillas and degus. Species eating less abrasive diets, e.g. mice, rats, hamsters and prairie dogs have brachyodont cheek teeth.

The dental formula (Box 14.2) varies among the species, ranging from 16–22 teeth. However, all rodents have four incisors (one in each quadrant) and no canine teeth. A diastema separates the incisors from the cheek teeth.

At rest (Fig. 14.2A), the mandible is in a caudal position and the incisors are out of occlusion (Crossley 1995b). During gnawing, the incisors are held in occlusion (Fig. 14.2B).

As in lagomorphs, the enamel layer of the incisors is thickest on the labial surface, with almost none present at the palatal/lingual aspect, resulting in a chisel-shaped pattern of tooth wear. The enamel is usually orange-yellow in color. However, the guinea pig has white enamel.

Consequences of tooth overgrowth

Tooth overgrowth commonly results in malocclusion. Complications to malocclusion include:

With advanced disease, the animal is unable to eat and weight loss occurs. The oral discomfort is often associated with excessive salivation (‘slobbers’), which predisposes to moist dermatitis (wet dewlap).

General considerations

A full history should be taken. Husbandry details (housing, diet) need to be known. A full physical examination is required to assess general condition and anesthetic risk. Signs that may be due to dental disease include:

It must be emphasized that the above signs can occur with other disease processes. Anorexia is a very common sign of advanced oral disease, but it is also a sign of almost any disease in these animals. Rabbits and rodents in pain usually stop eating. Tooth grinding is more commonly associated with abdominal discomfort than with dental disease.

Since animals with oral/dental disease are presented late in the disease process, they are often emaciated, dehydrated and obstipated. In addition, they are usually severely stressed from chronic discomfort/pain. Extreme care must be used in selection of medications (antibiotics, antiinflammatory drugs, analgesics and fluid treatment) as lagomorphs and rodents have sensitivities and toxicities to many drugs. It is outside the scope of this chapter to cover these issues. Several excellent texts on rabbit medicine, with extensive drug information, are available and the reader is encouraged to refer to these.

It may be necessary to stabilize the patient before anesthesia. While some authors recommend hospitalization to achieve this, in our experience it is not wise to hospitalize these stress-sensitive animals for long periods. A common response to stress is anorexia. If hospitalization is required, all should be done to decrease stress, i.e. the area should be quiet, as odor-free as possible, and a hiding area should be offered.

Examination of the face and oral cavity

Inspection and palpation of the face and oral cavity is the next step (Box 14.3).

Valuable information can be gained from oral examination of the conscious animal. Although the mouth cannot be opened, a reasonable view can be achieved using an otoscope or a bivalve nasal speculum. Excessive amounts of saliva, overgrowth of cheek teeth, tongue lacerations and wounds of the buccal mucosa may be identified in this way. Less severe problems will not be identified.

As in other species, thorough intraoral examination requires general anesthesia. Sedation and anesthesia in pocket pets is covered in other texts and will not be dealt with here. Aids such as mouth gags and cheek dilators are necessary to open the mouth. We do not use mouth gags in rabbits. Instead, we use cheek dilators as shown in Figure 14.3. We do use a light model mouth gag for most other species. Risks associated with using a mouth gag are damage to the teeth and damage to the temporomandibular joint (if the mouth is opened excessively).

Additional tools include spatulas to depress the tongue or push it aside. Good lighting is mandatory, and often not easy to achieve – a pen torch or headlight is useful. Endoscopy using a rigid endoscope is useful to identify subtle lesions (Capello et al. 2005a). Crown elongation, spikes, lacerations of tongue and oral mucosa, and missing teeth should be noted and recorded. The sulcus of each tooth should be examined with a periodontal probe to identify pathologic periodontal pockets. Even under general anesthesia, it is estimated that only 50% of pathology will be detected (Crossley 2000). In other words, disease is underestimated and oral examination under anesthesia needs to be complemented by imaging (radiography or CT scan, ultrasonography and MRI for abscesses).

Radiographic examination

Radiographs are mandatory to identify type and extent of pathology. Without such information, accurate diagnosis allowing appropriate treatment is not possible.

Three basic skull views need to be taken, namely lateral, dorsoventral and rostrocaudal. Of these, the lateral view is usually the most informative. Additional oblique lateral views are necessary for most patients. When possible, additional intraoral views to avoid superimposition of adjacent structures are recommended. The techniques for intraoral radiography are outlined in Chapter 7. Detail is essential, so non-screen films are required.

Suggested exposure time for the rabbit, guinea pig and chinchilla is as follows:

Using a dental radiography unit, an exposure time comparable to that for radiography of the canine teeth in a medium to large breed dog (depending on the size of the animal) should be appropriate.

Further examination

Even with radiographic examination, a lot of the pathology will be missed. Radiographic interpretation by an experienced examiner will only reveal around 85% of the pathology present (Crossley 2000). CT-scan will give more information, especially for the detection of early cheek tooth pathology (Crossley et al. 1998; Van Caelenberg et al. 2010). Other examinations that may be indicated are ultrasonography (for retrobulbar abscesses) or MRI (for soft tissue lesions).

The healthy mouth

Incisor overgrowth

Incisor overgrowth is common in rabbits. The condition can be classified as primary or secondary, depending on its cause. Primary incisor overgrowth occurs early in life (within the first year) and is the consequence of an inherited skeletal malocclusion (maxillary brachygnathism resulting in a relative mandibular prognathism). In contrast, secondary incisor overgrowth occurs later in life (adult, usually more than 1 year old) and is the consequence of cheek tooth overgrowth. Primary incisor overgrowth is over-diagnosed. Most rabbits presented for treatment of incisor overgrowth have developed the incisor overgrowth secondary to cheek tooth overgrowth.

Primary incisor overgrowth is seen in young animals. It occurs regularly in dwarf rabbits. Due to the jaw length discrepancy (i.e. the mandible is too long with respect to the maxilla), normal incisor occlusion is not established. The mandibular incisors occlude either level with or rostral to the large labial row of maxillary incisors. The result is that normal incisor wear does not occur. The upper incisors may curl inward (Fig. 14.5A) or flare out laterally (Fig. 14.5B), and the mandibular incisors protrude from the mouth. If eruption of the maxillary incisors is hindered, e.g. mechanical obstruction by abnormal occlusal forces, then tooth growth will occur in an apical direction and may result in perforation of the palatine shelf. When significant incisor malocclusion has developed, the animal cannot close its mouth normally and secondary cheek tooth overgrowth will develop over time. Radiographic features of primary incisor overgrowth are shown in Figure 14.6. If the condition is identified early, i.e. before excessive secondary cheek tooth overgrowth has occurred, the prognosis is relatively good with appropriate treatment.

The first step in treating incisor overgrowth, whether primary or secondary in origin, consists of correcting any cheek tooth overgrowth. Overgrown cheek teeth should be shortened to a normal level. Once that is done, two options exist for the incisor teeth. The teeth can be extracted, or they can be trimmed down every 3–5 weeks, as necessary. It is essential that feeding regimens that ensure adequate tooth wear be instituted. If the incisors are extracted, food needs to be cut into small pieces since the rabbit can no longer cut it itself.

Cheek tooth overgrowth

Cheek tooth abnormalities are very common in pet rabbits. As already mentioned, most rabbits presented for treatment of incisor tooth overgrowth have the incisor overgrowth secondarily to the cheek tooth overgrowth, i.e. the cheek tooth overgrowth is the primary cause. Although calcium and vitamin D deficiency may be involved in the etiology (Harcourt-Brown and Baker 2001), the primary cause of cheek tooth overgrowth is thought to be feeding diets that provide insufficient abrasion (Crossley 1995a; Redrobe 1997).

Early cheek tooth overgrowth is not obvious without examination under general anesthesia and radiography. The incisors may still be normally occluding and wearing. Consequently, animals with cheek tooth overgrowth are usually presented late in the disease process. In fact, it is often when the animal is unable to close its mouth and secondary incisor overgrowth and malocclusion has occurred that treatment is sought. The owners assume that the problem is isolated to the incisor teeth. Client communication and education is essential.

Late-stage disease is easy to diagnose. On conscious intraoral examination with an otoscope, the massive overgrowth of the cheek teeth is usually clearly visible. The upper cheek teeth flare out buccally (Fig. 14.7A), causing buccal ulceration and wounds. The lower cheek teeth show spikes on the lingual side (Fig. 14.7B), often associated with wounds on the tongue. The rabbit at this stage is unable to use the normal lateral chewing movements. It may be anorectic but if it is still eating, then it will only be able to consume soft fresh food or dry food, which does not need much chewing.

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