Dentistry of rabbits (Proceedings)

Dental disease is common in pet rabbits and can produce a wide range of clinical signs and symptoms. Underlying causes of dental disease are still subject to debate, but can be divided in congenital and acquired disease. Congenital dental disease is usually limited to malocclusion of incisors, especially in dwarf or brachycephalic breeds. Acquired dental disease can be related to nutrition (lack of wearing, metabolic bone disease), trauma or age-related attrition. Treatment of dental disease is designed to return anatomy and function as close to normal as possible, and to control associated infection and inflammation.

Normal dental anatomy and physiology

Proper diagnosis and treatment of dental disease requires a thorough understanding of normal rabbit dental anatomy and physiology. The rabbit has continually growing, open rooted (elodont) incisors and molars. Premolars and molars are anatomically indistinguishable, and are therefore simply termed "cheek teeth". There are two pairs of maxillary incisors, the second smaller incisors located behind the larger, easily visible incisors. These are commonly called "peg teeth". The normal dental formula of rabbits is I2/1 C0/0 P3/2 M3/3 for 28 teeth. When the jaw is at rest, the mandibular incisors rest between the first and second maxillary incisor teeth. Maxillary first incisors are shorter than the mandibular incisors, and have a central longitudinal groove on the labial aspect that is not present on the mandibular incisors. Normal grinding motions during chewing produce sufficient wear to keep incisors at the proper length in the normal animal. Normal molars have longitudinal grooves on the buccal aspect of each tooth, and the occlusal surfaces are flat, but not smooth. Irregular surfaces help in the crushing of hard, fibrous food. Similar to incisors, normal crushing of food produces adequate wearing of continually growing cheek teeth in the normal animal. There is no significant interproximal space between incisor or cheek teeth.

Pathophysiology of dental disease

Dental disease is a result of anatomical or physiologic abnormalities of incisor teeth, cheek teeth, or both. Any process interfering with normal eruption of or wearing of continuously growing teeth will result in dental disease. Disease of incisors can be secondary to primary disease of cheek teeth, and vice versa. For example, primary congenital malocclusion of incisors prevents proper wearing of the incisal edges of the teeth, and subsequent overgrowth. Elongation of incisors prevents proper occlusion of cheek teeth, with subsequent overgrowth and possible development of sharp points or spurs.

Insufficient wearing has been proposed as a leading cause of acquired dental disease. This theory is based on evidence that the diet of pet rabbits often significantly differs from that of their wild European counterparts, who consume rough grasses with higher silica content. Jaw movements of rabbits consuming rough hay have a significantly wider horizontal component than rabbits consuming a concentrated pelleted diet. This decreased horizontal jaw motion in pet rabbits eating pellets may result in decreased normal wear of continuously erupting teeth.

Any process that interferes with eruption of elodont teeth can potentially impact normal occlusion. Francis Harcourt-Brown has proposed metabolic bone disease (MBD) as a potential cause of acquired dental disease. Studies have demonstrated demineralization of skull bones, lower serum calcium and higher PTH levels in severely affected rabbits. MBD produces demineralization of bone matrix and subsequent changes in skull and teeth structure, interfering with normal occlusion and wearing of continuously growing teeth.

Regardless of the initiating cause of abnormal wearing of elodont teeth, the result is elongation of crowns. Overgrown, elongated incisors may develop a number of abnormal growth patterns, including lateral deviation with or without subsequent damage to soft tissues. Elongated cheek teeth produce excessive pressure on tooth roots when the rabbit chews, causing deformation of the root, increased interproximal spaces and bending and rotation of the tooth itself. These processes contribute to increased cheek teeth malocclusion and worsening of dental disease. Ultimately bone is lost as roots perforate cortical bone. Fractures and tooth root abscesses are common sequela. In end-stage acquired dental disease of cheek teeth, eruption of teeth either is delayed or ceases altogether, and bone is reabsorbed. Causes of dental disease are summarized in table 1.

Table 1. Causes of dental disease.

Clinical signs and symptoms of dental disease in rabbits

Prey species such as rabbits commonly hide symptoms of illness; therefore, lack of typically reported symptoms does not reduce the possibility of dental disease. Signs may be directly related to dental disease, or processes secondary to dental disease. Signs and symptoms associated with dental disease include emaciation and general loss of condition, decreased food intake, dysphagia, anorexia, changes in fecal size, quantity or appearance, excessive grooming, excessive salivation and drooling, epiphora, dyspnea, obesity, exophthalmos, facial masses or swellings, epiphora, purulent nasal discharge, inability to close the mouth, and restricted or painful mandibular movements. Diseases secondary to dental disease include systemic disease due to emaciation and loss of condition, dermatitis, ocular disease, including dacryocystitis, conjunctivitis and corneal ulceration, gastric impaction and bloat, and periapical abscesses.

Diagnosis of dental disease

Diagnosis of dental disease is greatly enhanced by three modalities: thorough examination of the oral cavity, radiology and endoscopic evaluation. Examination of incisors is straightforward and can be accomplished in most rabbits during routine physical examination. Examination of cheek teeth, however, can be much more difficult, especially in dwarf breeds. A cursory examination of cheek teeth can be performed in a properly restrained patient with an otoscope or other similar instrument. However, subtle lesions are frequently missed with this technique. Therefore, when dental disease is suspected based on history or cursory oral examination, thorough examination under general anesthesia is essential. Examination is greatly facilitated with instruments designed for this purpose, including a tabletop dental positioner, mouth gags and check dilators. Several manufacturers now produce equipment especially for this purpose, and their value in enhancing examination of the oral cavity of rabbits and rodents cannot be over-emphasized.

Thorough examination of the oral cavity under anesthesia should include examination of all aspects of each tooth, the tongue, buccal and lingual mucosa. Abnormalities can include deviations of the cheek teeth occlusal plane, including wave mouth (undulating occlusal plane) and step mouth (stair-like differences in tooth height); sharp spurs or spikes on lingual or buccal aspects of cheek teeth, loose or fractured teeth, pus, widened interproximal spaces and damage to the tongue and adjacent soft tissues. A thorough understanding of normal gross dental anatomy is essential to properly diagnose abnormalities.

Many authors have found that the detection of subtle lesions is greatly enhanced by the addition of magnification and illumination provided by oral endoscopy. Documentation of lesions for comparison or client education is another added benefit.

Dental disease is frequently related to abnormalities of tooth roots, which cannot be evaluated without proper radiographic evaluation. The minimum database includes a lateral projection, right and left oblique projections, and a dorsoventral or ventrodorsal projection. Skyline projections can be valuable as well. Interpretation of poor quality radiographs is extremely difficult. Excellent technique is critical to prevent missing subtle lesions, or incorrect interpretation.

The true lateral projection (perfect superimposition of the tympanic bullae, ventral mandibular profiles and other landmarks) is useful to evaluate the occlusion of incisor teeth and the occlusal plane of cheek teeth, which appears as a slight zigzag pattern. An important tool for the detection of cheek tooth elongation is to evaluate the radiopaque lines produced by the maxillary and the mandibular diastema (space between incisors and cheek teeth). These lines should not be parallel, but slightly oblique and converge some distance in front of the incisor teeth.

Right and left oblique radiographs are extremely useful for evaluation of the roots of individual maxillary and mandibular arcades, which is prevented when roots are completely superimposed on the true lateral view. Ventrodorsal or dorsoventral views are more difficult to interpret, but may help detect other abnormalities.

Specialized equipment for diagnosis and treatment of dental disease (table 2)

As mentioned above, specialized equipment greatly facilitates examination of the oral cavity of rabbits. The tabletop dental positioner is a devise that that positions the anesthetized rabbit at a slight incline, with adjustable bars to raise the maxilla and lower the mandible, allowing the veterinarian to work without the need of an assistant to hold the head in position. The bars fit behind the incisors; therefore, the devise is not helpful in patients who have undergone extraction of the incisors. Cheek dilators are inserted into the oral cavity to laterally retract the cheeks and further enhance examination.

Table 2. Specialized equipment for diagnosis and treatment of dental disease

The same effect can be obtained with a rabbit and rodent adjustable mouth gag combined with cheek dilators, but requires an assistant to hold the patient's head in position, which can become difficult during longer, more complicated dental procedures. Flat or curved spatulas are designed to reflect the tongue and soft tissues. The Crossley incisor and molar luxators follow the natural curve of the rabbit tooth to facilitate extraction. Many practitioners fashion extractors out of slightly flattened and curved 18-22 gauge needles.

Reduction of the length of the crowns of incisors and/or molars should never be accomplished with cutting or rongeur-type instruments, as the risk of iatrogenic damage is unacceptably high. Amputation of incisors is best performed with high-speed straight or even higher contra-angle dental hand pieces (ideally greater than 25,000 rpm) and cutting burs. Cone-shaped burs allow proper reshaping of the incisor occlusal plane. Similarly, reduction of molar length, restoration of occlusal plane and removal of sharp spurs and spikes of cheek teeth should be performed with a higher end low-speed (ideally 20,000-25,000 rpm) straight hand piece and longer grinding-type burs that can be safely introduced into the small oral cavity of the rabbit. Traditionally, dremel-type hobby tools have been used for this purpose. Recently, however, a number of manufacturers have offered precision, higher rpm low speed straight hand pieces. The typical straight dental hand piece utilized in small animal medicine runs at speeds near 8000 rpm, which is too slow to be optimally effective, and may produce unacceptable thermal damage to teeth.

A number of small rasps are commercially available to file points and spurs from cheek teeth. These are useless for reduction of tooth height, and rarely effective except in the case of very small points. The small oral cavity of rabbits hinders effective use, and increases risk of injury to adjacent soft tissues.

Treatment of dental disease

The goal of treatment is restoration of dental anatomy to as close to normal as possible. In many cases, cure of dental disease is impossible, and owners must understand that the goal of treatment is management only. Treatment also includes control of associated inflammation and infection, and analgesia.

Except in rare circumstances, general anesthesia is required for safe and effective surgical treatment of dental disease. Anesthesia of rabbits has been covered extensively. In general, anesthesia is accomplished through a combination of injectable and inhalant agents. Inhalant anesthesia during dental procedures is delivered through an endotracheal tube, nasal catheter, or nasal mask, as rabbits are obligate nasal breathers. Analgesia is chosen based on the extent of treatment and likelihood the procedure will produce pain.

Incisors

Incisor teeth that cannot be restored to normal length and occlusion due to congenital malocclusion or long-term acquired malocclusion should be extracted. Rabbits are able to eat normally without incisor teeth, as food is manipulated by the lips and tongue into the oral cavity for crushing by the cheek teeth. Indications for extraction of incisors include malocclusion necessitating frequent burring to restore length and occlusal surface, fracture and abscessation. The patient is anesthetized and placed in dorsal or lateral recumbency, and the gingival scrubbed with dilute 2% povidone iodine or dilute chlorhexidine solution. A #11 or #15 scalpel blade is inserted into the gingival sulcus to separate the gingival attachment around the entire tooth. Care must be taken to insert the luxator between the sulcus and the tooth, and not the sulcus and adjacent soft tissues. Luxators help progressively loosen and sever the periodontal ligament, and must be used following the natural shape and orientation of the sub-gingival portion of the tooth (reserve crown). Excessive leverage much be avoided to prevent fracturing the tooth. The shape of incisor luxators makes them optimally useful on the mesial and lateral aspects of the tooth. A contoured 18-gauge needle is more useful for severing the periodontal ligament on the lingual and labial aspects. Once the tooth is free, it can be gently extracted using steady, slowing increasing force, taking care to follow the natural curvature of the tooth and reserve crown. Once extracted, the tooth is examined to be sure that the entire tooth and pulp have been removed. Tissue remaining within the alveolus should be destroyed by curetting the alveolar walls with a needle or curved luxator. The tooth socket can be flushed with saline, iodine or chlorhexidine, and the gingiva closed over the alveolus with 3-0 or smaller absorbable suture using a simple interrupted or purse-string pattern. The alveolus should not be sutured when infection is present. Second maxillary incisor teeth are thin but long, and can be removed using a 22-gauge needle as a luxator.

Incisor teeth that break below the gingival during extraction are extremely difficult to remove. Partial removal of teeth or failure to destroy pulp tissues can result in partial or complete regrowth. Post-procedure skull radiographs may help detect remaining tooth fragments.

Reduction of incisor length and restoration of normal shape can be considered in earlier cases where correction of primary disease of cheek teeth can be expected to subsequently allow the rabbit to maintain normal wear and occlusion. Reduction of incisor length must never be accomplished by amputation of teeth with clippers, rongeurs or any other manual cutting instrument, as this frequently leads to fracture, root damage and subsequent abscessation. This treatment also cannot restore the normal shape of the occlusal plane. Length reduction is accomplished in a properly anesthetized patient with high-speed precision dental hand pieces and burrs. Prevention of thermal injury is accomplished by cooling the tooth with saline during the procedure, either delivered through the hand piece, or dropped onto the tooth from a syringe by an assistant. Care must be taken not to introduce large amounts of saline into the mouth of a non-intubated patient.

Indications for repeated reduction of the length and reshaping of incisors are rare. Patients requiring repeated treatment for acquired dental disease of cheek teeth with incisors that elongate or malocclude more frequently than do the cheek teeth should undergo extraction of the incisors as described above.

Cheek teeth

The goals of treatment of cheek teeth include reduction of abnormal length, and restoration of near normal shape, although it should keep in mind that it is impossible to restore the natural zigzag pattern of the normal occlusal plane of the rabbit. Additional goals include removal of loose or infected teeth. Frequency of repeated dental treatments varies from rabbit to rabbit, and in the same rabbit over time. An individual rabbit may require more frequent treatment as the disease progresses. Conversely, as roots are destroyed and growth slows or teeth are lost, frequency of treatment may decrease.

Improvement of abnormal cheek teeth occlusal plane is accomplished with higher low-speed straight hand pieces, as is it impossible to safely introduce a high-speed contra-angle hand piece into the small oral cavity. Soft tissues are protected with dental spatulas, and accumulating tooth dust removed with moistened cotton-tipped swabs. Thermal damage to teeth is prevented by saline cooling administered through the hand piece or by an assistant. Extreme care must be taken not to introduce excessive fluid into the oral cavity, especially in a non-intubated patient.

Extraction of severely maloccluded, loose, fractured or abscessed teeth is facilitated with dental elevators and luxators designed for this purpose. Several manufacturers produce luxators specifically for either molars or incisor teeth (table 2).

Indications for extraction of cheek teeth include periapical infection and marked deformity or malocclusion of individual teeth not easily corrected with burring. Most authors agree that the goal of therapy should be retention of as many cheek teeth as practically possible to allow normal crushing and ingesting of food. However, there are many reports of rabbits surviving long term on liquid hay diets (Critical Care, Oxbow Hay Products, www.oxbowhay.com) after periodic extraction or natural loss of most or even all cheek teeth. It is not necessary to extract the opposing tooth after extraction of a single cheek tooth. The arrangement of the cheek teeth of rabbits is designed to allow occlusion of one tooth with more than one opposite tooth. Each situation must be judged independently, however. Opposing teeth may be diseased and incapable of allowing normal natural occlusion.

Extraction can be performed intra or extra-orally, and differ in difficulty with location, degree of deformity and size of the patient. Choice of technique depends on tooth location and accessibility. For the intraoral approach, a Crossley cheek teeth luxator is used to sever the periodontal ligament on all four aspects of the tooth. The tooth is gently extracted, and in the case of diseased teeth, following the abnormal curvature of the root to prevent fracture. An extraoral approach is used any time intraoral extraction is impractical or too difficult. The most common indications are dental ankylosis, retained root tips, severe root deformity, periapical abscessation and osteomyelitis. Some teeth are extremely difficult to approach in the rabbit, for example, the most caudal maxillary cheek teeth, in particular C4-6. In these cases, extraoral access may require techniques such as maxillotomy.

Extraoral extraction is performed with the rabbit anesthetized, and the site surgically prepared and draped. Dissection of skin, subcutaneous and muscle layers reveal cortical bone, which is perforated with a luxator, needle, or dental bur on a water-cooled dental hand piece. The tooth is loosened with luxators or needles, and then extracted. Post operative radiographs help confirm complete removal. Multiple approaches are required for multiple teeth. The site may be closed routinely after curetting of bone fragments and flushing unless infection is present. In case of infection, the site should be marsupialized and treated as a dental abscess.

References:

Capello V, Gracis M. In: Lennox AM, ed. Rabbit and Rodent Dentistry Handbook. Ft. Worth: Zoological Education Network, 2005.

Harcourt-Brown F. Dental Disease. In: Textbook of Rabbit Medicine. Butterworth Heinemann; 2002, 165-205.

Reprinted in part (with permission) from the Proceedings of the Rabbit and Rodent Dentistry Wet Lab, Association of Exotic Mammal Veterinarians (www.aemv.org), 2005.

Special thank you to Vittorio Capello, DVM, Clinica Veterinaria S. Siro, Clinica Veterinaria Gran Sasso, Milano, Italy