Introduction

The dental formula for the pet rabbit (Oryctolagus cuniculus) is I ²/₁ C ⁰/₀ PM ³/₂ M ³/₃. Small mammal dental disease remains one of the most frequent presentations in exotic pet practice, and unfortunately it can be one of the most frustrating conditions for veterinarians to treat. Full details can be found in texts dedicated to the subject.^1,2,4,8

Classification of Malocclusion

There are potentially six classes of malocclusion based upon etiology.

1.  Dietary-induced. Insufficient dental wear due to lack of fibrous grass hays in the diet have been strongly implicated as predisposing factors for dental overgrowth and malocclusion. Calcium and vitamin D₃ issues have also been suggested in the UK but appear less common in the US.

2.  Congenital malocclusion is also a very commonly cited cause of dental anomalies seen within the brachycephalic breeds, particularly the dwarf and lop-eared rabbits. The malocclusion becomes apparent within 6-12 months of age and is permanent.

3.  Tooth root infection (and osteomyelitis) is a common cause of malocclusion which can occur at any age. In rabbits over a year of age malocclusion due to infection must be considered the primary differential, although it can sometimes be difficult to determine whether infection caused the malocclusion, or vice versa. Physical evidence such as the presence of purulent material emanating from the gingival margin, and tooth discoloration should alert the clinician to the possibility of incisor root infection. Tooth root infection due to Pasteurella, Bacteroides, Fusobacterium and Staphylococcus are most common, and there may be involvement of both incisors and (pre)molars. Affected rabbits obviously experience tooth pain and become anorectic and debilitated. Where only the incisors are affected, the treatment of choice is complete incisor removal with appropriate antibiotic therapy for 4-6 weeks. In some cases radical debridement may be necessary where osteomyelitis is present. The prognosis must be considered poor to guarded depending upon the extensive nature of the infection.

4.  Dental trauma as a cause of malocclusion is primarily seen as a result of inappropriate clipping of the incisors. During the clipping procedure the incisor(s) may fracture longitudinally down into the root causing the tooth to re-grow out of alignment. Dental trauma is also seen in solitary rabbits kept in hutches with large gauge wire screens. These bored animals develop a habit of pulling on the wire screens of their hutch with their teeth which over several months can result in a permanent incisor malocclusion. Husbandry improvements along with incisor removal represent the best long term solution for these animals.

5.  Head trauma is an uncommon cause of dental malocclusion but which is seen in cases involving interspecific aggression and falling injuries. Radiographic examination of the skull is often necessary to determine the extent of the damage to the teeth, mandible and maxilla. In general, fractures of the mandible or maxilla tend to cause most of the post-traumatic malocclusions as individually damaged teeth can be removed. The main difficulty when dealing with traumatized rabbits is medical stabilization prior to anesthesia and surgery. Warm, stress free surroundings are essential and nutritional/fluid support should be considered. Analgesia is of prime importance and the use of an opioid or non-steroidal anti-inflammatory drug should not be overlooked. Steroids are best avoided but may be employed as part of the therapeutic regime for acute shock.

6.  Primary pre-molar and molar malocclusion is not an uncommon condition and may be either congenital or acquired due to trauma, (pre)molar root infection and inflammation. In cases of incisor malocclusion (pre)molar involvement must be ruled out because although incisor removal may be beneficial it will certainly not be curative. In cases of (pre)molar malocclusion repeat dental surgery will be required. It is therefore essential that all the teeth are thoroughly examined both visually and radiographically before dentistry is undertaken.

Diagnosis

History & Physical Exam

An accurate diagnosis is essential in order to provide the most appropriate treatment and accurate prognosis. A detailed history may indicate reduced food intake, ptyalism, selection of concentrates and cereals over fibrous hay, dropping food from the mouth, and weight loss. Every rabbit owner should be encouraged to keep an accurate log of their rabbit's weight as 5% weight loss will often occur before the rabbit shows obvious clinical signs. The physical examination should be thorough. Palpation of the skull, particularly the maxilla and mandible may indicate swellings or asymmetries associated with tooth elongation, periapical penetration and swelling of the jaw. Apical penetration often, but not always, leads to facial abscessation. General gastro-intestinal complaints including intermittent anorexia, abnormal palpation and auscultation of the intestinal tract, diarrhea, constipation, poor fecal production etc, may all have their origins in the mouth. Poorly masticated and prepared food can cause problems for the intestinal processing and digestion of food.

For examination of the incisors, the rabbit can be restrained in a towel and the lips retracted to permit examination. The conscious examination of the premolars and molars is not as simple. Many feel that such an examination using an otoscope or similar device is helpful. However, such examination is often uncomfortable and stressful for a rabbit, and is of little benefit unless part of an annual wellness exam. When dealing with any rabbit showing any of the historical or physical signs of dental disease, conscious examination may or may not indicate a problem. It is impossible to examine all dental surfaces in the conscious rabbit. In any case a complete examination under general anesthesia is justified and so the benefit of conscious examination is dubious.

Dacryocystitis (due to elongation of upper premolar 2 impinging on the nasolacrimal duct) and facial abscesses (due to infected tooth root) are indicators of dental disease and even if the rabbit is showing no other signs, a dental investigation constitutes an essential part of the case work-up.

Radiography

There can be little doubt that an accurate radiographic interpretation is probably the key to successful diagnosis, maximizing treatment success, and providing an accurate prognosis. Survey skull radiographs include left and right laterals, left and right 30-45^o oblique views, and dorsoventral or ventrodorsal views. Slow speed, fine detail film-cassette combinations are required (e.g. mammography).

Good quality radiographs can be used to stage the degree of dental disease^1,3:

Grade 1 - normal rabbit. The mandible has a smooth ventral border, tooth roots are of optimal length, normal skull and tooth structure and radiographic density, interdigitation (zigzag) of the occlusal surfaces of the premolars and molars, parallel smooth linear pattern of the premolar and molar teeth.

Normal incisors malocclusion.

Grade 2 - subclinical disease. Thinning of the bone along the ventral border of the mandible, root elongation and early root divergence from the usual parallel array, hard bony swellings at the site of periosteal reaction become palpable along the mandible, incisor malocclusion variable, infection unlikely. Prognosis good.

Grade 3 - weight loss, mild clinical signs. Further thinning of the bone along the ventral border of the mandible, continued root elongation causing increase in diastema diameter, further deviation of the teeth from the normal parallel array, hard bony swellings at the site of periosteal reaction easily palpable along the mandible, further deterioration of the normal occlusal zigzag pattern, variable incisor malocclusion, infection more likely. Prognosis guarded.

Grade 4 - weight loss, restricted food intake, obvious clinical signs. Further thinning and perforation of the bone along the ventral border of the mandible, pronounced deviation of the teeth from the normal parallel array, obvious bony swellings at the site of periosteal penetration, complete lack of occlusal zigzag pattern, cessation of root growth, broken crowns, dysplastic appearance of the teeth and loss of linear pattern, blurred dental outlines due to loss of enamel, variable incisor malocclusion, infection and abscessation likely. Prognosis poor.

Grade 5 - Pronounced weight loss, poor ingestion, ptyalism, severe clinical signs. Gross changes to the mandible and/or maxilla associated with osteomyelitis and facial abscessation, pronounced deviation of the teeth from the normal parallel array, complete lack of occlusal zigzag pattern, cessation of root growth, broken crowns and lost teeth, remaining teeth are obviously dysplastic with loss of linear structure and blurred outlines due to enamel loss, variable incisor malocclusion. Prognosis poor to grave.

Computed Tomography

Radiographs are not always able to identify the tooth/teeth responsible for an abscess, nor can they definitively exclude dental disease. With the use of CT, excellent detail is provided for the fine bone structure of the skull, teeth, and adjacent soft tissue, allowing for the detection of small changes. This provides an opportunity for early intervention and thus more effective treatment. CT is recommended prior to any extensive surgical undertaking, including tooth extraction.⁶ CT scans of normal animals can be used as comparisons to clinically ill individuals.

Endoscopy

While radiography is key for evaluation of any dental issue in rabbits, nothing can compare to endoscopy for the examination of the supra-gingival crowns. The use of a 2.7 mm telescope with a 30^o oblique view provides unparalleled visualization of both the mandibular and maxillary arcades.⁵ Recording of still images and video are also a practical means of illustrating the problems encountered to the client, and greatly assist with ensuring client compliance. More extensive reviews of endoscopy equipment are available in the literature.

By far the most common dental endoscopy procedures involve the insertion of the telescope into the oral cavity. The rigid endoscope is ideally suited to examine the oral cavity in these small mammals, and offers considerable advantages including up to 20x magnification using focal illumination, and the ability to display and record the images. The limited access to the oral cavity may preclude the use of endotracheal tubes; however, anesthetic gas and oxygen can be supplied via nasal intubation, or by placing a small face mask over the nostrils. It is important to give consideration to the use of active scavenging from the area to avoid anesthetic gas exposure to staff. Alternatively, injectable anesthetic agents may be used, and although this does not negate the need for supplying oxygen via nasal line or mask, it does reduce staff exposure to inhalant agents.

The anesthetized patient is positioned in sternal on a heated surface with the head and neck extended, and the mouth held open using a mouth gag or dental restraint device (Rodent table retractor restrainer, Sontec Instruments, Inc., Englewood, CO). A cheek spreader is placed inside the mouth to restrain the buccal mucosa laterally (Sontec Instruments). The telescope, within a sheath, may then be inserted into the oral cavity to perform a detailed examination. The 30^o angle afforded by the telescope has advantages over 0^o endoscopes. With the telescope in a normal position (light guide connector facing down), the 30^o angle favorably permits detailed examination of the upper arcades, and with the telescope (but not the camera) rotated 180^o around its longitudinal axis (light guide connector facing up) the lower arcades are easily visualized.

The lingual, buccal, and occlusal aspects of every tooth should be evaluated using appropriately sized and curved dental probes. Tooth laxity, exudates, and gingival changes should be noted. In the vast majority of the small herbivores, the most commonly encountered malocclusions involve overgrowth to the lingual aspect of the lower arcades and buccal aspect of the upper arcades. Once identified the malocclusion should be trimmed with either rongeurs, or preferably, a motorized dental hand piece that is less likely to result in dental fracture. During dental trimming it is vital that the telescope is either protected using a guard, or temporarily removed and then be re-inserted to evaluate the teeth following reduction of the malocclusion. The telescope can also be used periodically to evaluate the intraoperative progress of premolar or molar extractions, or to examine the cavity left following extraction. Indeed, the telescope has also been used to target flushing and antimicrobials into dental cavities via the oral cavity. These techniques have been used to successfully treat retrobulbar abscesses in rabbits via the oral cavity, thereby avoiding enucleation.⁷ Although rare, soft tissue masses may be biopsied using the 5 Fr biopsy forceps, while foreign bodies may be removed using retrieval forceps.

There are occasions when dental disease requires an extra-oral approach, either alone or in conjunction with intra-oral surgery, and the telescope can serve as a useful surgical aid. The extension of hypsodont roots into the nasal cavity may warrant rhinoscopy via the nostrils or surgical rhinotomy. Dental abscesses affecting the maxilla may enter the paranasal sinuses and the telescope can provide evaluation via a small (4-5 mm) osteotomy. Even when extensive surgical osteotomy or rhinotomy are performed, surgical access is often very limited in small herbivores, but the telescope enables detailed evaluation including those areas cranial and caudal to the surgical site. It is important to use copious warm sterile saline to flush and clean the area prior to introducing the telescope, otherwise debris and mucus can obscure the view. The recent advent of 2 and 3 mm rigid instruments also permits biopsy and debridement within the nasal or paranasal sinuses via limited surgical access.

Treatment

The decision to proceed with treatment should follow accurate diagnosis and discussion with the owner regarding prognosis. Most rabbits present with a history of anorexia and weight loss. When the patient's health status is questionable it is often wise to temporarily improve the subject's condition by incisor burring and permitting a temporary return to normal feeding before incisor removal is attempted. Alternatively, anorexic rabbits can be fed Herbivore Critical Care Formula (Oxbow Pet Products) for a few days before surgery. A preoperative complete blood count, biochemistry panel, and urinalysis are also recommended prior to prolonged anesthesia and surgery.

Incisor Dentistry

Historically incisor malocclusion has been treated by regular clipping of the incisors. Clipping incisors is at the very least uncomfortable and probably a painful, stressful procedure for the rabbit. Moreover, the effects of clipping are invariably short-lived with the clipping procedure repeated at 3-6 weekly intervals, depending upon the degree of malocclusion and rate of tooth growth. In addition, there is a real danger of the incisors splitting which often worsens the malocclusion and may lead to subsequent tooth root infection. A more effective approach is to use a dental burr or cutting disc to cut the teeth and shape the bite, and this can often be accomplished in the conscious patient, although sedation/anesthesia is often preferred. The rabbit is restrained in dorsal recumbency and a protective guard (e.g. plastic sheath of a hypodermic needle) is held behind the incisors and in front of the lips and tongue. The incisors are then cut back using a circular saw burr and shaped to permit a normal (or as normal as possible) bite. This method typically requires less frequent visits to the veterinary surgery, but again the effects are temporary with dental surgery required every 4-8 weeks.

The only permanent cure for chronic incisor malocclusion is surgical removal of all 6 incisors under general anesthesia. Rabbits tolerate this procedure very well and in all cases treated by the author there has been a significant improvement in both appetite and demeanor within a few days of surgery. It is important for both clinician and owner to appreciate pre-existing premolar/molar disease, as incisor removal will not correct these problems and abnormal cheek teeth are likely to need repeated attention.

Appropriate anesthesia is essential and in small rabbits naso-intubation may provide more room than an endotracheal tube. A small dental elevator (or bent 18-20 gauge hypodermic needle) is used to break down the periodontal ligaments around the lower incisors. The elevator is inserted down the medial surface of the incisor and gently rotated. This process of elevator advancement and rotation is continued bearing in mind the orientation and position of the tooth root until the tough medial ligament is broken down along the entire length of the tooth. Once the medial ligament is completely broken the tooth will become obviously loose. The weaker lateral ligament is then broken down in a similar manner and the soft tissues are separated from the tooth as deeply as possible. The incisor is then grasped using tooth extractors (or hemostats) and pulled in the direction of the natural curve of the tooth. The tooth root should be examined to ensure that the entire root has been removed (a completely removed incisor has a soft root) and that no infection is present. If there is any suspicion of tooth root infection a root swab should be taken for culture and sensitivity. The curvature of the main upper incisors is much more pronounced than that of the lowers incisors, so it is important to extract the upper incisors with a more pronounced curve. The small secondary upper incisors (peg teeth) must also be removed. If an incisor breaks, it may be necessary to wait 6-8 weeks for regrowth before a second attempt can be made. There is usually little bleeding during the procedure. The tooth roots can be flushed with an oral antiseptic solution and if possible the gingival margins are sutured closed. Antibiotics are not usually required unless clinical concern warrants their use. If tooth root infection is suspected during surgery antibiotic therapy (subsequently based upon culture and sensitivity) should be continued for 4-6 weeks.

The rabbit should be placed in a warm environment until fully recovered, typically within 1-3 hours. Continued fluid therapy is advisable for old or debilitated animals. The use of post-operative analgesics, such as meloxicam should be considered routine.

Premolar and Molar Dentistry

A metal probe should be used to protect the soft tissues when using a high speed dental burr to cut, burr and reshape the premolars and molars. Loose teeth should be gently removed using forceps. Diseased, but firmly attached maxillary teeth will often require an extra-oral facial approach to aid removal, while mandibular teeth can be approached from the ventral mandible.

Cheek tooth infection and mandibular abscessation is not uncommon. To date the author's preferred method of treatment is radical surgical excision (including tooth removal). Initially, with the rabbit orally intubated, an extra-oral approach is used to dissect the facial abscess free and identify and remove as much of the diseased tooth (or teeth) and bone as possible. The placement of methyl-methacrylate antibiotic impregnated beads into the debrided jaw has also been successful. Antibiotic beads commonly used include ampicillin, metronidazole, ceftazidime and amikacin. Marsupialization of dental abscesses is also generally effective and promotes further post-operative flushing and wound care. Oral surgery may also be required to complete crown removal and undertake general dentistry. Systemic antibiotics should also be continued for 6 weeks, and injectable penicillin is often effective given the involvement of anaerobic bacteria. The removal of one tooth does not require the removal of the opposite tooth, and rabbits can perform well following the removal of one or two teeth. As more cheek teeth are removed, so the chances of opposite tooth overgrowth and post-operative complications increase.

Post-Operative Care

Rabbits will often start eating within a couple of hours of surgery. There are no special dietary considerations and all rabbits can be offered their usual pellets and ad lib hay. However, shredded, chopped or grated soft foods are often preferred for the first few days following surgery and should be offered in abundance. It is normal for the rabbit to quickly regain appetite. Owners typically report a major improvement in their rabbit's appetite, general activity and demeanor following successful surgery.

Incomplete tooth removal will usually result in tooth re-growth within 6-10 weeks of surgery, however the author has had no complications to date. Lateral and dorsoventral radiographs 6-8 weeks after surgery can be taken to confirm complete incisor removal and a lack of tooth re-growth. There will often be increased radiodensity in the region of the extracted incisors on radiographs taken 6-8 weeks after surgery. This is a surgically induced periosteal reaction within the tooth socket which may be misinterpreted as tooth re-growth. Repeat radiographs taken 2-3 months after surgery will indicate a resolution of this reaction, and can again be used to confirm the absence of tooth re-growth.