14 Rodent, Lagomorph, and Ferret Dentistry
The name rodent comes from the Latin word rodere, meaning “to gnaw.” The order Rodentia is the largest order of the class Mammalia and contains a great diversity of species. Smaller animals within this order that are kept as pets are the rat, mouse, guinea pig, hamster, gerbil, chinchilla, gopher, squirrel, and prairie dog. Most adult pet rodents have between 12 and 22 teeth.
The order Lagomorpha includes the domestic rabbit, hare, and cottontail. Lagomorphs were once grouped with rodents; a separate order was later created due to distinct differences in the number of incisor teeth. The rabbit is the only member of this order commonly kept as a family pet. Adult rabbits have between 26 and 28 teeth.
Most rodents and all lagomorphs are herbivores, eating leaves, grass, and other lush green plants; however, some rodents are omnivores. Rodents and lagomorphs have a dental formula that features variation in tooth size and shape among the incisors, premolars, and molars, known as a heterodont dentition. Rodents and lagomorphs do not have canine teeth but instead have a long diastema (toothless area) between the incisors and cheek teeth. Ferrets are carnivores with heterodont dentition with incisors, canines, premolars, and molars. Lagomorphs differ from rodents in that they have two rows of maxillary incisor teeth. The rostral row of incisors consists of two larger functional teeth; a second row of two rudimentary incisor teeth (sometimes referred to as “peg” teeth) sits immediately behind the first row.
Two basic types of teeth are found in the rodent: the aradicular hypsodont (continuously growing tooth) and, in some species, brachyodont cheek teeth. A tooth that grows continuously throughout life is known as an aradicular hypsodont tooth, also referred to as elodont. The brachyodont tooth is the same type of tooth found in humans, cats, and dogs: a tooth with true distinction between crown and root structure and a root that does not grow once erupted. Guinea pigs and chinchillas have aradicular hypsodont teeth for both the incisors and cheek teeth, whereas mice and rats have aradicular hypsodont incisors and brachyodont premolars and molars.
Both the incisors and cheek teeth of lagomorphs are the aradicular hypsodont type. Although these teeth have a clinical crown and root, they do not have a well-defined root structure. The so-called reserve crown is submerged below the gingival margin, waiting to erupt as the tooth undergoes normal attrition and abrasion.
Ferrets have long, thin canine teeth that form a tight dental interlock when the mouth is closed. Incisor teeth are small with three incisor teeth in each quadrant. The maxillary fourth premolar and mandibular first molar teeth are sectorial, closing in a scissor-like fashion (Figure 14-1). Hourglass-shaped maxillary first molar teeth are a common dental finding of the family Mustelidae. The relatively small size of the teeth and oral cavity, coupled with the animal’s active nature, makes it difficult to thoroughly assess oral health in conscious ferrets, and therefore oral pathology can easily be overlooked in the conscious patient. Ferrets, being carnivores, have a highly specialized brachyodont dentition.
The upper and lower incisor teeth of rodents and lagomorphs, with the exception of the peg teeth of lagomorphs, come to a chisel-like point. These teeth have enamel on the front and lateral sides but typically just cementum and dentin on the palatal/lingual surface. The cementum and dentin wear much faster than enamel, resulting in a chisel edge that is longer labially. The enamel of incisors in most rodents takes on a yellow-orange color—an exception is seen in the guinea pig. Incisor teeth are very long and curved. The location of the apex of these teeth varies with the species of animal. In most species the apex of the maxillary incisor teeth lies in the area of the diastema. However, in rats and mice, the mandibular incisor apex is distal to the roots of the last cheek tooth, whereas rabbits and chinchillas usually have their incisor apices near the mesial surface of the first cheek tooth.
Because of their unique function, hypsodont cheek teeth often have an angled rather than flat occlusal table. The highest point of the chisel tip of the maxillary cheek teeth is on the buccal side, angling dorsally toward the soft tissue of the hard palate. The bevel of the mandibular cheek teeth goes in the opposite direction. The chisel point of the mandibular cheek teeth is on the lingual side, and the tooth’s occlusal table angles toward the soft tissue of the cheek. These wear patterns are due to the fact that the maxillary cheek teeth are spread wider apart from the midline than are the mandibular teeth. This is known as an anisognathic, or naturally unequal, jaw relationship, in which the upper dental arch is slightly wider than the lower. Healthy rabbits have only a very slight angulation to the occlusal plane, healthy guinea pigs have a relatively steep angulation, and healthy chinchillas have a level occlusal plane.
The periodontal ligament (PDL) of permanent aradicular hypsodont teeth may differ from that of permanent brachyodont teeth due to the presence of an intermediate plexus of the PDL between its tooth and bone attachments. This plexus may allow continuously growing teeth to move upward as they grow. This adaptation of hypsodont teeth is thought to explain how the PDL allows for continued eruption, though the presence of the intermediate plexus has been debated.
Anesthesia is available in two forms: injectable and inhalant. Injectable medications such as ketamine and acepromazine usually result in a longer recovery time than inhalants. Some injectables are reversible. Inhalants such as isoflurane and sevoflurane are preferred for maintenance of anesthesia for most procedures. Anesthesia masks can be used for induction of anesthesia prior to intubation or for the maintenance of anesthesia using an off-and-on mask approach during the actual dental procedure. Induction may also be administered in an anesthesia chamber, though small mammals may hold their breath due to the odor of the inhalant. Anesthesia and oxygen may also be delivered through an endotracheal tube. A laryngoscope with a small pediatric straight blade is used for endotracheal intubation to maintain the airway and administer anesthesia. Endotracheal tubes in sizes 1.5, 2.0, 2.5, or 3.0 Cole or a standard endotracheal tube may be used depending on the patient’s size. A wire stylet placed in the tubes makes passage much easier (Figure 14-2).
Intravenous (IV) catheters (12 to 14 gauge) can be used as an endotracheal tube for smaller rodents if the stylet is removed. These catheters can often be passed with the use of an otoscope as a laryngoscope.
Burs are used for removal of hard tissue. The most common types are diamond burs, carbide round burs, carbide tapered fissure burs, and white stone points. A cylindrical diamond bur works well for cheek teeth and incisor teeth. Carbide burs are more aggressive than diamond burs and white stones. Of the carbide burs, the 701L tapered fissure and #1 and #2 round ball burs are helpful. Of the white stone abrasive points, the flame-shaped point works well. Friction grip (FG) burs may be used on high-speed handpieces when working in the rostral oral cavity, but straight (HP) burs are necessary for use on the cheek teeth of most rabbits and rodents due to poor accessibility with the high-speed handpiece. A bur guard can be used to protect soft tissues from being traumatized by the bur.
Files/rasps (sometimes referred to as “floats”) are instruments used to level an abnormally uneven occlusive table of the teeth. The hand floats used in the small mouths of rodents and lagomorphs are typically either modified bone files or rasps. The rasps cut both on the push and pull stroke, whereas the files typically cut only on the pull stroke. The BF4 bone rasp (Cislak, Inc., Glenview, IL) and the Howard 12 bone file (Butler-Schein, Inc., Port Washington, NY) are two instruments that can be used for floating cheek teeth (Figure 14-6). The rabbit molar rasp is a float specifically developed for use in rabbits. Most have a large handle for better control and a working end that is similar to a bone rasp. The J-41r (Jorgensen Labs, Inc., Loveland, CO) is a smaller diamond-coated rasp that works well for final smoothing of jagged tooth surfaces after use of a bur or larger file to reduce an overgrown tooth.
Molar or cheek teeth cutters may be modified hard-tissue nippers, pin and wire cutting pliers, or side-cutting rongeurs. However, some have been developed specifically for use in rabbits (Figure 14-7).
Elevators are used to work circumferentially around a tooth in the PDL space to aid in the tooth’s removal. Blunted injection needles, sizes 18- to 25-gauge, can often be used for this function in rodents and lagomorphs. However, standard elevators can also be used, such as No. 1 and 2 winged elevators, 301 apical elevators, and Crossley Rabbit Luxators (Jorgensen Labs, Inc., Loveland, CO).
Extraction forceps are used to grasp teeth loosened by elevation or severe disease. Most incisors can be handled with small animal extraction forceps. For the cheek teeth a small 90-degree angled Halstead mosquito forceps or an angled root tip forceps can be useful. Some extraction forceps are specifically designed for use in rabbits.
Bone substitutes are placed into areas in which periodontal or endodontic disease has resulted in bone loss. These grafts may be combined with an antibiotic or other medicament when placed in a bony void.
Antibiotic-impregnated beads may be placed for treatment of refractory infections. They may be nonabsorbable such as polymethylmethacrylate beads. Calcium phosphate antibiotic impregnated beads are typically absorbed within weeks after placement.
Calcium hydroxide is commonly used in either a powder or paste form. It is placed over the exposed pulp of a tooth in an attempt to maintain its vitality. Its use has also been described as a packing material in infected draining abscesses, though this use can cause severe soft tissue necrosis due to its high pH.
Restoratives used in small mammal and exotic pets are ordinarily a temporary filling material such as Cavit (3M ESPE, St. Paul, MN), reinforced zinc oxide–eugenol (ZOE) cements, or glass ionomer restorative materials.