Dental Corner: Canine orthodontics: Providing healthy occlusions
The idea of orthodontic correction for dogs frequently elicits snickers and causes uninformed eyes to roll.
The idea of orthodontic correction for dogs frequently elicits snickers and causes uninformed eyes to roll. People often assume that orthodontic procedures are purely cosmetic and are reserved for show dogs. But nothing could be further from the truth. In fact, the American Kennel Club does not permit dogs that have had their heritable oral anatomy altered to be shown in its conformation classes.
In people, orthodontic correction is used to treat abnormal occlusions and to improve aesthetics. In dogs, the goal of orthodontic treatment is to provide a healthy and functional occlusion—something to which every dog is entitled.
NORMAL OCCLUSION AND BREED STANDARDS
The normal occlusion in dogs is the scissor bite (Figures 1A & 1B). This occlusion is found in wild Canis species and in many domestic breeds, such as German shepherds. Any deviation from the scissor bite is called a malocclusion. However, in some domestic breeds (e.g. English bulldogs), people have selectively bred for an occlusion other than a scissor bite. In fact, as of January 2005, 50% (79 breeds) of all 159 American Kennel Club breeds were allowed occlusions other than a scissor bite.1 Thus, many of our canine patients have normal-for-the-breed-standard occlusions that by health standards would be considered malocclusions. But while 50% of purebred dogs have malocclusions, not all of them require orthodontic therapy.
Figure 1A. A normal occlusion, or scissor bite. The maxillary incisors overlap the mandibular incisors, and the mandibular canine teeth are positioned midway between the maxillary third incisor and canine teeth on the same side when the mouth is closed. 1B. The mandibular premolar cusps point to the midpoint of the interproximal space of the maxillary premolar of their counterparts (i.e. the central cusp of the mandibular second premolar is pointing up at the interdental space between the maxillary first and second premolars).
There are three skull types in dogs: dolichocephalic (long, narrow head; e.g. collies), mesocephalic (medium-proportioned head; e.g. German shepherds), and brachycephalic (short, wide head; e.g. pugs). Each of these types is predisposed to certain orthodontic problems. With so much variation in what is considered normal occlusion for a breed standard, it is important to focus on what is healthy for an individual patient.
One classification of malocclusion is based on the length of the maxilla in relation to the length of the mandible. A class I malocclusion is one in which one or more teeth are incorrectly aligned but the mandible-to-maxilla relationship is normal. A class II malocclusion—also termed an overbite, overshot, or, more accurately, overjet—occurs if the mandible is relatively shorter than the maxilla. A class III malocclusion—also termed an underbite, undershot, or, more accurately, underjet—occurs if the maxilla is shorter than the mandible.
Another way to classify malocclusions is as traumatic or nontraumatic. In other words, if the malocclusion causes problems for the dog, it is classified as traumatic. The problems associated with malocclusions include trauma to oral soft tissues or teeth, inability to close the mouth, and difficulty eating. Some of these problems may become serious, such as oronasal fistula formation. A severely undershot (class III) malocclusion may or may not be traumatic. A condition such as linguoversion (base narrow) of the mandibular canine teeth, in which the teeth are displaced to the lingual side of the proper occlusal position and can puncture the hard palate, is traumatic.
ETHICAL CONCERNS ABOUT ORTHODONTIC CORRECTION
Correcting malocclusions can entail ethical considerations related to breeding and participation in dog shows. Almost all malocclusions have a genetic basis, and the owners of dogs with malocclusions must be counseled regarding the implications of breeding these animals. And as mentioned above, the American Kennel Club prohibits dogs that have received orthodontic treatment from competing in its dog shows.2
BASIC ORTHODONTIC PRINCIPLES AND ERRORS
Orthodontic movement is achieved by creating a force on the targeted tooth or teeth. The direction and magnitude of the force is controlled and can be either sustained or intermittent. Over time, this force helps move the teeth through the jawbone and into the desired location. This movement, which is influenced by the compressive or tensile forces transferred through the periodontal ligament, is accomplished by the actions of osteoclasts and osteoblasts.
The course of orthodontic treatments in dogs is much shorter than in people, generally one to three months. In general, most dogs tolerate properly installed and managed orthodontic appliances well. Only veterinarians who are trained in the principles and practice of orthodontics should perform orthodontic treatments.
Many problems can arise during the course of an orthodontic treatment. These problems often result from technical errors, such as too much force or improper direction of force applied to the tooth or inadequate anchorage of teeth with subsequent movement of teeth that were not intended to be moved. Other problems can arise as a result of management errors, such as failure to keep the orthodontic appliance in the dog's mouth or failure to maintain good oral hygiene.
COMMON PROBLEMS REQUIRING ORTHODONTIC TREATMENTS
Following is a discussion of common orthodontic problems in dogs and their typical treatment. Other types of malocclusion may also warrant treatment, and other forms of management are available.
Persistent deciduous teeth
As a rule of thumb, an erupted permanent tooth and its deciduous predecessor should never exist together. When persistent (retained) deciduous teeth is diagnosed, the persistent deciduous teeth should be extracted immediately. Persistent deciduous teeth can prevent permanent teeth from occupying a correct position in the mouth (Figures 2A & 2B), which can lead to potentially traumatic malocclusions. Food and bacterial deposits can also accumulate between the retained deciduous tooth and its permanent counterpart and cause periodontitis. When extracting persistent deciduous teeth, be careful to avoid root tip fracture and other complications.
Figure 2A. A persistent deciduous mandibular canine tooth (arrow) is occupying the space where the permanent mandibular canine tooth should be. A persistent deciduous maxillary canine tooth is occupying the space where the permanent maxillary canine tooth should be, resulting in crowding and leaving little space for the mandibular canine tooth. 2B. Lingually displaced mandibular canine teeth (arrows) will often migrate into their normal positions if persistent deciduous canine teeth are extracted in a timely fashion.
Extracting deciduous teeth is also warranted in cases of dental interlock. During growth and development in puppies, the mandible and maxilla grow independently. Sometimes during growth spurts, one jaw grows faster than the other, and the deciduous teeth can become interlocked, interfering with the normal growth of the shorter jaw. Extracting the deciduous teeth will not promote jaw growth in a dog that is genetically destined to have malocclusion. But in cases in which the interlock has resulted from a temporary discrepancy in jaw growth, relieving dental interlock by extracting the interfering deciduous teeth may allow the shorter jaw to lengthen to its genetic potential. Careful case selection, client education, and meticulous extraction technique to avoid iatrogenic damage to the developing permanent tooth are the keys to successfully removing persistent deciduous teeth.
In many breeds, especially in dogs < 22 lb (10 kg), the teeth are large in relation to the jawbones they occupy. In these instances, crowding can occur. Crowded teeth predispose dogs to periodontal disease and may even interfere with jaw closure (Figure 3). In cases of crowding, treatment may include orthodontic movement of crowded teeth into healthier positions. But if there is not enough space in the mouth to accomplish such orthodontic correction, selected teeth should be extracted.
Figure 3. A chihuahua with crowded teeth. A mandibular canine tooth is directly impacting the maxillary third incisor (arrow) and preventing the dog from closing its mouth. Extracting the maxillary third incisor will allow this dog to close its mouth and will give it a healthy, nontraumatic occlusion.
Linguoversion of the mandibular canine teeth
Linguoversion of the mandibular canine teeth can be observed in deciduous teeth as well as in permanent teeth. The trauma to the palate caused by the abnormally positioned lower canine teeth not only is painful (these dogs are often head shy) but also can lead to oronasal fistula formation.
Deciduous mandibular canine teeth that exhibit linguoversion should be extracted. In cases of permanent mandibular canine tooth linguoversion, orthodontic tipping of the canine teeth into a healthy position is usually successful. In certain cases, especially if this condition is combined with a class II malocclusion, there may be nowhere to move these teeth to because the maxillary canine teeth are positioned just lateral to the malpositioned mandibular canine teeth. In these cases, crown reduction with vital pulp therapy is indicated. If movement into a healthier position is possible, several options are available.
Rubber ball therapy
One study showed that having young dogs with lingually displaced mandibular canine teeth hold an appropriately sized rubber device (e.g. a lacrosse ball or a spherical hard rubber chew toy) in their mouths for a minimum of 15 minutes three times a day can correct many of these malocclusions.3 The ball acts like an inclined plane, causing tipping forces to be placed on the affected teeth every time the dog bites down on the ball. The elegance of this physical therapy option is its simplicity and lack of need for anesthesia. However, it is imperative for the clinician to choose a rubber device that is the correct size and shape for each individual patient and to carefully instruct the pet owner in its use.3
In mild cases of linguoversion of the mandibular canine teeth, a wedge of gingival tissue can be removed from the maxilla between the third incisor and the canine tooth.4 This procedure is relatively simple and requires only a single anesthetic episode.
Inclined planes are fixed acrylic or metal orthodontic devices that act as a ramp to direct teeth into a healthier position (Figures 4A-4D). Inclined planes can be fabricated and installed directly while the patient is anesthetized, or they can be fabricated in a dental laboratory from a laboratory stone model created from impressions of the dog's occlusion and cemented into place at a subsequent appointment. Once installed, the inclined plane places tipping forces on the displaced tooth in a direction determined by the slope and direction of the fabricated appliance. The correction usually occurs within two to four weeks, at which time the appliance can be removed.
Figure 4A-4D. Normal occlusion on the right side (4A) and malocclusion on the left side (4B) in a young standard poodle. In 4C, the linguoversion of the left mandibular canine tooth is causing trauma to the hard palate (arrow). An inclined plane appliance was directly fabricated (4D) to correct this problem. This dog had a normal and healthy occlusion in just two weeks.
Another method used to treat linguoversion of the mandibular canines is so-called camouflage orthodontics, also called tooth extensions. Camouflage orthodontics involves building up the height and changing the shape of a tooth with dental plastics, so the newly created crown tip occludes normally. In the case of linguoversion of the mandibular canine teeth, if the crown tip is diverted buccally into a normal position, the forces acting on the tooth when the dog closes its mouth will tip it into a more proper occlusion. When the natural tooth is in proper position, the plastic built-up crown tip can be removed.
Figure 5. Treating an anterior crossbite in a Scottish terrier by using an upper expansion device and an elastic power chain on the mandibular incisor teeth. The upper expansion device is anchored to the maxillary canine teeth, and the owners are instructed to use a key to crank the expansion screw at a predetermined interval (once a day to once every four days). The mandibular incisors shown in this photograph have already been tipped caudally by the elastic power chain, which is anchored to buttons cemented on the mandibular canines and fixed to the incisors by orthodontic buttons. The elastic chain also functions as a retainer to prevent inadvertent rostral tipping of the incisor teeth.
An anterior crossbite is a malocclusion of incisor teeth in which one or more of these teeth are not aligned in the healthy scissor orientation. The incisor teeth may be evenly aligned, where they impact each other's incisal edge, or the mandibular incisors may be rostral to the maxillary incisors in a reverse scissor orientation. An anterior crossbite can result in abnormal tooth-on-tooth contact or, possibly, in abnormal soft tissue contact. In these cases, treatment is warranted to prevent periodontal or endodontic problems.
The basic tenets of therapy are to move the maxillary incisors rostrally and to move the mandibular incisors caudally. Camouflage orthodontics works well in mild cases; more severe cases will require the use of more sophisticated orthodontic appliances, such as an upper expansion device combined with lower orthodontic buttons and elastic (Figure 5).
Rostroversion of the maxillary canine tooth
Rostroversion of the maxillary canine tooth, also called the lance tooth, is an orthodontic problem encountered most commonly in Shetland sheepdogs. A common method of correction involves using a cheek elastic fixed to orthodontic buttons that have been cemented in specific places on selected teeth. Success depends on the ability to supply sufficient anchorage in the caudal part of the dentition to accomplish movement of the large-rooted canine tooth. To achieve sufficient anchorage, the carnassial tooth (maxillary fourth premolar) and the maxillary first molar must be stabilized together (Figures 6A & 6B).
Figure 6A. Orthodontic correction of rostroversion of the maxillary canine tooth in a Shetland sheepdog. Note the malposition of the lance tooth (arrow). Orthodontic buttons have been cemented to the maxillary canine tooth, the maxillary fourth premolar, and the maxillary first molar (under the composite). An elastic power chain is attached to these buttons and is being used to create a caudal tipping force on the malpositioned canine tooth. Composite bonding has been used to stabilize the maxillary fourth premolar and the maxillary first molar together to provide sufficient anchorage. 6B. Three weeks into therapy, the canine tooth (same tooth as in 6A) has moved into an almost normal position.
The critical steps in dealing with orthodontic problems in dogs include properly diagnosing the problem, determining the most appropriate method to achieve the goal of a healthy and comfortable occlusion, counseling owners about breeding and future showing potential, and selecting the proper orthodontic treatment technique. Clients need to be taught how to provide good oral hygiene and must commit to frequent follow-up visits during the treatment. Some therapies such as deciduous tooth extraction and rubber ball therapy can easily be incorporated into general veterinary practice. The more advanced techniques involving appliance fabrication and application are best referred to a veterinary dental specialist. A list of veterinary dental specialists is available at www.avdc.org
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The information and photographs for "Dental Corner" were provided by Daniel T. Carmichael, DVM, DAVDC, Veterinary Medical Center, 75 Sunrise Highway, West Islip, NY 11795.
Dr. Daniel T. Carmichael