"Orthodontics" is the area of dentistry concerned with the supervision, guidance, and correction of dento-facial structures. It includes the diagnosis, prevention, interception, and treatment of all forms of malocclusions of the teeth. There are three main divisions of orthodontic treatment for pets:
1. Movement of secondary teeth--passive or active force applied to teeth to correct their position or eruption angle
2. Crown modification--shortening or modifying crowns to remove traumatic occlusal interference
3. Interceptive orthodontics--selective extraction of primary teeth to guide or affect the eruption position of secondary teeth for an improved occlusion
Most malocclusions are inherited problems that resulted from selective breeding for other traits, such as body size, shorter face, or longer nose. Small breed dogs have a much larger tooth-to-bone ratio (the size of their jaw bones has decreased more than the size of their teeth) resulting in tooth crowding and rotation. Genetic elongation or shortening of the face affects the maxilla more than the mandible, resulting in disproportion between the upper and lower jaws and malocclusion.
An important aspect of treatment planning is whether the malocclusion causes discomfort; many malocclusions may be completely functional and comfortable. When this is the case, intervention is unnecessary. However, a traumatic occlusion may also be uncomfortable, causing soft tissue trauma and interfering with normal mastication. In these instances, orthodontic movement or other corrective measures should be taken. The ethics of intervention should be considered. It is unethical to correct a malocclusion for the purpose of fraudulently representing a genetically flawed animal as being genetically normal. Most malocclusions are genetic. However, acquired malocclusions can also occur as a result of trauma to the developing tooth bud or alveolar region surrounding developing tooth buds.
A common classification system for normal and abnormal occlusions separates them into 4 main groups:
Class 0--Orthoclusion, or normal occlusion
Class 1--Neutroclusion with normal jaw lengths but teeth maloccluded. This includes anterior crossbite ("reverse scissors"), posterior crossbite (upper 4th premolars occlude lingual to lower 1st molars), and individual tooth malposition, malversion or rotation.
Class 2--Mandibular distoclusion (mandible occludes distal to normal position with upper jaw. Also called overbite, brachygnathism, parrot mouth)
Class 3--Mandibular mesioclusion (mandible occludes mesial to normal position with upper jaw. Also called underbite or prognathism)
Mandibular mesioclusion is common in brachycephalic breeds. This condition often does not require intervention and is considered normal for many breeds. As long as there is no traumatic occlusion, these animals do quite well. If the lower canine teeth contact the upper 2nd or 3rd incisor teeth, the affected incisors can be extracted to provide comfort and to preserve the lower canine teeth. In this case, selective extractions of a few healthy, but relatively unimportant, teeth can resolve the occlusal interference.
Mandibular distoclusion often presents a much more serious problem. In this condition, the maxilla is longer than the mandible, and the space between the upper and lower incisors is increased in width. These patients have their lower canine teeth positioned too far back to occlude in their normal position in the diastemma between the upper 3rd incisor and the upper canine teeth. Instead, the lower canines often ride up the palatal surface of the upper canine teeth, resulting in a tipping inwards towards the oral cavity (linguoversion). The lower canines develop an eruption profile that is very vertical rather than the natural condition of emerging in a mesio-labial angle. As they erupt further, they can create a periodontal defect on the palatal aspect of the upper canine teeth, or they may puncture directly into the palate. If this trauma extends deeply into the palatal tissues, it can result in an oro-nasal fistula. Another condition that has a similar clinical presentation is that of base-narrow lower canine teeth. In this condition, the lower jaw is too narrow to allow the outward eruption profile of the lower canines to carry them far enough labially to clear the upper gingiva. As a result, the teeth are in a correct position mesio-distally, but are positioned too far orally (palatally) and they create a pit on the palatal mucosa in the area of the diastemma. These conditions require treatment since they cause discomfort, inflammation, and interference with function. The best treatment is orthodontic movement of the lower canines into either a normal position, or into an abnormal but comfortable and functional position. One method of moving these teeth, for selected cases, is the use of passive force (as opposed to active force that uses elastics, screws or activated wires) with an inclined plane. This device is constructed on the patient using an acrylic plastic, composite resin, or cast metal appliance placed on the upper canines and adjacent teeth. They appliance is designed with an inclined plane that guides the lower canine teeth into the desired position through intermittent force each time the animal closes its mouth. This passive force works best on relatively young patients that spend a good portion of their time with their mouth closed. The patient should be young enough that the teeth can readily move and before serious injury has occurred, but old enough that the upper teeth have completely erupted. Otherwise the appliance could interfere with further eruption of the upper teeth on which the appliance is based. This usually makes the ideal age for treatment between 6 and 10 months, depending on the individual case. Older dogs that frequently pant would be poorer candidates since the dental attachment is less labile and no force is applied when the mouth is open. For cases that are expected to take more than 6 to 8 weeks for movement, a laboratory can make a cast metal appliance built on models of the patient's mouth. The appliance is cemented onto the upper canine teeth. One advantage of this method is that the appliance is completely supported by hard tissue and does not contact the soft tissues, thereby avoiding the gingivitis that can occur with the direct-placement appliances. These cast appliances are a little challenging to design and construct just right, and can be even more challenging to remove.
An alternate treatment for linguoverted or base-narrow lower canine teeth is to shorten the crowns to a level that relieves contact with the opposing soft tissues. This involves crown reduction with a sterile bur, partial coronal pulpectomy, and vital direct pulp cap of the pulp stump. After bleeding is controlled, mineral trioxide aggregate (or calcium hydroxide) is placed over the pulp stump prior to being covered with a bonded hermetically sealed restoration. Sterile flush, burs, and technique are used to prevent bacterial contamination of the exposed pulp. Whenever the pulp is exposed and treated in this manner, follow up radiographs should be taken in one year, and then every three-to-five years, to monitor the root health in case definitive root canal treatment ever becomes necessary.
A relatively common single-tooth malocclusion is a "lance-projection" canine tooth. This condition has some breed predilection for Shelties, Collies, and Dachshunds. In this condition an upper canine tooth projects mesially, often impacting against the upper 3rd incisor and lower canine teeth, pushing them both labially. Extraction of the abnormally positioned canine tooth is the best treatment. The tooth can also be repositioned orthodontically, but orthodontic repositioning requires multiple anchor teeth, blocking the bite open for a period of time to allow the tooth to move past the lower canine (that usually locks it in place and begins to tip labially itself), multiple anesthetics, and many months. Other occlusal abnormalities can also be treated with these "active force" techniques, using elastic bands or orthodontic chain and direct bond buttons on the teeth, and sometimes labial or lingual archwires, all used to slowly move the teeth into position. It is critically important that the anchor teeth have a larger root surface area than the target teeth, or the anchor teeth will move out of position rather than the target tooth moving into position. Labial or oral arch-wires, palatal expansion devices, and activated orthodontic wires can all be used to move teeth.
Interceptive orthodontic procedures, i.e., strategic extractions of primary ("deciduous" or "baby") teeth, can help prevent the need for moving secondary ("permanent" or "adult") teeth later. Whenever a secondary tooth has erupted and the deciduous tooth remains firm in its socket, the deciduous tooth should be extracted. This will prevent the possibility that the persistent tooth root is diverting the eruption of the succedaneous secondary tooth. Another condition in which strategic extractions of primary teeth can be attempted is when a puppy has an upper-to-lower jaw mismatch. The primary canine teeth or incisors may cause a dental "interlock". This occurs when the teeth engage those of the opposing dentition to hold the jaws in an abnormal position and preventing them from correcting their relative positions if that were otherwise genetically possible. During maturation, the relation of the upper-to-lower jaw lengths can change back and forth until a dog is over 8 months of age if there are no mechanical blocks.
A major goal of veterinary orthodontics is to relieve malocclusive trauma and discomfort for our patients; all pets have the right to have a pain free and functional dentition. However, they do not have a "right" to a perfect occlusion or "show bite". Since most malocclusions are genetic, we have an ethical responsibility to avoid performing procedures that could mask a genetic defect on intact purebred animals.
1. Proffit WR, Fields HW. Contemporary Orthodontics. St. Louis: Mosby-Year Book. 1993
2. Wiggs RB, Lobprise HB. Domestic feline oral and dental disease. In: Veterinary Dentistry: Principles and Practice. Philadelphia: Lippincott-Raven; 1997