By definition, a malocclusion is an abnormality in position of the teeth. Malocclusions are common in the dog, but they also occur in cats.
Malocclusions can result from jaw length and/or width discrepancy (skeletal malocclusions), from tooth malpositioning (dental malocclusions) or a combination of both. The development of the occlusion is determined by both genetic and environmental factors. It is known that jaw length; tooth bud position and tooth size are inherited (1). It is also known that the development of the maxilla, the mandible and the teeth are independently regulated genetically (1). Disharmony is the regulation of these three structures result in malocclusion. Alteration of jaw growth by hormonal disorders, trauma or functional modifications may result in a skeletal malocclusion (2). Although tooth bud position is inherited, various events during development and growth may alter the definitive tooth position. It has been claimed that at least fifty percent of all malocclusions are acquired and have no genetic cause (3,4). There is no data to support such a claim in dogs or cats. Little research has been performed and there are no large epidemiological studies available. To date, there is no precise list of inherited or acquired malocclusions. Specific genetic mechanisms regulating malocclusions are unknown. However, a polygenic mechanism is likely and explains why not all siblings in successive generations are affected by a malocclusion to the same degree, if affected at all. With a polygenic mechanism, the severity of clinical signs is linked to the number of defective genes.
The most reasonable approach suggested (5,6) to evaluate whether a malocclusion is hereditary or acquired is as follows: Skeletal malocclusions are considered inherited unless a developmental cause can be reliably identified. Pure dental malocclusions, unless known to have a breed or family predisposition, should be given the benefit of a doubt and not be considered inherited.
Prevention is always better than treatment. Early recognition of a problem is essential to avoid discomfort and pain to the animal and prevent the development of severe pathology. Malocclusions affecting the primary dentition may require interceptive orthodontics. Malocclusions affecting the permanent dentition may need no treatment at all if they are not causing the animal discomfort or causing any oral pathology. Malocclusions causing discomfort and pathology need treating. The treatment options include orthodontics; but in many instances tooth shortening (endodontic therapy must be performed if the pulp is involved in the shortening) or extraction are preferable. The aim of any treatment is primarily to make the animal comfortable; aesthetics are a secondary consideration.
In man, medical (predisposition to periodontal diseases), functional (alteration of mastication or speech) and psychological (alteration of aesthetics) problems relating to a malocclusion are the primary reason for orthodontic treatment. In human orthodontics the aetiology, i.e., whether hereditary or acquired, of the malocclusion is not a consideration when making decisions regarding treatment. In pet animals, aesthetics and ethical concerns are linked, and treatment for the sole purpose of being able to 'show' dogs or cats cannot be encouraged. It is essential to determine if the presenting malocclusion is hereditary or not. Orthodontic correction of a malocclusion is contra-indicated where the malocclusion is hereditary, unless the animal is also neutered. The rationale for this is to avoid spread of inherited malocclusions within a breed. Some countries already have legislation relating to this issue.
Orthodontic movement of teeth can be described as prolonged application of pressure to the tooth, resulting in movement of the tooth as the bone around it remodels. Bone on the compression side undergoes lysis allowing the tooth to move and bone formation on the tension side ensures that the tooth stays in the new position. In the ideal situation bone lysis and bone formation should be in equilibrium. Usually, there is an imbalance and lysis occurs more rapidly. A retention phase maintaining the tooth in the new position while allowing time for bone formation is, therefore, necessary in many cases.
The optimal orthodontic force is one that moves teeth rapidly without resulting in structural damage as well as one that causes the least amount of discomfort or pain. Factors that need to be considered for any orthodontic appliance are the magnitude of the force, the distribution of the force and the duration of the force. The ideal force is a light continuous force. Heavy continuous forces are most damaging and should be avoided. Apart from the orthodontic forces applied, normal growth processes, and forces from the lips, cheeks and tongue resting on the teeth will determine the outcome of the treatment. Possible complications to orthodontic movement of teeth include pulpal disease, external root resorption, tooth mobility and pain. In short, the outcome of an orthodontic procedure is rarely predictable and needs frequent monitoring based on clinical signs and radiography.
Correction of common malocclusions will be described in the form of clinical case presentations.
References
1. Stockard CR (1941): The genetic and endocrinic basis for differences in form and behaviour. The American anatomical memoirs no 19, The Wistar Institute of Anatomy and Biology, Philadelphia, USA.
2. Hennet PR, Harvey CE (1992a): Craniofacial development and growth in the dog. Journal of Veterinary Dentistry 9 (2), 11.
3. Beard G (1989): Anterior crossbite: interceptive orthodontics for prevention, Maryland bridges for correction. Journal of Veterinary Dentistry 6 (2), 14.
4. Shipp AD, Fahrenkrug P (1992): Practitioner's Guide to Veterinary Dentistry. Dr Shipp's Laboratories, Beverley Hills, USA.
5. Hennet PR, Harvey CE (1992b): Diagnostic approach to malocclusions in dogs. Journal of Veterinary Dentistry 9 (3), 23.
6. Hennet PR (1995): Orthodontics in Small Carnivores. In; Crossley DA, Penman S (eds) Manual of Small Animal Dentistry, British Small Animal Veterinary Association, Gloucestershire, UK, pp182-192.