22 Malocclusion – an introduction
By definition, malocclusion is an abnormality in the position of the teeth. Malocclusion is common in the dog, but also occurs in cats. The clinical significance of malocclusion is that it may cause discomfort and sometimes pain to the affected animal. In some cases, it may be the direct cause of severe oral pathology. It is consequently important to diagnose malocclusion early in the life of the animal so that preventative measures can be taken.
Malocclusion can result from jaw length and/or width discrepancy (skeletal malocclusion), from tooth malpositioning (dental malocclusion), or a combination of both. The development of the occlusion is determined by both genetic and environmental factors. Specific genetic mechanisms regulating malocclusion are unknown. A polygenic mechanism, however, is likely and explains why not all siblings in successive generations are affected by malocclusion to the same degree, if at all. With a polygenic mechanism, the severity of clinical signs is linked to the number of defective genes.
In the mandibular prognathic bite, often called ‘undershot’, the mandible is longer than the maxilla and some or all of the mandibular teeth are rostral to their normal position. If the dental interlock prevents the mandible from growing rostrally to its genetic potential, lateral or ventral bowing of the mandible may occur to accommodate the length. This results in an open bite and is characterized by increased space between the premolar cusp tips. In addition, the caudal angle of the mandible is caudal to the temporomandibular joint to accommodate the extra length of the mandible.
A wry bite occurs if one side of the head grows more than the other side. In its mildest form a one-sided prognathic or brachygnathic bite develops. In more severe cases, a crooked head and bite develop with a deviated midline. An open bite may also develop in the incisor region so that the affected teeth are displaced vertically and do not occlude.
In some animals, the mandible is too narrow with respect to the upper jaw. The result is that the lower canines impinge on the maxillary gingivae or the hard palate instead of fitting into the diastema between the upper third incisor and upper canine on either side. The animal may not be able to close its mouth and injury to the gingivae or palatal mucosa commonly occurs. In untreated severe cases, an oronasal communication may develop over time. This condition is seen in both the primary (deciduous) and permanent dentition. Persistent primary canines will further exacerbate the condition, as the permanent canines erupt medially to their primary counterparts in the mandible. The incorrect dental interlock will interfere with the normal growth in width and length of the developing mandible. The condition can also be caused by persistent primary mandibular canines in a mandible of normal width.
This is a clinical term used to describe a reverse scissor occlusion of one, several or all of the incisors. The condition is thought to be secondary to persistent primary incisors. However, there is probably a skeletal origin as well, since affected animals often develop a mandibular prognathic bite. In other words, an anterior crossbite in an immature animal may be the first sign of a developing mandibular prognathism. Anterior crossbite is common in medium- and large-breed dogs, where persistent primary teeth are less common. The cause can be either a dental malocclusion (i.e. linguoversion of the upper incisors) or a skeletal malocclusion (i.e. mandibular prognathism or maxillary brachygnathism).