Occlusion and malocclusion

Introduction

Occlusion is the term used to describe the ‘bite’, i.e. the relationship of teeth in the same jaw as well as the relationship of teeth in opposing jaws. Occlusion is determined by the shape of the head, jaw length and width and the position of the teeth. By definition, malocclusion is an abnormality in the position of the teeth. Malocclusion is common in dogs, but it 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. It is known that jaw length, tooth bud position and tooth size are inherited (Stockard 1941). It is also known that the development of the upper jaw, mandible and teeth are independently regulated genetically (Stockard 1941). Disharmony in the regulation of these structures results in malocclusion. Alteration of jaw growth by hormonal disorder, trauma or functional modification may result in skeletal malocclusion (Hennet and Harvey 1992a). Although tooth bud position is inherited, various events during development and growth may alter the definitive tooth position.

It is claimed that at least 50% of malocclusions are acquired and have no genetic cause (Beard 1989; Shipp and Fahrenkrug 1992). There are no data to support such a claim in dogs or cats. Not much research has been done and there are no large epidemiologic studies available. 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 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 (Hennet and Harvey 1992b; Hennet 1995) to evaluate whether malocclusion is hereditary or acquired is as follows:

• Skeletal malocclusion is considered inherited unless a developmental cause can be reliably identified.

• Pure dental malocclusion, unless known to have breed or family predisposition, should be given the benefit of doubt and not be considered inherited.

Normal occlusion

When evaluating occlusion it is important to look at all parameters and not to base judgment solely on the positioning of the incisor teeth. In fact, the canine and premolar relationships often give a better guide to the occlusion.

The shape of the head affects the positioning of the teeth. Malocclusion occurs in any of the three head shapes (dolichocephalic, mesocephalic and brachycephalic), but is more common in brachycephalic breeds. The normal head type in the feral dog and cat is the mesocephalic head. We breed for brachycephalic or dolichocephalic, which leads to abnormal occlusion.

Skeletal malocclusion

Brachycephalic dogs have a shorter than normal upper jaw (Fig. 5.6) and dolichocephalic dogs have a longer than normal upper jaw (Fig. 5.7); in both cases the mandible is not responsible for any rostrocaudal discrepancy.

Fig. 5.6 Brachycephalic
Brachycephalic dogs have a shorter than normal upper jaw. A short jaw results in reduced interdental spaces with rotation and/or overlap of teeth.

Fig. 5.7 Dolichocephalic
Dolichocephalic dogs have a longer than normal upper jaw. The increased jaw length results in interdental spaces that are wider than normal.

Mandibular prognathic bite

In the mandibular prognathic bite, often called ‘undershot’ (Fig. 5.8), the mandible is longer than the upper jaw and some or all of the mandibular teeth are rostral to their normal position. The degree of malocclusion varies as follows:

Fig. 5.8 Mandibular prognathic bite
The mandible is longer than the upper jaw. This is normal for brachycephalic breeds.

• Normal incisor occlusion, but the mandibular canines touch the upper 3rd incisors and the mandibular premolars are rostrally displaced, which disrupts the ‘pinking shear’ effect.

• Level bite: the upper and lower incisors meet at their incisal edges; the lower canines touch the upper 3rd incisors and the mandibular premolars are rostrally displaced.

• Reverse scissor bite: the lower incisors are rostral to the upper incisors by 0.5 mm to 5 cm or more; the lower canines may be caudal to but touching the upper 3rd incisors, or may be rostral to the upper 3rd incisors; the mandibular premolars are rostrally displaced to a similar degree.

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. In addition, the caudal angle of the mandible is caudal to the temporomandibular joint to accommodate the extra length of the mandible.

Mandibular brachygnathic bite

A mandibular brachygnathic bite, often called ‘overshot’, occurs when the mandible is shorter than normal (Fig. 5.9). The degree of malocclusion varies as follows:

Fig. 5.9 Mandibular brachygnathic bite
The mandible is too short in relation to the upper jaw. This is normal for dolichocephalic breeds.

• The upper incisors are rostral to the lower incisors by 0.5 mm to 5 cm or more.

• The upper canines are caudal to but touching the mandibular canines, level with the lower canines or rostral to the mandibular canines.

• The mandibular premolars are caudally displaced relative to the maxillary premolars, disrupting the ‘pinking shear’ effect. The degree of displacement is similar to that of the incisors and canines.

Wry bite

A wry bite (Fig. 5.10) 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. The space between the upper and lower incisors can vary from 0.5 mm to 2 cm.