The genetics of occlusion - Occlusal problems
The etiology of dental malocclusion in dogs and cats has been explored to a large degree in recent years due to the strong
emphasis on veterinary dentistry.
Veterinary dentists receive much of their information from human orthodontics literature and the modifications proposed by
veterinary studies and theories. It was believed by early orthodontists that malocclusions in general were diseases of civilization
and improper jaw function. With the development of genetic research, malocclusions were found to be the result of inherited
dento facial proportions that are altered by developmental variations, trauma, or altered function.
Certain bites or occlusal relationships are influenced by years of established breeding within a family or breed. If we consider
normal occlusion to be one where there is normal molar premolar relationship, the line of occlusion is a smooth curve, and
the incisors in scissor relationship, then all other relationships are in malocclusion to a greater or lesser degree.
Malocclusions in small animals are either inherited from disproportionate tooth to jaw size, or from disproportionate size
of upper to lower jaws. Therefore, small teeth and large jaws or large maxilla and small mandible or any combination of the
above could be inherited.
These disproportions in size are usually not seen in the wild, primitive, or pariah specimens. Malocclusions were breed out
due to improper function and survival-of-the-fittest specimens. Genetic isolation of species usually produces uniformity
in occlusion with disturbances in occlusion bred out and eliminated from the species.
Malocclusion as primarily a genetic problem was developed by breeding experiments with animals carried out in the 1930s.
Experiments by Professor Stockard, involved the crossbreeding of dogs and the interesting effects of these crosses on body
structure. Professor Stockard crossed dogs of diverse sizes and types to determine the effects on jaw and tooth size. Stockard's
experiments indicated that dramatic malocclusions did occur in his crossbred dogs, more from jaw discrepancies that from tooth
size-jaw size imbalances. His experiments concluded that independent inheritance of facial characteristics could be a major
cause of malocclusion and often the result of increased outbreeding.
A misleading factor of many breeds of small dogs is their gene for achondroplasia. Animals or humans affected by this condition
have deficient growth of cartilage. The result is extremely short extremities and an underdeveloped midface. The dachshund
is the classic achondroplastic dog, but most terriers and bulldogs also carry this gene. Achrondroplasia is an autosomal
dominant trait. Like many dominant genes, the gene for achondroplasia sometimes has only partial penetrance, meaning simply
that the trait will be expressed more dramatically in some individuals than in others. The unusual malocclusions produced
in Stockard's breeding experiments can be explained, not on the basis of inherited jaw size, but by the extent to which achondroplasia
was expressed in that animal.
These resultant outcrossed animals produced severe skeletal, dental relationship, and soft tissue problems as well. The brachycephalic
and extreme brachycephalic breeds have restricted airway problems with resultant cardio-pulmonary complications. When we bred
for the micrognathic maxilla, the genetic make up fails to carry the gene to produce compatible soft tissue size. The soft
palate, the lateral nasal cartilage, and the tracheal rings, to name a few, all compromise and restrict the airway.
The premolars and molars of the maxilla rotate out of alignment due to crowding, with complicating periodontal disease seen
in teeth out of alignment.
There is a predominance of accompanying wry in the extreme brachycephalic breeds, often with cranial-facial asymmetry.
Many forms of malocclusion lend themselves to orthodontic therapy. However ethics in orthodontic therapy must be considered.
In essence only those cases of soft tissue trauma must be treated.
People in dog show conformation competition largely influence the continuance of malocclusal problems. The average time spent
in show competition is three to five years. In that time span much damage can be produced. Breeders seem to breed to what
is winning rather that what is correct. Often what wins is not correct.
Periodontal disease is seen in both dogs and cats. There is an eighty percent incidence of periodontal disease in dogs past
three years of age. Early diagnosis of advanced areas of pocket formation, bone loss, and treatment planning are essential
to oral diagnosis.