Malocclusion is the deformation of a dog's jaw structure, leaving either the mandible (lower jaw) or the maxilla (upper jaw) out of line, or the increased growth of a single side of a jaw, causing twisting of the jaws (wry mouth).
Malocclusion is mostly hereditary although retaining the baby teeth can force adult teeth to erupt out of line, resulting in gum injury and painful chewing. This form of malocclusion can be corrected easily by pulling the baby teeth if caught early enough.
Also known as parrot mouth or brachygnathism : the upper jaw extends beyond the lower jaw, causing the upper teeth to overlap the lower teeth, preventing them from aligning snugly as they should be. An overshot bite can create difficulty grasping with the jaws and in severe cases, the growth of adult teeth can cause significant injuy to the soft tissues of the roof of the dog's mouth. This makes it difficult to chew and often causes a reluctant eater.
Prognathism, or an underbite is common and expected in brachycephalic breeds (Boxers, Bulldogs, etc...), where the lower jaw protrudes out past the upper jaw. It should be noted, however,that as common as this condition is, in a breed where the jaws are not formed to accomodate this condition, problems can arise resulting in needed corrective surgery.
When one side of the jaw (right or left, not upper or lower) grows at a more rapid pace than the other, this causes a twisting of the mouth known as wry mouth. Wry mouth can cause a severe handicap when it comes tograsping items or chewing food, and gives a very lopsided appearrance. This condition can be temporary, correcting itself as the othr side catches up in growth.
Puppies should be examined around three or four months of age for early detection of bite problems. A dog's jaw finishes growing at approximately ten months of age, and until that time a mild under or overshot should be left alone as it may corrct itself spontaneously during the growth period. A severe bite structure problem is unlikely to be corrected with aging, but should not be surgically corrected until the puppy is older and has finished growing.