Orthodontics

It is important to understand the head shape when determining normal bite relationships in various breeds. There are three basic head shapes for dogs. Those with long and narrow muzzles (Rough Collies, Borzoi, Doberman, Greyhound, Saluki); those with a short and wide muzzles (Bulldog, Pug, Pekingese, Boxer, Boston Terrier, Shih Tzu); and dogs with medium length and width muzzles (Labrador Retriever, German Shepherd Dog, most Spaniels, Terriers, and Hounds). Cats also have different head shapes which vary from the short muzzled Persian to the longer muzzled Siamese.

Normal scissors bite, notice the midline of upper and lower jaws are aligned

Normally, the lower canine should intersect the upper lateral incisor and upper canine
The normal bite of dogs and cats with medium or long length and width muzzles is called a scissors bite. The upper incisors are located in front of the lower incisors when the mouth is closed, and there is a smooth curve from canine to canine without misplaced or rotated teeth. The lower canines should lie exactly between the upper lateral incisors and upper canines, yet touching neither. Premolar crown tips should point to a space between the crowns of the opposing premolars. In dogs that have a short, wide muzzle, a reverse scissors bite is considered normal where the lower incisors are in front of the upper incisors. The lower canines and premolars will also be shifted forward. While the reverse scissors bite is an acceptable breed standard, at times the upper incisors cause trauma and pain to lower jaw tissues.
 

Retained Deciduous Teeth

Retained upper canine tooth
Normally the deciduous tooth's root is resorbed, making room for an adult tooth. Should this fail, the adult tooth may deviate from it's normal position, producing malocclusion. The resulting double set of teeth overcrowds the dental arch, causing food to become trapped between the teeth, leading to early periodontal disease. A double set of roots may also prevent normal development of the socket, and erode periodontal support around the adult tooth, resulting in early tooth loss. A retained deciduous tooth should be extracted as soon as an adult tooth is noted in the same area as the baby tooth. If extraction is performed early, the abnormally positioned adult tooth usually moves to it's normal location.

A procedure performed by some breeders is to trim or cut deciduous teeth in hopes that they will be shed early preventing orthodontic problems. By cutting the tooth in half, pulp is exposed to oral bacteria causing infection, pain, and tooth loss. Unfortunately, the remaining infected root can interfere with the emerging adult tooth, which may not come in normally.

Dental Interlock
Jaws do not grow at equal rates. If deciduous teeth erupt during an accelerated growth phase of one jaw, an interlocking of both sets of primary teeth can maintain the abnormal bite relationship. Even genetically normal dogs can occasionally develop abnormal bites due to the interlock of primary teeth. If an under bite is noted before the permanent teeth erupt, treatment may be helpful. Removal of primary teeth from the shorter jaw that interfere with forward growth, if performed by ten weeks of age may allow the upper jaw to lengthen unimpeded. This procedure called interceptive orthodontics will correct about 50% of minor jaw length malocclusions, by the time permanent teeth erupt. Extraction does not stimulate jaw growth, it only removes a mechanical barrier to genetic control of the growth process.

Teeth that are crowded, rotated, or tilted at abnormal angles can result in:

 Early onset and increased severity of oral infection
 Damage to the soft tissues of the mouth, due to sharp teeth that penetrate the unprotected gum and mouth tissues. In addition, the lower canines can erode through the hard palate, causing food to enter the nasal cavity
 Excessive wear when abnormally aligned teeth grind against each other. Such abrasion frequently wears through enamel, causing a weakened tooth to fracture and expose the root canal system.
 Pain in joints of the jaw

Breeders, show judges, veterinarians, and others who wish to describe specific dental conditions in dogs and cats need to use proper orthodontic terms. Over bite, open bite, over jet, level bite, overshot, under bite, anterior cross bite, posterior cross bite, wry bite, and base narrow canines are orthodontic terms that are at times confusing.
Occlusion
The way which teeth align with each other is termed occlusion. Normal occlusion in most medium and long muzzled breeds consists of the upper (maxillary) incisors that just overlap the lower (mandibular) incisors (scissors bite). The lower canine should be located equidistant between the corner (lateral) incisor and the upper canine tooth. Premolar tips of the lower jaw should point between the spaces of the upper jaw teeth.

Malocclusion
 

Open bite - incisors do not meet

Malocclusion refers to an abnormal tooth alignment. Over bite (parrot mouth, over shot, class two, over jet, mandibular brachygnathism) occurs when the lower jaw is shorter than the upper. There may be a gap between the upper and lower incisors when the mouth is closed. The upper premolars are displaced at least twenty-five percent toward the front, compared to the lower premolars. An over bite malocclusion is never considered normal in any breed and is a genetic fault. The most commonly affected breeds are those with elongated muzzles (Collies, Shelties, Dachshunds, and Russian Wolfhounds).

Underbite, undershot prognathic skelatal malocclusion

An under bite (under shot, reverse scissors bite, prognathism, class 3) occurs when the lower teeth protrude in front of the upper jaw teeth. Some short muzzled breeds (Boxers, English Bull Dogs, Shih-Tzus, and Lhasa Apsos) normally have an under bite, but when it occurs in medium muzzled breeds it is abnormal. When the upper and lower incisor teeth meet each other edge to edge, the occlusion is considered an even or level bite. Constant contact between upper and lower incisors can cause uneven wear, periodontal disease, and early tooth loss. Level bite is considered normal in some breeds, although it is actually an expression of under bite.

Level Bite   Anterior Crossbite   Wry bite