Pet Orthodontics
PET ORTHODONTICS
by Jan Bellows, D.V.M.
Diplomate, American Veterinary Dental College
9111 Taft Street Pembroke Pines, Florida
954-432-1111
Just as one would seek to cure a painful skin condition, or treat a sore
joint in their pet, one should also consider relieving pain brought on by
orthodontic abnormalities. Poorly aligned teeth can lead to serious
periodontal disease. In most breeds, teeth are arranged "shoulder to
shoulder" in an arch. A self cleaning mechanism occurs in the arch to push
food away from the teeth and gums. If the teeth are not aligned normally,
food may be retained between the teeth causing inflammation and infection.
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.
The normal bite of dogs and cats with medium or long length and width
muzzles is called a scissors bite (picture 1a and 1b). 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 (picture 2) 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.
GENETIC OR NOT
Occlusion is controlled by genetics, nutrition, environment, and by
mechanical forces generated by the interlock of the upper and lower teeth.
Some abnormal bites (malocclusions) have been proven to be genetically
influenced, such as severe over or under bites, and wry bites. Other bite
abnormalities are known to be acquired (non genetic). Acquired malocclusions
can result from tug of war games played with towels or ropes which move
teeth into abnormal position. Traumatic birthing can also responsible for
some acquired abnormalities.
To help define whether the malocclusion is genetic in origin,
interdigitation of the premolars is studied. In breeds that have medium and
long muzzles, the premolars should meet in a saw toothed fashion. For
example, the tip of the lower third premolar should be positioned equally
between the crowns of the upper third and fourth premolars (picture 3). If
the tip of one premolar points to the tip of another premolar there may be a
genetically induced malocclusion (picture 4). This only holds true in those
breeds that do not have shortened muzzles.
Some genetic bite problems do not show up in each litter because they are
recessively passed on. The goal of selective breeding is to mate one animal
to another that has superior occlusion.
RETAINED DECIDUOUS TEETH
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 produces 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 (picture 18). 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:
1. early onset and increased severity of oral infection
2. 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.
3. 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.
4. 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.
Missing or Extra Teeth
Dogs and cats may be born without the proper number of teeth. Extra
(supernumerary) teeth can cause periodontal disease from over crowding.
The American Kennel Club sets standards concerning minimum
number of teeth accepted for each breed to be considered for show. Dental
x-rays can be taken as early as ten weeks of age to evaluate if the correct
number teeth are present. X-rays are recommended as a part of the
prepurchase examination in certain breeds. Missing teeth (hypodontia)
usually occur in the premolar area, but any tooth in the mouth may not erupt.
Missing or extra teeth are considered genetic faults. Collies
and Doberman Pinchers are most commonly affected. Sometimes the missing tooth
is trapped below the gum line. A dental x-ray can be taken to determine if
an unerupted tooth is present.
MISDIRECTED CANINE SYNDROME
Misdirected canine syndrome is a bite abnormality where retention of the
deciduous tooth tilts the erupting permanent canine tooth into abnormal
location.(picture 7). The opposing canines may not have room to occlude
properly, resulting in abnormal wear, periodontal disease, or early tooth
loss.
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
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).
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 (picture 8).
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.
Anterior cross bite occurs when canine and premolar teeth on both sides of
the mouth occlude normally but one or more of the lower incisors are
positioned in front of the upper incisors (picture 9). This condition can be
caused by tug-or -war games, retained primary teeth, or impacted roots.
Anterior cross bite is a common malocclusion. It is not considered a
genetic or inherited defect. Posterior cross bite occurs when one or more of
the premolar lower jaw teeth overlap the upper jaw teeth (picture 10). This
is a rare condition that occurs in the longer nosed dog breeds. A wry mouth
or wry bite occurs when one side of the jaw grows more than the other. Wry
bites show as triangular defects in the incisor area (picture 11). Some of
the incisors will meet their opposing counterparts while others will not. Wry
bite is a severe inherited defect. Base narrow canines occur when the lower
canine teeth protrude inward often producing damage to the upper palate
(picture 12). This condition is either due to retained deciduous teeth, or
by a overly narrow mandible. Base narrow canines may be corrected through
orthodontic devices that push the teeth into normal occlusion. An open
bite occurs when some incisors are displaced vertically and do not touch.
Often the tongue will protrude.
Rotated teeth, commonly affecting the upper third premolar, occurs mostly in
short muzzled breeds. Selective breeding has created undersized mouths that
cannot accommodate forty two teeth in normal alignment. The rotated tooth
root closest to the palate is prone to periodontal disease. Strict tooth
brushing may be helpful in saving a rotated tooth, but frequently the tooth
cannot be saved.
ORTHODONTIC CARE
Many abnormal bites can be corrected. Orthodontic care should be performed by
veterinarians familiar with tooth movement principles. Breeders at times use
rubber bands which may move teeth but compromise the gum tissue around teeth
leading to periodontal disease, pain, and early tooth loss. Orthodontic care
should be reserved to ease pet discomfort by realigning teeth in those
animals that will not be used for show. Tooth movement is accomplished by
employing brackets, acrylic retainers, springs, and elastics (picture 17).
Fortunately, in animals orthodontic movement can usually be accomplished in
months rather than years.