The
Horse's Teeth
The horse, like most other mammals, have two sets of teeth during
their life. They are born with one set of baby or decidious
teeth (or get them from the age of a few weeks/months), and
until a new and expanded set of permanent teeth replaces them
gradually. In
the full set of teeth in an adult horse each quadrant of the
mouth contains 3 incisors, 1 canine (not always), 1 wolf tooth
(may or may not be present), 3 premolars and 3 molars. The temporomandibular
joint is also marked as this joint can be affected by dental
problems and also cause them. The premolars are molars that
had precursors before them (baby teeth or decidious teeth),
the molars come in addition in the grown horse.
The
names:
Incisors are the front teeth of the horse,
then there is a space we call the bars where the bit rests,
and then behind the angle of the mouth and towards the back
of the mouth back there is a row of premolars and/or molars
or cheek teeth.
Decidious
teeth (baby teeth, milk teeth):
The full set of baby teeth in the horse
counts 3 incisors and 3 cheek teeth in each quadrant of the
mouth (a total of 24 teeth). The central incisor and the 3 cheek
teeth are usually present at birth or right after, the middle
incisor at 4-6 weeks and the corner one at 6-9 months.
The baby and weanling have only 3 cheek
teeth (the premolars). These "baby" teeth are called
decidious teeth, and they will all shed and be replaced by permanent
teeth.
Permanent
teeth:
Between the age of 2,5 yrs and 4,5 yrs
the normal horse will shed all these baby teeth to be replaced
by permanent ones. From the age of 1 year and upwards, the young
horse will also get 12 new additional cheek teeth, possibly
also 4 canine teeth (mostly in stallions and geldings) and from
0-4 wolf teeth.
So at the age when we ask the most of our young horses, especially
racehorses that are trained and raced early in life, they are
in the process of shedding 24 teeth and aquire from 36-44 new
ones (a full set of permanent teeth). It is not hard to understand
it can and will cause the horses pain and discomfort when we
put a bit in their mouth, tighten the nose band, use a tongue
tie and pull on the reins. Sharp enamel and discomfort from
shedding and erupting teeth is something that should seriously
be considered a potential problem.
We teach out horses to yield to pressure, it is not surprising
they get confused when they try to be good and get mixed signals
from the pain inflicted on them when we pull on the bit and
expect them to move towards pressure (pain).
Wolf
teeth:
The wolf tooth, if the horse gets any
at all (this varies), is a sharp little pointy tooth just in
front of the row of cheek teeth. They are rudimentary premolar
teeth (named pm1) with no real function in the modern horse.
The size of both the crown and root of the wolf tooth varies
a great deal. It is considered to be a permanent tooth as we
don't see any shedding. A horse can get from 0-4 of these teeth,
and they are usually removed as they are often sharp, and because
of the bit, they can cause problems in the modern sports horse.
The wolf teeth can also be unerupted or "blind" and
can be felt as little bumps in the gum. Blind wolf teeth can
also cause soreness in the gums and cause problems as they are
often dislocated a little further into the space between the
incisors and the cheek teeth where we put the bit. We most often
see them in the upper jaw, but in some horses (especially the
standardbred horse) it is not uncommon to also get wolf teeth
in the lower jaw (and often unerupted). Eruption of the wolf
teeth usually occur from 5-6 months of age. They are usually
removed to avoid problems later.
Canine
teeth (tushes):
The canine teeth are located in the space
beween the incisor row and the molar row (this area is called
"the bar"), and the canines in the lower jaw are located
a little further towards the incisors than they are in the upper
jaw. They are not in occlusion with each other. It is mostly
the male horses that get canine teeth, but you can see them
in the occasional mare also (25-30%), but then usually very
small or as a bump on the bar unerrupted. In the male these
teeth can get quite long and razor sharp, getting to be a tremendous
weapon, especially around the age when the stallion is on top
physically and sexually mature (best sperm count and libido),
and would normally defend the rights to a herd. In the older
stallion the canine teeth will get worn and more blunt, and
is not as efficient as before. There are no decidious set of
canine teeth, only permanent. They erupt between the age of
4-5 years. Sharp and pointy canine teeth are usully rounded
off to prevent accidents to happen.
The
incisors:
The incisor teeth are not as folded as
the molars are, but are to some degree. The mass of the tooth
is made up by dentin, and this is covered by enamel and an outer
layer of cement. When the teeth erupt they have a yellowish
look because there is a layer of cement covering the enamel.
This will wear off. There is a folding at the top, making up
an infundibulum. The top of this is open and is called a
cup, the lower portion is filled and is known as the
mark. The insides of the infundibulum is because of the
folding, also covered by cement and enamel. The pattern that
shows with wear on the occlusal surface of the incisors because
of this anatomy, is one of the techniques that has been used
for a long time in determining age in horses. The structure
known as the dental star, is in fact the pulp cavity filled
with secondary dentin, brown-yellowish in colour. The dentin
fills out the pulp cavity to protect the pulp's sensitive nerves
and vessels from external factors. The whole tooth is curved
and is approximately 7 cm long. The cement on the outside of
both incisors and molars also works as a fastening point for
fibres of periodontal membranes and is neccessary for holding
the tooth in the bone socket of the skull.
Adult incisor teeth seen from the front.
They are named from the middle and towards the back. The central
incisor will be I1, the middle incisor I2 and the corner incisor
I3.
The
molars:
The
horse has so called hypsodont teeth. Roughly explained they
are folded teeth covered by cement and enamel, and there will
be rings and ridges of enamel surrounded by cement and dentin
on the occlusal surface. The former pulp chambers will be filled
with secondary dentin as the tooth wears down. The
reserve crown is very long in the young horse, and it lies embedded
in the bones of the skull. When the erupted crown wears by a
2-3 mm every year, it is refilled with new tooth. Eventually
the horse will run out of reserve crown. With normal wear, and
no disease, the teeth will usually last until the age of 30-35.
The molars in the lower jaw are narrower than it its counterparts
in the upper jaw, and they have 2 main pulp caveties. The molars
in the lower jaw usually has two roots (M3 has 3). The
molar from the upper jaw has five pulp cavities filled with
secondary dentin. The molars of the upper jaw usually develops
3 roots, some times 4.
The
dention in the jaws seen from above. The molars form a continous
row that works as a unit leaning against each other to grind
food and avoid spaces between them. Note the difference inn
size and shape of both teeth and jaws.
The
jaws:
The horse has a wider upper jaw than lower
jaw, this will affect the teeth in a way that makes them wear
more on the inside of the upper molar and the outside of the
inside where there is occlusion. Sharp points of enamel will
develop all along the outside of the upper cheek teeth and the
inside of the lower cheek teeth and cause pain, ulceration and
infections in the horse's mouth. More about this in the molar
problems section. In the picture above you can see the difference.
Reserve
crown and roots:
The
reserve crown is very long in a young horse, but gets worn down
with age. The real roots develop after the permanent tooth has
been in wear for a while. In the pictures below, you can see
the state of the roots and reserve crown at different stages
in life. The roots in the pictures is where the ends separate
into individual branches.
This
is a 2-3 year old horse. The yellow colour represents decidious
teeth. As you can see the permanent cheek teeth are pushing
their way downwards on top of the baby teeth, which in turn
are resorbed because of the pressure from the new teeth. They
will get thinner and thinner and eventually shed off as so called
caps. The 3rd molar has not yet erupted as this usualy happens
at the age of 3 1/2 yrs. (There are also of course buds for
the permanent incisors lying above the decidious, but this hasn't
been marked on this drawing.)
Picture
below: This
horse is a bit older, maybe 6-7 years. The length of the reserve
crown is almost at its maximum, 7-8 cm, and as you can see,
most of it is embedded in the jaw. The teeth have been in wear
for a while before the development of real roots occur.
With
age, the cheek teeth erupt as they are worn, and the length
of the reserve crown will be shorter by the year. The roots
are now well developed.
An
old horse. The reserve crown is almost worn down to the roots,
and there is very little, if any, enamel left. The teeth in
an old horse like this will look more yellow and feel smooth,
as there is mosty dentin and sement left on the occlusal suface.
It is important to be aware of this and to feed older horses
special diets to compensate for the reduced ability to chew
and grind its food.
