Equine Teeth
In the dog and cat (and humans), the tooth erupts over a relatively short period
of time. The exposed part of the tooth is the crown and is covered by a layer of the
hardest substance in the body: enamel. The embedded part of the tooth is the root,
which is covered with a bonelike substance, the cementum, that is united to the wall
of alveolar bone chiefly by collagenous fibers. This collagenous connective tissue
forms the periodontium, which anchors the tooth to the jaw.
The tooth of the Equidae (horses, mules, donkeys) is different. It erupts over the
life of the animal. It is a hypsodont, high, (hypsos = height) tooth in which embedded
parts of the tooth are gradually erupted and come into wear. Throughout most of the
horse’s life (a horse will live 30 to 35 years or longer if health is maintained), eruption
of the tooth keeps up with wear of the tooth, and the tooth maintains an even height.
In later life, eruption ceases and the height of the exposed part of the tooth gradually
decreases. The body of the tooth grows over the first five to seven years and the
roots of the permanent cheek teeth are formed over a period that extends from the
fifth through the seventh or eighth year. The extended growth period and continuous
eruption of the hypsodont tooth is functionally related to the herbivorous diet; that is,
the diet is more wearing on the teeth, and growth and continued eruption assure a
dental table for grinding the food.
Figure 1.
Equine incisor
(longitudinal section,
modified from Sisson)
just breaking through
the gum. Note that
cementum covers the
entire tooth and that
enamel extends deeply
onto the embedded part.
In an erupting tooth, the
root canal and pulp
cavity are large, but are
gradually narrowed as
the animal grows.
gum
cementum (orange)
enamel (black)
dentine
pulp cavity
root canal
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The hypsodont tooth is also characterized by the presence of one or two
infundibula or “cups” on its occlusal surface. Incisor teeth have a single cup, the
cheek teeth have two (the infundibula of the lower cheek teeth are formed as medial
infoldings of the surface enamel; that is, they are not cuplike invaginations of the
enamel of the occlusal surface; see Figure 10). With wear, the single cup of the
incisors is ultimately lost; a small enamel spot, the base of the cup, remains until
about the 16th to 17th year. In the newly erupting tooth, both exposed and embedded
parts of the tooth are covered with cementum, and the enamel extends the length of
the embedded part of the forming body of the tooth. The covering of the embedded
part by cementum and enamel is functionally associated with the continuous eruption
of the tooth in which embedded parts are ultimately exposed to wear.
Figure 2. The tooth “in
wear.” After eruption,
the tooth continues to
emerge, contacts the
tooth of the opposing
arcade, and begins to
wear off the cementum
and then the enamel of
the occlusal surface.
lingual surface
enamel “cup”, filled
with cementum
labial surface
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With wear, much of the cementum and enamel of the occlusal surface is lost.
This exposes the dentine internal to the perimeter enamel and creates an undulating
surface in which the softer dentinal surface and cementum are interrupted by hard
enamel ridges. With the chewing movements of the animal, such surfaces are very
efficient in breaking down the roughage of the herbivorous diet.
Figure 3. Longitudinal section
through a cheek tooth at the level
of an infundibulum. The tooth is in
wear and shows its grinding
surface: ridges of enamel
interrupted by softer cementum
and dentine.
enamel
dentin
e
cementum
cementum
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Dental formulae of the deciduous and permanent teeth, and their dates of
eruption, are given in the attached chart, which is taken from the Merck Veterinary
Manual:
Table 1.
Fully developed canine teeth are present only in the male (stallion and gelding)
and appear only in the permanent set. They are absent or vestigial in the female and,
if present, do not usually erupt (the mare’s vestigial teeth may be seen on
radiographs).
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The lower incisor teeth are frequently used to estimate the age of the horse, and
with experience this can usually be done accurately in a horse up to about 15 to 17
years of age. In estimating the age of a horse, it is first necessary to distinguish the
permanent from the deciduous incisors. At eruption, the deciduous incisors are about
as large as the permanent incisors but are distinguished from the permanent teeth by
the presence of a distinct neck (often not apparent in the live animal as it is usually
below the gum line) and a fine grooving on their labial surface. The permanent incisor
lacks a neck and exhibits a pronounced longitudinal groove parallel to the long axis of
the tooth; it is near the medial (mesial) side of the tooth’s labial surface. With the
important deciduous/permanent dentition decision made, the method of ageing the
horse may proceed as follows:
Figure 4.
Deciduous and
Permanent
Teeth
neck
(Di2)
fine grooving on
labial surface of
deciduous incisor
teeth
deep sulcus on labial
surface of permanent
incisors
Figure is from Sisson and Grossman’s
The Anatomy of the Domestic Animals;
1975, Getty, Ed. (Saunders)
Incisors of a horse about three years of age. The first incisors (I1) in this
figure are permanent teeth. The second and third (Di2, Di3) are deciduous. The
deciduous in this case are easily distinguished as they are well worn, and the
permanent first incisors are only newly erupted. When first erupted, the deciduous
incisors are large. Note especially the deep groove on the labial surface of the
permanent incisors. Although difficult to tell in this figure, the labial surface of the
deciduous incisors shows a fine grooving; they lack the prominent mesial groove of
The permanent incisors, from medial (I1) to lateral (I3), erupt at 2 ½, 3 ½ ,
the permanent set.
and 4 ½ years of age, and come into wear about six months later. The permanent
canine erupts at 4 ½ to 5 years of age. It is present as a well-developed tooth only in
the male; it is lacking or vestigial in the deciduous dentition.
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All lower incisors are “level” at seven years of age. A level tooth is one in which
the oval ring of enamel at the worn occlusal surface of the infundibulum is completely
surrounded by dentine.
Figure 5. The changes with wear of the
occlusal surface of an incisor tooth. B-B of this
figure is a “level” tooth. Continued laying down
of dentine by odontoblasts gradually narrows
the pulp cavity as indicated by the dotted line
within the lower part of the cavity.
A-A
dental star
B-B
cementum
m
cup
enamel
C-C
dentine
enamel “spot”
The outline of the occlusal surface of I1, I2, and I3 becomes “round” (its
width and depth are approximately equal) at 9, 10, and 11 years.
The outline of the occlusal surface of I1, I2, and I3 is “triangular” about 4
½ years later (13 ½, 14 ½, 15 ½ years, respectively);
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The outline of the occlusal surface of I1, I2, and I3 is “biangular”
(described as “oblong” in some texts) about 3 years later (16 ½, 17 ½, and 18 ½
years respectively).
Note: The transition of the occlusal surface from oval (young horse)
to round, to triangular, to biangular (old horse) is gradual, and the intermediate stages
are usually easily distinguished and utilized in making the estimation of age.
Figure 6. Diagrammatic view of an
incisor tooth showing the different
levels of wear as reflected by the
outline of the occlusal surface.
oval
round
triangular
(equilateral)
biangular
(isosceles triangle)
The deep sulcus on the labial side of upper I3 (Galvayne’s groove) may
be utilized in older horses. It should be regarded as a rough guide. The groove
appears at the gumline at 10 years, is halfway down the exposed surface of the tooth
at 15 years, reaches the occlusal surface at 20 years, is lost on the upper half of the
exposed surface of the tooth at 25 years, and is lost entirely at 30 years.
Figure 7.
A = Galvayne’s groove. In this
specimen, it extends the full length
of the exposed surface of the tooth
= about 20 years.
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The profile of the angle formed by the meeting of the upper and lower incisors,
observed in lateral view, is very useful in making a quick decision as to whether you
are dealing with a young or old horse. The angle formed is obtuse in the young horse,
more acute in the aged animal.
Note: There are many systems for estimating the age of the horse and all do not
agree. In the last few years, it has become popular to claim that horses cannot be
aged accurately beyond seven years. In the writer’s experience, horses (donkeys are
different) with a normal bite and diet can be aged fairly accurately up to 18 years or
so.
Most reliable are the eruption dates of the lower incisors and the outline of the
occlusal surface of those teeth. The corner incisor (I3) does not follow the pattern so
well but with other clues will not interfere with the accurate assignment of age. Up to
seventeen years of age, you can usually assess the age of the horse within two
years, within one year or so in younger horses. Diet and environmental factors (sandy
soil, etc.) will affect the rate of tooth wear. Vices such as “cribbing” will give an
abnormal form to the teeth.
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Figure 8 (from Sisson). Lower and upper incisor and canine teeth of a five
years old male horse. Except for Galvayne’s groove, only the lower incisors are
used for ageing. This is a young horse and the occlusal surface of these teeth is
oval in outline. How many teeth in the lower jaw are level? The photograph is
from Sisson. The photograph of Galvayne’s groove (Figure 7) is by Dr L E St
Clair of the 1975 edition of Sisson and Grossman’s The Anatomy of the
Domestic Animals (Saunders).
Figure 9. Incisor teeth of a horse about ten years old. The outline of the
occlusal surface of the second incisors is “round.” The width and depth of
I2 are not equal but about as close as they get; it is what is designated a
“round” tooth and this horse is about ten years old. I1 is heading toward
triangular. Giving yourself a year’s leeway, the horse is between 9 and 11
years. The first incisors are starting to wear to the “triangular” outline. A =
bottom of the infundibulum; B = dental “star.” Note the mesial groove
(arrows) of these permanent teeth. This photograph is also by Dr L E St Clair of
the 1975 edition of Sisson and Grossman’s The Anatomy of the Domestic Animals.
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The “dental star.” Figure 2 (see also Figure 5) is a diagrammatic longitudinal
section of an erupted incisor “in wear.” This is a young tooth and the root canal and
pulp cavity are large. Note that, toward the occlusal surface, the pulp cavity narrows
and extends rostral to the infundibulum, nearer the labial surface of the tooth.
Throughout life, the layer of dentine-forming odontoblasts remains applied to the wall
of the canal and cavity. The odontoblasts continue to form dentine. The pulp cavity
becomes smaller and smaller and the root canal becomes a very narrow passage. As
the tooth wears, the newer dentine that now fills what was the pulp cavity appears as
a dark streak on the occlusal surface of the tooth. It lies labial, rostral, to the
infundibulum. This dark streak is popularly designated the “dental star.” Note: The
increase in the dentine within the cavity and canal places the dental nerves farther
from the tooth’s surface. The teeth become less sensitive with age. A child truly is
liable to feel more pain at the dentist than does the child’s parent.
Figure 10 (From Sisson). Upper and lower dental arcades of a
horse (female) about 4 years old. Using the lower incisors, the corner
incisor is deciduous (Di3) and the permanent middle incisor (I2) is “in wear”
(erupts at 3 ½ yrs. + 6 mos. to come “into wear”; that is, to contact the
opposing tooth).
upper jaw
2 infundibula
dentine
Diagram of the occlusal
surface of cheek teeth of the
right side of the upper arcade.
2 infundibula
cementum
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lower jaw
Diagram of the occlusal
surface of cheek teeth of the
left side of the lower arcade.
arrows
(2 enamel
infoldings)
2 “infundibula”,
(enamel
infoldings)
Why do we think that this is a mandible of a female horse? The alveolus that
we see in the lower jaw (mucosa covers the upper jaw at canine tooth level) for the
canine tooth is not an alveolus of a robust canine tooth of a male horse. Sometimes,
as here, a small canine tooth is visible in a female horse or remains deep to the
mucosa.
Note the third molar tooth. It is well erupted in the upper jaw, only slightly
worn in the lower jaw in which its occlusal surface is still largely covered by
cementum. Before coming into wear, the tooth was completely covered by cementum.
Some clinical considerations:
“Wolf” tooth. The first premolar in the upper arcade (it sometimes
appears in the lower arcade as well) is a part of the permanent dentition. It
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is a simple tooth and not present in the deciduous set. This PM1 is much
smaller than the other premolars and is called the “wolf” tooth (see Figure
10, upper jaw, and Figure 11, upper and lower jaws). It erupts at about six
months and owing to its small root and consequent instability often has to
be removed. The instrument used is a “wolf tooth elevator.”
Caps. The permanent premolars of both arcades erupt at 2 ½ to 4 years.
The deciduous premolars often fail to be extruded but sit as “caps” on the surface of
the emerging permanent tooth. This occurs at about two to three years of age and
interferes with the animal’s chewing. The caps must be removed by the attending
veterinarian. In the live animal, the caps can be remarkably difficult to detect; you
must look closely if you suspect them as the cause of the horse’s chewing problem.
Fig. 11. Horse about three years old (it has permanent first
incisors erupted and coming into wear). This horse has
“caps.” It also has “wolf” teeth in upper and lower jaws. Note
how high the newly erupting cheek teeth (M1 – M3) of the
upper jaw project into the maxillary sinus. From Sisson.
“Floating” Teeth. In conformity with the mandible, the lower arcade is more
narrow than the upper (this is also true of human beings). The result is that, with
wear, the occlusal surface of the cheek teeth of the upper arcade slopes
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lateroventrally (the tooth is high laterally, low medially), developing if unattended a
sharp lateral edge that frequently lacerates the buccal mucous membrane. The lower
arcade if unattended develops an occlusal slope that is high medially, low laterally.
Lacerations of the tongue often result from this condition of the lower arcade. An
instrument called a “float” (there are others) is used to remove the sharp edges of
these teeth.
Fig. 12. Diagrammatic
cross-section through the
cheek teeth, showing the
more narrow arcade of
the mandibular teeth and
the developing sharp
edge of these teeth if left
unattended.
sharp
edge
sharp
edge
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Fig. 13. This is a five years old male horse. The figure’s
numbering of the cheek teeth from 1 – 6 is only indicative of the
number of large cheek teeth. The wolf tooth is PM1, then PM 2, 3,
4; then M1, 2, 3. Note that roots are beginning to form. Compare
Fig. 11. Figure is from Sisson.
In an animal in which dental disease is being considered, a thorough
examination of the teeth is indicated. Use a good light to get the necessary visibility of
the caudal cheek teeth.
Main things to know and understand:
1. The permanent teeth of the horse erupt over a long period of time.
Associated with this phenomenon is the fact that the cementum and enamel cover
embedded as well as exposed parts of the tooth; for parts that are embedded are
later exposed to wear. Roots of the permanent cheek teeth form from the fifth to the
seventh or eighth year.
2. The incisors have a single infundibulum; the cheek teeth have two. The
enamel “cups” are functionally associated with the improved grinding ability of the
teeth when the tooth comes into wear.
3. The eruption dates of the three permanent incisors, from medial to lateral
(I1 to I3) are 2 ½, 3 ½, and 4 ½ years. The canine tooth is fully developed only in the
male (stallion and gelding) and erupts at 4 ½ to 5 years. Cheek teeth erupt
(approximately): PM 2, 3, 4 at 2, 3, 4 years; M1, 2, 3 at 1, 2, 3 years.
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4. With the exception of Galvayne’s groove, only the lower incisors are used
in ageing horses. Know “round” at 9-10-11; “triangular” about 4 1/2 years later;
“biangular” 3 years after that.
5. Know “wolf” tooth, “caps”, and what is accomplished in “floating” the
horse’s teeth.
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