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DENTISTRY
Relationship Between Equine Cheek Tooth
Occlusal Morphology, Apparent Digestibility, and
Ingesta Particle Size
James L. Carmalt, MA, VetMB, MVetSc, Diplomate ABVP, Diplomate ACVS;
and Andrew Allen, DVM, MVetSc, PhD
In some herbivorous species, dental morphological variables have been significantly correlated with
fecal particle size. This study does not support this finding in the horse. Further research is needed
to determine the role of equine dentition in subsequent gastrointestinal feed processing. Authors’
addresses: Department of Large Animal Clinical Sciences (Carmalt) and Department of Pathology
(Allen), Western College of Veterinary Medicine, 52 Campus Drive, University of Saskatchewan,
Saskatoon SK S7N 5B4, Canada; e-mail: [email protected]. © 2008 AAEP.
1.
Introduction
Floating (rasping or filing) is the most common
equine dental procedure performed.1 Some veterinary texts suggest that weight loss, ptyalism, and
colic, among other medical abnormalities, can be
caused by dental disease.2– 4 Published clinical
trial data, barring two abstracts,5,6 refute these
statements and currently, whereas the majority of
published evidence suggests that disease of the hard
dental tissues does not lead to either weight loss or
colic,7,8 a single retrospective study claims that dental disease was the cause of weight loss.9
There is a significant body of research into the
effect of chewing in the ruminant and other herbivorous species.10 The morphology of teeth and their
role in chewing effectiveness has not been widely
studied. In human studies, molar occlusal surface
area has been used, but in herbivorous species in
which the anisognathic jaw results in lateral translocation of the mandible during mastication, the
NOTES
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2008 Ⲑ Vol. 54 Ⲑ AAEP PROCEEDINGS
enamel ridge perimeter distance may be more
useful.11–15
We conducted a prospective, observational study
to investigate the effect of various equine cheek
tooth occlusal morphological variables and oral pathology score (OPS) on subsequent ingesta particle
size reduction and apparent feed digestibility in the
horse.
2.
Materials and Methods
Seventeen registered horses (six Clydesdales, three
Percherons, and eight Quarter horses) of known age
were used. They had been maintained solely on
one of three different hays, which was fed ad libitum
for ⱖ14 days before death. They were euthanized
using an intravenous mixture of pentobarbitala and
embutarateb administered as a bolus into the left
jugular vein.
Heads were disarticulated post-mortem at the atlanto-occipital joint, and the mandibles were separated from the crania by sectioning through the
DENTISTRY
vertical ramus of the mandibles using a band saw.
Teeth and oral mucosa were cleaned in running
water and dried. The cheek teeth were examined,
and the presence of malocclusions, missing teeth,
diastemata, gingivitis, or periodontal disease (defined as gingival recession with hyperemia and feed
packing) was also noted. Based on these findings,
an oral pathology score (OPS) was assigned to the
mouth, as previously described.16 Briefly, each abnormality was assigned a single point, and the OPS
was the sum of all abnormalities found. No weighting was given to any one malocclusion or soft tissue
abnormality.
Maxillary and mandibular cheek teeth were photographed using a digital camerac with a scale positioned at the level of the occlusal surface to account
for any size distortion of the image. A computerbased photographic programd linked to a personal
computer with an attached graphic pade was used to
trace the outlines of the teeth and their internal
morphology. By this method, a black and white
replica of the occlusal surface was created (Fig. 1).
Morphological variables were measured and calculated from these images for the maxillary and mandibular arcades (Table 1). The maxillary and
mandibular measurements were combined to give a
total value representative of the complete amount of
enamel and surface area within the mouth of each
animal.
Gastrointestinal samples were collected within 30
min of death. A 1-l representative sample of the
agitated contents of the stomach was obtained in
addition to entire fecal balls from the small colon/
rectum. Both samples were frozen immediately at
⫺20°C. Feed and gastrointestinal samples were submitted to a commercial laboratoryf for digestibility
analysis, including dry matter, crude protein, aciddetergent fiber, neutral detergent fiber, energy, and
acid insoluble ash content (AIA). Apparent digestibility was calculated using the acid insoluble ash
method.17–19
Mean forage particle size was measured using a
purpose-built forage particle separator in accordance with American Society of Agricultural Engineers (ASAE) Standards 1998 (R2007).20 Fecal
particle size analysis was similarly undertaken, in
accordance with ASAE Standards 1998.21
Statistical analysis was performed using a Windows-based computer package.g A one-way analysis
of variance (ANOVA) was used to determine
whether there were significant differences in the
apparent digestibilities of ingested hays and
whether there were hay group effects on hay (preingestion), stomach, and fecal particle size, independent of dental variables. A linear regression was
used to examine the effect of oral pathology score on
stomach and fecal particle size before and after controlling for feed group. The level of significance
was set at p ⬍ 0.05. A Spearman rank correlation
was used to examine the effect of dental variables on
feed digestibilities. Interpretation was as follows:
Fig. 1. (A) A graphic representation of the mesial and distal infundibular surface area and enamel ridge perimeter distance in maxillary teeth in a maxillary specimen. (B) A graphic representation of
the outer enamel ridge perimeter distance and inner surface area of
the maxillary cheek teeth in a maxillary specimen. (C) A graphic
representation of the outer surface area of the maxillary cheek teeth
in a maxillary specimen. (D) A graphic representation of the
enamel ridge perimeter distance and inner surface area of the mandibular cheek teeth in a mandibular specimen. (E) A graphic representation of the outer surface area of the mandibular cheek teeth
in a mandibular specimen.
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DENTISTRY
Table 1.
Explanations of Measured and Calculated Maxillary, Mandibular, and Total Horse Cheek Tooth Occlusal Variables
Data Variable
Max-TSA
Max-ISA
Max-OSA
Max-TERPD
Max-IERPD
Max-OERPD
Man-TSA
Man-ISA
Man-OSA
Man-TERPD
Horse-SA
Horse-ISA
Horse-OSA
Horse-ERPD
Horse-IERPD
Horse-OERPD
Explanation
Maxillary total surface area
Maxillary inner surface area ⫽ area of the tooth bounded by the OERPD (Incl. infundibulae)
Maxillary outer surface area ⫽ area of external cementum (OSA ⫽ TSA-innerSA)
Maxillary total enamel ridge perimeter distance (TERPD ⫽ IERPD ⫹ OERPD)
Maxillary infundibular enamel ridge perimeter distance ⫽ mesial and distal
Maxillary outer enamel ridge perimeter distance
Mandibular total surface area
Mandibular inner surface area ⫽ area of the tooth bounded by the ERPD
Mandibular outer surface area ⫽ area of external cementum (OSA ⫽ TSA-innerSA)
Mandibular enamel ridge perimeter distance
Max-TSA ⫹ Man-TSA
Max-ISA ⫹ Man-ISA
Max-OSA ⫹ Man-OSA
Max-TERPD ⫹ Man-TERPD
Max-IERPD
Max-OERPD ⫹ Man OERPD
␳ ⬍ 0.1, trivial correlation; 0.1 ⱖ ␳ ⬎ 0.3, slight
correlation; 0.3 ⱖ ␳ ⬎ 0.5, moderate correlation;
0.5 ⱖ ␳ ⬎ 0.7, substantial or large correlation; ␳ ⬎
0.7, very large correlation.22,23
3.
Results
The mean age of the horses was 7.6 ⫾ 6.31 (SD) yr
(range, 2–21.2 yr). There was no significant effect
of body size (0, standard horses; 1, large horses) on
dental morphology. Horses had a mean OPS of
5.75 ⫾ 5.07 (range, 0 –17). There was no statistical
increase in OPS with age.
Nutrient analysis indicated that there was no significant difference in apparent digestibility between
the three hays fed to the horses (p ⬎ 0.05). Despite
a significant difference in mean particle size of the
ingested hay (p ⬍ 0.0001), there was no significant
difference in ingesta particle size within the stomach. Additionally, particle size of the feces (1.36 ⫾
0.16 mm; range, 1.15–1.72 mm) was not significantly different from that of the stomach (1.29 ⫾
0.26 mm; range, 0.69 –1.76 mm).
There was no significant correlation between OPS
and particle size of stomach contents or feces. There
was also no significant correlation between OPS and
the difference in feed and stomach particle size, even
when controlling for diet. There was no correlation
between any of the dental variables and apparent feed
digestibilities.
4.
Discussion
This study did not find any correlation between dental morphological variables and feed digestibility.
There was also no correlation between mean particle
size of the stomach contents and feed digestibility, a
finding that is in agreement with the digestibility
studies in red deer.12
As horses age, the amount of residual crown is
reduced, until it equals that of the exposed crown.
Further attrition will result in loss of occlusion and
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2008 Ⲑ Vol. 54 Ⲑ AAEP PROCEEDINGS
eventually tooth loss. Despite this loss, we did not
see clinically apparent weight loss associated with
the loss of occlusion (in most cases), nor did we see
evidence of insufficient mastication or poor apparent
feed digestibility. Similarly, although there are
many influences on growth of young horses from
weaning to 4 yr of age, there is no clinically apparent
saltatory increase in weight or condition associated
with the eruption of more cheek teeth and the associated increase in enamel or surface area for mastication. The findings of this study may corroborate
these observations in that there was no difference in
apparent feed digestibility between immature
horses with incomplete dentition and mature horses
with a complete set of 24 cheek teeth.
The results of our study indicated that, despite
being fed one of three different hays (with attendant
variability in feed particle length), there was surprisingly little variation in the size of particles removed from the stomach. Interestingly, fecal
particle size was not significantly different from that
of the stomach, indicating that, despite the digestive
process, there was no further size reduction as the
ingesta passed down the gastrointestinal tract.
This would suggest that mastication is the most
important determinant of feed particle size reduction in the horse, as in the ruminant.10
More research is needed to determine the role of
equine dentition on the subsequent gastrointestinal
processing of ingested feed. There was no evidence
that dental abnormalities in our sample of clinically
normal horses contributed to mal-digestion or
weight loss, as shown by one of the horses in this
study who was 30 yr old. His OPS was 17, included
in which were missing teeth, diastemata, and advanced periodontal disease. This horse did not
have statistically larger stomach or fecal particle
sizes than other study horses, nor was there a reduction in apparent digestibility of feed in this
horse.
DENTISTRY
There is obviously a critical point at which oral
pain related to dental disease is sufficient to limit
voluntary feed intake. Tamzai9 published a study
on horses with chronic weight loss. In this case
series, a significant proportion could be linked to
dental disease, which, when treated, resulted in the
resolution of the weight problem. Discussions with
the author have shown that this series included six
cases of dental overgrowth impinging on soft tissue,
two severe wavemouths, one tooth fracture, and two
apical infections.h It is possible that these abnormalities were such that oral pain was the limiting factor. It may be that feed intake contributes
relatively more to forage feeding value than digestibility per se and hence effects of poor dentition
reducing voluntary feed intake have more of an impact on animal performance or growth than does a
reduction in digestibility. In addition to this, there
must be a minimum amount of tooth necessary for
efficient mastication. Below this limit, the reserve
capacity of the gastrointestinal tract is exceeded,
and mal-digestion or weight loss results. This morphological limit is currently unknown.
References and Footnotes
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Euthanyl Forte, Bimeda-MTC, Animal Health Inc, 420 Beaverdale Road, Cambridge, ON, N3C 2W4 Canada.
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