Comparison of mandibular motion in horses chewing hay and pellets

Abstract

Previous studies have suggested that temporomandibular joint (TMJ) kinematics depend on the type of food being masticated, but accurate measurements of TMJ motion in horses chewing different feeds have not been published.
The temporomandibular joint has a larger range of motion when horses chew hay compared to pellets.
An optical motion capture system was used to track skin markers on the skull and mandible of 7 horses as they chewed hay and pellets. A virtual marker was created on the midline between the mandibles at the level of the 4th premolar teeth to represent the overall motion of the mandible relative to the skull during the chewing cycle.
Frequency of the chewing cycles was lower for hay than for pellets. Excursions of the virtual mandibular marker were significantly larger in all 3 directions when chewing hay compared to pellets. The mean velocity of the virtual mandibular marker during the chewing cycle was the same when chewing the 2 feeds.
The range of mediolateral displacement of the mandible was sufficient to give full occlusal contact of the upper and lower dental arcades when chewing hay but not when chewing pellets.
These findings support the suggestion that horses receiving a diet high in concentrate feeds may require more frequent dental prophylactic examinations and treatments to avoid the development of dental irregularities associated with smaller mandibular excursions during chewing.

Full text

258 EQUINE VETERINARY JOURNAL
Equine vet. J. (2007) 39 (3) 258-262
doi: 10.2746/042516407X157792
Summary
Reasons for performing study: Previous studies have
suggested that temporomandibular joint (TMJ) kinematics
depend on the type of food being masticated, but accurate
measurements of TMJ motion in horses chewing different
feeds have not been published.
Hypothesis: The temporomandibular joint has a larger
range of motion when horses chew hay compared to
pellets.
Methods: An optical motion capture system was used to
track skin markers on the skull and mandible of 7 horses
as they chewed hay and pellets. A virtual marker was
created on the midline between the mandibles at the level
of the 4th premolar teeth to represent the overall motion of
the mandible relative to the skull during the chewing cycle.
Results: Frequency of the chewing cycles was lower for hay
than for pellets. Excursions of the virtual mandibular
marker were significantly larger in all 3 directions when
chewing hay compared to pellets. The mean velocity of the
virtual mandibular marker during the chewing cycle was
the same when chewing the 2 feeds.
Conclusions: The range of mediolateral displacement of the
mandible was sufficient to give full occlusal contact of the
upper and lower dental arcades when chewing hay but not
when chewing pellets.
Potential relevance: These findings support the suggestion
that horses receiving a diet high in concentrate feeds may
require more frequent dental prophylactic examinations
and treatments to avoid the development of dental
irregularities associated with smaller mandibular
excursions during chewing.
Introduction
The importance and complexity of the equine
temporomandibular joint (TMJ) were highlighted by Ramzan
(2006), who emphasised the need for information describing the
dynamics of mastication in relation to the burgeoning interest in
equine dentistry. The anatomy and relationships of the equine
TMJ have been described (Weller et al. 1999, 2002; May et al.
2001; Rosenstein et al. 2001; Rodriguez et al. 2006), but less is
known about TMJ dynamics, an essential for understanding
function and dysfunction in the masticatory apparatus
(Okeson 1998).
Evolution of the modern day horse has involved striking
adaptations in dentition. The coarse, fibrous nature of the feral
horse’s diet and the silica particles it contains, wear the occlusal
surface of the teeth. Continuous growth of the hypsodont teeth
compensates for tooth wear during mastication. As in other
herbivorous mammals, horses have a mastication cycle consisting of
3 phases: the opening, closing and power strokes (Baker and Easley
1999). It is during the power stroke that the lower cheek teeth sweep
across the upper cheek teeth to crush and grind the food.
Maintenance of a normal occlusal pattern requires the entire occlusal
surface of the cheek teeth to be worn, which implies the need for
adequate lateral excursion of the mandible. The lower dental arcade
is straighter and narrower than the upper arcade (anisognathism) and,
consequently, the mandible must move a considerable distance
laterally for the mandibular cheek teeth to make occlusal contact
across the entire surface of the maxillary cheek teeth. When the
mandibular and maxillary teeth do not completely abrade against
each other, enamel overgrowth tends to develop on the lingual aspect
of the mandibular teeth and on the buccal aspect of the maxillary
teeth. This may contribute to inefficient chewing, decreased
digestive efficiency, buccal and lingual trauma, bit discomfort and
reduced performance (May et al. 2001). One of the variables likely
to affect the chewing cycle is the nature of the food, especially its
fibrous content and particle size (Leue 1941; Baker 2002).
Several techniques have been used to study equine masticatory
kinematics. One of the more ingenious was a mechanical recorder
attached to the upper and lower jaws that used a pen to trace the
motion of the mandible relative to the skull on a stationary notepad
(Leue 1941). Two-dimensional video analysis (Collinson 1994;
Baker and Easley 1999) has been used to track the pattern of
mandibular motion but, since the lips are not rigid bodies, their
movements do not accurately represent motion of the skull and
mandible. Recently, optical motion capture has been used to measure
mandibular motion more accurately and in 3 dimensions in horses
chewing sweet feed (Bonin et al. 2006).
The objective of this study was to use an optical motion capture
system to compare mandibular kinematics in horses chewing hay vs.
pellets. The experimental hypothesis was that mandibular motion is
greater when chewing hay than pellets.
Comparison of mandibular motion in horses chewing hay
and pellets
S. J. BONIN, H. M. CLAYTON*, J. L. LANOVAZ and T. JOHNSTON
McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State
University, East Lansing, Michigan 48854, USA.
Keywords: horse; temporomandibular joint; mastication; kinematics
*Author to whom correspondence should be addressed.
[Paper received for publication 05.06.06; Accepted 01.09.06]

S. J. Bonin et al. 259
Materials and methods
The study protocol was approved by the institutional committee
for animal care and welfare.
Horses
The subjects were 7 horses (one Hanoverian, one Quarter Horse,
2 Thoroughbreds, one Morgan and 2 Appaloosas), age 4–15 years,
wt. 480–655 kg and height 155–175 cm at the withers. A
veterinary dentist (T.J.) had performed routine prophylactic dental
care of the teeth 1–6 months prior to the study and had verified
that no malocclusions were present.
Kinematic data collection
The marker locations were clipped and the skin cleaned with
alcohol. Twelve tracking markers (Fig 1) were attached to the skin
overlying palpable bony landmarks with cyanoacrylate glue
(Bonin et al. 2006).
Kinematic data were collected using 6 infrared cameras1
placed in a semi-circle around the volume that would be occupied
by the horse’s head. The data collection volume, which measured
approximately 2m x 1m x 1m, was calibrated by the motion
analysis system1. Errors in calculating linear measurements were
less than 0.88 mm (Lanovaz et al. 2002).
Data collection began with the recording of 2 stationary files.
During the first, the horse stood with its head in the calibrated
volume with the 12 tracking markers in place and 2 temporary
markers overlying the articular tubercles of the skull. Data were
collected for 1 s at 60 Hz while the horse was not chewing. The
temporary markers on the articular tubercles were removed and
attached to the condylar processes of the mandible for the second
stationary file. Again, the horse stood in the calibrated volume
while data were collected for 1 s at 60 Hz. The temporary markers
were then removed. These stationary files were used to calculate
the locations of the articular tubercles and condylar processes
from the measured locations of the tracking markers during post
processing of data. The reasons for using temporary markers on
these locations were that the proximity of the articular tubercles
and condylar processes made it difficult to attach and
differentiate markers over these bony landmarks; and also there is
more skin displacement relative to the underlying bones at these
locations.
Chewing files were recorded at a sampling frequency of
120 Hz. A data set was considered acceptable if the horse did not
move its head out of the calibrated volume, did not make any rapid
head movements and was not in the process of ingesting feed. The
horses were fed hay and pellets2in random order while the head
was in the calibrated volume. The hay was a mature, first cutting
with 84.4% dry matter and 44.6% fibre. The pellets and were a
commercial concentrated feed with 90.0% dry matter and
30.0% fibre.
When the feed type was changed, the horse was allowed to
consume the new feed type for at least 2 min before commencing
data collection to ensure that the prior feed type was no longer in
the mouth and a consistent mastication cycle had been established.
A minimum of 4 chewing cycles per trial and a minimum of
6 trials per feed type were recorded in each horse.
Data analysis
The stationary and tracking files were analysed through a
customised MATLAB3programme to determine mandibular
motion relative to the skull (Bonin et al. 2006). Using the
3D coordinate systems for the skull and mandible, described by
Bonin et al. (2006), the x axes were oriented rostrocaudally and
designated positive in a rostral direction, the y axes were oriented
mediolaterally and designated positive to the left and the z axes
were oriented dorsoventrally and was positive upwards. The
origin of the skull’s coordinate system was the midpoint between
the articular tubercle markers. The origin of the mandibular
coordinate system was the midpoint between the left and right
mandibular condyles (Fig 2). Angular motion was expressed in
terms of pitch around the x axis, roll around the y axis and yaw
Fig 1: Location of skin markers relative to the bones of skull and
mandible. Temporary markers (shown in black) were located: 1) over the
articular tubercle of the skull; and 2) over the condylar process of the
mandible on each side of the head during recording of the stationary files.
Tracking markers (shown in grey) were located as follows: 3) dorsally on
the caudal aspect of the mandibular ramus; 4) ventrally on the caudal
aspect of the mandibular ramus; 5) the notch where the facial vessels cross
the ventral edge of the mandible; 6) middle of the facial crest; 7) rostral
end of the facial crest; 8) dorsal midline of the face at the level of the orbit;
and 9) dorsal midline of the face, approximately 10 cm rostral to the level
of the orbit. Markers 1–6 were placed bilaterally, markers 7 and 8 were on
the dorsal midline. The black square indicates the position of the virtual
marker on the midline between the mandibular rami at the rostrocaudal
level of the rostral facial crest markers and at the height of the markers
placed over the mandibular notch for the facial vessels.
Z’ Z
Y
Y’
X’ X
Fig 2: Orientation of local coordinate systems for the skull (X,Y,Z) and
mandible (X’,Y’,Z’).

260 Comparison of mandibular motion in horses chewing hay and pellets
around the z axis of the skull. Translations were measured along
x, y and z axes.
Movements of the rostral part of the mandible during the
chewing cycle were described by creating a virtual marker on the
midline between the mandibular rami at the rostrocaudal level of
the rostral facial crest markers and at the height of the markers
placed over the mandibular notch for the facial vessels. This
virtual marker was tracked relative to the skull’s coordinate
system in the chewing files as a general indicator of mandibular
displacement.
Data were processed for the first 3 acceptable trials per feed
type for each horse. Each trial consisted of 3 complete,
consecutive chewing cycles, with a cycle starting and ending
when the mouth was in the fully closed position, which
corresponded with the minimum pitch angle. As the mouth
opened, the pitch angle increased, reaching a maximum value
when the mouth was fully opened.
In order to normalise data, the output from the stationary files
was subtracted from the tracking file output for all trials. The x, y
and z locations of the midline-mandibular marker were averaged
over the 60 frames of standing file data and subtracted from the x,
y and z values of the tracked files. This referenced the midline-
mandibular marker location to the standing positions with the
mouth closed. These data are referred to as ‘normalised’.
From the normalised data, the total displacement of the
midline mandibular marker in the x, y and z directions was
calculated by subtracting the minimum value from the maximum
value in each direction. The chewing cycle was described in
3 phases: the opening, closing and power strokes (Fig 3). The
opening stroke was from the fully closed position (minimum pitch
angle) to the fully open position (maximum pitch angle). The
closing stroke was from the fully open position to the most lateral
position. The power stroke was from the most lateral position to
the fully closed position. Displacement of the midline mandibular
marker in the Y-Z plane during each cycle was determined and
used to calculate the average speed of the marker in this plane
during the entire chewing cycle. The duration of each cycle was
measured and used to calculate the chewing frequency.
Statistical analysis
Three trials, each consisting of 3 chewing cycles, were evaluated
for the 2 feed types in the 7 horses. The measured variables were
averaged over the 3 cycles and the 3 trials to determine mean ±
s.d. for the horse. These values were used to calculate group
means for comparing mandibular motion variables when chewing
hay and pellets using a 2-tailed paired samples t test4with a
probability of 0.05 alpha.
Results
During the opening stroke, the virtual marker on the mandibular
midline moved ventrally, laterally away from the chewing side
and slightly caudally. During the closing stroke, the marker moved
dorsally, laterally in the opposite direction and therefore crossed
the midline, toward the chewing side and slightly rostrally. During
the power stroke, the mandible slid medially and dorsally as the
mandibular cheek teeth moved across the occlusal surface of the
maxillary cheek teeth and returned to the starting position. If the
horse’s head was viewed from in front of the nose, as in Fig 3, the
mandible moved in a clockwise direction for horses chewing on
TABLE 1: Mean ± s.d. of variables for horses (n = 7) chewing hay and
pellets. Displacements describe the motion of a virtual markers located
on the midline between the mandibles at the level of the 4th premolar
tooth
Variable Hay Pellets
Range of rostrocaudal displacement (mm) * 6.1 (1.0) 4.6 (0.7)
Range of mediolateral displacement (mm) * 19.9 (2.5) 15.4 (3.2)
Range of dorsoventral displacement (mm) * 15.8 (3.4) 13.9 (2.9)
Chewing cycle duration (s) * 847 (101) 678 (31)
Chewing cycle frequency (Hz) * 1.18 (0.14) 1.48 (0.11)
Mean speed of marker during chewing (mm/s) 77.6 (7.3) 79.0 (19.6)
* Indicates pairs of variables that differ significantly (P<0.05).
Fig 3: Path of a virtual marker located on the midline between the
mandibles at the level of the fourth lower premolar tooth in the frontal
(Y-Z) plane (above), in the sagittal (X-Z) plane (middle) and in the dorsal
(X-Y) plane (below). The traces are for a single chewing cycle of the same
horse chewing hay and pellets, with arrows indicating the direction of
motion. Grey, solid line: opening stroke; black, solid line: closing stroke;
black, dashed line: power stroke.
0
-5
-10
-15
Z axis (mm)
-15 -10 -5 0 5 10
Pellets
Hay
Z
Y
Y axis (mm)
Pellets
Hay
Z
X
0
-5
-10
-15
Z axis (mm)
-15 -10 -5 0 5 10
X axis (mm)
-15 -10 -5 0 5 10
Hay
Pellets
X axis (mm)
X
Y
6
4
2
0
-2
Y axis (mm)

S. J. Bonin et al. 261
the right side and in an anticlockwise for horses chewing on the
left side. For the chewing cycles recorded in this study, each horse
chewed only on one side: 4 horses chewed on the right and
3 on the left.
Comparisons between the chewing cycles for hay and pellets
showed that displacements of the midline mandibular marker in
the x, y and z directions were significantly larger when horses
chewed hay compared with pellets (Table 1, Fig 3). The duration
of the chewing cycle was significantly longer and, consequently,
chewing frequency was reduced when the horses chewed hay
(Table 1). The average velocity of the virtual midline marker
during the chewing cycle was not significantly different when
chewing hay vs. pellets, indicating proportional increases in the
distance travelled by the marker and the duration of the chewing
cycle.
Discussion
The results of this study support the experimental hypothesis that
mandibular motion is greater when chewing hay than when
chewing pellets. The masticatory cycle in herbivorous mammals is
generally considered to consist of 3 events: the opening, closing
and power strokes. A recent study (Bonin et al. 2006) described in
detail the rotational and translational movements at the TMJ during
each phase of the chewing cycle. During the opening stroke, there
is a downward hinge movement (a pitch rotation about the
transverse horizontal axis) combined with a rolling motion around
the rostrocaudal axis, that separates the upper and lower dental
arcades on the chewing side. At the same time, a yaw motion
around the dorsoventral axis swivels the mandible (crosses the jaw)
away from the chewing side. During the closing phase, a small
amount of roll brings the upper and lower arcades into apposition
on the chewing side, while the yaw swivels the mandible across the
midline. In the power stroke, the lower dental arcade slides across
the upper arcade in a lateral to medial direction. The slope of the
molar table angle probably dictates the extent of roll angle during
the power stroke: a steeper molar table angle results in more
mandibular roll and a more level molar table will have less
mandibular roll. Variations in table angles in the range of 6.3–19.3°
have been reported in horses age 3–19 years (Carmalt et al. 2005).
Ingestion of feed involves prehension and grasping of food by
the lips and incisors (Baker 2002); and mastication does not begin
until the rostral part of the oral cavity is filled with food. In this
study, horses ingested each food type for at least 2 min prior to
starting data collection, allowing time to fill the rostral oral cavity
and establish a regular chewing pattern. Mastication and
movement of food materials within the horse’s oral cavity has
been likened to the action of an auger. The tightness of the cheeks
holds feed within the intra-dental part of the oral cavity, the
occlusal surfaces of the cheek teeth crush the food and the cusps
on the occlusal surface direct the crushed material into the palatine
ridges in the interdental space. Pressure from the tongue squeezes
the crushed particles against the palatine ridges and, through the
rotary action of mastication, combined with tongue and cheek
compression, food is moved caudally in a spiral fashion (Baker
2002). Therefore, the tongue and cheek muscles also play a crucial
role in the masticatory process.
It has been suggested that masticatory movements change
according to the physical properties of the ingesta with the
mandible undergoing more lateral excursion when horses chew
food with a high moisture content, a high fibre content or a large
particle size (Leue 1941). Since the dry matter contents of the
2 feeds used in this study were similar (pellets 90.0%, hay 84.4%),
the larger mandibular displacement when chewing hay may have
been associated with the higher fibre content (hay 44.6%, pellets
30.0%) or larger particle size of hay. Bundles of fibrous food, such
as hay or grass, are more easily maintained on the occlusal surface
of the mandibular cheek teeth as the jaw moves laterally to its full
extent, thereby allowing a complete horizontal grinding
movement. When chewing particulate food, such as pellets, a
relatively small quantity can be positioned on the narrow
mandibular occlusal surface and its retention is facilitated when
mandibular movements are primarily vertical in direction, with
minimal lateral excursion.
Based on a figure published by Becker (1962), depicting the
results of an earlier study (Leue 1941) that compared mandibular
motion in horses chewing different feeds, lateral excursions of the
mandible can be estimated to be 60, 38 and 23 mm while chewing
grass, oats and bran, respectively. These values should be regarded
as approximations, since the published curves are too smooth to
represent raw data and the sketch may have been resized for
publication purposes. Based on these approximations, lateral
excursion while chewing oats was 63% of the lateral excursion
while chewing grass. In the present study, lateral excursion while
chewing pellets was 77% of the lateral excursion while chewing
hay. Lateral excursion might be expected to be greater for grass
than hay due to the higher moisture content of grass. Therefore,
the relative lateral excursions estimated from Leue’s study are not
dissimilar from the values reported here. In further support of an
association between feed characteristics and mandibular range of
motion, Baker (2002) concluded, from observations of video
recordings, that drier feeds were associated with smaller lateral
excursions of the mandible.
The amount of lateral displacement of the virtual midline
mandibular marker while chewing hay (19.87 mm) is considerably
less than the 44–45 mm of incisor displacement reported by
Collinson (1994), but this is not surprising since the measurements
were made from different parts of the mandible in the 2 studies.
The virtual mandibular marker was located at the approximate
level of the 4th premolar tooth, and would undergo less motion
than the incisors since it is closer to the point of TMJ rotation.
Distance along the rostrocaudal axis from the mandibular
condyles to the virtual midline mandibular marker is
approximately half the distance between the condyles and the
incisors. Consequently, lateral excursions of our virtual marker
would be expected to be approximately half of the excursions
measured at the incisors, which makes the displacement reported
by Collinson (1994) consistent with our measurements.
The mean width of the lower fourth premolar, the largest
tooth in the molar arcade, was reported to be 16.4 mm in a sample
of 16 horses (Collinson 1994) with the upper cheek teeth being
slightly wider than the lower ones (Dyce 1987). The lateral
excursion of 19.9 mm while chewing hay would be sufficient to
wear almost the entire occlusal surface. The 15.4 mm lateral
excursion while chewing pellets, would however, be insufficient
completely to wear the opposing occlusal surfaces, therefore
facilitating the development of sharp enamel points if the horse’s
diet comes primarily from concentrates. Under these
circumstances, regular dental prophylaxis may be needed to
prevent malocclusions from developing.
Rostrocaudal position of the mandible varies with head
position; the distance between the rostral edges of the upper and

262 Comparison of mandibular motion in horses chewing hay and pellets
lower first incisors is smaller when the mandible is horizontal than
when the poll is fully flexed. Rostrocaudal mobility is reduced by
dental lesions, and the amount of rostrocaudal motion increased
significantly after floating the teeth (Carmalt et al. 2003).
Interestingly, the range of rostrocaudal motion produced by
manual manipulation after floating (5.89 mm) was similar to that
of horses chewing hay in this study (6.1 mm).
Average velocity of the mandibular marker during the entire
chewing cycle was 77.6 mm/s for hay (44.6% fibre) and
79.0 mm/s for pellets (30% fibre). Although the frequency of the
chewing cycles decreased significantly when chewing hay, the
greater distance moved by the mandible resulted in there being no
change in average speed of this marker. Collinson (1994) reported
the speed in the frontal plane measured at the 4th premolar to
range 104 mm/s for high fibre hay (75% fibre) to 85 mm/s for low
fibre hay (55% fibre), which is somewhat faster than the values
reported here. However, a number of assumptions were made that
may have introduced errors: distances used to estimate speed of
the incisors were based on lip motion and these values were then
used to estimate speed attained by individual molar teeth during
mastication. Movements of the lips relative to the incisor teeth
could have introduced errors that were propagated through the
calculations.
Electromyographic measurement of masseter muscle activity
indicated similar chewing frequencies for foods with different
fibre content: 1.14 Hz for high fibre (75%) forages, 1.15 Hz for
medium fibre (65%) forages and 1.16 Hz for low fibre (55%)
forages (Collinson 1994). In the study reported here, mean
frequency of 1.18 Hz while chewing hay is similar to Collinson’s
findings, but pellets were chewed with a significantly faster
chewing frequency (1.48 Hz).
As a consequence of the smaller volume of feed and the more
rapid chewing frequency, horses that receive diets high in
concentrates spend less time chewing than horses that eat more
forage, which may also be a factor in the development of enamel
overgrowths. Gobel and Duffner (1954) noted that overgrowths
could be reduced by a high roughage diet, but their development
was encouraged by feeding a high concentrate diet. Enamel
overgrowths are found rarely in species that are constantly
grazing, such as zebras, Przewalski’s horses, mules or African
donkeys and are very slight when they do occur (Becker 1962).
No evidence of enamel overgrowths was found in fossil equine
skulls, whereas 92.2% of 1000 domesticated horses had sharp
enamel overgrowths and 78.8% had buccal trauma (Becker 1962).
A later study of 32,000 cavalry horses found enamel overgrowths
in 91.7% and shear mouth in 0.3% (Becker 1962). Overall, the
dental health of domesticated horses appears inferior to that of
nondomesticated equids. Two contributing causes confirmed by
this study are a smaller mandibular excursion and more rapid
chewing cycles when horses chew pellets vs. hay.
The results of this study support the suggestion that an ample
diet, high in the type of roughage consumed by nondomesticated
horses, promotes dental health through a greater range of
mandibular motion and a slower chewing frequency. However, the
calorific requirements of high performance horses are more easily
met by feeding more energy-dense concentrate feeds at the expense
of fibrous foods. Under these circumstances, it may be necessary to
perform dental prophylactic examinations and treatments more
frequently in order to avoid the development of dental irregularities
associated with reduction in chewing excursions.
Acknowledgement
This study was funded by the McPhail Endowment.
Manufacturers’ addresses
1Motion Analysis Corporation, Santa Rosa, California, USA.
2Purina Mills, St. Louis, Missouri, USA.
3The Mathworks, Natick, Massachusetts, USA.
4SPSS, Inc., Chicago, Illinois, USA.
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Author contributions All authors contributed to the initiation,
conception, planning, execution and writing. Statistics were by
S.J.B., H.M.C. and J.L.L.