A Look at the
Dressage Horse’s Back
New research helps uncover equine spinal structure and function
By Hilary M. Clayton, BVMS, PhD, MRCVS
t
he dressage horse’s neck and
back play a crucial role in his
locomotion and performance.
injuries and diseases of the neck and
back occur frequently but can be difficult to diagnose because they commonly manifest as a reduction in
performance rather than as overt
lameness.
behavior. Tese studies are supervised
by physical therapist narelle Stubbs,
who has already performed a detailed
analysis of the thoracolumbar spine
in Toroughbred racehorses as part
of her doctoral research. She has now
turned her attention to horses of other
breeds and occupations, with special
emphasis on dressage horses.
Figure 1. Te vertebrae of the horse. C1: first cervical vertebra. C7: seventh cervical vertebra.
T1: first thoracic vertebra. T5: fifth thoracic vertebra. T10: tenth thoracic vertebra. T18: eighteenth thoracic vertebra. L1: first lumbar vertebra. L6: sixth lumbar vertebra.
equine limb injuries are relatively
straightforward to diagnose and treat,
but much less is known about the
causes, diagnosis, and treatment of
injuries to the horse’s neck and back.
Terefore, in the McPhail Center last
year we decided to study the structure,
function, and diseases of the horse’s
back. Tis year, we hope to continue
and expand this research to include the
horse’s neck. our goals are to provide
a better understanding of neck and
back injuries and their effects on range
of motion of the intervertebral joints,
dressage performance, and horse
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The Vertebral Formula
Te horse’s back includes the thoracic,
lumbar, and sacral regions of the vertebral column and the associated soft
tissues. What we call the “backbone” is
actually a series of bones or vertebrae
(singular vertebra; plural vertebrae):
usually eighteen in the thoracic (t) region, six in the lumbar (L) region, and
five in the sacral (S) region (Figure 1).
Te vertebral formula indicates
the number of vertebrae in each
region, with the typical formula for
the horse being t18, L6, S5. However,
these numbers may vary in individual
horses. Te most common aberration is for the last lumbar vertebra to
be fused to the front of the sacrum
(sacralized), resulting in t18, L5, S6.
it’s generally known that many
Arabians have only five lumbar vertebrae, but narelle’s study has shown
that, regardless of breed, about 30
percent of horses have only five lumbar vertebrae. Te exception is the
Standardbred breed, in which sacralization of L6 has not yet been seen.
Te five sacral vertebrae are fused
together to form a single bone called
the sacrum. Te bony fusion precludes any movement between adjacent sacral vertebrae and stabilizes the
spine in the area where the hind limbs
and pelvis are connected via the sacroiliac joint to the sacrum.
Anatomy of a Vertebra
each vertebra shares the same general features (Figure 2), but there are
Figure 2. Te parts of a single vertebra as seen from the front (left), and the relationship between two articulated vertebrae as seen from the side (right).
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regional differences in vertebral shape
and size that allow an anatomist to
recognize which part of the spine a
vertebra belongs to.
Body of the vertebra. A vertebra is built on a block of solid bone
called the body. Te thoracic vertebrae have short bodies; the bodies are
much longer in the lumbar region.
Conformational differences in the
lengths of horses’ backs are a result
of differences in either the number of
vertebrae or the length of the bodies
of the individual vertebrae. Longbacked horses usually have longer
vertebral bodies than short-backed
horses.
Te length of the vertebral body
affects the amount of deviation resulting from a change in angle at the
joint(s). Figure 3 shows this diagrammatically. each rectangle represents
a vertebral body, and there are 24
vertebrae (eighteen thoracic and six
lumbar). Te angle at each intervertebral joint bends the spine by 1 degree.
Clearly, the amount of displacement
to the side is greater when the vertebrae are longer.
Figure 3. Effects of the length of the vertebral
bodies on the horse’s ability to bend. Each gray
rectangle represents the body of a thoracic or
lumbar vertebra, as seen from above. Each
successive intervertebral joint is angled by
one degree. Te deviation (representing the
amount of lateral bending) is larger when the
vertebral bodies are longer.
Applying this principle to the
riding of lateral movements in dressage, a long-backed horse requires
less change in angle at the intervertebral joints than a short-backed
horse to achieve the same amount
of displacement of the shoulders
when performing a shoulder-in or
of the haunches when performing
travers or renvers. Put another way,
a short-backed horse needs to show
more suppleness to achieve the same
amount of lateral displacement of the
shoulders or haunches as his longerbacked competitor.
A short-backed horse
needs more suppleness
to achieve the same
result in lateral work
as his longer-backed
competitor.
Te bodies of the adjacent vertebrae are joined by the intervertebral
discs. Human discs consist of a central nucleus composed of a jelly-like
material, surrounded by rings of fibers
that attach into the bodies of the vertebrae. Te entire disc functions as a
shock absorber to disseminate forces
between adjacent vertebrae while
allowing some movement in all directions. Te movements are caused and
controlled by the core (trunk) muscles, especially the transverse abdominis and the deep spinal muscles. Tese
muscles are activated in anticipation
of movement to support and stabilize
the spine. equestrians need to develop
strength and control of their core
muscles in order to ride effectively
with an independent seat.
Compared with the vast amount
of human spinal research, very little
attention has been paid to the horse’s
intervertebral discs. Tere is doubt
as to whether horses’ discs have a
nucleus in the center or if they are
fibrous throughout, and this is one
of the issues that we are currently
investigating. We do know that equine
discs are relatively narrow, a finding consistent with there being only
a small amount of motion at these
joints. Te exception is the lumbosacral joint, which has a wider disc
and a larger range of motion.
At the McPhail equine
Performance Center, we tested the
mechanical properties of horses’ lumbar discs in a machine that is usually
used to study human intervertebral
discs. it was immediately obvious that
the equine disc is considerably stiffer
and less compressible than the human
disc. Tis is further confirmation of
our clinical impression that there is
little intervertebral motion in the lumbar area, and is consistent with the
notion that stability of the horse’s back
during locomotion is more important
than mobility.
Vertebral arch. Te arch extends
upward from the vertebral body to
enclose the vertebral canal. Te spinal
cord runs through the vertebral canal,
Figure 4. Normal facet joint (large white
arrow) and intervertebral foramen for passage of the spinal nerve (small white arrows).
Compare the normal joint with the arthritic
facet joint on the left (large black arrow),
which shows extensive new bone formation
uniting the vertebrae and occluding the intervertebral foramen (small black arrows).
protected by the body and arch. At
each vertebral level, the spinal cord
gives off a pair of spinal nerves, which
leave the vertebral canal through the
intervertebral foramen on each side. A
foramen (plural foramina) is an opening; many foraminae transmit nerves.
Te intervertebral foramen is located
between the vertebrae, just above the
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disc (Figure 2). Sometimes the foramen is partially occluded by bony outgrowths that are part of an arthritic
syndrome resulting in compression
of the nerve as it passes through the
foramen (Figure 4).
Bony processes project outward
from the arch and the body, providing
a place for attachment of the numerous muscles, tendons, and ligaments
that move and stabilize the spine.
Spinous process. Te spinous
process, which is sometimes referred
to as simply the spine of the vertebra,
projects upward from the middle of
the arch. You can feel the tops of the
spinal processes along the midline
of your horse’s back. When the back
muscles are poorly developed, the
spinous processes are prominent;
when the muscles are well developed
w
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or the horse is excessively fat, they lie
in a groove.
Te spinous processes increase in
height from t1 to t5, which usually
corresponds with the highest point
of the withers. From t5 to t10, the
spinous processes become shorter and
form the contour of the back of the
withers. if you look at Figure 1, you’ll
appreciate that, as the spinous processes get shorter, the vertebral bodies get closer to the surface. Te tips
of the spinous processes are covered
only by a ligament and the skin, which
makes them vulnerable to pressure
and abrasion from an ill-fitting saddle
or blanket. (For more on the relationship between blankets and back pain,
see my “Horse-Health Connection”
column in the May 2009 issue.) Te
presence of white hairs on the withers
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is evidence of previous injury that was
serious enough to damage the hair
follicles.
Adjacent spinous processes should
be separated by a space occupied by
the interspinous ligament. Sometimes
this space is absent, especially in
short-backed horses, allowing the
processes to rub against each other—
the so-called kissing spines (Figure
5). evidence of kissing spines can be
seen on radiographs and ultrasound
images. Te associated clinical signs
are highly variable, ranging from
asymptomatic to exquisitely painful.
Usually the signs are most evident
when the horse is ridden because the
rider’s weight extends the horse’s back
a little in the area under the saddle,
thereby pressing the spinous processes closer together. Affected horses
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Figure 5. At left, the spinous processes are separated in this postmortem specimen (large white arrows), but there is evidence of bony changes
(small white arrows) showing that the spines were impinging (“kissing spines”) when the horse was working, likely from the rider’s weight causing the
back to hollow slightly. At right, the spinous processes are very close and are obviously impinging at the top (black arrows).
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Figure 6. Normal separation of transverse processes (white arrows) and areas of impingement
of transverse processes, with new bone bridging across between the transverse processes (black
arrows).
may resent being saddled, refuse to
stand to be mounted, appear coldbacked, or show bad behavior (such as
bucking or rearing) when ridden.
Transverse process. Te transverse processes project outward on
the left and right sides of the vertebra
in the area where the arch joins the
body. Tey are particularly large in
the lumbar region, where they provide attachment for the back muscles
on top and the sublumbar muscles
underneath. (Tese muscle groups
separated by the transverse process
are the meat on a t-bone steak.)
normally, there is a space between
adjacent transverse processes that
is filled by the inter-transverse ligament, but we have seen a variety of
structural variations. Quite often
there is impingement or overriding of
the transverse processes, and a joint
may form between the transverse
processes. in other cases, new bone
forms, which may eventually fuse and
unite the two transverse processes
(Figure 6). it is reasonable to assume
that these lesions are painful and may
cause the horse to resist bending to
one or both sides or to be reluctant to
raise his back behind the saddle.
Articular processes and facet
joints. Te articular processes protrude above the transverse processes
on each side, and the articular processes on adjacent vertebrae form
synovial joints known as the facet
joints. Te facet joints vary in their
orientation in different regions of
the spine, and this affects the type
of movement that can occur. in the
lumbar region, the orientation of the
facet joints virtually precludes lateral
bending.
Te facet joints are frequently a
site of problems, especially arthritic
changes with formation of new bone
that causes pain, stiffness, or both
during movement. Veterinarians
usually diagnose facet arthritis ultrasonographically. treatment options
include nSAiDs (nonsteroidal antiinflammatory drugs) or local injections into or around the affected facet
joints. s
Meet the Expert
H
ilary M. Clayton, BVMS, PhD,
MRCVS, is a world-renowned
expert on equine biomechanics
and conditioning. She is the Mary Anne
McPhail Dressage Chair in Equine Sports
Medicine at Michigan State University’s College of Veterinary Medicine. Her
research focuses on the performance, health, and welfare of sport horses.
Dr. Clayton has earned her USDF gold, silver, and bronze medals, and she is
a member of the US Equestrian Federation Dressage Committee.
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