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It will be easier to understand and comprehend the
traditional treatment of under-run heels using egg bar
shoes (with or without wedges) if we again first review
the causes and pathogenesis of compromised heels. The
causes of under run heels that were discussed earlier
are a genetically weak foot, individual hoof / limb conformation,
breed, type of terrain on which horse is ridden, amount
of daily work, amount of turnout, moisture content of
feet, diseases of the feet and finally (and very important)
are the farrier practices applied to the horse along
with the intervals between hoof care. It is generally
a combination of the above causes that contribute to
under-run heels. For example, a horse with a genetically
week foot, which is not ridden, lives in a dry terrain
and is given unlimited access to pasture will maintain
a healthy heel whereas if the same horse is exercised
hard with a rider, has a high moisture content in its
feet along with limited turnout will ultimately develop
compromised heels. If we consider these causes of under
run heels and the chronicity that often accompanies these
cases which leads to severe damage, treatment aimed at
maintenance rather than resolution of the condition may
be all the can be accomplished.
The palmar/plantar section of the foot is made up
of horny (weight bearing) structures and soft tissue
(supporting) structures. These structures compliment
each other and interact with each other; if one structure
is damaged, it will invariably affect the other structures.
The weight bearing structures are the hoof wall, the
bars and a thin strip of sole immediately adjacent to
the hoof wall. The soft tissue supporting structures
are the frog, the digital cushion and the deep digital
flexor tendon. One can see the importance of the combined
structures in supporting the weight of the horse and
dissipating the energy of impact. A healthy functional
heel base as depicted in Figure 1 shows the hoof wall,
buttress, angle of the sole and bars intact as one solid
structure.
Figure 1
Before discussing the traditional treatment
of under run heels or trying to formulate a rational
approach to treating this problem, it may be helpful
to understand the process in which the heel is damaged
thus showing the dilemma encountered by veterinarians
and farriers when trying to improve this structure.
The hoof wall at the heel is immature horn making
it less rigid and hence more flexible when compared to
the hoof wall in the toe area. This increased flexibility
allows the normal physiology of the foot in the form
of expansion to take place but in turn makes the
heels more vulnerable to damage. As this syndrome begins,
the heels stop growing and over time the horn tubules
angle forward such that the most caudal extent of
the bearing surface of the wall is no longer at the base
of the frog but has moved forward toward the widest
part of the foot. The heels become lower; the horn
tubules lose their angle and bend until they reach
the point where they are parallel (horizontal) to
the ground. The hoof wall at the heels becomes thin,
separates and rolls under the heel. The rolled under
hoof wall at the heel puts pressure on the bar which
pushes it downward and laterally. This coupled with the
lack of growth in the heel area make the bar non-functional
and basically non-existent which in turn allows the
heels to become unstable. As the heels grow forward,
the frog and digital cushion move backwards. Figure 2A & 2B
show a classic under-run heel.
The heels at this point lose their mechanical strength
and can no longer bear weight and the weight bearing
function is shifted onto the soft tissue structures (frog,
digital cushion, deep digital flexor tendon) in the palmar
portion of the foot.
The traditional treatment for under-run heels as I
was taught in veterinary school was to use an egg bar
shoe which was often accompanied by a wedge pad to raise
the angle of the heels and correct the broken back hoof
pastern axis that is usually associated with this condition.
These shoes are fitted back to the bulbs of the heels
with the thought being that the increased ground surface
of the shoe will support the palmar section of the foot
and the bar would stabilize the heels and thus correcting
the problem. But how can one support compromised
structures (heels) that no longer have the ability to
bear weight? We can see right off that the egg bar
shoe places the bulk of the weight bearing on the supporting
structures of the heel which over time will damage these
structures further. The egg bar shoe may also act as
a moment arm or lever on the heels as shown on the radiograph
in Figure 3.
Figure3 – note the length
of the shoe behind the center of rotation
The extra length of the shoe will force the horse to
land heel first and by doing so, the length of the shoe
will act as a lever upon the heels which causes increased
pressure on the already damaged heels during weight bearing
thus preventing any chance of re-growth and actually damaging
them further. Any positive effect of the bar in stabilizing
the heels will be negated by this increased pressure placed
on the heels. I have seen minimal encouraging results from
using egg bar shoes in treating this condition over the
years in our podiatry practice and subsequently I rarely
use them. In a future essay, we will discuss some rational
treatments of under-run heels based on the extent of the
damage to the heels. |
