run heel foot configuration is one of the most important and common
foot abnormalities facing the horse industry today. This type of foot
conformation is so common that it is often thought to be within
long toe-under run heel has been defined (by Dr Tracy Turner) as
occurring when the angle of the heel is 5 or more degrees lower than
the angle of the toe. Although this is a good guide, the heel angle
will always be somewhat lower than the angle at the toe as the heel
continually moves against the shoe during expansion causing wear
whereas the toe is static. The under run heel is generally
accompanied by a broken-back hoof pastern axis, which means that the
slope of the foot exceeds the slope of the pastern (Figure 1). This
condition could also be referred to as a dorsopalmar imbalance of the
toe-under run heel is of major concern to both veterinarians and
farriers. For veterinarians, this abnormal hoof conformation is known
to contribute to many foot related problems such as navicular
disease, chronic heel pain (bruising), DIP joint (coffin joint)
synovitis, quarter and heel cracks, and interference problems. The
farrier is confronted with the essential task of prevention,
correcting and/or maintaining this type of foot conformation.
Many causes may
lead to the development of a long toe-under run heel. There may be a
genetic basis for this problem, since many offspring appear to be
born with or acquire the same foot conformation as one or both
parents. In recent years, it appears that an increasing number of
foals are born with a low heel. Over or under trimming foals or the
frequency of trimming foals may contribute to this type of foot.
Environmental factors may predispose a horse to long toe-under run
heel. Well known farrier Eddie Watson has often stated, "A
horse's feet are the product of its environment". The type of
climate and footing a young horse is raised in whether dry and hard
or wet and soft may play a role. On a hard surface, a youngster will
wear its feet normally as opposed to soft footing where the foot
sinks in the ground. Horses that are subjected to continuous or
excessive moisture may be affected due to softening of the hoof.
Horses with forelimb conformation such as long pasterns or short
straight pasterns may be more likely to develop this type of foot.
This problem is particularly common in Thoroughbred horses but is
present in all breeds. At the racetrack, there is a tendency to
maintain a low heel coupled with a long toe with the erroneous
thought that this increases stride length. The use of a toe grab
increases the severity of the long toe-low heel. Farriers, in an
attempt to prevent shoe pulling in the front feet will use a shoe
that is smaller than necessary. This places the bearing surface of
the foot in front of the vertical axis of the limb, creating the same
mechanical effect. Over time the foot grows in this configuration.
Finally, if the toe is continually allowed to grow excessively long,
the heels will follow in the same direction. The long toe I am
referring to is the one seen when viewed from the bottom of the foot,
where the distance from a line drawn across the widest part of the
foot to the toe is markedly increased from a line drawn from the
widest part of the foot to the base of the frog. This distance can
and does increase in many horses over time as a result of farrier
of this process is logical. Many under run heels have their genesis
in a long toe. Direction of heel growth follows that of toe growth
i.e. as the toe is allowed to become long, the heels grows forward
and hence lower. This causes the pastern to move forward, creating a
broken back hoof-pastern axis (Figure 2).
As the heels become low, the horn tubules at the heels bend until they
reach the point where they are parallel to the ground. At this point
they are unable to support weight and will begin to thin, separate,
collapse and roll underneath the foot. The heels are further damaged
during expansion where the heel of the foot moves against the shoe.
Evidence of this can be seen by examining the solar surface of the
shoe and a noting a trough that is worn in the heel area. This
prevents any growth of hoof wall in the heels between shoeing
intervals yet toe growth is unaffected. The compromised heels lose
the ability to support weight causing more weight bearing to be
transferred to the frog, deep digital flexor tendon (DDFT), and
digital cushion. If the digital cushion becomes damaged, the frog
atrophies and is pushed out of position toward the rear of the foot.
The long toe sets up a mechanical moment arm (lever), causing bending
of the horn tubules and laminar tearing in the dorsal hoof wall. This
causes the soles to descend and flatten. Bending of the horn tubules
seen in the toe area results in a concavity of the dorsal hoof wall
("dish"), often accompanied by a full thickness toe crack.
The severity of the long toe-under run heel conformation depends on
the anatomical changes that have occurred within the foot.
It should be
easy to see how the altered mechanics of the foot can lead to
lameness. The acute hoof angle, which occurs with a long toe-under
run heel, increases the tension in the deep digital flexor tendon.
Increased tension in the deep digital flexor tendon in turn will
increase the pressure on the navicular bone and bursa. This is
accentuated by the lack of ground surface area in the palmar portion
of the foot as a result of the heels moving forward. The compromised
heels lose both the ability to support weight and to transfer this
function to the supporting structures above the heels, leading to
bruising within the heels and the adjacent soft tissue structures.
Hoof wall separations, corns, quarter and heel cracks may further
compromise the damaged heels. The long toe appears delay breakover,
which causes further tension to develop in the deep digital flexor
tendon. The delayed breakover keeps the foot on the ground longer,
often leading to interference problems such as overreaching, forging,
or scalping. The long toe sets up a mechanical lever arm, which
exerts an abnormal bending force causing the hoof wall at the toe to
deform hence the appearance of a concavity or "dish".
Internally, the lamina stretches or tears allowing the sole to
flatten. The sole now becomes more susceptible to bruising,
especially if heel pain is present as it promotes toe first landing.
This toe first landing often accounts for stumbling. Because of the
toe first landing, the sole wall junction (white line) becomes
widened, decreasing protection and allowing easier penetration of
organisms, which may lead to abscesses.
foregoing discussion, one can see the significance of maintaining
proper toe length, a normal hoof angle and a parallel hoof pastern
axis in order to prevent lameness associated with this type of foot.
In order to treat this problem, it is essential to understand the
mechanism, which leads to this abnormal hoof conformation.