Introduction
The hoof capsule is comprised of the hoof wall, sole,
frog, and bulbs of the heels, which, through the
unique continuous bond between its components,
form a casing on the ground surface of the limb that
affords protection to the soft tissue and osseous
structures enclosed within the capsule.1 The hoof
wall is a viscoelastic structure that has the ability to
deform under load and then return to its original
shape when the weight is removed.1 It is well accepted that abnormal weight distribution on the foot
or disproportionate forces placed on a section of the
hoof will, over time, cause it to assume an abnormal
shape.1–5 These abnormal stresses within the foot
will also predispose the foot to injury or disease.
Increased stress or weight bearing placed on a section of the hoof capsule may originate from a single
source or it may be from multiple contributing factors, such as abnormal limb conformation, strike
pattern, amount of work, type of footing, and inappropriate farrier practices. Excess stress placed on
one section of the hoof capsule can manifest itself in
a variety of ways, such as compressed growth rings,
flares or under running of the hoof wall, dorsal migration of the heels, and either focal or diffuse displacement of the coronary band.5–7 Distortion of
the hoof capsule of the forelimbs appears to be related to limb alignment and load, whereas deformation in the hind feet seems to be different and
related to propulsion. As the hoof capsule distortion of the forelimbs is commonly associated with
lameness and various disease processes, only the
forelimbs will be considered in this paper. As the
“normal” foot has never been defined, an attempt
will be made to describe what the author perceives
to be an ideal, good, or healthy foot.1,9–11 Palpation
of the hoof capsule often complements the visual
examination and the areas where palpation is relevant will be included. Any evaluation of the hoof
capsule seeks to identify deformation and changes in
the growth pattern, which indicate abnormal distribution of forces (stresses) on the foot. The most
common hoof capsule distortions encountered in
equine veterinary practice are the long toe-low
heel conformation, clubfoot, sheared heels, and
mismatched feet. As hoof capsule distortion and
abnormal loading usually accompany lameness, farriery will form part of or sometimes the entire
treatment.
Mechanism of Distortion
The hoof capsule morphology will indicate where the
hoof wall is unduly stressed; however, the clinical
evaluation has to be coupled with an understanding
of the abnormal distribution of forces that lead to
hoof capsule deformation. Increased load or weight
bearing on a portion of the hoof wall has three consequences: (1) it may cause deviation of the wall outwards (flares) or inwards (under running) from
its normal position, (2) it may cause the wall to move
proximally, or (3) it may decrease hoof wall growth.
A reduction in load or weight bearing would generally
have the opposite effect (i.e., the flared side). Understanding the biomechanical forces leading to hoof capsule distortions is paramount for the clinician
(veterinarian/farrier) in applying the appropriate
farriery to modify these stresses. Biomechanical
principles in the standing horse have been studied
extensively, and there are many excellent reviews of
basic biomechanics of the hoof in the veterinary
literature.1–7 Using biomechanical principles combined with a thorough clinical examination afford
the clinician an added dimension when formulating
a farriery plan to address a hoof capsule distortion.
Briefly, in the standing horse, the weight of the
horse passes through the center of rotation (COR)
and is borne by the limb that is supported by the
ground that opposes the weight with an equal and
opposite force. The force exerted on the foot by the
ground is termed the ground reaction force (GRF).
The term center of pressure (COP) is the point on
the ground surface of the foot through which the
GRF acts on the foot; however, this point is not
positioned directly under the COR of the distal interphalangeal (DIP) joint. Rather, it is positioned
dorsally, away from the COR of the joint, which
creates a lever or what is referred to as a “moment
arm.” The action of the GRF and its moment arm
creates a torque, that is, a force that produces or
tends to produce rotation or torsion. This torque is
the extending moment of the DIP joint. The extending moment of the DIP joint is balanced by an
equal and opposing flexing moment generated by the
deep digital flexor tendon
(DDFT) (Fig. 1).1–7 The
COP varies among horses but is approximately located in the center of the solar surface of the foot in
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Fig. 1. Moments about the distal interphalangeal joint
(DIP). Green arrow is the center of rotation, red arrow is the
GRF, and the blue arrow is the DDFT. (Courtesy of Andrew
Parks). |
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the standing horse. However, when the horse is
moving, the location of the COP changes dynamically. The position of the COP at any point in the
stride determines the distribution of forces between
the medial and lateral, and the dorsal palmar aspects of the foot. When the COP is moved to one
side of the foot, that side of the foot will be subject to
increased forces. If the COP is moved in a palmar
direction, the weight bearing or load on the palmar
hoof wall is increased. Relating this to hoof capsule
distortions, for example, if the COP is located more
dorsally due to increased tension in the DDFT,
the hoof capsule will develop a higher heel with a
flare in the dorsal hoof wall. Farriery is used to
change the location of the COP (to some extent)
and change the distribution of forces on the
ground surface of the foot. Therefore, when applying therapeutic farriery, biomechanical principles can be used to redistribute the load on the
bottom of the foot, change the COP, and decrease
the length of the extending moment arm (rollover).
Limb Conformation
When evaluating hoof capsule deformation, limb
conformation should be considered. Abnormal limb
conformation affects the landing pattern and stance
phase of the stride. Few horses have ideal limb
conformation and any change in conformation may
lead to a change in the distribution of forces within
the hoof capsule, leading to deformation. In the frontal plane, the forelimbs should be of equal length and
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Fig. 2. The distal phalanx within the hoof capsule will be offset
laterally. The coronary band will be displaced proximally on the
media quarter/heel. |
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Fig. 3. Horse with one heel bulb displaced proximally (sheared
heel conformation). Note the compressed contour of the pastern
above the displaced heel bulb. |
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size and bear equal weight. A line dropped from the
scapulohumeral joint to the ground should bisect the
limb. Certain types of abnormal limb conformation
have been described.
9 In the frontal plane, abnormal conformation is described as valgus (the limb’s
segment distal to the affected joint will deviate laterally) or varus (the distal segment of the limb will
deviate medially). The joint most often affected is
the carpus and, to a lesser degree, the metacarpophalangeal joint. Here, there will be excess load
placed on the hoof opposite the direction of the deviation. If a line dropped from the metacarpophalangeal joint through the digit to the ground does
not bisect the hoof capsule, the foot is considered
offset to one side (usually laterally) and, therefore,
increased load is placed on the opposite side of the
foot
(Fig. 2). In the transverse plane, conformation
abnormalities are characterized by axial rotations of
the limb or its segments, either laterally or medially.
For example, a horse with a narrow chest and a
lateral axial rotation will land on the lateral side of
the hoof and then load the medial side, resulting in
proximal displacement of the quarter/heel on the
medial side and causing the hoof deformation
termed sheared heels
12,13 (Fig. 3). A limb with a
medial (inward) rotation of the digit relative to the
third metacarpal bone (toed-in) may develop a hoof
with a diagonal asymmetry, with a narrow lateral
toe and medial heel and a wide medial toe and
lateral heel. The altered distribution of forces leading to hoof capsule deformations follow a logical
pattern where the overloaded sections of the hoof
are less developed and the under-loaded sections are
overdeveloped. In the sagittal plane, abnormal
conformation can best be described by the position of
the DIP joint, either a flexural deformity or marked
dorsiflexion (i.e., extension) of the joint. The shape
or conformation of the hoof in the sagittal plane will
be dependent on the tension in the DDFT, the integrity of the laminar apparatus, and the digital cushion, all of which determine the angle of the solar
margin of the distal phalanx. A flexural deformity
will overload the toe, whereas marked dorsiflexion of
the DIP joint will overload the palmar section of the
foot.
Healthy Foot
A morphological description of what is considered to
be a good or ideal foot can be used as a model not
only to relate or compare a hoof capsule distortion
but also as a template or guideline when formulating a farriery plan.11 Examination of the foot begins with the horse standing square on a firm, flat
surface and the foot viewed from all sides. Following visual examination of the feet, the horse should
be observed in motion, both going away from and
toward the examiner, on a firm flat surface to note
the landing pattern of the foot as it strikes the
ground. Finally, the ground surface is examined
with the foot off the ground. Additionally, small
changes in the shape of the hoof capsule (such as the
coronet and the digital cushion) may be further appreciated by careful palpation of the foot than by
visual inspection.
Dorsal Aspect
When the foot is viewed from the dorsal aspect, the
ideal hoof should be approximately symmetrical.
An imaginary line drawn between any two comparable points on the coronary band should be parallel
to the ground. The medial wall should be the same
height as the lateral wall, but because it is often
slightly steeper, it may be slightly shorter. An
imaginary line that bisects the third metacarpal
should bisect a line drawn between any two comparable points on the coronary band or the ground
surface of the hoof. Similarly, the hoof should be
symmetrically related to the distal limb such that an
imaginary line that bisects the third metacarpal
bone bisects the pastern and the hoof, allowing for
the slight asymmetry due to the different angles of
the medial and lateral wall (Fig. 4A).1 When the
foot is viewed from the dorsal aspect, the shape of
the forefeet may be asymmetrical, with one hoof
being narrower than the other (“mismatched feet”).
On palpation, the coronary band of a healthy hoof
should feel thick and spongy. There should be no
evidence of a “ledge” or “trough” behind the proximal
margin of the hoof capsule when palpated. A depression in the coronary band indicates that the
distal phalanx has displaced within the hoof capsule, a finding that can be present in sound horses.12
This palpable depression will generally be accompanied by a thin, flat sole, narrow frog, and contracted heels. The dorsal aspect of the coronary
band should also be palpated for effusion of the
DIP joint. This is often seen with horses that
have a broken back hoof pastern axis (HPA) and
synovitis of the DIP joint.
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Fig. 4. A, Dorsal view. A line drawn between any two comparable points on the coronary band should be parallel to the ground.
The hoof should be symmetrically related to the distal limb such that a vertical line should bisect the third metacarpal bone, the
pastern and the hoof. B, Lateral view. A line drawn down the dorsal surface of the pastern and hoof is the HPA. A vertical line that
bisects the third metacarpal bone should intersect the ground at the palmar aspect of the heels of the hoof capsule. |
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Lateral Aspect
When viewed from the lateral aspect, the angle the
dorsal hoof wall forms with the ground is variable
and typically related to the conformation of the digit.
The heel tubules of the hoof capsule should form an
angle with the weight-bearing surface similar to the
angle of the horn tubules in the toe region. Tradition has it that the angle of the wall at the heel
should match that of the dorsal hoof wall at the toe;
however, it varies and is generally a few degrees
less. The length of the dorsal hoof wall is similarly
variable, but is determined by the amount of sole
depth present. There are two guidelines that relate
the proportion of the foot to the rest of the distal
limb. First, the foot pastern axis describes the relationship between the angles made by the dorsal
hoof wall and the dorsal aspect of the pastern with
the ground. Ideally, the dorsal hoof wall and the
pastern form the same angle with the ground so that
the angle between them is 180° and the axis is considered straight. Second, an imaginary line that
bisects the third metacarpal should intersect the
ground at the most palmar aspect of the ground
surface of the hoof (Fig. 4B). The healthy coronary
band should have a gentle, even slope from the toe to
the heels and the hair should lie flat against the hoof
capsule; hair projecting horizontally may indicate
excessive forces on the associated hoof wall.1 The
width of the growth rings below the coronet should
be equal from toe to heel. A disparity in the width
of the growth rings between the toe and the heels is
indicative of nonuniform circulation of the coronary
corium or excessive forces below, because wall
growth is generally inversely related to load at the
bearing border of the foot.
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Fig. 4. C, Palmar view.
Illustration shows various contours of the junction of the heel bulbs, with the skin ranging from a wide heel to a severely contracted
heel with a poor digital cushion. D, Solar view. The ground surface of the foot should be approximately as wide as it is long (red lines)
and approximately symmetric about the long axis of the frog (blue lines). The heels should not project dorsal to the frog. (Courtesy
Dr. Andrew Parks). |
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Palmar Aspect
The heels are evaluated from the palmar aspect for
their overall width and height. The heels frequently
become narrower when the foot itself is narrow. The
overall height of the heels is readily assessed from the
lateral aspect but viewing from the palmar aspect is
useful to compare the relative heights of the two heels
when measuring from the hairline at the bulbs to the
ground. For example, in the case of the sheared heel,
one heel is displaced proximally relative to the other
heel. Another example is mismatched feet where
there is a marked disparity in heel height between
feet. The contour of the junction of the heel bulbs
with the skin can be evaluated relative to the width of
the hoof wall at the heels and the thickness of the
digital cushion (Fig. 4C).
Distal or Solar Aspect
When viewed from the distal surface, the ground
surface of the foot should be approximately as wide
as it is long.1,2 The foot should be approximately
symmetrical about the long axis of the frog; the
lateral side of the sole frequently has a slightly
greater surface area that corresponds with the difference in wall angles at the quarters described in
the dorsal view. The width of the frog should be
approximately 60–70% of its length.14 The ground
surface of the heels should not project dorsal to the
base of the frog, and the hoof wall at the heels and
the frog should be on the same horizontal plane.
Imaginary lines drawn across the most palmar
weight-bearing surface of the heels and across the
heel bulbs at the coronary band should be parallel
and both lines should be perpendicular to the axis of
the frog (Fig. 4D).1
The author further evaluates the solar surface of
the hoof capsule by drawing a line across the widest
part of the foot. This line forms a consistent landmark and is located just dorsal to the COR (of the
DIP joint). Using this line as a starting point, there
should be approximate proportions from this line to
the perimeter of the toe and to the base of the frog.
Hoof balance or a balanced foot has been used
historically to describe the ideal hoof conformation.
However, hoof balance has no consistent definition
and remains a concept. The author prefers to use
the term “proportional foot” to describe an acceptable foot conformation. This term can be used to access foot conformation from the lateral side as well
as the ground surface of the foot by using the COR as
the intersection between the proportions (Figs. 5A
and 5B).
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Fig. 5. A, Lateral view shows the proportions (red arrow) on
either side of the middle of the foot (black line). 5, Solar view
shows the proportions of the foot on either side of the widest part
of the foot. Yellow dot denotes the COR. |
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Fig. 6. A, Note the broken back hoof pastern axis. The yellow
arrow is the end of the ground surface of the hoof wall at the
heels. Note the proportions on either side of the middle of the foot
and the thickening of the coronet at the heels with the “knob”-
shaped appearance. B, Shows a marked decrease in the structural
mass of the digital cushion |
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Long-Toe, Low-Heel Foot Conformation
A low-heel conformation can readily occur with or
without excessive toe length. This type of foot configuration is so common in equine practice, especially in Thoroughbred horses, that it is thought to
be normal.15 A long-toe/low or underrun heel conformation (LT-LH) is defined as the angle of the
heels being considerably less than the angle of the
dorsal hoof wall. When this difference in angles is
considerable, it is characterized by a broken back
HPA where the angle of the dorsal hoof wall is lower
than the angle of the dorsal pastern. It is often the
result of leaving the heels to migrate dorsally when
trimming, which allows them to grow forward and
lose their angle. When evaluating the foot from the
lateral aspect, there will be disproportionate distances on either side of the middle of the foot to the
toe and to the heel. There may or may not be a flare
in the dorsal hoof wall. The coronet will reveal an
acute angle from the toe to the heel and the coronet
at the heels will thicken and begin to form a “knob”-
shaped appearance. The angulation of the horn tubules will decrease from toe to heel and may often be
parallel with the ground at the heel (Fig. 6). The
ground surface of the foot will again show a disproportionate distance from the widest part of the foot
to the perimeter of the toe and to the base of the frog.
The heels of the hoof capsule will have migrated
dorsally while the soft tissue structures are located
palmar to the end of the heels, and, in many cases,
the frog is situated distal to the bearing border of the
hoof wall. Interestingly, when observed in motion
on a firm flat surface, a horse with LT-LH conformation may have a markedly heel-first landing due
to the lack of ground surface in the palmar foot, the
horse may land flat, or the horse may land toe first
if they are experiencing discomfort in the palmar
foot.
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Fig. 7. A, Shows the increased pressure on the navicular bursa with a low heel combined with a negative angle of the solar border
of the distal phalanx (Courtesy Dr. Andrew Parks). B, Shows the effects of a LT-LH on the DIP joint (Courtesy Renate Weller). |
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A low hoof angle results in dorsiflexion of the DIP
joint, which concentrates weight bearing on the palmar section of the foot and increases strain on the
DDFT. This excess load, in turn, may cause increased stresses on the navicular apparatus and the
soft tissue structures associated with the palmar
foot and the joints proximally16 (Figs. 7A and 7B).
If low or underrun heels are allowed to progress,
this condition can be readily observed both visually
and radiographically; the point at which the angle
that the hoof capsule or the distal phalanx forms
with the ground is lower palmarly/plantarly than it
is dorsally. A negative angle of the solar border of
the distal phalanx, as noted radiographically, means
that the soft tissue structures (frog, digital cushion)
are underdeveloped or have decreased in mass, usually due to damage, or they have prolapsed palmarly
which allows the distal phalanx to descend distally
(Fig. 8). Biomechanically, it changes the angle of
insertion of the DDFT on the distal phalanx, increases the peak force on the navicular bone
bursa, and moves the GRF dorsally toward the toe
(Fig. 9).4,5
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Fig. 8. Radiograph shows a negative angle of the solar border of
the distal phalanx. Also note the alignment of the bones of the
digit, the lack of structural mass in the palmar foot allowing the
DP to descend, and the “knob”-shaped appearance of the soft
tissue structures. |
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Fig. 9. The descent of the distal phalanx changes the angle of
insertion of the DDFT on the DP and increases the tension in the
DDFT. This increase in tension in the DDFT increases the flexor
moment, which in turn increases the force of the extending moment
moving the GRF dorsally toward the toe (Courtesy Dr. Andrew Parks). |
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Farriery
The treatment of low or underrun heels is difficult,
and often the conformation of the heels can only be
maintained rather than improved. Farriery seeks
to reduce the length of the dorsal wall and redistribute the weight on the ground surface of the foot.
The traditional farriery for low or underrun heels is
to use an egg bar shoe to support the heels, often
accompanied by some form of heel elevation to raise
the angle of the heels and correct the broken back
HPA. However, it is questionable whether “support”
can be applied to compromised structures (heels) that
no longer have the ability to accept weight, and the egg
bar shoe may do little more that apply leverage to the
palmar foot.
The ability to improve the soft tissue structures in
the palmar foot and to produce new hoof wall growth
at the heels may be limited. In the author’s experience,
it appears that some form of structural framework
is necessary to support renewed hoof wall growth at
the heels. In the palmar foot, this “framework”
seems to be the digital cushion, frog apparatus, and
ungual cartilages, and when these structures are
compromised, renewed hoof wall growth is poor or
absent. Various techniques have been successful,
depending on the amount and integrity of the structures present. When possible, the author has had
success leaving the shoes off and allowing the horse
to be barefoot for 30–60 days.17 This method is
useful if the frog is located distal to the ground
surface of the foot, as it will put the frog back on the
same plane with the hoof wall. Otherwise, when a
prolapsed frog is present, the shoe should be removed, excess horn trimmed from the frog, and the
horse stood on a hard surface such as a rubber stall
mat for 24–48 hours before the trim.
Foot preparation begins in all feet by visualizing
the two basic landmarks on the ground surface of the
foot: the widest part of the foot and the base of the
frog. The palmar section of the foot is trimmed appropriately by using the widest part of the foot as a
starting point, and the heels are trimmed to healthy
horn when possible, making sure that all of the structures of the heel and the frog are on the same plane.
The toe is shortened accordingly, again using the widest part of the foot as a guideline. Many horses with
a LT-LH will have decreased sole depth, so, when
necessary, the toe length can be reduced by using the
nippers in a vertical plane across the toe (which preserves sole depth) rather than using them in the usual
horizontal plane. Following the trim, the heels are
assessed with regard to the structural integrity and
the structural mass present.
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Fig. 10. A, A Colleoni® shoe with a heel plate and impression
material across the palmar foot. B, A steel shoe with a spider
plate that will redistribute the load across the solar surface of the
foot. |
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A shoe that attempts to place the load over the
entire palmar foot complements the trim, not just on
the hoof capsule at the heels (load sharing concept)
and provides heel elevation to improve the HPA
when possible. If the structures of the heel are
intact and the hoof wall angle simply needs to be
raised, an open aluminum wedge shoe or an open
steel or aluminum flat shoe with a wedge pad can be
used to achieve the desired heel angle. Depending
on the severity that the heels are damaged or compromised, a load-sharing effect can be accomplished
by using a straight-bar shoe with a pad or degree
pad placed between the shoe and the foot. The
author is reluctant to use a heart-bar show if the
frog and digital cushion lack sufficient structural
mass. An open-steel or aluminum shoe with a heel
plate welded between the branches of the shoe or a
plastic bar wedge placed between the shoe and the
foot can also be used. Holes can be drilled in the
heel plate or bar wedge and some form of silastic
material placed underneath the plate or wedge to
create a deformable interface to spread the weightbearing function over all the structures in the
palmar foot. The same effect can be achieved by
placing a “spider” plate between the shoe and the
foot. Impression material is placed on the ground
surface of the palmar foot, starting in the middle of
the frog and extending palmarly as far as desired.
The shoe and pad or plate are placed on the foot,
and the heel of the shoe is pressed into the impression material, forming a slight wedge. Two nails
are placed in the toe of the shoe and the foot is held
off the ground until the impression material cures
(Figs. 10A and 10B). Glue-on technology may be
helpful to improve the structures in the heel but
should not be applied long-term because damage to
the hoof wall is thought to occur from the heat generated by the composite, and they have been shown
to decrease expansion at the heels.18
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Fig. 11. Shows the biomechanical effects of moving breakover in a palmar direction. |
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Farriery for low heel conformation is often based
on trial and error and combinations of various methods but depends on assessing the structures present,
the footing, the athletic pursuit of the horse, and
client expectations. Any form of farriery for LT-LH
should be accompanied by enhancing breakover.
Moving breakover palmarly can be accomplished in
a variety of ways, such as rolling or rockering the toe
of the shoe or creating a rolled toe in the shoe by
using a hand grinder where the breakover begins at
the inner branch of the shoe. Moving breakover
palmarly/plantarly decreases the moment applied to
the DIP joint and appears to decrease the maximum
tension in the DDFT, which occurs toward the end of
the stance phase at the beginning of breakover (Fig.
11).1–7
Upright or Clubfoot Conformation
Flexural deformities have been reported as a cause
of decreased athletic performance and chronic lowgrade lameness in the mature horse.19–22 A clubfoot is defined as an upright conformation of the foot
associated with a flexural deformity of the DIP
joint.19,21 It is characterized by a broken forward
HPA, which is a reflection of a hoof capsule where
the angle of the dorsal hoof wall is higher than the
angle of the dorsal pastern. This broken forward
HPA or flexural deformity is created by some degree
of shortening of the musculotendinous unit (DDFT
and associated muscle bellies), causing the DIP joint
to be drawn into a flexed position. Biomechanically, as the tension in the DDFT increases, the COP
moves dorsally in the toe (Figs. 12A–C).19,22
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Fig. 12. A, Shows a clubfoot with a broken forward hoof pastern axis, the coronary band has lost the slope, and a flare in the dorsal
hoof wall. B, A radiograph with a flexural deformity and C shows the biomechanics of a clubfoot; when the tension in the DDFT
increases, the COP moves dorsally in the toe (Courtesy Dr. Andrew Parks). |
Examination from the lateral side generally reveals a broken forward HPA, the coronet assumes a
more horizontal position, poor hoof wall consistency,
a disparity of hoof wall growth with more growth at
the heel than at the toe is generally present, and
there will be some degree of a flare in the dorsal hoof
wall. Looking at the ground surface of the foot,
there will be disproportionate ground surface on
either side of the widest part of the foot, with the
palmar section showing less surface area, a thin
sole, separations at the toe, and the frog generally
receding due to excess hoof wall growth at the heels.
Observing the horse in motion, depending on the
severity of the flexural deformity, the horse will
either have a toe first landing pattern or will land
flat.
Farriery
High hoof angles with no or mild phalangeal misalignment can generally be improved by gradually
trimming the heels in a tapered fashion from the
apex of the frog palmarly to the heels. The solar
surface of the foot dorsal to the frog should not be
trimmed so all sole thickness is maintained. This
increases the ground surface of the foot and attempts to reestablish weight bearing on the entire
solar surface of the foot. The upright foot will often
have a thin sole, so toe length is reduced accordingly
from the outer dorsal hoof wall with an attempt to
remove any concavity in the wall. A flat-steel or
aluminum shoe fitted so it extends beyond the heels
of the hoof capsule and breakover moved palmarly to
the first nail hole to compensate for any increased
tension in the DDFT created by lowering the heels is
adequate. The polyurethane shoe (Polyflex®) provides another option to use with mild upright foot
conformation due to its compliance of the flexible
shoe with any DDFT tension and the mild heel elevation present in the shoe, which is enhanced by
creating breakover in the shoe with a grinder (Robert Hunt, DVM, personal communication).
Farriery for a high hoof angle with concurrent
phalangeal misalignment is a greater challenge.
The object of farriery is to realign the distal phalanx
within the hoof capsule, load the heels, and compensate for the shortening of the DDFT, all of which will
improve the HPA. Therefore, farriery is directed at
lowering the heels, but the amount to remove can be
hard to determine. In mild to moderate clubfeet,
the amount of heel to be removed can be estimated
by placing the thick end of a 2° or 3° pad under the
toe of the foot and allowing the horse to stand on it.
If the horse does not resent the tension placed on the
DDFT, the thickness of the degree pad can be removed in a tapered fashion starting at the widest
part of the foot. The toe is shortened by backing up
the dorsal hoof wall with a rasp. The trimmed foot
is fitted with a shoe that has the breakover forged or
ground into the shoe starting just dorsal to the apex
of the frog and tapering toward the toe to further
decrease the stresses on the DDFT.
A clubfoot with a marked flexural deformity
should still have the heels trimmed in order to load
the heels and unload the toe. However, heel elevation must be added to the shoe or incorporated into
the shoe to compensate for the shortening of the
musculotendinous unit. The necessity of adding
heel elevation will also be evident if the horse had a
toe-first landing pattern noted during the initial examination prior to the farriery. This can be determined following the trim by placing the trimmed
foot on the ground palmar to the contralateral limb
to observe for any space between the heels of the foot
and the ground (Fig. 13A). The author uses a wedge
shoe or places a wedge pad or a bar wedge between
the heels of the foot and the shoe to compensate for
the shortening of the tendon unit. If a wedge shoe
is selected, silastic material should be placed over
the solar surface between the branches of the shoe to
support the sole which will be higher off the ground.
This method allows the heels to be weight-bearing
but at the same time decreases the stresses in the
muscle tendon unit. Breakover is applied as described above (Figs. 13B and 13C). Severe flexural
deformities that result in chronic lameness can be
treated by performing an desmotomy of the accessory ligament of the DDFT combined with the appropriate farriery described above.9,22
 |
|
Fig. 13. A, Clubfoot placed on the ground behind the contralateral forefoot to test for tendon shortening after heels are trimmed.
Note the space under the heels. B, Radiograph of a clubfoot with a marked flexural deformity. Note the lucency in the palmar section
of the image that denotes the receded frog. C, The same foot seen in B after being shod with heel elevation to compensate for the
shortened muscle tendon unit and consequently be able to load the heels. Note the hoof pastern axis. |
 |
|
Fig. 14. Movement of heels in opposite directions when the
landing pattern is asymmetrical is responsible for the formation
of a fissure in the soft tissue at the base of the frog (yellow arrow). |
|
Sheared Heels
Sheared heels is a hoof capsule distortion resulting
from displacement of one heel bulb proximally relative to the adjacent heel bulb.12,13,23 This disparity
between the lateral and medial heel bulb is generally 0.5 cm or more. The displaced bulb is predominately seen on the medial side but can be seen on
the lateral side. Sheared heels appear to develop
as an adaption-distortion of the hoof capsule as a
consequence of limb conformation that results in an
abnormal strike and loading pattern of the foot on
the ground. The author believes that when the
adaptive ability of the hoof capsule is surpassed by
the excessive load on one section of the foot, the
ensuing type of hoof conformation can be a source of
unilateral palmar foot pain and predisposes the foot
to subsolar bruising, corns, quarter cracks, fracture
of the bar, and deep fissures within the base of the
frog (Fig. 14). This foot conformation is readily observed by standing behind the horse and noting the
relative distances measured from the heel of the
hoof capsule to the hairline at the bulbs of the heels
and noting any difference between the lateral and
medial heel. Observing the displaced heel/quarter
from the side, the wall will be straighter due to
increased load and may start to roll under; the coronet will be displaced proximally and assumes a
more horizontal appearance, and there will be
tightly packed growth rings below the coronet (Figs.
15A and 15B). Biomechanically, the position of the
coronary band is related to the balance between hoof
wall growth at the coronary band and the rate of
migration of the hoof wall distally (Andrew Parks,
personal communication 2017).
 |
|
Fig. 15. A, Dorsopalmar view of a horse with sheared
heels. Note the disparity between the length of the two heels
and the deformation of the structures of the medial side of the
foot. Black arrow shows compression of soft tissue. B, The medial side of the foot that shows proximal displacement of the
coronet, the tightly packed growth rings, and the heel beginning
to roll under. |
|
Furthermore, the
rate of migration of the hoof wall is a balance between an active process occurring in the lamellae to
cause them to move distally and the force on the wall
from the GRF. The increased load on a given side
of the foot over time appears to result in biological
remodeling rather than the heel being pushed proximally, in other words, the heel is “growing” out of
shape rather than being pushed out of shape
(Figs.
16A and 16B). There is a correlation between limb
conformation in which the limb has a rotational
deformity that changes the flight pattern of the limb
and ultimately the manner in which the foot lands.
When observing the horse in motion on a firm surface, it will be noted that the horse lands or contacts
on one side of the foot (generally laterally) and then
loads the other side of the foot. Looking from behind the horse, the direction of the carpus at break-over will be lateral, causing the horse’s foot to strike
the ground prior to impact at an arc rather than flat.
 |
|
Fig. 16. A, An illustration showing the balance between the growth of the hoof wall and its migration toward the ground, which is
countered by the force on the hoof wall. B, Shows the COP moved to the overloaded side of the foot, which can be addressed by the
appropriate farriery (Courtesy Dr. Andrew Parks). |
Farriery
Farriery is directed toward unloading the hoof wall
and decreasing the forces on the displaced side of the
foot. Because many horses with sheared heels will
have a toe-out conformation, traditionally, farrier
practices have advocated trimming the horse’s heels
so the ground surface is lower on the opposite side
from the side being displaced proximally. Intuitively, if the heel is longer on the displaced side
(measured ground surface to hairline), it is reasonable to trim the displaced side. When possible, the
author likes to remove the shoes and stand the horse
on a hard surface for 24 hours prior to the farriery,
as this allows the affected side of the foot to settle
into a more acceptable conformation.
 |
|
Fig. 17. A proportional foot with approximate distances (black
line) on either side of the widest part of the foot (red line). Note
increased ground surface trimmed under displaced heel (yellow
arrow). |
|
Farriery is initiated by removing the shoes and
again observing the horse walking on a hard surface,
noting the strike pattern of the foot. The author
will use a double-trimming method in an attempt to
improve and unload the distorted quarter/heel.
As described previously, the trim begins with a line
drawn across the widest part of the foot with a magic
marker. The frog is trimmed to where it is pliable
and the quarters and heels of the hoof capsule from
the middle of the foot are rasped palmarly so the
heels of the hoof capsule and the trimmed frog are on
the same plane if possible. An attempt is made to
create as much ground surface under the affected
heel as possible, which will often result in more
ground surface on the displaced side, which may
make that side marginally lower than the other side
of the foot. The toe and quarters are reduced appropriately so when the trim is completed, the surface area on either side of the line drawn or the
widest part of the foot will approximate each other,
resulting in a proportional foot (Fig. 17). Trimming
the quarter/heel on the displaced side of the foot is
logical, as it is the taller heel and it increases the
ground surface of the foot on that side. Following
the trim, the horse is again walked on a hard surface
and some improvement in the landing pattern is
generally noted.
 |
|
Fig. 18. A, A steel straight bar shoe with the nailing pattern for a sheared heel. B, Shows the platform created under the heels by
the bar shoe with the medial heel unloaded. C, Shows the space created under the displaced heel at breakover. Note the displaced
coronet and the hoof wall defect present. |
If the displacement is significant, the author’s
choice is a wide, web steel, straight-bar shoe fitted
symmetrically to the trimmed foot (Fig. 18A). Bar
shoes effectively increase the surface area of the
foot, allow the palmar/plantar section of the foot to
be unloaded, and decrease the independent vertical
movement at the bulbs of the heels. If the displacement of the quarter/heel is marginal, an open-heel
shoe can be used, but the trim remains the same.
Before applying the shoe, a second trim is performed
under the proximally displaced quarter heel, which
goes from 0 mm at the ipsilateral toe (e.g., inside toe
for medial sheared heel) to an average of 7 mm at
the affected heel. The amount of heel that can be
taken off in the second trim depends on the sole
depth at the seat of corn and on the severity of the
proximal displacement of the coronary band at
the sheared heel. The amount of horn, under the
sheared heel, which can be taken off with this second trim, ideally corresponds to the difference in
length/height between the two heels. Lowering the
hoof wall at the quarter/heel will create a space
between the shoe and the hoof wall on the displaced
side of the hoof (Figs. 18B and 18C). This improves
the landing pattern, unloads the affected heel, and
allows the heel bulb to settle down and assume a
more acceptable position. Feet with a low palmar/
plantar angle rarely have enough sole depth under
the affected heel for the second trim; in these cases,
the rest of the hoof wall can be raised with a full
leather or synthetic pad and impression material.
Impression material is placed in the palmar section
of the foot from the apex of the frog palmarly except
under the displaced heel/frog sulci where the second
trim was performed. After the shoe is attached to
the foot, the affected heel will rapidly descend onto
the shoe, making the original space created by the
second trim between the hoof wall and the shoe
disappear. As most horses with a sheared heel
have a predisposing limb conformation (e.g., a rotational deformity), these feet have a tendency to continue to deform the affected heel proximally and the
double trim method usually has to be applied to
some degree at each consecutive shoeing. Horses
with this type of hoof conformation should be reset
at 4–6-week intervals.
 |
|
Fig. 19. A, Shows the difference in dorsal hoof wall angles in a pair of mismatched forefeet. B and C, A side view of a pair of
mismatched forefeet where the LF has a low angle while the RF has an upright angle with a mild broken forward hoof pastern axis. |
Mismatched Feet
The management of mismatched hoof angles remains a controversial subject for both the farrier
and veterinarian. Mismatched feet could be defined as forefeet conformation that have a high or
upright hoof angle on one foot and a low hoof capsule
angle on the contra lateral foot (Figs. 19A–C).21,24
The difference between the forefeet could range from
a high hoof angle with a straight HPA to a clubfoot
with a flexural deformity and an overloaded low heel
on the contra lateral limb. The mass, integrity, and
difference in heel height will be the contributing
factors to the mismatched dorsal hoof angles (Fig.
20). Limb length disparity has been suggested as a
cause for mismatched feet, although it has not been
scientifically proven. Mismatched feet may contribute to poor performance, subtle lameness, and a
shortened anterior phase of the stride on the upright
foot. Traditional farriery seeks to elevate the heel
on the low foot and therefore match the forefeet.
However, this practice should be discouraged and
treatment should be based on farriery principles to
improve the structures and function of the individual foot.
 |
|
Fig. 20. Palmar view of mismatched forefeet. Note the difference in soft tissue structures in the palmar foot and the frog
receded in the foot with the high heel. |
|
Observation again begins by standing the
horse on a firm flat surface. Looking from the
front, the hoof with the upright hoof angle will be
narrower than the hoof with the low hoof angle.
The HPA should be evaluated from the side with the
third metacarpal bone always perpendicular with
the ground. It should be determined whether the
foot with the high hoof angle has a straight or broken forward HPA and whether the foot with the low
angle has a straight or broken backward HPA (Figs.
21A and 21B). If the foot with the high hoof angle
has a broken forward HPA, it should be considered
to have a flexural deformity or a clubfoot (Fig. 21C).
Looking at the heels from behind, the integrity of
the structures in this area should be evaluated and
the difference in the height of the heels should be
noted. It is also important to note whether the frog
is recessed between the heels of the hoof wall on the
foot with the high heel and whether the frog is
prolapsed distal or palmar to the hoof wall on the
hoof with the low hoof angle. Looking at the bottom of the foot, it is helpful to visualize a line drawn
across the widest part of the foot and look at the
proportions of the foot on either side of the line.
Again, it is important to note if the frog is distal to
the hoof wall or whether the frog is recessed below
the heels of the hoof wall. The frog being recessed
causes impaired function in the palmar section of
the foot and places the entire load on the hoof wall.
The horse should be evaluated in motion at both the
walk and the trot. First and foremost, it is important to rule out any lameness. It is especially important to evaluate the landing pattern of the
forefeet, as the foot with the high hoof angle will
often land toe first rather than flat. Lastly, the
horse should be trotted to note whether the horse
has a shortened stride on the high-heeled foot.
It must be remembered that a shortened stride on
one limb will cause the opposite foot to be on the
ground longer, which, over time, may create further
damage to the heel structures of the foot, with the
low heel resulting in a flat “panned out” low foot
angle.
 |
|
Fig. 21. Mismatched feet. A, Upright foot with a straight HPA. B, A foot with a low hoof angle with a broken back HPA. Note the
small shoes on both A and B. C, An upright foot with a broken forward HPA, defined as a flexural deformity or clubfoot. |
Farriery
Horses with a disparity between dorsal hoof wall
angles will generally have a straight HPA and the
hoof wall growth below the coronet from the toe to
the heel will be even. In this case, the author suggests using good farriery principles to apply the
appropriate trim and shoe for each foot on an individual basis. These basic farriery principles include the following:
Using the HPA
Using the COR as a guideline for trimming
and placement of the shoe.
Trimming the heels to the base of the frog or to
the same plane as the frog.
Managing horses with mismatched feet where one
foot has a high hoof angle with a flexural deformity
or a clubfoot becomes more complex. This type of
case will often present with a shortened stride on
the limb with the upright foot. Low or compromised heel structures may be noted on the opposite
foot from overloading the heel on that side due to the
shortened stride placing excess weight on that foot
over time. Managing these horses can be difficult
and the proper shoeing protocol may not be inherently obvious. Again, it must be emphasized that
each foot should be approached on an individual
basis (Figs. 22A, 22B and 22C). It is common to see
horses with mismatched feet shod with two different
size shoes; often a smaller shoe is used on the upright foot. This practice should be discouraged, as
the ground surface on both forefeet should be the
same.
 |
|
Fig. 22. Mismatched forefeet shod according to their individual conformation. A, A low heel with a spider plate and impression
material. B, A high heel with a wedge insert for heel elevation. C, A palmar view of A and B, showing the appropriate farriery for
the foot conformation |
Farriery for the clubfoot has been discussed previously in this paper. When approaching the foot
with the low angle, the clinician is often inclined to
wedge up the heels to improve the HPA. However,
this will place more stress on the already compromised heel structures. Although the HPA will appear improved immediately following the shoeing,
the long-term effect is exacerbation of the low angle,
further crushing of the heels, and prolapse of the
frog below the ground surface of the foot. Alternatively, the heels should be trimmed back to the widest point of the frog if possible, or an attempt should
be made to get the hoof wall at the heels and the frog
on the same plane. As much toe length as possible
should be reduced, generally using the dorsal hoof
wall, as there is usually decreased sole thickness in
the foot with a low heel. It should be emphasized
that when possible, it is extremely important to
obtain good quality radiographs prior to trimming to
determine the amount of heel and especially sole
that can be removed. Breakover, such as a rolled
toe or a rocker toe if there is adequate sole thickness
to allow it to be trimmed into the foot, is very beneficial, as it will further decrease the leverage at the
toe on the low-angle hoof. The COP on a low-angle
foot is further palmar than that of the upright or
normal hoof, therefore, the shoeing protocol is directed at moving the COP away from the overloaded
heels. Additionally, redistributing the load or load
sharing with the weight-bearing structures of the
low-angle foot may help to decrease the forces directed to the heels. This can be accomplished with
impression material, a pour-in pad, and a spider
plate or a heel plate added to the palmar aspect of
the shoe.
Conclusion
The clinical examination of the equine foot has been
well described and is generally performed in lameness cases.14 Evaluation of the hoof capsule during
the lameness examination will not only provide additional information as to the etiology and treatment
of the lameness but will also serve as a guideline to
apply therapeutic farriery and other preventive
measures to maintain a healthy hoof. The morphology of the hoof capsule reveals deformation and
changes in growth that occur following increased or
reduced force. The relationship between the limb
and the foot indicate conformations that predispose
the foot to abnormal weight bearing. Inversely, using the abnormal distribution of forces and the subsequent hoof capsule distortion as a template,
appropriate farriery or therapeutic farriery will
form at least part of the treatment plan. Here, it is
essential for the clinician to be familiar with the
biomechanics of the foot and how these forces can be
altered to change the distribution of forces or the
focal stresses on a given section of the foot.
ACKNOWLEDGMENTS
Declaration of Ethics
The Author has adhered to the Principles of Veterinary Medical Ethics of the AVMA.
Conflict of Interest
The Author has no conflicts of interest
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