Evaluating for equilibrium of the equine foot
Pete Healey, Farrier
P.O. Box 704, Los Olivos, Ca. 93441
info@balancedbreakover.com
Introduction
Horses are shod for a variety of reasons; protection, enhancement of gait, traction or
lameness. Subsequently these feet are trimmed and reshod or left barefoot, why? To restore
balance to the foot. Balance embraces both shape and function of the foot in relation to the
ground as well as to skeletal structures of the limb, both at rest and at exercise (1). The
definition of Balance is an even distribution of weight or amount; one with a central pivot; to
bring into equilibrium. The wild horse offers the model for natures design as it simultaneously
wears and grows (2). Research estimates that the hoof goes through mitosis or cell division
every eight hours (3). Domestic horses are under a whole different set of circumstances as their
feet are influenced by breed type, stabling, feed and ridding disciplines. Domestic feet go
through a period of distortion that is corrected by manually trimming. The goal of this paper is
to outline specific parameters to evaluate equilibrium in the distal limb. Since the foot is
physiologically the same in every horse we can use numbers and angles that apply to the
biomechanics of the foot. A combination of radiograph and physical floor measurements are
used for optimum evaluation.
Radiograph Evaluation
Lateral radiographs are an effective tool to evaluate balance within the foot as we can
measure soft tissue as well as bone angles. Although the radiograph is static we are evaluating
for mechanical dynamics. A basic principle of mechanics is that every force, to be in equilibrium,
must have an equal and opposite force, hence for every dermal mass there is an equal
epidermal mass.
If the horse being evaluated is shod, leaving the shoes on for the x-rays offers the best
assessment as the shoe becomes the mechanical bottom of the foot(4) ( fig 1).
Common Areas of Interest
Hoof-Lamella Zone (H-L): The H-L zone is measured just distal to the extensor process and at
the tip of P3. The H-L zone is approximately half keratinized horn and half soft tissue
components. A healthy H-L zone measurement usually corresponds to 25% of the measurement
of the palmer cortex of P3 (3). A normal proximal measurement with a wider distal
measurement would indicate distal leveraging of the hoof wall. A normal proximal
measurement with a narrowing distal measurement is an indication of over rasping of the hoof
wall or a caudal rotation of P3. Thick measurements in the proximal and distal H-L zone are
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indicative of horses with acute or chronic laminitis. A foot that has an estimated H-L zone of 20
mm and measures 25 mm is significant as the extra 5 mm is in the lamella side which is a 50%
increase due to swelling or distortion.
Sole: The sole is measured in two places, under the tip of P3 and under the palmer rim or
wing of P3. A good rule of thumb for the distal sole would be the same thickness as the H-L
zone. This allows equal epidermal sole to solar corium and circumflex vessels. Surface area
under the palmer rim of P3 is usually deeper considering a healthy foot has a positive palmer
angle to some degree. Most horses that have less than 20 mm of foot under the wing have
collapsed heels and often show signs of wall buckling in the quarter-heel region. It is often
recommended to maintain at least 20 mm of depth for these reasons (5,6).
Break-over: Break-over is a point on the foot or shoe distal to Center of Rotation (COR) that
makes a radius and where the foot starts to roll from as the heels lift off during stride (7).Breakover of the hoof capsule is usually measured to its relationship to the tip of P3 as the Deep
Flexor Tendon (DFT) is connected to P3, the point of break-over in any foot would be the apex
of P3. Any part of the hoof distal
to that point that doesn’t roll as
the bone rotates would be out of
equilibrium with the bone.
Break-over is also a
consideration in the
craniocaudal balance of the foot.
Placing the break-over radius
closer to (COR) as with a rollermotion shoe helps with the
cranial rotation of the foot. This
affects the foot in several ways;
tension in the DFT as it affects P3
and the navicular bone; The
Hoof-Pastern Axis (HPA), the P1
vector and the Palmer Angle (PA)
of P3. Tension in the DFT is
greatest during the break-over
phase of the stride (8). This is
regulated by the Ground
Figure 1
Reaction Force (GRF) to the
leverage of the foot distal to the
apex of P3 and the dorsiflexion of the fetlock joint (9). Cranial rotation of the foot in soft ground
or through the use of a roller motion shoe produces a smaller fetlock joint angle (10). This can
ease the tension in the DFT, Superficial Flexor Tendon (SFT) and the Suspensory Ligaments (SL).
Consideration of where the foot is in the shoeing cycle is important when evaluating the
break-over distance as the foot migrates distally on an average of 2.5 mm per week (11).
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Although radiographs offer a good assessment of break-over, posturing of the horse on the xray blocks and the direction of the beam may distort the measurement (4)).
Palmer Angle: This is called the Plantar Angle in the hind feet. The palmer Angle (PA) is the
bottom angle of P3 to the ground, also referred to as the sole plane. The PA can be from 2 to 10
+ degrees in a sound horse (12). The PA is relative to the amount of foot mass under the palmer
rim of P3 and the HPA as the angle of one will regulate the angle of the other. A positive PA
usually ensures a healthy digital cushion- frog support system. This is necessary during optimum
load when the flexion of the coffin joint and the dorsiflexion of the fetlock produce
compression on the caudal hoof (13). The PA is largely regulated by the contraction of the DFT
and the mechanics of the foot distal to COR, this is important to know as not maintaining the
conformational PA can upset the equilibrium in the foot.
P1-P3 Axis: The P1-P3 axis is the
angle of dorsal P1 as it bisects dorsal
P3 (this is measured as the HPA on the
floor measurement), because of the
irregular shape of P1 this can be
measured by a line that bisects P1 and
crosses a line that is parallel to the
dorsal face of P3 (14). To use this
measurement as a basis, the cannon
bone should be at 90 degrees to a
level floor. Unless a horse is positioned
properly on the x-ray blocks this
measurement can be distorted. The
P1-P3 axis is a valuable measurement
in determining the proper PA as a 2
degree palmer angle change will move
the P1-P3 axis 5 degrees (unpublished
data). How the horse postures himself
on the blocks can offer information
about pain in the feet, noting an
extreme vertical or horizontal P1.
Figure 2: Red line positions COR at 90˚ to ground; Green line positions COR
at 90˚ to palmer P3: Yellow line marks a quarter crack, note position to
COR.
A normal foot has somewhat of a negative P1-P3 axis as viewed with the foot in the resting
position, which flexes into a zero degree axis as the foot is loaded. The P1-P3 axis is important
as it influences the tension on the DFT, its’ relationship with the navicular bone and the vector
of P1 to weight bearing on the foot through COR (15).
Center of Rotation: Also known as Center of Articulation, Center of Rotation (COR) is located
in the center of the condyle of P2 (16). Literature denotes that a perpendicular line dropped
from COR should equally bisect the weight bearing portions of the foot (17). Although a more
accurate evaluation would be that COR bisects the foot at a 90 degree axis to the palmer angle
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of P3 (fig2). This would take into consideration the areas of soft tissue compression or strain
relative to the DFT attachment to P3, the ground reaction force, the HPA and the dorsiflexion of
the fetlock. Since the soft tissue parameters usually stay constant to the bone, measurements
can be transferred from the radiograph to the foot and vice versa.
COR can be described as the center pivot, it is the nucleus to equilibrium in the foot. To
evaluate COR measure across the center of the condyle of P3 and mark a point half way, draw a
line from this point distally at a 90 degree angle through palmer P3 to the ground surface of the
foot or shoe. Next draw a parallel line from the tip of P3 to the ground surface, the distance
between these two lines is the COR measurement. An equal measurement palmer from the
COR line will denote proper caudal support; this geographical point on the solar surface of the
foot is where the central frog sulcus terminates at the bulb of the heel and hair line. This will be
described later in the floor measurements and is an important aspect in transcribing
radiographs to the foot. Important measurements about COR concerning equilibrium would be:
•
•
•
COR to the tip of P3 vs. COR to break-over
COR to break-over vs. COR to caudal heel support
Caudal heel support to COR vs. caudal heel support to the drop of the fetlock (fig3)
Floor Measurements
The floor evaluation consists of physical
measurements and visual characteristics of
the foot. The visual characteristics give us our
first impression of equilibrium in the foot.
Most importantly is the growth of the hoof
wall just distal to the coronary band. A foot
that maintains good equilibrium throughout
the shoeing or trimming period has an equal
growth of new horn wall distal to the coronet
band. Buckling or diminished growth are signs
of compressive loading (18).
There are four areas addressed for
equilibrium; palmerodistal balance,
mediolateral balance, craniocaudal balance
and depth of foot.
PalmeroDistal: Palmerodistal balance is the
Figure 3: Leverages and support about Center of Rotation (COR); Tip of
P3, palmer frog sulcus (PFS), break-over (BO), palmer shoe (PS).
relationship of the toe and heel of the hoof
capsule as it relates to P3 and the mechanics
of COR. This aspect of balance is the most deliberated on as it can affect every plane of balance
in some way. The common goal being to restore equilibrium to the foot for movement. Comfort
and prevention of lameness. Using COR as a guide we can easily map out the solar surface of
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the foot for the position of P3 and evaluate and trim the foot accordingly. COR on the solar
surface of the foot is a point just distal to the central frog sulcus, often referred to as the
‘Bridge’ (19). Palpating the coffin joint by placing an index finger directly behind the coronet
band at the dorsal aspect of the foot and then grasping the solar surface with a thumb so as
trying to touch the index finger will also identify COR. This is useful in evaluating feet that are in
pads or have no visible frog sulcus. This also gives one an immediate sense of balance of the
foot according to the center. To use COR as a guide, measure its’ position on the solar surface
of the foot to the palmer junction of the central frog sulcus at the heel bulb. This measurement
when reversed distally from COR will denote the tip of P3. This is the same measurement used
on the radiograph from COR to the tip of P3. In a current study, feet that were measured
physically and then measured on the radiograph, the floor measurement was in an average of 3
mm to the radiograph. This is a
very accurate way of identifying
COR to map out the solar surface
of the foot. This is important as we
can take measurements from the
radiograph to the foot. One simply
has to measure distally from the
junction of the central frog sulcus
at the heel bulb and make a mark
that coincides with the radiograph
measurement from COR to the tip
of P3. Once COR is established the
foot can be evaluated for breakover, caudal heel support and
palmerodistal hoof growth around
COR. A foot in equilibrium would
Figure 4: A foot that measures 65 mm from COR.
have break-over at the tip of P3
and heel support where the frog
sulcus terminates into the heel bulb; these would be equal measurements from COR (fig4).
MedioLateral: Mediolateral balance is described in the square standing horse as a line that
bisects the limb longitudinally is intersected at 90˚ by a transverse line across the heels (20).
The inside to outside level of the foot can dictate how the foot lands and loads medial-laterally.
Traditionally this is simply done by sighting down the hoof in the solar position using the leg as
a reference. Using a T-square with the handle of the ‘T’ positioned over the tendon bundle
along the long axis of the cannon bone is consistently accurate and offers more information to
the solar surface of the foot as it can be assessed at the heels, quarter and toe. Conformation of
the limb proximal to the hoof capsule can determine how the foot grows or distorts to the
weight of the horse producing an asymmetric hoof. Although they appear asymmetrical often
these feet are symmetrical medial-laterally when the lengths of the hoof walls are measured,
the more vertical medial wall giving the optical allusion of being longer. It has been published
that most feet tend to land laterally first (21) and often dorsal-palmer radiographs show
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remolding on the lateral rim of the coffin bone. Depending on the flexibility and vertical wall
distortions of the foot, some feet will relax once the shoes have been pulled and trimmed and
lose their medial-lateral balance
during the shoeing procedure.
Using a T-square before applying
the shoe can determine which
areas need to be rebalanced.
When evaluating a foot for mediallateral balance a T-square can be
used as a point of reference to
measure from, for example a foot
may be high in the lateral toe
quarter 10 mm or perhaps a foot
may be low in a medial heel
quarter 5 mm. It is important to
note that when using a T-square
doesn’t always mean removing
Figure 5
foot to balance to the ‘T’, often
artificial hoof repair needs to be added to correct balance. A foot in equilibrium is level
mediolaterally to the long axis of the cannon bone and is contoured for palmerodistal balance
(fig5).
Craniocaudal: Craniocaudal balance is the angle of the hoof and its relationship to the pastern
(22). A caudal rotation would be a low heel and broken back hoof-pastern axis (HPA) and a
cranial rotation a normal to high heel and straight or broken forward HPA. Often this is
assessed as a visual alignment of the
pastern to the hoof capsule. This can
be misleading as some feet will have
a relatively high hoof angle and
palmer angle (PA) of P3 and a
broken back HPA or a relatively
normal hoof angle with a flat to
negative PA and a straight to broken
forward HPA. An accurate evaluation
of the PA of P3 can be done by laying
an object such as a rasp in the
shallow cup of the central frog
sulcus on the long axis of the frog,
the angle difference between this
and the ventral angle of the foot is a
consistent way of estimating the
palmer angle of P3 (23). To
accurately measure the HPA a
Figure 6: Static position, this foot would flex to -5˚, a 2˚ increase in palmer
angle would adjust this to zero when flexed.
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goniometer can be used (a). One part of the goniometer is placed on the dorsal surface of the
hoof just below the coronet, the second part is adjusted parallel to dorsal P1 and the angle is
read. To use the HPA measurement as a basis the horse should be standing on level ground
with the cannon bone at 90 degrees to the ground. The HPA is then read with the foot static
and then loaded by picking up the other foot. The difference between the two readings is the
amount of flexion and is usually 5 – 10 degrees in the standing horse, the steeper the hoof
angle the less flexion. In the flexed position the angle of the HPA would be 0 degrees to be in
equilibrium. A 2 degree hoof angle change will change the HPA 5 degrees. This can be used to
develop a precise strategy in balancing the foot (fig6).
Depth of Foot: Evaluation of sole depth and wall length coincides with the palmerodistal,
mediolateral and craniocaudal balance of the foot. A foot that is out of equilibrium has too
much foot or is lacking foot in one or all of these areas. Equilibrium of the sole would consist of
an equal dermal to epidermal ratio. Soles that give under moderate thumb pressure lack
sufficient epidermal sole to protect the solar corium against the force of the ground to the
weight of the horse.
A healthy foot has a prominent digital cushion and frog which would measure from the point
of COR through the heel bulb at about 65 mm. When looking at the solar surface of the foot the
wall growth is equal around COR and the plane of the frog. A foot that is over weighted palmer
to COR will have more wall growth distal to COR and a foot that is over weighted distal to COR
will have more foot growth palmer to COR. A heel or heels that are shorter than the solar depth
of the frog are signs of a caudal rotation or have inadequate wall length.
Equilibrium in the Shoeing Cycle
The foot of a normal healthy horse
grows an average of 10 mm of hoof per
month (2.5 mm/ week). Unless the horse
is barefoot and in an environment that
consolidates wear for growth the feet are
in a constant state of distortion until
manual trimming of the foot restores
equilibrium and the process starts over
again. Once applied the horseshoe
becomes the mechanical bottom of the
foot. Its’ application can either enhance
equilibrium or detract from it.
Biomechanical standards for the shoeing
industry are vague. The widest part of the
foot is often used as a reference point
(24) but may be distal to the center of the
foot due to capsular distortion (fig 7),
combined with a flat shoe fitted to the
perimeter of the foot the mechanics of
Figure 7
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the hoof become quite distant from that of the coffin bone. The shoe or shoeing package
should take into consideration the elements of equilibrium to maintain a healthy foot or to
establish health to the foot. Most horses revolve around a six week shoeing cycle. An average
foot will migrate distally about 15 mm; lose 2 degrees in the palmer angle and a corresponding
5 degrees in the HPA (25, 26). This is an acceptable amount of distortion for a healthy foot with
ample sole depth. Although a foot with weak heels and has to have more than 2 degrees of wall
removed distal to COR to regain equilibrium may need to be shod earlier as to not over load the
heel area.
Using measurements for a Shoeing Prescription
Example: Left Front Foot
Radiograph Measurements
Floor Measurements
Proximal HL
23 mm
COR
65 mm
Distal HL
21 mm
M-L
+5 mm Lateral
PC
19 mm
B-O
25 mm
Distal Sole
18 mm
HPA
-7 degrees
Palmer Sole
20 mm
PA
-1 degree
B-O
18 mm
COR
64 mm
COR/PS
68 mm
P1-P3
-31 degrees
Summary of Measurements
This foot is in a caudal rotation evident of the negative 1 degree PA. Using 25% of the PC
measurement as a guide for the HL zone we can see that the proximal HL is distorted due to the
caudal rotation and the distal HL is distorted due to the long break-over, 25 mm on the floor
measurement. The COR estimate on the floor was very accurate to the radiograph by 1 mm so
the floor B-O measurement would be more accurate than the radiograph although they are
within 7 mm. Both sole measurements are minimum to maintain equilibrium. The HPA
measures -7 degrees with the foot flexed which would indicate a 3 degree PA increase to
balance the foot, this is close to the P1-P3 measurement which is -31 degrees; flexing he foot
would reduce this to 20 degrees and divided by 5 would indicate a 4 degree PA increase.
The goal of trimming and shoeing is to reestablish balance ( figs 8,9). First would be to address
the break-over by rolling the toe back to COR. Reducing the toe wedge would move the PA to
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Figure 8: Example radiograph
Figure 9: Mark-up on radiograph to simulate shoeing Rx.
0+ degrees, no sole is removed as there is no surplus. The HPA is then measured with a
goniometer to determine the amount of wedge needed to balance the HPA to 0 degrees when
flexed. A 2 0r 3 degree wedge shoe is rockered into a roller-motion and applied. The belly of the
shoe is at COR which lets the Deep Flexor Tendon pull the foot into a cranial rotation; this will
help relieve the palmer sole. The belly of the shoe puts break-over palmer to the tip of P3 which
relieves the distal sole and allows distal movement of break-over during the shoeing cycle
without leveraging P3. Post shoeing radiographs would reveal a PA of 2-3 degrees. It is
important to note that a foot should not be wedged in a negative PA or without ample sole
mass. These feet should be rebuilt with an acrylic to regain foot mass before wedging.
Summary
Balance or equilibrium has a center axis. In the equine foot this is Center of Rotation, located
in the center of the condyle of P2. There are four components that pertain to equilibrium of the
foot; Palmerodistal, mediolateral, craniocaudal and depth of foot. Every trimming or shoeing
practice whether a simple trim or a pad package on a gaited horse has some kind of balance
strategy in mind. Radiographs combined with a floor measuring system offer an accurate
evaluation analysis that can be written into a specific prescription. Currently there is no
standardized measuring system for the equine foot. Most balance paradigms concentrate on
the hoof alone without consideration of bone and soft tissue inside and above it. Using an
evaluation system that identifies the biomechanical needs of the foot can greatly enhance the
communication between the veterinarian, farrier and owner or trainer as to the management
of the horses’ feet.
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a) Balanced Break-over Management, Los Olivos,Ca.
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