1

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

2

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).Break-

over 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 roller-

motion 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

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).

Figure 1

3

Although radiographs offer a good assessment of break-over, posturing of the horse on the x-

ray 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.

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

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.

4

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

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

Figure 3: Leverages and support about Center of Rotation (COR); Tip of

P3, palmer frog sulcus (PFS), break-over (BO), palmer shoe (PS).

5

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 break-

over, caudal heel support and

palmerodistal hoof growth around

COR. A foot in equilibrium would

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

Figure 4: A foot that measures 65 mm from COR.

6

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 medial-

lateral 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

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 5

Figure 6: Static position, this foot would flex to -5˚, a 2˚ increase in palmer

angle would adjust this to zero when flexed.

7

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

8

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

9

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.

Figure 8: Example radiograph Figure 9: Mark-up on radiograph to simulate shoeing Rx.

10

References:

1) Parks AH. Foot balance, conformation and lameness: In: Ross MW., Dyson SJ editors.

Diagnosis and management of lameness in the horse. Saunders, 2003; 250-261.

2) Ovnicek G., Erfle JB. Wild horse hoof patterns offer a formula for preventing and treating

lameness. In: Proceedings, American Association Equine Practitioners. 1995; 41: 258-260

3) Pollit C., Equine Laminitis, RIRCC Publication 2001

4) Redden RF., Radiographic views of most value to farriers. Equine Podiatry, Saunders 2007;

199-202

5) Redden RF., Clinical and radiographic examination of the equine foot, in Proceedings. 49th

American Association of Equine Practitioners Convention 2003; 169-178

6) Morrison S., Foot management. Clinical techniques in equine practice, Elsevier 2004; 71-82

7) Curtis S., Corrective Farriery, A Textbook of Remedial Horseshoeing, Volume 1, 2002 R&W

Publications, Newmarket, U.K.

8) Clayton HM., Effects of hoof angle on locomotion and limb loading. In White NA., More JN.,

eds. Current techniques in equine surgery and lameness, 2nd

ed. Philadelphia; W.B. Saunders

co., 1998; 504-509

9) Wilson AM., Sedig TJ., Shield RA., Silverman BW., The effect of foot imbalance on point of

force application in the horse. Equine Vet J 1998; 30: 540-545

10) Rooney JR., The Lame Horse, 1974 Barns and Co. Cranbury, New Jersey; 120-121

11) Butler KD., The effect of feed intake and gelatin supplementation on the growth and quality

of the equine hoof. Ph.D. Thesis, Cornell University, Ithaca, New York.

12) van Heel MC., Barneveld A., van Weeren PR., Back W., Dynamic Pressure measurements for

the detailed study of hoof balance: the effect of trimming. Equine Vet J 2004; 36: 778-782

13) Denoix JM., Functional anatomy of the equine interphalangeal joints, in Proceedings, 45th

Annual American Association of Equine Practitioners Convention 1999: 174-177

14) Ovnicek GD., Page BT., Trotter GW., Natural balance trimming and shoeing: its theory and

application. Vet Clin North Am Equine Pract. 2003; 19: 353-377

15) Barrey E., Investigation of the vertical hoof force distribution in the equine forelimb with an

instrumented horseboot. Equine Vet J, 1990; (suppl): 35-38.

16) O’Grady SE., Poupard DE., Proper physiological horseshoeing. Vet Clinic North AM (Equine

Pract) 2003; 19: 333-334.

17) Colles C., Interpreting radiographs 1. The Foot. Equine Vet J. 1983; 15: 297-303.

11

18) Rooney JR., Functional anatomy of the foot; Equine Podiatry, Saunders 2007: 57-73.

19) O’Grady SE., Guidelines for trimming the equine foot: A review, In Proceedings, 2009

American Association of Equine Practioneers Convention.

20) Caudron I., Meisen M., Grulke S., Vanschepdace P., Serteyn D., (1997a) Clinical and

radiological assessment of the corrective trimming in lame horses. J. Equine Vet Sc. 17: 375-

379.

21) Chateau H., Deguence C., Denoix JM., Evaluation of three dimensional kinematics of the

distal portion of the forelimb in horses walking in a straight line. AM J Vet Res 2004; 65: 447-

455.

22) Clayton H., Bach W., Equine Locomotion; Saunders 2001: 138.

23) Redden RF., Understanding Laminitis; 1998, The Blood Horse Inc., Lexington Ky.

24) Bach O., White K., Butler D., Matcalf. Hoof balance and lameness: foot bruising and limb

contact. Compend Contin Educ Pract Vet 1995; 17 (12): 1505-1506.

25) van Heel MCV., Moleman M., Barneveld A., van Weeren PR., Back W., 92005) changes in

location of center of pressure and hoof- unrollement pattern in relation to an 8-week shoeing

interval in the horse. Equine Vet. J 37: 536-540.

26) Moleman M., van Heel MCV., van Weeren PR., Back W., Hoof growth between two shoeing

sessions leads to a substantial increase of the moment about the distal, but not the proximal,

interphalangeal joint. Equine Vet J. (2006) 38 (2) 170-174.

a) Balanced Break-over Management, Los Olivos,Ca.