Objective Assessment Hoof Balance
Tracy A. Turner, DVM, MS, Dipl. ACVS
St. Paul, Minnesota 55108
The importance of a balanced foot in the treatment
of equine lameness is well documented.1-4
However, the assessment and choice of options for correction of an
imbalanced foot can be quite subjective.1-3,5,6
Balance is defined as the harmonious adjustment of parts. For the
hoof, balance has been defined as the equal distribution of weight
over the foot.2,5,6 This must be
more precisely defined as equal medial to lateral distribution of
weight since more weight is normally placed on the caudal half of
the foot.5 Caudal to cranial
imbalance has been defined as deviation in the hoof alignment or
as problems with heel support.
Effective communication between the veterinarian and farrier is
imperative for proper treatment of most lamenesses in the horse.
Unfortunately, it is often difficult for both to attend the horse
at the same time. There can also be a difference in semantics
between the professions which can lead to misinterpretation of
clinical findings and recommended corrections which can in turn
lead to resentment. It is important therefore, to develop
acceptable definitions of problems and methods of communicating
precisely.
Six hoof balance abnormalities have been described7:
broken hoof axis, underrun heels, contracted heels, shear heels,
mismatched hoof angles, and small feet. Some authors have
attempted to define these hoof abnormalities objectively.2,3,7
A broken hoof axis exists when the slopes of the pastern and hoof
are not the same. This condition is further defined as
broken-back, when the hoof angle is lower than the pastern angle,
and as broken-forward when the hoof angle is steeper than the
pastern angle. Underrun heels have been defined as angle of the
heels of 5o less than the toe angle. Contracted heels
was defined as frog width less than 67% of the frog length.
Sheared heels was defined as a disparity between the medial and
lateral heel lengths of 0.5cm or more. Small feet (small feet to
body size) was defined as a weight to hoof area ratio of greater
than 78 pounds per square inch. The purpose of this paper is to
show how measurements of the feet and radiographs can be used to
graphically document the presence of hoof abnormalities.
Eleven measurements are made of each foot. The horse's weight is
determined with a weight tape or scale. Seven measurements are
made of the hoof length with a tape measure: medial and lateral
heel lengths, medial and lateral quarter lengths, dorsomedial and
dorsolateral toe lengths, and sagittal toe length. These
measurements are recorded on a graph to illustrate the general
shape of the foot. In addition, the frog's length and width are
measured at their longest and widest points. The hoof
circumference immediately below the coronary band, and the hoof
angle (using a hoof gauge) are also measured. From these
measurements, two additional measurements can be calculated: the
frog ratio (frog width divided by length) and the body size to
hoof area (horse's weight (pounds) X 12.56 / square of the hoof
wall circumference(C) (inches)).
A dorsopalmar(plantar) and a lateral radiograph of the hoof can
also be used to determine valuable information about hoof balance.
The horse must be standing with the metacarpus (tarsus)
perpendicular to the ground which can most easily be determined by
either the use of a level placed against the cannon bone or the
use of a weighted string to align the leg. The radiographic beam
should be horizontal and centered on the hoof. Resting the horse's
foot on a block to raise the hoof off the ground facilitates these
exposures (the opposite limb should be similarly elevated). For
easy identification of landmarks, radiopaque markers can be
attached to the hoof. A wire placed sagittally (midline) along the
toe from the coronary band to the ground, a thumb tack in the apex
of the frog and thumb tacks in the most caudal point of the ground
contact of each heel emphasize these areas on the radiographs,
making their identification much easier.
After plotting the hoof wall lengths, one should have a curve that
reflects the shape of the hoof. For a hoof of average hoof angle
(48o-55o), flattening of the plotted curve
indicates that the heels are underrun. A flat curve would also be
expected for very upright hooves (>60o). Generally
speaking, the 3 measurements at the toe should be equal. The
measurements at the quarter are usually 1-2 cm shorter than the
toe (for the average hoof). The heel length should generally be
about one-third of the toe length. The remaining measurements are
used in the previously described formulas to determine weight to
body size and frog ratios for determination of a contracted foot.
Examination of the lateral and dorsopalmar(plantar)(DP)
radiographs provides excellent pictorial evidence of imbalance.
The lateral radiograph should be evaluated for P2 and P3 alignment
which gives insight into the presence of a broken-hoof axis. In
addition, the alignment between P3 and the hoof wall should be
assessed. If the hoof wall and dorsal surface of P3 are not
parallel the functional hoof angle can be determined by measuring
the angle of the dorsal surface of P3 with the ground. Usually the
slope of the heels can be seen on the radiograph and can also be
used to determine whether the heels are underrun.
The DP radiographic projection should be assessed for joint
alignment, medial and lateral hoof wall lengths, and foot
symmetry. Joint alignment is determined by examining the symmetry
of the joint space. Malalignment is present if one side of the
joint is more narrow. This phenomenon can also be caused by poor
positioning in which case all three of the lower leg joints
(fetlock, pastern, and coffin) will be affected. The hoof wall
length can be measured directly from the film. The symmetry of
weight bearing can be predicted in a similar manner.
Numerous factors contribute to the balance of an equine hoof.1-7
Toe length is important because it determines the length of the
lever arm over which the limb rotates and the timing of hoof lift.4
Hence, a long toe, that would delay breakover, could be expected
to increase the pressure of the deep flexor tendon over the
navicular bone, increase the tension on the proximal suspensory
ligament of the navicular bone, and increase the dorsal rim
pressure on the joints of the leg.4,8
The optimal toe length has not been determined. Toe length to a
certain extent will be dictated by the horse's use as well as the
horse's height and weight. Guidelines have been described that
relate toe length to body weight: 3 inches (7.6cm) for 800 to 900
pound horses, 3.25 inches (8.25cm) for 950 to 1,050 pound horses,
and 3.5 inches (8.9cm) for horses weighing 1,150 to 1,250 pounds.4
A graph of hoof measurements will document disparities in hoof
wall length between feet. Lengthening one hoof over its opposite
has been suggested as a treatment for limb length disparity in the
horse.9 However, this condition
has not been scientifically documented in the horse. It has been
the author's experience that apparent limb length disparities are
more commonly due to mild flexural deformities (contracted tendon)
rather than actual differences in limb length and that this
condition is most commonly manifested as mismatched hoof angles.
One study indicated that 28% of normal performance horses may be
affected in this manner.3 This
can most easily be documented utilizing the lateral radiographs.
The hoof angle should be the same as the hoof axis.2-7,10,11
Utilizing the lateral radiographic projection, the ideal hoof
angulation to properly align the second and third phalanges can be
measured accurately. The appropriate correction can be determined
by measuring the degree of malalignment (flexion or extension)
present in the coffin joint and raising or lowering the hoof angle
that amount.11 For instance, if
the lateral radiographic projection showed 4o flexion
of the coffin joint, then the hoof angle should be lowered 4o.
In most cases, the aligned hoof axis is 52o+2o
for the front feet and 55o+2o for the back
feet.2,4,5,7,8 Intentional
lowering of the hoof angle has been used to increase stride length
in racehorses but studies have shown that this is not true8;
therefore, there is no reason not to shoe for a correct hoof axis
and a broken hoof axis can predispose to lameness problems2,4,5,7
and it has been associated with a greater risk of breakdown in
racehorses.12
In addition to hoof axis deviations, the lateral radiographic
projection can be used to document problems of heel support, i.e.
underrun heels. In horses with hoof angles between 50 and 55
degrees, the hoof length graph also documents underrun heels if
toe length to heel length ratio is less than 3:1. The proper
position of the heels can be determined by either drawing a
bisecting line through the metacarpus to the ground.4,5
or measuring the appropriate position on the radiograph.7
Where these lines contact the ground is the point where the heels
should be. From a practical point, the heel-ground contact should
be even with the base of the frog.2
Underrun heels are the most commonly encountered hoof abnormality.
In one study of foot related lameness it was found in 77% of the
horses2 and in another study of
normal performance horses this condition was found in 52% of the
horses.3 The necessity of
correcting underrun heels has been well documented.1-7
If left uncorrected underrun can cause alterations in hoof wall
growth that can be very difficult to correct and it can predispose
to lameness problems that range from bruised heels to navicular
syndrome.1,2,7
One of the most difficult parameters to assess is the hoof's
ability to expand.13,14 Applied
clinical studies have shown that the frog length to width ratio is
useful for this purpose; when the frog's width is at least
two-thirds its length, the hoof has normal expansive abilities.2,3,7
When the frog is narrow, hoof expansion is thought to be reduced.
Whether this is a function of frog pressure is not known, although
both reduced and excessive frog pressure have been shown to cause
hoof contracture.13,14
Identification of a narrow frog should alert the clinician that
steps need to be taken to promote hoof expansion.2,3,7
These may vary from simply ensuring proper heel support to
encouraging hoof expansion through the use of slipper heels.
Medial/lateral imbalance or shear heels has been shown to cause,
or predispose to, a number of hoof related lamenesses.2-4,6,7,15
Medial/lateral balance can be assessed by both the hoof
measurements and the radiographic examination. The graph of hoof
wall measurements will clearly show if one side of the hoof is
longer than the other.a The obvious correction is to make the
walls equal, although it is not always that simple.2-4,6,7,15
The dorso-palmar radiograph will also clearly demonstrate any
imbalance. Since this projection will also show the effect the
imbalance has on the coffin joint, this radiograph can be used to
emphasize the need for correction. The magnification in most
radiographs makes even subtle disparities more obvious. It is
accepted that conformation can alter this balance.4-6,15
The radiograph will help determine if the imbalance is hoof
related or conformational. Hoof related imbalances will show
medial/lateral hoof length disparities, and the first and second
phalanges can be bisected equally. If the medial/lateral disparity
is conformationally related the first and second phalanges will
appear oblique on the DP radiograph.
The final assessment of balance is the weight of the horse in
proportion to its feet. Small feet have been a commonly described
problem, particularly in Quarter Horses, that predispose the horse
to lameness.2,3,7,16 One study
identified small feet as an indicator of poor prognosis in the
treatment of navicular syndrome.2
Most descriptions of what actually constitutes a small foot are
quite subjective. However, studies have been performed utilizing
simplistic formulas to make this assessment objective.2,3,7
These studies measured the circumference of the hoof immediately
below the coronary band. This was done to get a rough idea of the
hoof cross sectional area. This was then compared to the horse's
weight and statistical analysis was performed. Seventy-eight
pounds per square inch was determined to be the maximum weight to
hoof area ratio for a normal performance horse. The steps to
determine this number have been simplified to the following
formula:12.56 X wt(lbs)/C2(in2). Once identified, a high weight to
hoof area ratio can be used to show a client that their horse
should lose weight. In addition, it can be used to show the
necessity of fitting a shoe as fully as practical in order to
produce the largest surface area as possible for that particular
horse's hoof.
Assessment of a horse's hoof balance utilizing hoof measurements
is a method that can be easily incorporated into the routine
examination of the hoof. It actually takes very little time (about
2 minutes) to make the necessary measurements. Radiographic
examination of the feet is a routine diagnostic assessment already
performed. By assuring proper positioning and alignment of the
hoof during radiographic examination, additional information that
can be invaluable in the case assessment can be attained. The
information will help show problems of imbalance, can be used to
graphically illustrate the problem to an owner or trainer, and can
be used to clearly show the shoeing needs of the horse, thus
improving overall communications between veterinarian, owner, and
farrier.
REFERENCES
1. Moyer W, Anderson JP. Lameness caused by improper shoeing. J
Am Vet Med Assoc 1975;166:47-52.
2. Turner TA. Navicular disease management: shoeing principles, in
Proceedings. 32nd Annu Conv Am Assoc Equine Practnr
1986;625-633.
3. Turner TA, Stork C. Hoof abnormalities and their relation to
lameness, in Proceedings. 34th Annu Conv Am Assoc Equine
Practnr 1988;293-297.
4. Balch O, White KK, Butler D. Factors involved in the balancing
of equine hooves. J Am Vet Med Assoc 1991;198:1980-1989.
5. Butler KD. The prevention of lameness by physiologically-sound
horseshoeing, in Proceedings. 31st Annu Conv Am Assoc Equine
Practnr 1985;465-475.
6. Moyer W. Therapeutic principles of diseases of the foot, in
Proceedings. 27th Annu Conv Am Assoc Equine Practnr
1981;453-466.
7. Turner TA. Shoeing principle for the management of navicular
disease. J Am Vet Med Assoc 1986;189:298-301.
8. Clayton H. Comparison of the stride of trotting horses trimmed
with a normal and a broken-back hoof axis, in Proceedings.
32nd Annu Conv Am Assoc Equine Practnr; 1986;289-298.
9. Gonzales T. Proper balance movement: a diary of lameness.
Manassas: REF publishing, 1986;9-25.
10. Schryver HF, Bartel DL, Langrana N, et al. Locomotion in the
horse: kinematics and external and internal forces in the normal
equine digit in the walk and trot. Am J Vet Res
1978;39:1728-1733.
11. Bushe T, Turner TA, Poulos P, et al. The effect of hoof angle
on coffin, pastern, and fetlock joint angles, in Proceedings.
33rd Annu Conv Am Assoc Equine Practnr;1987;729-737.
12. Kobluk CN, Robinson RA, Clanton CJ, et al. Comparison of the
exercise level and problem rate of 95 Thoroughbred horses: a
cohort