Introduction
Lameness originating in the foot is common. In many instances
the cause can be ascribed to a "recognized" disease, such as
pedal osteitis or navicular disease; this is based on physical
examination supported by regional anesthesia and radiographs
with little regard for the conformation or balance of the
distal limb because, as useful they are, they do not change
the diagnosis. However, in many other instances a lameness may
be isolated to the foot, but further definition of the problem
by radiography and nuclear scintigraphy is lacking; the best
approach to treatment without a specific diagnosis is then in
part based symptomatically on the balance and conformation of
the distal limb.
The foot is anatomically defined as the structures within and
including the hoof, and the digit describes the limb distal to
the metacarpophalangeal joint. For practical reasons, this
distinction is frequently blurred. The hoof is the integument
of the foot and the hoof capsule is the cornified epithelium
of the hoof.
Conformation and balance, both terms used to describe the
distal limb, are unfortunately sometimes used interchangeably
because of some natural ambiguity. For the purposes of this
discussion, conformation is used to describe the size and
shape of the distal limb excluding the hoof; conformation of
the distal limb is therefore the sum of the shape and size of
the individual anatomical structures and the relationship
between them. Balance is considered to be the relationship
between the hoof and the other anatomical structures of the
distal limb, and between the hoof and the ground. Conformation
is determined with the horse at rest and does not change much
and then only slowly. In contrast, balance is both a static
and functional concept that applies to the horse at rest and
in motion, and is subject to rapid as well as slow change.
The form and function of the hoof can change in response to
several influences. Rapid distortion of the hoof capsule
occurs in response to prolonged steady alteration in
intramural stresses because of its viscoelastic nature.
Imbalance between hoof wear and growth directly alters the
length of the hoof capsule and secondarily influences its
angle and shape. Uneven growth around the circumference the
circumference of the coronary band occurs in response to
uneven stresses within the wall. Identification of these
changes in form of the foot require an appreciation for what
constitutes a normal foot.
The "normal" foot
The notion that there is a single pattern for the ideal foot
is probably flawed because the ideal may vary with breed and
use. Given this limitation, the size of the normal foot is
related to the size of the horse, and although systematic
documentation is lacking some practical guidelines are
available for the forefeet. 1,2
The shape of the foot is defined by a series of ratios that
relate the size, and angles that relate the position of any
one structure to the other structures of the distal limb.
Though conformation and balance are three dimensional
concepts, they are described from three viewpoints, laterally,
dorsally and from the solar surface.
From the lateral aspect, the pastern and dorsal hoof wall are
parallel, that is, the foot-pastern axis is straight. The
angle of the foot-pastern axis is approximately 50-54 degrees.
The angle of the heels is within 5 degrees of the pastern. The
length of the toe is approximately related to the weight of
the horse: 7.6 cm for a 360-400 kg horse, 8.25 cm for a
425-475 kg horse, and 8.9 cm for a 525-575 kg horse. An
imaginary line that bisects the third metacarpus reaches the
ground at the palmar border of the weight bearing surface of
the foot. The coronary band slopes evenly towards the heels,
at which point it may dip less acutely distally.
From the dorsal aspect, an imaginary line that bisects the
third metacarpus bisects the pastern and foot. This same line
is perpendicular to the ground and another line drawn between
any two corresponding points on the coronary band. The highest
point of the coronary band is at the center of the hoof and
slopes gradually both medially and laterally. The medial wall
is frequently slightly steeper than the lateral wall.
From the solar surface, the foot should be approximately as
wide as it is long. The contour of the foot is smooth and
almost circular. The foot should be approximately symmetrical
about the axis of the frog; the lateral side may be slightly
(5%) wider than the medial side. The width of the frog is 50 -
66% of its length. Both heels are of equal length. A line
drawn across the weight bearing surface of the heels is
perpendicular to the axis of the frog.
History
The signalment is useful to raise or lower the index of
suspicion for a specific problem. The duration, speed of
onset, progression of disease, treatment if any, and response
to treatment are standard questions to be addressed for any
lame horse.
The trimmin and shoeing history is important. In any horse,
the shoeing intervals and the time since the last shoeing are
important because the foot length and angle change between
shoeing, and lameness soon after the last shoeing may be
related to nail placement. Equally important is whether the
onset of lameness coincides with a change in the way the horse
is shod or with a change in farriers. The age the horse was
first shod may give an indication to the relationship between
shoeing and the lameness in young horses.
Physical Examination
Visual examination of the feet and distal limbs are made in
conjunction with an assessment of the overall conformation of
the horse to identify angular deformities, from base
narrow/wide to varus/valgus, and rotational deformities
arising at any level in the limb before more specifically
examining the digit and foot. The appearance of the distal
limb is compared to the ideal "normal" foot. In addition to
examination from the three perspectives, the heels are
evaluated from the palmar aspect with the foot on the ground,
and the relationship between the ground surface of the foot
and the axis of the limb examined by holding the metacarpus
horizontally and sighting along the limb and down across the
solar surface of the foot. Though balance and conformation are
most frequently subjectively evaluated, evaluation by accurate
measurement and recording enhances awareness, emphasizes small
discrepancies and provides objectivity for future comparison.
Foci of pain are identified by careful palpation, methodical
application of hoof testers, and flexion and extension of the
interphalangeal joints. Paring the foot to expose clean horn
allows evaluation of the white line for areas of separation,
and the sole for hemorrhage in unpigmented areas. Inspection
of the wall identifies the pattern of growth rings, regional
decreases or increases in convexity, vertical and horizontal
fissures, and hemorrhage originating from the coronary band.
The shoes themselves, and their positioning and attachment to
the foot are examined. The type of shoe is evaluated for
protection to the ground surface of the foot, amount of
traction it is likely to provide, position of breakover,
presence of supporting devices such as extensions and pads,
and for potential effect on animation. The shoes should be
examined for wear to indicate the direction of breakover, and
the size and fit are examined in relation to the size and
shape of the foot.
Observation at a walk should determine the manner the horse
lands and breaks over, as well as the path of the foot during
the flight phase of the stride. Toe first landing or
excessively heel first landing indicate compensation for pain
or dorsopalmar imbalance. Similarly, medial or excessively
lateral heel/quarter first suggest mediolateral dynamic
imbalance. The flight of the foot during the stride is
correlated with rotational deviation of the limb and imbalance
of the foot. The horse is observed at a trot for lameness
including lunging, but evaluation of balance at gaits faster
than a walk necessitate videorecording with or without a
treadmill. Further evaluation with regional anesthesia may be
required before radiography or nuclear scintigraphy.
Lameness originating in the foot is frequently associated with
secondary foci of pain caused by compensation for the primary
problem, e.g., suspensory desmitis in the forelimbs, and
tarsal and back pain with the hindlimbs. These primary and
secondary foci of pain need to be correlated.
Radiology
The radiographic findings of diseases affecting the osseous
structures of the digit are well documented. Of equal
importance, but given less consideration, is the relationship
between the hoof capsule and the osseous structures. A range
of parameters has been evaluated in a detailed study of young
thoroughbreds, 3 and a second study has evaluated
smaller numbers of a spectrum of different breeds. 4
A radiodense marker centered on the dorsal hoof wall and
another at the apex of the frog aid correlation of the soft
tissue features of the radiographs and the foot. A linear
radiodense marker in the block the horse stands on for
radiographs delineates the ground surface of the foot. To
consistently evaluate this relationship accurately, a
consistent radiographic technique is essential. There should
be even weightbearing between both forelimbs. At least one
lateral radiograph should be centered on the solar margins of
the distal phalanx so that the 2 palmar processes are
superimposed and. To evaluate the interphalangeal joints other
laterals may be necessary. The most salient observations
include, the relationship between the dorsal hoof wall and the
parietal surface of the distal phalanx, the thickness of the
soft tissues between the dorsal hoof wall and the parietal
surface of the distal phalanx, the thickness of the sole, the
angle of the sole of the distal phalanx to the ground, and the
relationship between the center of rotation of the distal
interphalangeal joint and the weight bearing surface of the
foot.
A true dorsopalmar radiograph must be similarly taken centered
on the solar surface of the distal phalanx with both forelimbs
bearing weight evenly. The marker on the dorsal hoof wall at
the center of the toe should be superimposed on the central
sulcus of the frog. A block with a built in swivel has been
recommended to allow the horse to stand in a natural position
5 because twisting the foot after it is planted on
the block has been shown to introduce error in interpretation.
The most salient features include the symmetry of the distal
interphalangeal joint space, the relationship between the
articular surface of the distal phalanx and the ground, and
the relationship between the distal phalanx and the medial and
lateral hoofwalls.Scintigraphy
Scintigraphy is most useful to confirm the clinical
significance of ambiguous radiographic findings such as pedal
osteitis or ossified collateral cartilages, or to detect foci
of inflammation that is otherwise occult, such as deep digital
flexor insertional tenopathy.
References
1. Balch, O., K. White, and D. Butler, Factors involved
in the balancing of equine hooves. J Am Vet Med Assoc, 1991.
198(11): p. 1980-9.
2. Turner, T.A., The use of hoof measurements for the
objective assessment of hoof balance, in American Association
of Equine Practioners. 1992. p. 389-395.
3. Linford, R.L., T.R. O'Brien, and D.R. Trout,
Qualitative and morphometric radiographic findings in the
distal phalanx and digital soft tissues of sound thoroughbred
racehorses. Am J Vet Res, 1993. 54(1): p. 38-51.
4. Cripps, P.J. and R.A. Eustace, Radiological
measurements from the feet of normal horses with relevance to
laminitis. Equine Veterinary Journal, 1999. 31(5): p. 427-432.
5. Caudron, I., et al., Clinical and radiological
assessment of corrective trimming in horses. J Eq Vet Sci,
1997. 17(7): p. 375-379. |
