THE EQUINE FOOT:
FORM AND FUNCTION
Andrew H. Parks, MA, Vet MB, MRCVS, Diplomate ACVS
College of Veterinary Medicine
University of Georgia, Athens Georgia
Reprinted with permission from the North
American Veterinary Conference.
Original printed in the 2001 NAVC conference proceedings
The foot is interesting not only because it is the dominant
site of lameness in performance horses, but also because it
has an intrinsically fascinating structure that on the surface
departs radically from the rest of the integument and
functions as an extension of the musculoskeletal system.
Unfortunately, the terminology used to refer to the distal
limb varies so it is prudent to begin by restating the
anatomical definitions, taken from the Nomina Anatomica
Veterinaria wherever possible. The digit is that part of the
limb distal to the metacarpophalangeal joint. The foot is the
part of the distal limb encased by the hoof. The hoof (Ungula)
is, by definition the integument of the foot. The hoof capsule
is formed by the cornified layer of the epidermis of the hoof.
The gross morphologic differences between distal limb and the
rest of the musculoskeletal system cause some intimidation.
However, to overview the distal limb as a cause of lameness,
there are only so many structures in the foot and there are
only so many pathological processes that may occur in each
structure: degenerative, anomalous, autoimmune, metabolic,
nutritional, neoplastic, inflammatory, infectious, and
traumatic (DAMNIT). By understanding how these structures
function normally and considering all the potential processes
that occur, a specific diagnosis may become more readily
apparent or the diagnostic soup can be reduced through the
process of elimination. This reductionism simplifies the
process of the diagnosis of disease, but as will be discussed
later, is limited by our inability to isolate pain to a
precise structure or to image such a structure.
The proximal and middle phalanx are structurally unremarkable
compared to the long bones of the limb. The distal phalanx
however is unique. It has three surfaces, one of which is the
articular surface. The parietal surface forms a large surface
area that is remarkably porous - ideally adapted for a broad
attachment to soft tissue. The solar surface is smooth with no
vascular foramina. Attached to the palmar process of the
distal phalanx are the collateral (ungual) cartilages. These
cartilages are flat rhomboids in shape that extend proximally
from the palmar process of each palmar process so that
approximately 50% of each cartilage is proximal to the
Both interphalangeal joints are ginglymus joints so that
motion is primarily restricted to extension and flexion in the
saggital plane. The distal interphalangeal joint has a much
greater range of motion than the proximal interphalangeal
joint. The distal interphalangeal joint has three separate
articulations: 1) between the 2 phalanges, 2) between the
middle phalanx and the distal sesamoid bone, and 3) between
the distal phalanx and the navicular bone.
The articular surfaces of the phalanges of both
interphalangeal joints are maintained in apposition by paired
collateral ligament. In the proximal interphalangeal joint the
palmar aspect of the joint is supported by 2 pairs of palmar
ligaments. In the distal interphalangeal joint, the position
of the distal sesamoid bone is stabilized by the paired
collateral ligaments of the distal sesamoid proximally and the
impar ligament distally. In addition the deep digital flexor
tendon supports the distal interphalangeal joint palmarly. In
addition to the these ligaments, there are 5 small ligaments
associated with each ungual cartilage.
It is the integument of the foot that really separates the
foot from the rest of the musculoskeletal system. Like the
skin, the integument of the foot, that is the hoof, is
composed of 3 principle layers: epidermis, dermis (corium),
and subcutaneous tissue (hypodermis). Like the skin, the
epidermis is further subdivided into layers: the stratum
basale and the stratum spinosum, which are collectively known
as the stratum germinativum, and the stratum corneum. The
stratum corneum forms the hoof capsule.
Unlike the skin, which is relatively uniform over the surface
of the body, the hoof is divided into 5 distinct regions based
on their gross appearance: coronary band, wall, sole, frog and
heel bulbs. Underlying the hoof, the germinal layers of the
epidermis, the dermis and the subcutaneous tissues are highly
specialized and are named after the tissues they generate or
support: perioplic (limbic), coronary, lamellar, solar and
cuneate (frog). The terms used to describe the region and
epithelial type are not necessarily interchangeable because
the surface does not necessarily reflect the type of
The limbus or periople is a narrow band of modified skin that
bridges the gap between the skin of the pastern and the
coronary band, and forms the stratum externum of the hoof
The corona is the band like proximal segment of the hoof
frequently called the coronary band. The coronal subcutaneous
tissue forms the coronary cushion, the coronal dermis follows
the curve of the coronary cushion from which the dermal
papillae project and the germinal layers of the coronary
epithelium, which follow the contour of the dermal papillae,
generate the tubular and intertubular horn of the stratum
medium of the hoof capsule.
The laminar integument covers the parietal surface of the
distal phalanx and the ungual (collateral) cartilages. The
laminar subcutaneous tissue forms the modified periosteum or
perichondrium of the distal phalanx and ungual cartilages. The
lamellar dermis forms the primary and secondary ridges that
run in a proximal to distal direction to form lamellae that
interdigitate with the epidermal lamellae. The primary
epidermal lamellae are keratinized, the secondary lamellae are
not. The lamellar horn forms the stratum internum of the hoof
The solar integument covers the solar surface of the distal
phalanx. The subcutaneous tissue likewise forms the modified
periosteum of the solar surface of the distal phalanx, the
dermis forms the dermal papillae and the overlying epithelium
forms the tubular and intertubular horn of the sole.
The cuneate integument forms the digital cushion from the
subcutaneous tissue, the dermis ,the dermal papillae, and the
epidermis, the tubular and intertubular horn of the frog. The
cuneate integument differs from that of the sole in gross
appearance, in texture and because there are occasional
adnexal structures arising from the epidermis.
Hoof growth and replacement, like that of skin, is a constant
process. However, whereas the most superficial layers of the
skin arise from the immediately underlying tissues, different
parts of the hoof have differing relationships with the
underlying epithelium. The stratum corneum of the sole and
frog reflect the underlying basal layers of the epithelium.
However, because of the unusual structure of the wall, the
most superficial layer, the stratum externum, doesn't
necessarily reflect the underlying type of basal epithelium,
which for the wall is the laminar epithelium. As the stratum
medium and stratum externum of the hoof wall constantly
regenerated from the coronary and perioplic epithelium, the
basal layers of the lamellar epithelium proliferate just
sufficiently to allow the hoof wall to glide distally.
The hoof capsule is viscoelastic; that is, when subjected to a
sudden high stress, it deforms elastically. In contrast, when
subjected to a constant stress it deforms slowly in a viscous
manner which will reverse when the stress is removed. In fact
the hoof wall is so resistant to sudden high stresses that it
is more fracture resistant than bone.
In addition to discussing the individual elements of the
distal limb it is necessary to discuss how they relate to each
other. In order to simplify this discussion the author
suggests 2 definitions: Conformation describes the size and
shape of the musculoskeletal structures and the way in which
they are spatially arranged. Balance on the other hand
describes the way in which the hoof capsule relates to the
skeletal structures of the limb.
In the resting horse, these relationships can be examined by
viewing the foot form the lateral, dorsal and solar aspects.
From the lateral aspect, the foot pastern axis should be
straight and in the forelimb is about 50-62 degrees to the
ground. There are ideal lengths of for the foot-pastern axis
and the ground surface of the foot. In addition, a vertical
line that bisects the third metacarpal should intersect with
the ground at the most palmar aspect of the weightbearing
surface. When these three are taken together, it is evident
that there is a triangular relationship between the length and
angle of the foot pastern axis, the location of the third
metacarpal and the length of the foot, that should hold
regardless of the size of the horse. This relationship defines
static dorsopalmar balance and conformation.
When viewed form the dorsal aspect the axes of the metacarpus
and pastern are in the same plane. A vertical line that
bisects the metacarpus and pastern should be perpendicular to
a horizontal line drawn between any 2 comparable points on the
coronary band or the ground surface of the wall. The medial
wall of the foot may be slightly steeper that the lateral
wall. Growth rings should be equally spaced around the
circumference of the foot. This relationship defines static
mediolateral balance and conformation.
When viewed from the solar surface of the foot, the medial and
lateral sides should be approximately symmetrical about a line
bisecting the frog. The length of the foot should approximate
the width. The frog width should be at least half as much as
the frog length.
The distal limb is functionally a set of levers and pulleys.
The load against which they are working is the force (mass x
acceleration) down the limb and the equal and opposite force
from the ground on the limb.
At rest, the weight borne by the foot, the position of the
structures in the foot and the tension in the tendons all
remain constant. The mechanisms that allow the feet to bear
weight are best understood by examining the forces on the
distal phalanx which flexes and extends about the distal end
of the middle phalanx and the phalangeal axis (considering all
3 phalanges acting as one) which flexes and extends about the
distal end of the metacarpus.
Forces that act on the distal phalanx: Force is transmitted
from the ground to the hoof over the area of contact. The area
of hoof-ground contact varies with the surface the horse is
standing on and the balance/conformation of the hoof. The
majority of the ground-hoof interaction force is transmitted
from the ground to the wall and then to the distal phalanx
through the laminae; that is , the laminae suspend the distal
phalanx from the hoof. The frog and parts of the sole also
bear some weight. Combining all the forces on the distal
phalanx form the laminae produces a resultant force. Without
any other forces on it, the position of the distal phalanx on
the ground is inherently stable. The weight of the horse from
the proximal limb is transmitted to the distal phalanx through
its articulation with the middle phalanx. The resultant
vertical force on the distal phalanx is in the opposite
direction to the ground reaction force and positioned palmar
to the ground reaction force. Unless opposed, the distal
phalanx would rotate. Through the insertion of its tendon on
the distopalmar aspect of the distal phalanx, the deep digital
flexor muscle aided by the inferior check ligament opposes
rotation of the distal phalanx.
Forces acting on the phalangeal axis as a single unit (assume
the phalanges are all fused): The ground reaction force is
applied to the distal phalangeal axis through the hoof. The
weight of the horse from the proximal limb is transmitted to
the proximal articulation of the proximal phalanx through the
metacarpus. Because these 2 vertically opposed forces are not
aligned, they create a moment that would rotate the phalangeal
axis, and the metacarpophalangeal joint would drop to the
ground. This moment is opposed by the digital flexor muscles
and associated check ligaments through their tendons of
insertion, and the suspensory ligament (interosseous).
Consequently, at rest the distal limb is in an unstable state.
In motion, the weight borne by the limb, the position of the
foot, the joint angles of the phalangeal axis and the tension
in the flexor tendons are constantly changing. The stride can
be divided into 4 phases: Impact / Landing, horses usually
land heel first or flat footed; Stance / Support phase, the
foot is flat on the ground; Breakover, the heel is no longer
in contact with the ground, but the toe still is; Flight /
Swing, the foot is off the ground.
During the landing phase and the first part of the stance
phase, the mass of the body is accelerating towards the
ground. To decelerate mass of the body as it descends to the
ground (and also breaks forward momentum) as the foot lands
and bears weight several events occur. The fetlock
hyperextends, and the DIP and PIP joints flex (the latter only
slightly), so that the fetlock drops towards the ground as the
tendons absorb and store energy. The distal phalanx rotates
slightly within the foot about its dorsal solar margin so that
the palmar processes move towards the ground. The articulation
between the distal phalanx and navicular bone opens up. The
hoof expands (the exact mechanism is unknown).
During the second half of the stance phase and the breakover
phase the horse must be accelerated forwards and the limb
lifted off the ground. Contraction of digital flexor muscles
and release of stored energy in the tendon and inferior check
ligament, flex the fetlock, and extend the DIP and PIP joints.
The hoof acts as an extension of the distal phalanx, the
leverage about the DIP joint may change.
During the flight phase, the distal limb flexes and then
extends to prepare for landing as it is protracted.
Proprioceptive receptors appear to determine the angles of the
joints in preparation for impact with the ground. The way the
foot lands is described as dynamic balance. A horse is said to
be in dorsopalmar dynamic balance when the foot lands flat. A
horse is said to be in mediolateral dynamic balance when the
foot lands with both heels simultaneously.