Equine Foot Wounds:
General Principles of Healing and Treatment.
Andrew H. Parks, MA, Vet MB
Reprinted with permission of the American
Association of Equine Practitioners.
Original printed in the 1999 AAEP convention proceedings.
1. Introduction
Most equine clinicians are familiar with the healing and
treatment of traumatic wounds to the distal limb proximal to
the foot because the general principles of wound healing are
well taught in veterinary schools. This receives constant
reinforcement as these wounds are a weekly if not daily
occurrence in general practice. Foot wounds, though less
frequently encountered, are not uncommon. Unfortunately, they
are often treated hesitantly by practitioners, presumably
because of misunderstanding or unfamiliarity, so that the
treatment is often ineffective. This is probably because
little time is spent relating the principles of healing to
treatment foot wounds in already overcrowded college
curricula, because of the morphologic differences between the
integument of the foot and skin is confusing, and experience
with management of such wounds in practice is less common.
It is unfortunate that the gross morphologic differences
between the integument of the foot and the skin belie the
overwhelming similarities, both in structure and response to
injury. To overcome any misunderstanding based on the
morphological differences, the author finds it simplest to
relate the less familiar foot wounds to the well understood
more proximal limb wounds, to emphasize the similarities and
then point out the differences. To this end, it is pertinent
to review the anatomy and general classification of wounds
before discussing wound healing and treatment. Unfortunately,
there is little or no information from research on the way
foot wounds heal, so that practical treatment of foot wounds
must be based on experience and extrapolation from research on
skin wounds.
2. Anatomy of the Integument
A. Skin
The skin is composed of three layers, epithelium, dermis and
tela subcutanea (subcutaneous tissue). The epithelium is
subdivided into 5 layers: stratum basale, stratum spinosum,
stratum granulosum, stratum lucidum and stratum corneum that
reflect the progression from the germinal cells at the base to
the fully differentiated cornified cells superficially. The
epithelial layer is sharply demarcated from the dermis between
which is interposed the basement membrane. Dermal projections
of various lengths and widths, called papillae, extend
superficially into the epidermis. The epidermis that
interdigitates with the dermal papillae forms the epidermal
pegs. The deeper layers of the dermis blend with the
subcutaneous tissue, which in turn is loosely attached to the
underlying fascia. Epithelial elements, namely glands and hair
follicles, extend into the dermis and subcutaneous tissue. The
general architectural pattern of the skin described here is
similar across most of the body surface though the relative
thickness of the layers and density of the adnexal structures
varies. The stratum corneum of the skin is continually
replaced by multiplication and differentiation of cells from
the deeper layers of the immediately underlying epidermis to
replace the stratum corneum as it exfoliates superficially.
B. Integument of the foot1,2
Like the skin, the integument of the foot is also composed of
3 layers: the epidermis, dermis and subcutaneous tissue.
Similarly the epithelium is subdivided into layers, though
over most of the surface of the foot, only three layers are
recognized: the stratum basale, stratum spinosum and stratum
corneum. The stratum basale and stratum spinosum are
frequently referred to together as the stratum germinativum.
It is the stratum corneum of the foot that forms the hoof
capsule.
Within this common framework of the integument, the structure
of the integument of the foot is highly specialized both
because of its difference as a whole to the skin and because
of the differences between the regions within the foot. Unlike
the relative topographical uniformity of skin, the structure
of the integument of the foot is divided topographically into
several regions: periople, coronary band, wall, sole, frog and
heel bulbs. These divisions reflect the differences in
appearance, structure and function of the different portions
of the integument of the foot rather than the nature of the
underlying germinal epithelium and dermis. The integument of
the foot is divided into 5 types, perioplic, coronary,
laminar, solar and cuneate that are continuous with one
another but the boundaries are abrupt.
Just as the dermal papillae of the skin interdigitate with the
epidermal pegs, so the dermis and epidermis of the integument
of the foot interdigitate. However, the interdigitation
between the dermis and epidermis of the integument of the foot
is more regular and highly specialized. The perioplic,
coronary, solar and cuneate dermis all form long thin
filament-like papillae that project distally into
corresponding pockets in the epidermis. The germinal
epithelium on the sides of the dermal papillae forms the horn
tubules. The germinal epithelium that abuts the dermis between
the base of the dermal papillae forms the epidermal pegs that
generate the intertubular horn. The laminar dermis is folded
into longitudinal ridges, called the primary dermal laminae,
that extend from the junction of the laminar dermis with the
coronary dermis proximally to its junction with the solar
dermis distally. The surface of each dermal laminae is further
folded into many secondary ridges, called secondary laminae,
on both sides and on the tips of the primary dermal laminae.
The infolding of the epidermis that follows the contour of the
dermis forms the primary and secondary epidermal laminae. Only
the primary epidermal laminae are keratinized.
The subcutaneous tissue of the integument of the foot is also
highly specialized and forms the modified periosteum of the
distal phalanx and the perichondrium of the collateral
cartilage. The cuneate subcutaneous tissue forms the digital
cushion. The stratum corneum of the integument of the foot is
replaced by the germinal layer as it is in the skin. However,
while the relationship between new stratum corneum formation
and the immediately underlying epidermis is obvious for the
sole and frog, this relationship is not immediately perceived
as being as straightforward for the wall because the wall is
derived from epithelium of three types: the outer layer, the
stratum externum, is the stratum corneum of the perioplic
epithelium; the middle layer, the stratum medium, is the
stratum corneum of the coronary epithelium; and the inner
layer, the stratum internum, is the stratum corneum of the
laminar epithelium. However, the plane of the perioplic and
coronary epithelium is almost perpendicular to the plane of
the laminar epithelium so that all 3 cannot proliferate and
increase in thickness perpendicular in the same manner as
skin.
The stratum externum and stratum medium are replaced in a
similar manner to skin, i.e. the direction of growth or
replacement is perpendicular to the plane of the epithelial
layer. In contrast, the laminar epithelium undergoes limited
proliferation which is concentrated near its junction with the
coronary epithelium.3
3. Classification of Wounds
Open wounds have been classified in various ways including
etiological appearance, depth and degree of contamination.4
For the purpose of comparing skin wounds to foot wounds with a
later view to comparison of the mechanisms of wound healing is
beneficial to discuss both the depth and etiological
appearance of wounds in the skin, followed by comparison with
the foot.
A. Wounds involving the skin
Wounds that cause defects characterized by tissue loss may be
classified as partial or full thickness wounds. Partial
thickness wounds of the skin involve loss of the epidermis and
partial loss of the dermis, but truncated adnexal epidermal
elements are still present in the dermis. Full thickness
wounds extend through the epidermis and dermis to the
subcutaneous tissue and may involve any other underlying
structures.
By etiology, wounds are subdivided into abrasions, incisions,
lacerations, avulsions and punctures. Abrasions involve loss
of the epidermis and portions of the dermis. The deeper
portions of epidermal adnexal structures persist across the
surface of wound in the dermis and in the subcutaneous
tissues. Incised wounds in the skin, typified by a scalpel
incision, have smooth margins that gape, sever all soft
tissues in the wound, and extend to varying depths.
Lacerations are caused by the tearing of the skin and
underlying soft tissues so that they have irregular margins
and are often accompanied by surrounding soft tissue trauma.
Avulsions are similar to lacerations except that more
superficial tissues are torn from their attachments to the
deeper structures and the blood supply is more likely to
compromised than a straightforward laceration.
Puncture wounds are characterized by a small entry wound that
gives no indication to the depth or direction of the
underlying trauma and are highly likely to be contaminated.
B. Wound involving the foot
The thickness and the biomechanical properties of the stratum
corneum of the foot affect the pattern of injury seen in the
equine foot. The thick stratum corneum deflects many minor
blows that might injure the skin, and any small defects that
occur in the outer layers of the stratum corneum are of no
clinical significance. The biomechanical properties of the
tubular and intertubular horn of the wall are such that cracks
in the stratum corneum caused by more serious injury are
deflected outwards to the exterior surface of the foot away
from the sensitive dermis. It is also the structural nature of
the stratum corneum of the foot wall that causes it to
fracture along predetermined planes rather than tear
irregularly.
Stump asserts that when the horny layer of the epidermis is
separated from the foot, either by maceration, accident or
surgery, the separation occurs between the stratum corneum and
stratum germinativum;2 a clinical example would be
the defect caused by hoof wall ablation with a cast cutter.
This would not classify as a partial thickness wound because
the germinal layers of the epithelium would not be breached.
In fact, because the integument of the foot does not have
adnexal epidermal structures that project into the dermis and
subcutaneous tissue except for the merocrine glands present in
the frog, a direct corollary of the classical partial
thickness skin wound does not exist in the foot. However, in
an injury that removes most of the epidermis, but leaves
islands of the stratum germinativum of the epidermal pegs or
secondary laminae attached to the dermis might be expected to
heal in a similar manner to partial thickness skin wounds. In
contrast to Stump's assertion, it is the author's experience
that most surgically created defects, except for the drainage
of foot abscesses, and many naturally occurring foot wounds
are full thickness injuries.
Abrasions to the coronary band are the most likely injury to
resemble partial thickness wounds occur because the integument
is thinnest at the coronary band. Where the integument is
thicker, in the author's experience, any injury of sufficient
force to damage the full thickness of the wall frequently
creates a full thickness wound.
Incised wounds to the integument of the foot are very
uncommon, but when they occur the rigidity of the stratum
corneum of the foot prevents the margins from gaping. An
example of such a wound would occur is a horse kicked against
an edge of sheet metal. Lacerations of the integument of the
foot are less common than they are in skin because the rigid
nature of the stratum corneum causes the hoof to fracture
rather than tear, which usually causes the tissues to avulse
from the underlying structures.
Lacerations to the pastern, however, are quite common, most
commonly involve the heels, and may extend distally through
the coronary band. Avulsions of the hoof wall are not uncommon
and usually involve the wall at the quarters or heels. They
may extend proximally to involve the coronary band and / or
distally to involve the sole.
Avulsions may be complete or incomplete. In complete
avulsions, the affected wall and attached structures are
completely separated from the foot. Incomplete hoof wall
avulsions remain attached to the foot along at least one or
more margin, usually proximally.
Puncture wounds are common injuries to the ground surface of
the foot and occasionally occur at the coronary band, but are
rare in the wall. They may only be apparent as a small dark
spot on the horn of the sole, and if they occur in the frog,
the softer horn may close over the original injury.
4. Wound Healing
A. General principles as applied to skin wounds
The healing of full thickness wounds is classically divided
into 4 phases: Inflammation, debridement, repair and
maturation.5 In the inflammatory phase an initial
vasocontriction to minimize hemorrhage is followed by
vasodilation and white cells migrate and plasma proteins leak
into the wound. The protein together with the red and white
cells from the wound forms a clot which later contracts to
form a scab that protects the surface of the wound. In the
debridement phase bacteria, blood clots, and necrotic and
foreign material are removed from the wound through
phagocytosis by white cells, sloughing or surgical debridement.
The repair phase is itself a combination of three processes,
contraction, fibroplasia and epithelialization. Wound
contraction occurs through the action of myofibroblasts which
actively develop tension in the periphery of the wound that
causes centripetal movement of the wound margins over the
granulation tissue surface. Fibroplasia occurs at the surface
of almost all healthy debrided wound surfaces deep to the
epithelium. Fibroblasts, differentiated from mesenchymal
cells, proliferate at the surface of the wound and migrate
along the fibrin lattice in the wound. This is rapidly
followed by the development of new capillaries by endothelial
budding. Together the fibroblasts and capillaries form
granulation tissue. Although granulation tissue forms on the
surface of almost any healthy tissue in the wound, it proceeds
more rapidly from highly vascular tissues such as the
subcutaneous tissue and more slowly from less vascular tissues
such as tendon or bone. Collagen secretion by the fibroblasts
during this phase of wound healing causes to the rapid gain in
wound strength. Epithelialization occurs by proliferation of
epithelial cells at the periphery of the wound which then
migrate across the surface of the granulation tissue. As the
margin of the newly formed epithelial surface encroaches on
the center of the wound, the epithelial cells towards the
periphery of the wound proliferate so that the epithelium
increases in thickness.
In the maturation phase the vascularity and cellularity of the
wound decreases as wound continues to gain in strength, though
at a slower pace, as the collagen reorganizes and forms
crosslinks.
The healing of partial thickness skin wounds follows a
somewhat different pattern to full thickness wounds.6
Because full thickness of the dermis has not been lost and
severed epithelial adnexal structures persist in the dermis,
the epithelium proliferation occurs at the margin of the wound
and at the margins of the truncated epithelial structures.
Because dermal tissue persists across the surface of the
wound, the proliferating epithelium is able to migrate across
the surface of the dermis without granulation tissue forming
in the wound and it is not until the wound is completely
epithelialized that a fibroblastic reaction occurs in the
dermis. Wound contracture is limited compared to full
thickness wounds.
B. General as applied to foot wounds
Foot wounds follow the same four basic processes. The
differences that are apparent primarily relate to the rigid
nature of the stratum corneum and the variety of epithelial
types found in the foot and the pattern or the continuous . In
the inflammatory phase, wounds below the coronary band are
contained within the rigid stratum corneum of the foot so the
tissues within the hoof are unable to swell. In the
debridement phase, the rigid nature of the hoof may impede
drainage of exudate and the sloughing of necrotic or foreign
material from wounds.
In the repair phase, contraction does not occur.7
The most apparent difference between healing of skin and foot
involves epithelialization. Epithelium migrates across the
surface of a wound in the foot just as it does in the skin.
However, the epithelial margin of a skin wound is usually
relatively homogenous whereas the epithelium at the margins of
a foot wound may be of one or more types. The simplest example
would be a defect confined to a single epithelial type, either
the sole or frog. The epithelial margins wound then be of a
uniform type so that epithelial coverage of the wound would be
very similar to a full thickness skin wound except for the
greater thickness of stratum corneum to be replaced.
A full thickness wound to the wall that does not involve
either the sole or the coronary band forms a defect in all
three layers of the wall but only involves the germinal layer
of the laminar epithelium. Therefore, the defect will be
epithelialized by laminar epithelium. However, because the
laminar epithelium is only capable of limited proliferation,
the defect in the stratum internum is closed, but the defects
in the stratum externum and stratum medium persist moving
distally with normal hoof wall growth until the wall proximal
to the defect has reached the weight-bearing surface of the
hoof.
If the margins of a wound are composed of more that one source
of epithelium, it appears that the nature of the newly formed
stratum corneum reflects the source of epithelium that
migrated to fill the defect. Nowhere is this more apparent
than wounds that involve the skin of the pastern and the
coronary band; it is not uncommon for epithelium to migrate
proximally to the previous margin of the coronary band to for
a spur of horn in the distal pastern. Likewise if a defect in
the coronary band is not filled with coronary epithelium, a
persistent defect in the hoof wall may develop.
5. Treatment of Wounds
A. General treatment of skin wounds
Many factors, both local and systemic, influence the way
wounds heal.5 The systemic influences on wound
healing are beyond the scope of this discussion. It is the
clinician's task to manipulate the local factors affecting
wound healing to achieve the fastest wound closure compatible
with the best possible functional outcome. To this end, wound
debridement, closure, bandaging, and medical treatment must
all be considered.
The objective of wound debridement is to remove as much
necrotic tissue, foreign material, and infection as possible.
This is usually accomplished by wound lavage and surgical
excision. If tissue viability is questionable and it can be
excised without endangering other nearby structures it should
be removed, but if tissue of questionable viability is
adjacent to a vital structure, such as a joint capsule, it is
better left and observed and may be debrided at a later date.
If wound contamination and tissue injury do not preclude it,
wound closure almost always provides the most rapid and
functional result. The wound closure may be primary or delayed
primary.
Bandaging serves many functions including wound protection
from trauma and desiccation, applying pressure to the wound
surface, wicking away exudate, and maintaining dressings and
topical medication in place. Typically, distal limb bandages
are composed of 3 layers, a surface dressing, a layer of sheet
or roll cotton to distribute pressure and absorb moisture, and
an elastic layer to maintain the bandage in place and apply
pressure. Surface dressings influence, debridement,
contraction, fibroplasia and epithelialization of wounds
healing by secondary intention. Adherent dressings are
frequently used to lift necrotic debris off from a wound
surface. Once a wound is debrided, contraction proceeds faster
under a petrolatum impregnated gauze, but once wound
contraction is complete epithelialization proceeds more
rapidly under a Telfa pad.
Topical antibiotics and antiseptics are available as
solutions, ointments and creams. Ointments and creams are
easier to apply and maintain on a wound surface, but
gentamicin solution has been proven to be superior to a
gentamicin cream. The author favors solutions because they
dilute exudate and help to maintain a physiological
environment at the wound surface. Topical antiseptics are very
effective in low concentrations, for example, 0.1 - 1%
povidone iodine or 0.05% chlorhexidine but are deleterious at
higher concentrations. In general, astringents and caustics
are deleterious to fibroplasia and epithelialization and their
use should be avoided. Systemic antibiotics are seldom
warranted in the treatment of wounds that are confined to the
integument.
B. Treatment of foot wounds
Foot wounds warrant special consideration because of the foot
is in constant contact with the ground and because of the
structure of the stratum corneum of the integument of the
foot. In addition, there are several traditional practices
that are applied to foot wounds that most practitioners would
not consider doing to other wounds. The debridement of foot
wounds closely follows the general principles, but caution
must be taken when debriding deeper wounds because of the
close proximity of many vital structures. The practice of
soaking horse's feet in buckets of water with or without
antiseptics to lavage a wound is highly questionable under any
circumstance, but soaking feet with wounds to deeper
structures such as the navicular bursa cannot be recommended.
Wound closure is seldom possible or necessary so that most are
left to heal by secondary intention. The exception is injuries
to the coronary band which should be reconstructed wherever
possible to retain the greatest functional capabilities of the
digit. The coronary band can be readily sutured proximally
where it is thin, but the hoof wall may need to be shaved more
distally to allow the placement of sutures. The closure may be
primary or delayed primary. Any avulsed wall distal to the
coronary band will not reattach to the foot so it may be
removed unless it is required to maintain the normal
orientation of the coronary band.
Bandaging horse's feet follows similar principles to the rest
of the distal limb except that the middle layer is frequently
omitted. Any topical medication that serves to treat other
distal limb wounds may be applied with similar considerations.
The use of full strength povidone iodine or topical
astringents is usually deleterious and should only be used
with considerable caution, especially when the integument has
been breached. The author's preference is a dry or wet to dry
dressing with a Telfa pad. Wet to dry dressings are readily
applied by first applying the Telfa pad, cotton gauze backing
and roll gauze dry before applying the solution, either by
pouring or spraying it on top of the dressing. Because
moisture from stall bedding readily seeps into any foot
bandage, the application of piece of rubber inner tube or
treatment plate is advantageous; taping on the first wedge and
cuff of a Redden Modified Ultimate works very well. Once a
wound has completely epithelialized, topical astringents such
as 2% iodine may be applied with caution to toughen up the
newly formed stratum corneum. Bandages do not always provide
satisfactory stability to the foot capsule for wounds
involving the coronary band so that casting with a digital
cast is employed to decrease the likelihood of persistent hoof
wall defects.7,8
The use of antibiotics at all in foot wounds has been
questioned.9 However, most clinicians would
recommend systemic antibiotics if the wound breaches the
integument until the entire surface of the wound is covered by
granulation tissue, and most would consider systemic
antibiotics imperative if a joint, tendon sheath or bursa is
involved.
6. Complications
A. Distal limb wounds proximal to the foot.
The most likely complications to occur are a delay or
cessation of closure in wounds healing by secondary intention,
breakdown of sutured wounds, and a loss of function due to
fibrosis in the wound. Wounds left to heal by secondary
intention on the distal limb of horses heal notoriously
slowly. This is in part due to innate features of wound
healing in the distal limb of the horse. Wound contraction in
the distal limb is limited compared to wounds of the trunk
because of the tension in the skin. Also, exuberant
granulation tissue is common and it prevents wounds from
epithelializing satisfactorily. In addition, the presence of
infection, necrotic debris or foreign material will impede
wound healing. Inadequate protection of the wound will
desiccate the wound surface and may allow further wound trauma
to occur.
B. Foot wounds
Most of the complications seen in the healing of foot wounds
are the same that occur in skin wounds. Many full thickness
foot wounds also heal slowly. While wound contraction is
absent in foot wounds, exuberant granulation tissue is seldom
a problem. Being in constant contact with the ground the foot
is inherently predisposed to further injury. Occasionally
wound closure appears to be arrested for no apparent reason;
these wounds usually respond to light surface debridement and
bandaging with a wet to dry dressing.
After the wound has completely healed, the foot may not
function normally if the hoof is unable to evenly support the
distal phalanx or if injury to the deeper structures has
interfered with attachment of a structurally sound hoof to the
distal phalanx. Therapeutic trimming and shoeing with bars,
clips or extensions may be needed to minimize any functional
deficits.
7. Conclusion
By combining knowledge about the anatomy and function of a
horse's foot with knowledge about the principles of healing in
skin wounds, sound decisions can be made about the treatment
of foot wounds when specific information is not available that
result in satisfactory clinical outcomes.
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