I want to touch a bit on Diseases of
the Hoof, Distal Phalanx and Associated Structures. The hoof/foot is
very complex. "By definition, the foot of the horse includes the hoof
and all structures contained therein, including the sole and frog. This
hoof is only the cornified epidermis of the foot (wall, sole, frog),
is no-vascular in structure, and has no nerve supply. Nutrition for
the hoof is obtained from the combined coria".
The hoof is composed of the wall and it is approximately 25% water and
is a modified cornified epithelium. It has three layers; the first outer
layer is the periople, the second or middle layer composes the bulk
of the hoof wall and is the densest portion of the wall. This is the
layer that contains the pigment in pigmented feet. There is no difference
in the make up of a pigmented hoof and a non-pigmented (white) hoof
other then the pigments. The third layer or inner layer is the laminar
layer that forms the epidermal laminae of the hoof.
The ground surface of the hoof is divided into sections called the toe,
quarters and heel. Growth of the hoof wall is quite slow, about 6 mm
or 1/4 inch per month. It takes about a full year for the hoof to grow
down from the coronary band to the ground surface. This can be speeded
up by feeding additional protein. You see this kind of growth when you
feed supplements that have a protein base (bean meal, alfalfa meal,
sea weed) or when you turn your horse out on lush pasture. Any change
will be noted from the top of the hoof first. Not a the bottom.
The Bars, at the heels, where the wall turns anteriorly to from the
bars that converge towards one another to meet at the apex of the frog.
They run parallel to the collateral sulci of the frog. The sole then
conforms to the inner curvature of the wall and to the angles formed
by the bars. The sole comprises most of the ground surface of the hoof.
It is approximately 33% water. The structure is similar to that of the
wall and the tubules run vertically as formed by the papillae of the
sole corium. These tubules curl near the ground surface, which accounts
for the self-limiting growth of the sole and causes shedding of the
sole. The frog is a wedge-shape Inverted V mass that occupies the angles
bounded by the bars and sole. It is soft because it is about 50% water.
The frog is divided into the apex which is at the front (anterior angle
of the frog), base which is the hind (posterior aspect) and the frog
stay which is the central ride of the internal surface.
The White Line is the junction of the wall and the sole. It is visible
as a white line following the circumference of the wall around the hoof.
The white line is the junction between the laminae of the wall and the
tubules of the sole.
The Corium or Dermal layer, is modified vascular tissue that furnishes
nutrition to the hoof. It is divided into five parts; Perioplic corium,
coronary corium, laminar corium, sole corium and the frog corium.
The Digital Cushion is a fibroelastic, fatty, pale yellow, relatively
avascular, and yielding pyramidal structure containing areas of cartilage
in the posterior half of the foot. The primary purpose of this structure
is to concussion to the foot. There are also other mechanism that also
aid in the concussion such as the hydrology of the blood supply. The
Coronary Cushion is the elastic portion of the coronary corium; it aids
slightly in reducing concussion.
Lateral Cartilages are part fibrous tissue and part hyaline cartilage.
When ossification of these cartilages takes place it is called "Sidebone".
The Coronary is the combined perioplic corium, coronary corium and coronary
cushion; it is the primary growth and nutritional source of the bulk
of the hoof wall. Injuries to this structure are serious and usually
leave a permanent defect in the growth of the hoof wall/horn.
The Bulbs of the Heel are located in the posterior aspect of the foot
where the perioplic corium covers the angles of the posterior aspect
of the hoof wall. They are supported by the digital cushion.
The Distal Phalanx/third phalanx/p-3 bone/coffin bone, set on top of
and with in these structures of the hoof.
There are two types of Diseases for the hoof: Diseases with Physical
Causes and Diseases with Infectious & Immune.
I will cover in this first part, Diseases with Physical Causes.
Hoof Wall Cracks, especially those in the quarter and heel region, are
a common cause of foot lameness. Hoof cracks have a multitude of causes.
The horny hoof wall often fails internally before the crack propagates
externally. Central toe cracks almost always are the result of rotation
of the distal phalanx, such as in laminitis or deep flexor tendon contracture
syndrome. Unattended feet that get to long. Excessively wet or dry condition
or combinations of these may lead to hoof cracks. Conformation may also
lead to hoof cracks and not being trim or shod to the conformation of
the horse (Form To Function). Incorrect trimming and shoeing create
focal foot imbalance, causing abnormal impact that may lead to hoof
wall failure.
Keratoma Equine Keratoma is an uncommon hoof disease best described
as a tumor of the keratin-producing cells of the hoof wall. The mass
is interposed between the hoof wall and underlying third phalanx.
Sole Bruising is a common but often overlooked and underrated cause
of lameness. Lameness can be acute or chronic in onset and the severity
can vary from barely perceptible to non-weight bearing.
Coffin Bone/Distal Phalanx/Third Phalanx/P-3 Fracture are encountered
occasionally. There are two main categories of fractures: articular
fractures and nor articular fractures. Trauma is the predominant cause
of distal phalanx fractures. High speed impack, kicking a hard object,
stepping with high velocity on a nonyielding object.
Quittor is a lay term for necrosis of the collateral cartilage of the
third phalanx. It is characterized by an intermittent purulent discharge
and sinus tract formation at or proximal to the coronary band in the
proximity of the collateral cartilage.
Sidebone is a lay term describing osification of the collateral cartilages
of the third phalanx. This is a normal ageing process. The concern is
that premature or abnormal osification of this structure may lead to
lameness. Most likely this condition is the result of uneven foot impact
caused by many factors, including continual work on hard, uneven surfaces,
poor conformation and improper shoeing or trimming.
Pedal Osteitis is the demineralization of the solar margin of the distal
phalanx/coffin bone/third phalanx/p-3. It is not fully understood and
appears to be a secondary phenomenon that must be differentiated from
the primary problems.
Sheared Heels is a relatively common cause of foot lameness. The name
describes a structural breakdown and ultimate change in the shape of
the foot that occur between the medial and lateral heels. The condition
occurs as the result of single heel overload, either acutely or chronically.
It may be found as a single entity or in combination with other foot
problems.
Underrun Heels is the low heel/long toe configuration is perhaps the
most important and common foot abnormality. The configuration is so
common that is often thought to be with acceptable limits with many
people. It is not a lameness in itself, but rather a major cause of
foot problems and contributor to limb problems. The presence of the
abnormality is noteworthy, regardless of the other problems. It should
be corrected if possible.
If you have any of these problems they need to be addressed by your
professional farrier and equine lameness veterinarian.
Next month I will cover Inflammatory, Infectious and Immune Disease
of the foot.
Information for this article was taken from: O.R. Adams DVM, MS, "Lameness
in Horses, Equine Medicine and Surgery, Patrick Colaham DVM Dipi ACVS,
Alfred M. Merritt DVM, MS, James N. Moore, DVM, PhD, Dipi ACVS, I.G.
Mayhew, BVSc, PhD, FRCVS, Dipi, ACVIM, ECVN. Manual of Equine Practice,
Reuben J. Rose DVSc, DipVetAn, MaCVSc, PhD, FRCVS, David R. Hodgson,
BVSc, PhD, FACSM, Diplomate ACVIM.
