Diseases of the Hoof,
Distal Phalanx & Associated Structures -- Part I
By Ray Miller
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.
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