Keratoma is an uncommon tumor of the
keratin producing epidermal cells of the inner hoof wall (Lloyd
et al 1988). It is rare to find a keratoma located in the
dermis of the sole and there is little information reported in
the literature regarding occurrence in this region. Keratomas
result from proliferation of cornified tissue on the inner
side of the hoof wall (Chaffin et al 1989). They are
interposed between the stratum medium of the hoof wall and
underlying third phalanx and can be located any where from the
coronet to the solar surface of the foot at the white line.
They can be cylindrical or spherical in shape and are
generally found in the toe or the quarters of the hoof.
Keratomas are slow- growing tumors and its not until the
increasing size exerts pressure on the laminar dermis and the
surface of the third phalanx that lameness becomes apparent (Honnas
et al 1988). Aetiology remains obscure, but it is thought
to result from trauma or chronic irritation from direct hoof
injury or sole abscesses (Honnas et al 1994). This
report describes a keratoma found in the dermis of the sole
leading to an elusive lameness.
A 12-year-old Warmblood gelding was presented for
consultation with a history of having persistent right
hindlimb lameness. The lameness (Grade 2 to Grade 4 of Grades
1-5) had an insidious onset and had been present for the
previous two months. Focal discomfort was evident when hoof
testers were applied over the sole of the lateral quarter. At
the initial examination, haemorrhage was noted when the sole
was superficially pared away over the painful area. An initial
diagnosis of a foot abscess or subsolar bruising was made and
the horse was treated with hot soaks, poultice and
anti-inflammatory medications. As the lameness persisted,
periodic radiographs where taken, but no abnormalities were
identified. The lameness was abolished by lateral posterior
digital nerve block and by intra-articular anesthesia of the
distal interphalangeal joint. Various shoeing methods were
used, including a shoe with a rim pad, to remove any pressure
on the sole. Interestingly, lameness increased when the horse
was shod and improved when allowed to remain unshod.
When examined, the horse was unshod and walking well.
Grade 2 lameness was
observed when the animal was trotted on a hard surface. The
conformation of the foot was normal with respect to size,
shape and angle. There was a focal, discolored (haemorrhagic),
slightly convex section of sole medial to the white line in
the lateral quarter of the hoof. The architecture of the white
line in this area was not distorted nor were there any
separations or tracts. Again, pain was elicited to hoof
testers but the area was also sensitive to digital pressure.
At this juncture, examination of further radiographs showed a
small convex area of osteolysis or bone resorption on the
plantar surface of the third phalanx on the lateral view (Fig
1). The dorso-ventral view of the third phalanx was
unremarkable. Because these findings were indicative of a mass
causing the lysis of the third phalanx, it was decided to
surgically explore this section of the foot.
procedure was done standing with the horse sedated using
xylazine (1.1 mg/kg, IV) and butorphanol (0.1 mg/kg, IV). The
foot was desensitized with a biaxial plantar nerve block just
proximal to the sesamoid bones. An Esmarch's bandage was
applied to the lower limb to control hemorrhage and aid
visualization during surgery. The hoof was prepared
aseptically with povodine iodine and alcohol. A thin looped
hoof knife (Fig 2) was used to cut a trough around the
perimeter of the lesion (Fig 3). The dissection was continued
through the sole until a cavity was entered. The overlying
flap of cornfield sole was laid back and a mass removed (Fig
4). The cavity was curretted down to normal healthy bone and
the overlying flap of epidermis was removed. Silver
sulphadiazine ointment was placed in the wound and the cavity
loosely packed with gauze. An impervious bandage was placed on
The horse was treated with sulfamethoxazole/Trimethoprim
(30 mg/kg bwt per os b.i.d.) for five days following surgery.
Phenylbutazone (4.4 mg/kg bwt per os) was used only as needed
for pain relief. The bandage was changed at 4-day intervals
and the wound was packed with silvadine ointment. Granulation
tissue filled the cavity rapidly and was level with the
surrounding sole at the third bandage change, at which time a
bar shoe with a treatment plate, that had been constructed and
fitted prior to the surgery, was applied to the foot. The
treatment plate was made from high-density plastic 1
which is light and resists wear (Fig 5). At this point the
topical medication was changed to Lotagen Gel 2 to
increase toughness of keratinised epithelium and the dressing
was changed every other day. Forty-five days post surgery; the
horse was sound and started in light exercise.
The 2 x 1.5 x 1 cm. teardrop-shaped mass was submitted for
histopathology. The tumor was composed of lamellated keratin.
On cross section it had an "onion skinned appearance and was
covered with a thin layer of flattened orderly squamous
epithelium. Diagnosis was a keratoma.
The initial presentation, foot-related lameness, associated
with a focal area of pain, with the lameness abolished by
anesthesia of the palmar digital nerves, is a common
presentation in equine practice. Usual differential diagnosis
for this presentation should include subsolar abscess or
bruising, puncture wound, focal osteomyelitis and a marginal
fracture of the third phalanx. However, in this case there was
no tract or puncture found in the sole or white line nor was
there discoloration on the outer surface of the sole. In
addition, consecutive radiographs failed to reveal a fracture.
The diagnosis of a solar keratoma is uncommon. Although
palmar digital anesthesia confirmed the foot as the source of
pain, the block was not specific for the problem. It is
unlikely that anesthesia of the distal interphalangeal joint
would have provided additional information, as recent studies
have shown that local anesthesia injected in the DIP joint
will block the dermis of the anterior portion of the sole (Schumacher
et al. 1999). Indications that a mass might be present
were that when a shoe was applied the lameness increased from
Grade 2 to Grade 3 or more. Additionally, the lameness
subsided when the shoe was removed. This suggested that when
the horse bore weight, impingement of a mass interposed
between the shoe and the inner structures of the foot resulted
An insidious onset of lameness is often the presenting
complaint with keratoma (Honnas 1998). Over time,
diagnostic indications of a solar mass become more obvious. In
long standing cases of keratoma of the hoof wall, focal
thickening or inward deviation of the white line becomes
apparent. If a tumor enlarges over time, it causes increasing
pressure on the third phalanx leading to a focal area of bone
resorption (Lloyd et al. 1998). Pressure is also placed
on the overlying epidermis of the sole causing it to become
convex (bulge). This pressure also leads to thinning and
discoloration of the overlying sole.
In conclusion, unresolved lameness with pain localized to a
focal area of the sole can present a diagnostic challenge.
With persistent lameness localized to a small section of the
sole, keratoma within the dermis of the sole or other tumors
of the equine hoof should form part of the differential
1 Castle Plastics, Leominster, MA 01453
2 Essex Tierarznei, Munich, Germany
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