Page 1152 - Adams and Stashak's Lameness in Horses, 7th Edition

1118 Chapter 11

of limb conformation that results in an abnormal strike hoof capsule and sheared heels. Commonly, the distal
30
and loading pattern of the foot on the ground. When phalanx is offset laterally within the hoof capsule, which
VetBooks.ir the entire hoof during the landing and stance phase causing the medial heel to displace. The unequal distribu­
places additional load on the medial side of the foot,
the weight of the horse is not distributed uniformly over
tion of vertical forces on a given side of the foot over time
of the stride, one focal area of the foot, usually a heel or
heel and accompanying quarter, receives a disproportion­ appears to result in a biological remodeling rather than
ate amount of the total load during impact and displaces the heel being pushed proximally or “jammed” to use
proximally. Although a number of sound horses have the common farrier term (i.e. the heel is “growing out of
this type of hoof capsule distortion, the author believes shape rather than being pushed out of shape”). 4,5,22,23,30 .
that when the adaptive ability of the hoof capsule is sur­ Biomechanically, the position of the coronary band is
passed by the excessive load on one section of the foot, related to a balance between hoof wall growth at the cor­
this type of hoof conformation can be the sole source of onary band and the rate of migration of the hoof wall
unilateral palmar foot pain. This pain could originate distally. Furthermore, the rate of migration of the hoof
from the compression of the soft tissue located between wall is also a balance between an active process occurring
the middle phalanx and the hoof wall when the heel is in the lamellae to cause them to move distally and the
displaced proximally. The continual disproportionate force on the wall from the GRF. This appears to be the
loading and increased compressive stresses on one quar­ mechanism of displacement in horses with sheared heels/
ter/heel predispose the foot to subsolar bruising, corns, quarters (Denoix and Parks. Personal Communi cation,
quarter/heel cracks, fracture of the bar, and a deep fis­ 2012). Proximal displacement of the soft tissue structures
sure within the base of the frog that is susceptible to occurs palmar to the distal phalanx and appears to result
thrush (Figure 11.36). This type of foot conformation is from mechanical stress. To substantiate this statement,
readily observed by picking up the foot and noting the the author reviewed 50 DP radiographs on horses’ feet
relative disparity in distances from the heel of the hoof with at least one heel bulb that was displaced proximally
capsule to the hairline at the bulbs of the heels between 0.5 cm or greater. In each case, the solar surface of the
the lateral and medial heel (Figures 11.36 and 11.37). distal phalanx was basically horizontal (parallel) with the
The etiology of this condition is not completely ground, indicating the disparity in heel height did not
understood. Inappropriate lateral to medial orientation originate from the lamellae within the hoof wall located
(balance) of the foot where the foot does not land flat adjacent to the distal phalanx (Figure 11.36). 4,5
has always been implicated, but this has not been sub­ Farriery is directed toward unloading the hoof wall
stantiated. A correlation with rotational limb deformity and decreasing the forces on the displaced side of the
(lateral) that changes the flight pattern of the limb and foot. This is accomplished by improving the shape of
ultimately the manner in which the foot lands has also the hoof capsule and landing pattern by applying the
been suggested. 2,29 Furthermore, there appears to be a appropriate shoe. Intuitively, if the heel is longer on the
correlation between an offset distal phalanx within the displaced side (measured ground surface to hairline), it
is reasonable to trim the displaced side and create addi­
tional ground surface under the affected side.
Again, when possible, the author will remove the shoes
and trim the heels of the palmar foot such that the hoof
wall and frog are on the same plane. The horse can be left
without shoes for 24–48 hours or longer if necessary.
Leaving the horse barefoot for a brief period allows the
displaced heel to descend, the compressed area between the
coronet and middle phalanx will widen, and the palmar
section of the foot will assume a more relaxed position.
With the heels in a more acceptable position, the author
uses a double trim method to address sheared heels in an
attempt to improve and unload the displaced quarter/heel.
The foot is trimmed appropriately using the guidelines of a
parallel HPA, the COR, and the heels trimmed to the base
of the frog. 22,23 If the displacement of the heel is significant,
a wide‐web steel or aluminum straight‐bar shoe is fitted as
symmetrically as possible underneath the long axis of the
limb using the apex of the frog as a central marker. The
4,5
bar shoe effectively increases the surface area of the foot,
provides more expansion (ground surface) on the displaced
side that has a straighter wall, and decreases the vertical
movement of the heel bulbs. Before applying the shoe, a
second trim is performed under the proximally displaced
quarter heel, which goes from 0 mm at the ipsilateral toe
quarter (e.g. inside toe quarter for medial sheared heel) to
an average of 7 mm at the affected heel. The amount of heel
Figure 11.37. Sheared heel with a fissure through the base of that can be removed with the second trim depends on the sole
the frog (yellow arrow). Note the opposite direction of forces applied depth at the seat of corn and on the severity of the proxi­
to the walls during landing (red arrows). mal displacement of the coronary band at the sheared heel.