Page 1147 - Adams and Stashak's Lameness in Horses, 7th Edition

Foot Care and Farriery 1113

is removed due to its elastic nature. It is well accepted However, when the horse is moving, the location of the
that abnormal weight distribution on the foot or dispro­ CoP changes dynamically. The position of the CoP at
VetBooks.ir over time, cause it to assume an abnormal shape. 6,7,13,33–36 forces between the medial to lateral and the dorsal to
any point in the stride determines the distribution of
portionate forces placed on a section of the hoof wall,
palmar aspects of the foot. When the CoP is moved to
These abnormal stresses within the foot will also pre­
dispose the foot to injury or disease. Increased stress or one side of the foot, that side of the foot will be subject
weight‐bearing placed on a section of the hoof capsule to increased forces. If the CoP is moved in a palmar
may originate from a single source or from multiple con­ direction, the weight‐bearing or load on the palmar hoof
tributing factors such as weight of the horse, abnormal wall will be increased.
limb conformation, strike pattern of the foot, amount of Relating the CoP to hoof capsule distortions, if the
work, type of footing, and inappropriate farrier prac­ CoP is located more medially, over time, a medial hoof
tices. The responses of the foot to stress and differing wall flare (bending) and a lateral wall underrunning will
patterns of weight‐bearing are threefold: change in posi­ develop. If the CoP is moved dorsally due to increased
tion of the coronary band, change in hoof growth rate, tension in the DDF tendon (DDFT), the hoof capsule
and deviation(s) of the hoof wall. will likely develop a higher heel with a flare in the dorsal
Excess stress placed on a section of the hoof capsule hoof wall. Farriery is used to change the location of the
can manifest itself in a variety of ways including com­ CoP (to some extent) and change the distribution of
pressed growth rings, flares or underrunning of the hoof forces on the ground surface of the foot. When evaluating
wall, dorsal migration of the heels, and either focal or hoof capsule deformation, limb conformation should be
diffuse displacement of the coronary band. 34,36 As hoof considered as abnormal limb conformation affects the
capsule distortion of the forelimbs is commonly associ­ landing pattern and stance phase of the stride. Few
ated with lameness and various disease processes, only horses have ideal limb conformation, and any change in
the forelimbs will be considered in this chapter. As the conformation is likely to change the distribution of
“normal” foot has not been defined, each view of the forces within the hoof capsule leading to deformation.
foot will begin with what is perceived to be an ideal,
good, or healthy foot. 2,31,33–36,39 Palpation of the hoof Low‐Heel Foot Conformation
capsule often complements the visual examination, and
the areas where palpation is relevant will be included. The palmar section or heel area of the foot plays a
The goal of evaluating the hoof capsule is to identify critical role not only in soundness but also in the health
deformation and changes in the growth pattern that and maintenance of the horse as a whole. It is subject
may indicate abnormal distribution of forces (stresses) to almost incomprehensible external and internal biome­
on the foot. As hoof capsule distortion and abnormal chanical forces, which inherently predispose the palmar
loading usually accompany lameness, farriery will form foot to injury or disease. 2,19,20,22,31 However, the applica­
part of or sometimes the entire treatment to reverse the tion of farriery for this section of the foot, whether rou­
process. Farriery is used to help redistribute the load and tine, or for addressing abnormal conformation or
help improve or resolve the hoof capsule deformation. prevention remains controversial. Long toe, low heel is
the generality used to describe this foot conformation,
The Mechanism of Distortion but low heel accompanied by a long toe may be the
preferred term. A low or underrun heel conformation is
Evaluation of the hoof capsule morphology will indi­ defined as the angle of the heels being considerably less
cate where the hoof wall is unduly stressed; however, the than the angle of the dorsal hoof wall. When this differ­
evaluation has to be coupled with an understanding of ence is considerable, it is characterized by a broken‐back
the abnormal distribution of forces that lead to hoof HPA—the angle of the dorsal hoof wall being lower
capsule deformation. Understanding the biomechanical than the angle of the dorsal pastern. A common cause is
forces leading to hoof capsule distortions is also helpful not trimming the heels appropriately, which causes them
for the clinician when applying the appropriate farriery to grow forward and lose their angle. The low‐heel foot
to modify these stresses. There are many excellent conformation is so common in equine practice, especially
reviews of basic biomechanics of the hoof in the veteri­ in Thoroughbred horses, that it is thought to be normal.
nary literature. 6,7,13,33,35 Increased load or weight‐bearing In one study of foot‐related lameness, low‐heel confor­
by a portion of the wall has three potential consequences: mation was found in 77% of the horses, 47,48 whereas in
(1) deviation of the wall outward (flares) or inward a different group of horses that where lame without evi­
(underrunning) from its normal position, (2) movement dence of navicular disease, 73% had underrun heels.
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of the wall proximally, or (3) decreased hoof wall In another study of Thoroughbred racehorses, underrun
growth. 33–36 A reduction in load or weight‐bearing on heels were found in 52% of the horses. 14
the foot generally has the opposite effect. Briefly, in the A low hoof angle results in dorsiflexion of the DIP
standing horse, the weight of the horse borne by the joint, which concentrates weight‐bearing on the palmar
limb is supported by the ground, which opposes the section of the foot and increases strain on the DDFT. The
weight with an equal and opposite force. The force low‐heel conformation changes the angle of insertion of
exerted on the foot by the ground is termed the ground the DDFT on the distal phalanx, which places increased
reaction force (GRF). The term center of pressure (CoP) stresses on the navicular apparatus. When excessive toe
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is the point on the ground surface of the foot through length is present, the increased stresses in the palmar foot
which the GRF acts on the foot. The CoP varies among are potentiated at break‐over. This abnormal hoof
horses but is approximately located in the center of the conformation also removes the foot’s ability to absorb
solar surface of the foot in the standing horse. 6,7,13,33,35 concussion and dissipate the energy of impact, thus