Page 949 - Adams and Stashak's Lameness in Horses, 7th Edition

Principles of Therapy for Lameness 915

VetBooks.ir

Figure 8.4. Schematic illustration demonstrating the effect of flexor moment remains approximately the same, the force in the
moving the center of pressure in a palmar direction. The arm of the deep digital flexor tendon is decreased. (Vectors represented by
extensor moment becomes shorter, and because the arm of the arrows are for illustrative purposes and do not represent real values.)

5
of the distal phalanx. Therefore, extending the effective more smoothly, thereby decreasing the abruptness in
border of the hoof distally at any point around its cir- changes in stress in the underlying tissues. To a lesser
13
cumference increases the tendency that the foot will land extent the medial and lateral abaxial branches of the
on the lengthened wall first. Extensions on the medial shoe can be beveled to improve the ease of motion as a
side of the foot of a horse should be used with caution horse is turning. While this has not been explicitly stud-
because of the risk of interference. Similarly, toe exten- ied, it is known that a turning horse loads the side of the
25
sions should be used with caution because they increase hoof on the inside of the turn. Therefore, it is rational
the propensity of a horse to trip. Lateral extensions may to anticipate that rolling or beveling the abaxial margins
be used without such obvious side effects but are likely of the shoe smoothes out the changes in stress in the
to alter flight of the foot as the limb is protracted due to underlying tissues on the ipsilateral side of the foot.
increased air resistance, the magnitude, and significance
of which have not been determined. Changing the width Distribution of Force
of the web of one branch of the shoe causes the foot to
become asymmetrically weighted, changing the inertia The main rationale for distributing part of the load of
that must be overcome as the foot accelerates away from weight‐bearing over more of the sole and frog is to
the ground at breakover and decelerates at impact. The reduce load bearing by the lamellae and wall, which
significance of this effect has not been investigated. assumes that most of the load is normally born by the
wall. This is frequently done by recruiting the entire
Ease of Movement ground surface in horses with laminitis. In those with
underrun heels, reducing weight‐bearing by the heels is
Clinicians have long thought that making the breako- attempted by increasing weight‐bearing by the frog,
ver point more palmar improves the function of the foot bars, and angles of the sole.
and relieves mild lameness, particularly that associated However successful these manipulations appear clini-
with palmar heel pain. However, several studies have cally, the mechanical effectiveness of this manipulation
examined the effect of changing breakover in this man- is uncertain because the role of the sole and wall in
ner and have been unable to demonstrate a decrease in weight‐bearing is poorly understood. It has been clearly
the duration of the breakover or a change in the flight of demonstrated that when a horse that has been at pasture
the foot, 10,17 though it may cause the foot to breakover barefoot stands on a firm surface, it bears weight in a
15
slightly earlier. Additionally, it does not change the peak “4‐point” pattern , that is, on its heels and at the toe‐
moment about the distal interphalangeal joint or the quarter junctions. If the same horse is housed on con-
peak force on the navicular bone, even though it crete, the wear of the ground surface of its foot causes it
decreases the length of the moment arm of the ground to bear weight around more of the perimeter of its foot
reaction force about that the distal interphalangeal joint and frog. If the same horse is made to stand on sand, the
at breakover (Figure 8.5). 10 weight is distributed over a much greater area, which is
The most likely explanation for the improvement no longer distributed around the periphery of the foot
seen by clinicians is that the foot breaks over (unrolls) but over the sole in a band running from quarter to