Page 113 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 79
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Figure 2.12. The angle of the shoulder usually influences the
angle of the pastern.
in improved balance. The scapulohumeral (shoulder)
21
joint is supported entirely by the muscles and tendons
surrounding it. Because this muscle support is so impor­ adequate height and width of heel, a concave sole, and a
tant to proper movement, a horse should have well‐ robust frog. The bulbs of the heel should lie vertically
developed muscles in this region.
The length of the humerus (from the point of the
shoulder to the point of the elbow) can affect stride
length (Figure 2.12). A long humerus is associated with
a long, reaching stride and good lateral ability; a short
humerus is related to a short, choppy stride and poor
ceps muscle that facilitates a larger range of elbow
lateral ability. A long humerus corresponds to a long tri­
movements. In general, the steeper the angle of the
21
humerus, the higher the action; the closer the angle is to
horizontal, the lower the action.
The angle formed by the humerus and radius and
ulna at the elbow joint should be between 120° and
36
150°. A more flexed elbow together with a horizontal
scapula results in a longer stance duration, which is
thought to improve gait quality with more collection in
the forelimbs. 21,23 Straighter conformation (lesser angu­
lation) at this joint may result in a short, choppy gait
and increased concussion on the distal limb.
Ideally the limb should form a straight column from
the elbow joint to the fetlock. This conformation will
disseminate the axial compression forces to all bony sur­
faces equally. With malalignment, axial compressive
forces become focused on one side, and tensional forces
are created opposite to it, increasing the stress and strain
on musculoskeletal structures. 36
The carpus (knee) is a complex joint and functions in
flexion, absorption of concussion, and extension.
Flexion primarily occurs at the radiocarpal and middle
carpal joints, and extension occurs by a locking mecha­
nism while the horse is weight bearing during the exten­
sion phase of the stride. The carpal bones should be in
good axial alignment with the radius and third metacar­
pal bone and of sufficient size to support downward
concussive forces. The carpus should appear straight
and not deviate forward or backward (Figure 2.13).
Fetlock joints should be large enough and angled to
permit free movement. A more extended fetlock joint
resulted in more maximal extension, which correlated
6
with a good gait in the forelimbs in one study. It has
also been found that a straighter hindlimb fetlock joint
angle results in a longer stride and swing duration.
6
A study on Swedish Warmblood horses found the mean
front fetlock angle to between 146° and 155° and the
mean hind fetlock angle between 153° and 161°. 22
This is considerable variation in the literature regard­
ing what the “normal” hoof‐pastern axis of the forefoot
should be. Previous reports of angles less than 50° are
currently considered to be too low, and normal angles
are thought to be approximately 54°. 4,29,36 Although
these angles may vary considerably, the angle of the
dorsal hoof wall should be similar to the angle of the
pastern. Exceptionally long, sloping pasterns or a bro­
9
ken‐back hoof‐pastern axis are thought to increase
strain to flexor support structures of the fetlock and
phalanges (Figure 2.14). Short, upright pasterns or a
broken‐forward hoof‐pastern axis are thought to cause
greater concussive stresses to the fetlock, phalangeal
joints, and foot. 4,9,29
The hoof should be appropriately sized for the size
for the horse, well shaped (more round than egg shaped),
and symmetric. It should have high‐quality hoof horn,
below the central axis of the cannon bone in the sagittal
plane (Figure 2.11B). Normally trimmed hooves usually
impact the ground heel first or flat‐footed. 4,36 Studies
have documented that lower hoof angles predispose