Page 162 - Adams and Stashak's Lameness in Horses, 7th Edition

128 Chapter 2

Vertical head movement signal the sound limb. So why is this incorrect maxim “down
C on sound” propagated in our veterinary lexicon and still
VetBooks.ir Diff Max Head A vertical head movement as one (the most simple) type of
taught to veterinary students?
Examining the different shapes of the trajectories of
forelimb lameness is gradually increased suggests the
origin and context of this false maxim (Figure 2.119).
Considering a very mild lameness, the asymmetry of the
down and then up movement, twice per stride, is small,
so there is just a little less downward motion of the head
B (upper left quadrant, Figure 2.119) during lame limb
Diff Min Head D
stance. The head halts its fall at a slightly higher vertical
position during lame or more lame forelimb stance com­
pared with not lame or less lame forelimb stance. As the
lameness increases, this asymmetry is increased, but the
Right forelimb stance Left forelimb stance
overall shape of the vertical head movement trajectory
Figure 2.117. Overall vertical head trajectory pattern for horse (upper right and lower left quadrants, Figure 2.119) is
with pushoff type, in this case right forelimb lameness, or lameness retained. Ultimately, as the amplitude of lameness is fur­
with pain greatest in the second half of right forelimb stance. Peak ther increased, an inflection point in the trajectory is
(highest maximum) head vertical position occurs near beginning of reached, and the downward and then upward move­
stance of the not or less lame forelimb. Diff Max Head or first ment of the head is now upward in the first and then
maximum head height (A: near beginning of stance of the right downward in the second half of stance. This can appear
forelimb) minus second maximum head height (C: near beginning of as primarily upward movement with subjective evalua­
stance of the left forelimb) is a negative value. Lowest head position tion. This deficient version of “down on sound” seems
occurs during stance of the not or less lame forelimb. Diff Min Head correct only when the lameness is severe, so, in the prac­
or first minimum head height (B: during stance of the right forelimb) tical sense, it is not a useful explanation.
minus second minimum head height (D: during stance of the left With pain in the forelimbs that is maximum during
forelimb) is a positive value. Diff Max Head and Diff Min Head are of the first half of weight‐bearing, the horse will move its
opposite signs. For a right forelimb pushoff lameness Diff Min Head head upward so that it is decelerating upward at the
is positive (+) and Diff Max Head is negative (−).
beginning of stance and then accelerating downward
during the remaining first half of stance. Thus the head
A C will fall from a relatively higher position but then halt
Diff Max Head = 0 its descent prematurely so that it ends up at a local mini­
Vertical head movement signal
mum position higher compared with its position during
the contralateral (less lame side) stance. The common
B description of this “head nod” phenomenon as “down
on sound,” meaning moving down more on the sound
limb, is therefore also not always true, and the actual
length of downward descent may be greater than during
Diff Max Head the same period of contralateral (not lame or less lame)
D forelimb stance. The more accurate descriptive maxim
in this type of forelimb lameness would therefore be
“lowest on sound.”
Right forelimb stance Alternatively, if the pain is present during the second
Left forelimb stance
half of stance when the head is normally moving upward
Figure 2.118. Overall vertical head trajectory pattern for horse with the center of mass of the body, the head will move
with midstance right forelimb lameness or lameness with pain up more, decelerating at the end of stance. The head will
greatest in the middle of right forelimb stance. Peak (maximum) then reach its highest vertical position preceding the
head vertical position before right and left forelimb stance is beginning of stance in the contralateral, non‐lame or less
equivalent. Diff Max Head or first maximum head height (A: near lame, forelimb. There will be a difference in the maxi­
beginning of stance of the right forelimb) minus second maximum mum heights of the head with the head highest before
head height (C: near beginning of stance of the left forelimb) = 0. the stance phase of the not lame or less lame forelimb.
Lowest head position occurs during stance of the not or less lame This is opposite that of lameness that occurs during the
forelimb. Diff Min Head or first minimum head height (B: during
stance of the right forelimb) minus second minimum head height first half of stance, where the head will move up more
(D: during stance of the left forelimb) is a positive value. after pushoff of the non‐lame or less lame forelimb.
However, side of lameness remains determined correctly
by the maxim “lowest on sound.” “Lowest on sound” is
is really only true in horses with severe lameness. But, in a correct description of side of lameness, regardless of
horses that are trotting, the head follows the center of timing of lameness within stance.
mass of the body, moving down when the horse is land­ Thus the side of forelimb lameness is identified by the
ing on a forelimb and then up when the horse pushes off difference in the low or minimum positions of the head
that forelimb, even with moderate to severe lameness. during stance, and timing of lameness is determined by
The head does not just move up during the stance phase the difference in the high or maximum positions of
of the lame limb and down during the stance phase of the head occurring after pushoff of each forelimb.