Page 161 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 127

Therefore, vertical head movement in the perfectly Vertical head movement signal
“not lame” horse can be modeled as if there is only one A
VetBooks.ir the horse is trotting, which is that following the verti­ Diff Max Head C
predictable periodic signal at twice the stride rate when
cal movement of the center of mass of the body
(Figure 2.115). 13 B
Alternatively, when a horse is lame or more lame in
one forelimb, then another periodic factor, at the stride Diff Min Head D
rate, or ½ the frequency of the normal expected vertical
movement, additionally influences vertical head height
from the ground. The horse will always try to move the
head upward during pain with maximum effect at the Right forelimb stance
moment of peak pain. Overall head trajectory is the sim­
ple summation of these two components in the horse Left forelimb stance
with lameness. The phase or timing relationship between Figure 2.116. Overall vertical head trajectory pattern for horse
these two components determines the overall shape of with impact‐type right forelimb lameness or lameness with pain
vertical head trajectory. This phase relationship between greatest in the first half of, in this case, right forelimb stance. Peak
vertical movement components has been described (highest maximum) head vertical position occurs near beginning of
previously. 2,29,38 stance of the lame or more lame forelimb. Diff Max Head or first
When pain (or other dysfunction) is greatest during maximum head height (A: near beginning of stance of the right
the first half of stance, component summation results in forelimb) minus second maximum head height (C: near beginning of
an augmentation (a raising) both of the maximum head stance of the left forelimb) is a positive value. Lowest head position
position before lame (or more lame) stance and of mini­ occurs during stance of the not or less lame forelimb. Diff Min Head
mum head position during stance of the lame (or more or first minimum head height (B: during stance of the right forelimb)
lame) forelimb. Overall vertical head trajectory (a com­ minus second minimum head height (D: during stance of the left
bination of the two components) features differences in forelimb) is a positive value. Diff Max Head and Diff Min Head are
maximum and minimum head positions between right the same sign. For right forelimb lameness, both Diff Max Head and
and left sides of the stride that are of the same sign, Diff Min Head are positive.
meaning that the highest maximum head height occurs
before and the highest minimum head height occurs there is also augmentation in the maximum position of
during lame or more lame forelimb stance (Figure 2.116). the head after pushoff of the lame or more lame fore­
Conversely, when the pain is greatest during the sec­ limb, which is unlike an impact lameness. Overall verti­
ond half of stance, there is augmentation of the mini­ cal head trajectory features differences in maximum and
mum head position during stance of the lame or more minimum head positions that are of opposite sign, with
lame forelimb, as in an impact lameness, but, instead, highest minimum position during and highest maximum
position following lame or more lame forelimb stance
(Figure 2.117).
Vertical head movement signal In both of the above types of lameness, the entire
amplitude of forelimb lameness is reflected in a combi­
Diff Max Head nation of the difference in low and high heights of the
head. The classic description of “head nod” as only
“down on sound” or only as “up on bad” neglects con­
sideration of differences in both the minimum and maxi­
mum head positions and will therefore underestimate
amplitude of forelimb lameness.
When pain is greatest near the middle of stance, the
Diff Min Head
effect of the head moving upward during peak pain will
augment only the minimum head position during stance
of the lame or more lame forelimb so that there is a dif­
One stride ference in the minimum head positions between right
and left stance but no difference in maximum positions
Right forelimb stance Left forelimb stance of the head (Figure 2.118; Diff Max Head = 0). In this
case the amplitude of “head nod,” usually described as
Figure 2.115. Head movement trajectory in non‐lame trotting “down on sound” or “low on sound,” captures the entire
horse following trajectory of center of mass of the body. Trajectory is amplitude and will not underestimate forelimb lame­
similar to a sine wave at frequency of twice stride rate because ness. This is probably why midstance forelimb lameness
movement is down and then up twice per single stride. In a horse is easiest to detect subjectively and veterinarians infre­
without lameness, downward movement in first half of stance of one quently disagree on limb involved or amplitude with
forelimb is equivalent to that in the opposite forelimb, and upward this type of forelimb lameness.
movement in second half of stance of one forelimb is equivalent to
that in the opposite forelimb. Therefore, the differences in positions The classic “head nod” as “down on sound” implies
of the head from the ground at local minimums (during stance) or, that in a forelimb lame horse, the head moves down
Diff Min Head, is equal to 0. Likewise, difference in positions of head when the non‐lame or less lame forelimb is weight bear­
from the ground at local maximums (between the stance phases of ing, and then up when the lame or more lame forelimb
each forelimb), or Diff Max Head, is equal to 0. is weight bearing. This is a deficient simplification that