Page 160 - Adams and Stashak's Lameness in Horses, 7th Edition

126 Chapter 2

unlocking results in increased tension in the patellar the limbs experience greater vertical force and faster
ligaments at the end of limb extension, followed by a rates of force application at the trot compared to the
VetBooks.ir stored energy, as limb hyperflexion, when finally despite the presence of a single‐limb weight‐bearing por­
forced unlocking, and then an abnormal release of
asymmetric gait of the canter at equivalent speeds,
released. The resulting hyperflexion of the hindlimb
tion of the stride.
Decreased loading and weight
5,49,56
is not unlike that of stringhalt (see previous para­ shifting off a painful limb is more easily accomplished in
graph), but is different in that it is preceded by the symmetrically moving horse. All of these factors
delayed stifle extension with prolonged weight‐bear­ contribute to understanding and appreciating why lame­
ing and increased hindlimb retraction. Also, momen­ ness is not only easiest but also more accurately and
tary or incomplete upward fixation of the patella, more precisely evaluated at the symmetric gaits. It has
compared with stringhalt, is more intermittent (thus been shown that lameness even at a slow trot will be
the alternative name of the condition, intermittent detected earlier and more consistently than lameness at
upward fixation of the patella), from stride to stride. the asymmetric gait of the gallop. 34
In addition to these swinging limb abnormalities, Speed of movement at the trot has been shown to
momentary/incomplete/intermittent upward fixation affect amplitude of lameness, especially when using
of the patella causes hindlimb weight‐bearing lame­ limb movement as the indicator of lameness. It is rea­
ness, primarily decreased propulsion of the pelvis sonable to think that the amplitude of lameness dis­
upward after hindlimb pushoff. played by the horse will increase with speed of
4. Gastrocnemius muscle injury: Injury to the gastroc­ movement, but this is not the case. When relying on
nemius muscle insertion causes characteristic internal subjective evaluation, it frequently appears that lame­
rotation (calcaneus moves abaxially) of the hindlimb ness is greater at slower trotting speeds, but this is
36
at the end of weight‐bearing. Only a few cases with most likely because it is easier to see. When measuring
definitive diagnosis have been described. 52 lameness, it seems that, instead, there is an optimum
5. Peroneus tertius rupture: Rupture of the peroneus speed of movement when lameness amplitude is greatest
tertius is usually caused by the horse falling with the and stride‐by‐stride variation is least. Finding this
37
hindlimbs retracted, falling in a full hindlimb cast, optimal speed is most likely to be successful by allowing
or struggling to free an entrapped or restrained the horse to move naturally. This can be confusing when
hindlimb. Rupture of this dense fibrous ligament that blocking horses, since horses, feeling better, may want
spans two highly mobile joints (stifle and hock) dis­ to move faster after a successful block. To prevent
rupts the normal function of the hindlimb of coupled changes in amplitude of lameness due to speed of move­
stifle and hock flexion and extension. With this ment, one should attempt to keep speed of movement
injury, at the termination of hindlimb stance, stifle the same before and after block.
flexion initiates swing of the hindlimb forward, but The most sensitive body‐movement indicators of
the hock does not actively flex. Hock flexion is thus lameness in quadrupeds is the pattern of vertical move­
delayed, weight‐bearing and retraction of the affected ment of the midline of the horse or vertical movement of
hindlimb is prolonged, and the hock flexes only with the head for forelimb lameness and vertical movement
distal limb momentum as it swings forward. Although of the entire pelvis for hindlimb lameness. Limb move­
at rest the horse can seemingly bear full weight when ment parameters are useful in some cases, but in general
standing, decreased weight‐bearing during move­ they are too variable and inconsistent to be considered
ment occurs in the affected limb. more sensitive and specific indicators of lameness than
6. Shivers: Shivers is a presumed degenerative neuro­ vertical movement of the head and pelvis.
logic abnormality that causes a hyperflexion or lift­
ing of the hindlimbs, which is more like a spasm than
a gait abnormality. EVALUATION OF VERTICAL MOVEMENT
OF THE HEAD FOR FORELIMB LAMENESS
In order to understand the biomechanics of vertical
EVALUATION OF THE HORSE AT THE TROT head movement as it relates to forelimb lameness, some
In many disciplines and breeds of horses, the trot is simple principles must be understood. When a horse is
the most common gait used for observing lameness. In trotting, the body moves down and then up in one half
principle the other symmetrical gaits are similar to the of the stride and then down and then up again in the
trot with the head and pelvis falling and rising during other half of the stride, and the head normally follows
the first and second halves of stance, but with greater this movement pattern. A stride begins when a limb
(less commonly) or lesser (more commonly) amplitude. strikes the ground and ends when that same limb strikes
So, the basic biomechanical principles describing altera­ the ground again at the start of the next stride. In the
tion of head and pelvic height with lameness are the non‐lame horse, the downward movement in the first
same, but the amplitudes of asymmetries are different. half of the stance of one forelimb is equivalent to that in
Additional information on lameness in “gaited” horses the first half of stance in the other forelimb. Likewise, in
is explained in Chapter 9. the non‐lame horse, the upward movement in the second
Detection of lameness in quadrupeds is easiest during half of stance in one forelimb is equivalent to that in the
the symmetrical gaits, like the trot. A normal expected second half of stance in the other forelimb. Therefore,
symmetry provides a standard against which amplitudes horses without lameness will have no difference in the
of asymmetry between right and left halves of a stride lowest positions of the head during stance or in the high­
can be compared within the individual horse. However est positions of the head before and after stance.