The horse will attempt to maintain diagonal synchrony whether it is sound or lame. Stride-to-stride gait compensation is necessary at the walk and trot to support the horse’s weight and maintain balance. This is an important consideration during lameness assessment, because what affects the movement of one limb will also affect the movement of the contralateral limb on the other end of the horse (VL 10b). For example, application of a toe extension to the left hind foot as a means of delaying limb breakover and lengthening stride will generate a similar gait adjustment of the right forelimb. And as you might expect, the limbs constituting the other diagonal pair will adapt in reciprocal fashion so that the horse can remain stabilized during movement.
VL 10b Scan/Click to view video. www.getsound.com/tutorials/10b
During lameness evaluation, the practitioner must determine whether the origin of each gait deficit is primary in nature or a result of the horse’s attempt to maintain synchrony with its diagonal counterpart. In the latter case, lameness would be considered to be artificial or referred.
Referred lameness manifests dissimilarly between the front and hind limbs. This is due to a number of factors, most of which incriminate the horse’s general physique. Since horses are inherently front-end heavy, it is much easier for them to transfer weight in a hind-to-fore direction (fig. 10.2 A). Hind limb asymmetry, even when very mild, can dramatically influence how the horse loads the front end (VL 10c). In many instances, the degree of secondary/referred forelimb lameness exceeds that of its primary hind limb complement.
10.2 Dissimilar Transfer of Weight Between the Front and Hind End
A. Since horses naturally bear more load on their front end, it is relatively easy for them to transfer weight from the hind end in a forward direction (toward the front end).
B. Dissimilarly, horses are not very proficient at transferring weight in a backward direction (from the front end toward the back end).
VL 10c Scan/Click to view video. www.getsound.com/tutorials/10c
By contrast, horses are only marginally effective at transferring weight from the front to the back end (fig. 10.2 B). For this reason, forelimb weight-bearing lameness only generates visible hind limb asymmetry when it is pronounced (VL 10d). Moreover, the components of the lameness that get transferred tend to be those associated with stride length rather than load burden. Since the driving motive is usually to maintain balance (rather than shift weight), fore-to-hind referrals are almost always expressed in the contralateral hind limb. This is useful knowledge to the observer, who can surmise that any demonstrative weight-bearing lameness in the hind limb is probably authentic, since it is rare for referred deficits to manifest in this way.
VL 10d Scan/Click to view video. www.getsound.com/tutorials/10d
It is important to keep in mind that the severity of the referred component may exceed that of its parent source. In fact, recognition of secondary and/or artificial lameness is often what prompts the horse owner to solicit veterinary intervention in the first place. Examiners who visually separate front and back halves of the animal, evaluate each half individually and actively seek potential relationships between coexisting deficits, will both clarify their assessment and enhance the quality of their interpretation.
11 Axial versus Peripheral Lameness
We generally think of lameness affecting the horse’s limbs, but irregular movement can also be demonstrated between different sides of the horse’s axial skeleton, such as the neck and back. Axial lameness almost always occurs in conjunction with limb lameness because the head, neck, and trunk serve as powerful tools for transferring weight and maintaining balance (VL 11a). As a matter of fact, the adept observer will usually appraise movement of the horse’s median anatomy at the outset of assessment (chapter 20), since it can provide valuable clues with regard to the region(s) or limb(s) that should be evaluated next.
VL 11a Scan/Click to view video. www.getsound.com/tutorials/11a
Appropriately, many practitioners will presume that axial asymmetry is either artificial (referred) or compensatory if it coexists with peripheral lameness. In these instances, the median anatomy will assuredly display unnatural movement to adjust for gait deficits in one or more limbs, thereby making it extremely challenging for the observer to validate the primary contribution of any axial component.
Interestingly, horses with authentic axial lameness are more likely to avoid movement altogether as opposed to moving irregularly. As previously discussed (in chapter 5—p. 26), the relative proximity between contralateral axial structures in combination with the horse’s emphatic desire to safeguard painful areas can make effective assessment of the median anatomy (by itself) very perplexing.
For these reasons, it is important that we do everything at our disposal to simplify the process: observing the horse on a regular basis (from one day to the next), asking the horse to move in concentrically smaller circles and appraising the effect of a rider’s weight are all common techniques intended to clarify our visual assessment of head, neck, and back motion.
The Head and Neck
The horse’s head and neck are quite heavy, accounting for almost 10% of the entire body weight. The head is located a considerable distance from the trunk on account of the appreciable length of the neck. As such, it serves as an extremely effective means for transferring weight, particularly that associated with the front end. It is also a very large and visibly prominent structure that can be easily tracked with our eye. Based on these characteristics, the horse’s head can essentially be considered a peripheral structure that aptly awards the observer with an explicit marker for assessing lameness. In combination with the withers, the head and neck are evaluated foremost during visual lameness assessment (see chapter 23 and VL 11b).
VL 11b Scan/Click to view video. www.getsound.com/tutorials/11b
Notwithstanding, the horse’s head also embodies the aspect of anatomy most often utilized to emit emotion, such as anxiety or resistance. Abnormal movement related to these forms of expression can be very difficult to interpret, and should, therefore, be considered in conjunction with how the rest of the horse’s body is moving.