Characterizing the nature of the horse’s lameness is one of the key objectives of effective visual examination. Achieving this task in conjunction with identifying the lame limb(s) comprises the foundation of any satisfying assessment. This is because there tends to be a healthy correlation between the nature of a horse’s lameness and the general location of its source (fig. 9.1). For instance, we can usually rule out a foot problem in a horse exhibiting purely non weight-bearing lameness in a forelimb. On the other hand, if the same horse subsequently develops severe unilateral weight-bearing lameness a few days after being reshod, there’s a good chance that the issue can be successfully addressed with the help of the farrier. This region-specific information becomes invaluable as we navigate through the examination process.
9.1 Relationship Between Limb Region and Nature of Corresponding Lameness
Problems originating below the level of the fetlock joint(s) usually produce weight-bearing lameness. Pathology involving structures within the horse’s upper limb often generate non weight-bearing lameness. Issues affecting the horse’s mid-limb commonly manifest as combination lameness, comprising both weight-bearing and non weight-bearing components.
As you’ll learn in chapter 15 (p. 85), there is also a close relationship between the nature of a horse’s lameness and the physical design of the structure that is causing it: structures that undergo load-bearing stress (i.e. bear weight) have the capacity to produce weight-bearing lameness, whereas structures that change their shape during movement tend to generate non weight-bearing lameness (fig. 9.2).
9.2 Relationship Between Anatomic Role and Nature of Corresponding Lameness
A. Structures that “feel” the load of the horse’s weight have the potential to generate weight-bearing lameness.
B. Structures that change shape as the horse moves have the potential to generate non weight-bearing lameness.
Knowledge of this interrelationship enables the experienced observer to more easily decipher the true cause of a problem amongst a myriad of possibilities—all based solely on the way the horse moves. This can be achieved by using the nature of the horse’s lameness to reveal the primary function(s) of its source (fig. 9.3 and VL 9a). For example, the differences in the physical roles of the cannon bone and fetlock joint with regard to load bearing and movement will be reflected in the weight-bearing versus non weight-bearing characteristics of the horse’s gait, respectively. Appropriately, issues affecting these structures can often be discriminated without the assistance of local anesthesia (blocks) or diagnostic images.
Figure 9.3 Disclosure of Anatomic Function Based on the Nature of Lameness
VL 9a Scan/Click to view video. www.getsound.com/tutorials/9a
As you can see, determining the nature of a horse’s lameness is a critical part of our inspection methodology in view of the valuable diagnostic clues that this intelligence provides. Fortunately, this exercise is relatively simple for the informed observer. Several visual markers that are unique to both weight-bearing and non weight-bearing issues enable one to judge the nature of most gait deficits with confidence. The majority of these indicators, which will be highlighted in chapter 24 (p. 170), are relatively obvious once we know which aspects of the horse’s gait demand special attention.
10 Authentic versus Artificial Lameness
From a visual standpoint, all gait deficits are “real” in that they alter movement and produce lameness. Some deficits, however, exist for the sole purpose of helping the horse to adjust for a shift in body weight or balance that occurred as a result of a problem somewhere else. Without constant incitement from the primary issue, this secondary “adjustment” would instantly resolve. Since it is nothing more than a visible product of another lameness, we denote this type of gait deficit as artificial or referred.
Referred lameness is secondary by definition. Even though it isn’t considered to be “real” and doesn’t factor into the treatment strategy, its existence dramatically facilitates our ability to accurately assess the poorly performing horse. Like all secondary issues, referred gait deficits provide valuable insight into the nature of the primary problem.
Referred deficits, although seemingly obscure, are fairly easy to predict in the lame horse. Proper identification and classification of the primary component usually exposes the basis behind any referred elements. And, as previously mentioned, acute characterization of a referred component can correspondingly lead us to likely primary instigators.
It is important to note that primary deficits will typically generate referred deficits of comparable nature. In the event that referred lameness manifests, the observer will usually find that:
A primary weight-bearing lameness in the forelimb generates a referred weight-bearing lameness in the contralateral hind limb.
A primary non weight-bearing lameness in the forelimb generates a referred non weight-bearing lameness in the contralateral hind limb.
A primary combination lameness in the forelimb generates a referred combination lameness in the contralateral hind limb.
A primary weight-bearing lameness in the hind limb generates a referred weight-bearing lameness in the ipsilateral forelimb.
A primary non weight-bearing lameness in the hind limb generates a referred non weight-bearing lameness in the contralateral forelimb.
A primary combination lameness in the hind limb generates a referred combination lameness in the ipsilateral forelimb.
The Concept of Diagonal Synchrony
The walk and trot each comprise a two-beat stride pattern in which the horse’s weight is distributed evenly between diagonal pairs of limbs. The left hind and right front limbs comprise one diagonal pair, whereas the right hind and left front limb constitute the other (fig. 10.1). The horse maintains similar movement (i.e. synchrony) between the two limbs comprising each diagonal pair at these gaits. In other words, the diagonal pair of limbs move at the same time and in the same way (VL 10a). The left pelvic (LH) and right thoracic (RF) limbs maintain concurrent weight-bearing and non weight-bearing (flight) phases of the stride. The right pelvic (RH) and left thoracic (LF) limbs do the same. This form of coordinated movement is known as diagonal synchrony.
10.1 Diagonal Pairs of Limbs
The left hind and right front limbs comprise one diagonal pair (blue). The right hind and left front limbs constitute the other diagonal pair (orange).