and muscles attach to bone (directly and indirectly, respectively), they are most often implicated as sources for biomechanical lameness. Intermittent upward fixation of the patella (IUPF) and fibrotic myopathy of the hamstring musculature are two well-described biomechanical problems that occur in horses. Each are further characterized in chapters 16 and 19 (respectively) because, like most biomechanical issues, their corresponding gait deficits are distinctive (pp. 91 and 127).
Horses can also exhibit non-painful lameness in response to neurologic disease. Neurologic lameness often arises as a consequence of compromised motor innervation (in which nerves are supplying inadequate input to the muscles that move the body and limbs) and/or decreased proprioception (in which reduced sensory output from the limbs affects spatial awareness). Neurologic lameness can be weight-bearing and/or non weight-bearing in nature (see chapter 9—p. 43), depending on the nerves and structures affected. Circumduction is a gait deficit most evident at the walk and often attributed to neurologic disease (VL 8a).
VL 8a Scan/Click to view video. www.getsound.com/tutorials/8a
Non-painful issues usually produce non weight-bearing lameness. This is easily demonstrated via the application of a splinted brace to one of your knees. The splint, when properly positioned, should not be uncomfortable nor prevent you from bearing a normal amount of weight on the limb during full extension. Yet it will effectively prohibit flexion of your limb, thereby resulting in a visibly obvious gait deficit as you try to ambulate.
9 Weight-Bearing versus Non Weight-Bearing Lameness
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
