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Lameness in Horses: Introduction |  |
| Lameness is the result of a change from normal stance and gait caused by either a structural or a functional disorder of one or more of the limbs, the neck, or the trunk. It is not a disease, but a manifestation of either pain caused by an impediment in the musculoskeletal system or, if pain is not involved, of a mechanical lameness, although a combination of the two frequently exists. Mechanical lameness is best typified by fibrotic myopathy with its characteristic gait
abnormality, but can also be the result of a restriction (eg, tendon sheath restriction in annular ligament syndrome). It is important to correctly distinguish the type of mechanical lameness, eg, fibrotic myopathy from stringhalt. In fibrotic myopathy, the affected limb is pulled back and down before the end of the protraction phase, resulting in a lengthened weightbearing phase and a shortened cranial phase. The signs are most obvious at the walk. In stringhalt, the affected limb
is hyperflexed during the cranial or swing phase, while the stepwise caudal jerking movement before foot contact does not occur. |
| Pain-related lameness can be classified as weightbearing (supporting leg) or nonweightbearing (swinging leg) lameness, although lameness most often is composed of both. A supporting leg lameness is seen when the horse attempts to reduce the amount of time a particular limb is bearing weight. The horse elevates its head and shifts its weight away from a particular limb during weightbearing for a forelimb lameness, whereas the opposite is true for a hindlimb lameness. Hindlimb
lamenesses should be assessed from the side as well as from behind, because this provides an opportunity to assess arc of flight, duration of protraction and retraction phases, and length of weightbearing phase. In milder cases, the horse might fail to make a circle when lunged because it is trying to redistribute its weight to offload the lame leg, most commonly seen when the lame limb is the outside limb on a circle. A supporting leg lameness may originate from anywhere in the limb
(proximal or distal), while a swinging leg lameness, although often believed to represent a proximal problem, may originate either proximally or distally. |
| Factors that predispose to lameness include physical immaturity (eg, bones that are anatomically normal but biomechanically weak due to the age of the horse at the onset of training or bone that is abnormally weak due to developmental orthopedic disease), and monotonous repetitive stresses on bones (eg, stress fractures in racehorses continuously training around left-handed bends, or chronic imbalance of the feet resulting in repeated abnormal loading of a particular limb).
Inciting factors in lameness include direct or indirect trauma, incoordination of muscle action following fatigue in racehorses racing over long distances, or inflammation—more often than not without infection—of joints, tendons, and ligaments in particular. |
| Lameness in one part of a limb likely results in at least some secondary soreness from another area of the same limb as well as a small degree of lameness in the contralateral fore- or hindlimb. The former is regularly proved by the complexity in step-by-step elimination of lameness in the distal limb in particular, the latter by the almost invariable switch of lameness, often of a much lesser degree, to the contralateral limb. One dramatic example of this is the development of
traumatic inflammation and occasionally rotation or sinking of the distal phalanx in the contralateral limb to a severe, nonweightbearing lameness (eg, a fracture). |
