Definition

The clinical manifestations of lameness in the horse are well known, but an exact definition is difficult. The word lame is an adjective, meaning “crippled or physically disabled, as a person or animal … in the foot or leg so as to limp or walk with difficulty.”² A medical dictionary defines lameness as “incapable of normal locomotion, deviation from the normal gait.”³ The noun lameness can be, but infrequently is, used interchangeably with claudication, described as “limping or lameness.”³

Lameness is simply a clinical sign—a manifestation of the signs of inflammation, including pain, or a mechanical defect—that results in a gait abnormality characterized by limping. The definition is simple, but recognition, localization, characterization, and management are complex.

Localization of Pain

In certain conditions, characteristic gait abnormalities allow immediate and straightforward recognition and localization of the problem. Sweeny, fibrotic myopathy, upward fixation of the patella, stringhalt, shivers, and radial nerve paresis are examples. However, similar gait deficits exist for a variety of lameness problems, complicating recognition and localization. A fundamental concept in lameness diagnosis is the application of diagnostic analgesic techniques to localize the source of pain causing lameness. The sequence of properly determining the lame leg (recognition) and then abolishing the clinical sign of lameness by use of diagnostic analgesia (localization), only to have lameness return when the local anesthetic effects abate, is essential for accurate diagnosis. In essence diagnostic analgesia establishes clinical relevance, a most important concept to the lameness diagnostician. With experience and under certain circumstances, this step in lameness diagnosis can be omitted. The degree of lameness, certain gait characteristics, and palpation findings allow the clinician to strongly suspect a certain diagnosis. The next step may be diagnostic imaging. For example, a racehorse with prominent lameness after training may be suspected of having a stress or incomplete fracture. Performing radiographic and scintigraphic examinations before proceeding with diagnostic analgesia is a prudent choice. Trial and error also occasionally work and in some instances may be the preferred approach. Intraarticular analgesia can be performed in selected joints without disrupting distal-to-proximal perineural techniques later during the same examination. However, because pathognomonic signs are rare, proficiency in diagnostic analgesic techniques is mandatory for the lameness diagnostician.

Baseline and Induced Lameness

Baseline, or primary, lameness is the gait abnormality recognized when the horse is examined at a walk or trot in hand before flexion or manipulative tests are used. The clinician usually recognizes this abnormality by watching the horse on a firm or hard surface, while it is being trotted in a straight line. Diagnostic analgesia is used to abolish this lameness. Changing the surface or nature of the exercise by lunging, or circling the horse at a trot in hand, potentially changes the baseline lameness. The surface and exercise (gait and speed) must be consistent. In some horses no observable lameness is present at a walk or trot in hand. Lameness may be evident when the horse is ridden, and this lameness becomes the baseline lameness.

Flexion tests and other forms of manipulation are used to exacerbate baseline lameness or to induce lameness. An induced lameness is one that is observed after flexion or manipulative tests, but induced lameness may not be the same as the baseline lameness. Manipulative tests are expected to, and often do, exacerbate the primary lameness. However, flexion and manipulative tests can cause development of additional lameness, unrelated to the primary or baseline lameness, and test results must be interpreted carefully.

Coexistent Lameness

Horses often have several sites of pain, although one usually is most obvious and the cause of baseline lameness. In many horses, secondary or compensatory (sometimes referred to as complementary) lameness develops in predictable sites or limbs. Concomitant bilateral forelimb or hindlimb lameness is common, but horses often demonstrate more prominent clinical signs in one limb. In horses with palmar foot pain, initially pronounced single forelimb lameness that is abolished by palmar digital analgesia may be present, with subsequent recognition of contralateral forelimb lameness. In racehorses, bilateral lameness, such as in the carpi or metacarpophalangeal or metatarsophalangeal joints, is common. The clinician should carefully examine the contralateral limb. Predictable compensatory or secondary lameness often exists in the ipsilateral or contralateral forelimb when primary lameness is present in the hindlimb, or vice versa. In a Thoroughbred (TB) racehorse with left forelimb lameness, compensatory problems in the right forelimb and left hindlimb are not uncommon, because these limbs presumably are succumbing to excessive loads while protecting the primary source of pain. In a trotter, diagonal lameness often occurs (primary lameness in the left hindlimb and compensatory lameness in the right forelimb), whereas in pacers, ipsilateral lameness is most common (primary right forelimb and compensatory right hindlimb). When several limbs are involved, identification of the primary or major source of pain is important. If forelimb and hindlimb lameness exist simultaneously, diagnostic analgesic techniques should begin in the hindlimb (see Chapter 10). A common secondary lameness abnormality, proximal suspensory desmitis, can develop in the compensating forelimb or hindlimb.

Coexistent lameness can make assigning primary or baseline lameness to a particular limb during lameness examination difficult (see Chapter 7). Bilaterally symmetrical pain may cause a short, choppy gait, but primary or baseline lameness often cannot be seen when the horse is examined in a straight line in hand. Often, horses with coexistent lameness must be circled, lunged, or ridden for primary or baseline lameness to be observed. The lameness diagnostician may have to arbitrarily assign lameness to a limb and begin diagnostic analgesia in this manner. Often, once the primary source of pain has been identified, horses show pronounced lameness of much greater magnitude than expected in another limb, vivid clinical evidence that coexistent lameness exists.

Lameness Distribution

Among all types of horses, forelimb lameness is more common than hindlimb lameness. A horse’s center of gravity or balance, while dictated to a certain extent by conformation (see Chapter 4), is not located in the center of the horse but is closer to the forelimbs than the hindlimbs. Thus the forelimb/hindlimb (F/H) weight (load) distribution ratio is approximately 60% : 40% (Figure 2-1). Higher loads are expected on the individual forelimbs (30% each), predisposing the horse to greater injury.

Fig. 2-1 The center of balance (gravity) of the horse is located closer to the forelimbs, which accounts for the load distribution difference between the forelimbs and hindlimbs. Conformation, namely the angles of the shoulder and rump, and weight of the head and neck and gait can change this load distribution.

At certain times during the stride cycle of gaits such as the canter (three-beat gait) and gallop (four-beat gait), a single forelimb is weight bearing, which predisposes the limb to injury. The weight of a rider may shift F/H load distribution to 70% : 30% (Figure 2-2

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