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Assessment of the Lame Horse

Introduction

• Lameness is an indication of a structural or functional disorder in one or more limbs or the axial skeleton that is evident while the horse is standing or at movement.
• Lameness is a clinical sign, not a disease.
• Lameness can be caused by trauma (single event or repetitive work), congenital or acquired anomalies, developmental defects, infection, metabolic disturbances, circulatory and nervous disorders, or any combination of these.
• It is important to differentiate between lameness resulting from pain and nonpainful alterations in gait, often referred to as "mechanical lameness," and lameness resulting from neurologic (nervous system) dysfunction.
• Most lameness in horses is due to pain during weight-bearing, with shifting of load away from the source limb onto the other non- or less affected limbs.
• The detection of clinical signs of lameness is primarily the recognition of an asymmetric gait in the horse.
• The primary objectives of a lameness examination are to determine:
  1. Whether the horse is lame.
  2. Which limb or limbs are involved?
  3. The site or sites of the problem.
  4. The specific cause of the problem.
  5. The appropriate treatment and/or rehabilitation.
  6. The prognosis for recovery.
• The steps to perform a routine or traditional lameness examination include
  1. Complete history including signalment and use.
  2. Visual exam of the horse at rest.
  3. Palpation of the musculoskeletal system including hoof tester examination of the feet.
  4. Observation of the horse in motion (usually at a straight walk and trot/lope followed by circling) with or without objective lameness evaluation tools.
  5. Observation of the horse under saddle or in work if necessary.
  6. Manipulative tests such as flexion tests.
  7. Diagnostic anesthesia, if necessary.
  8. Diagnostic imaging.

Adaptive Strategies of Lame Horses

• With most lameness conditions, the horse attempts to "unload" the lame limb during weight-bearing or the stance phase of the stride. As an example, in horses with chronic lameness, the limb with the flatter hoof is thought to exhibit higher vertical loads because it is the non-lame limb (Figure 1.1).

  Figure 1.1 Chronic hindlimb lameness that has resulted in a wide flat foot on the sound limb (LH) and a narrow, upright hoof on the lame limb (RH).

• Horses accomplish this by abnormal movement of a body part (head nod or pelvic hike), weight shifting (to the contralateral or diagonal limb or torso), change in joint angles (lack of fetlock extension), and alterations in foot flight. Detecting these compensatory movements is an integral part of lameness diagnosis.
• The most consistent compensatory movements that are observed in lame horses are the vertical displacement and acceleration of the head in forelimb lameness and of the sacrum and tuber coxae in hindlimb lameness.
• More subtle lameness causes fewer compensatory changes, making lameness diagnosis more difficult. Although these gait changes may be difficult to appreciate during visual examination of a lame horse, it should be remembered that the primary adaptive strategy of the horse is to redistribute load to compensate for pain in a limb without causing an overload situation in other limbs.

Classification of Lameness

• In most cases, there is a primary or baseline lameness that contributes to the most obvious gait abnormalities. Compensatory, secondary, or complementary lameness results from overloading of the other limbs as a result of the primary lameness.
• Lameness may also be classified according to when it occurs (or is best observed) within the stride - i.e. stance, swing, pushoff, etc.
• Visual observation of the gait can usually determine whether the lameness is supporting limb, swinging limb, or mixed. However, because of the many adaptive strategies that occur in lame horses, some clinicians feel that mixed lameness occurs most commonly in horses.
• The different classifications of lameness are:
  1. Supporting limb lameness: Apparent when the foot first contacts the ground or when the limb is supporting weight (stance phase). This is by far the most common type of lameness identified in the horse.
  2. Swinging limb lameness: Evident when the limb is in motion. The majority of these problems are thought to involve the upper limbs or axial skeleton.
  3. Mixed lameness: Evident both when the limb is moving (swing phase) and when it is supporting weight (stance phase).
  4. Primary or baseline lameness: Most obvious lameness or gait abnormality that is observed before flexion or manipulative tests. This can be complicated by lameness in multiple limbs, but the most severe lameness is usually considered the primary lameness.
  5. Compensatory or complementary lameness: Pain in a limb can cause uneven distribution of weight on another limb or limbs, which can produce lameness in a previously sound limb. It is common to have complementary lameness produced in an opposite forelimb or hindlimb.

Anatomic Problem Areas

• The majority of lameness problems in horses occur in the forelimb due to the increased weight-bearing on the forelimbs (60%-65%) and the shock of landing that the forelimbs absorb during movement.
• Breed and occupation can alter these generalizations as horses that pull carts or perform events such as dressage, cutting, and reining, which place greater stress on the hindquarters, often have a higher percentage of hindlimb lameness.
• It is also estimated that approximately 95% of lameness in the forelimb occurs distal to the carpus and approximately 80% of lameness problems in the hindlimbs involve the hock or stifle.
• Although there is considerable overlap, specific lameness conditions are often associated with a specific type of work or occupation. Knowing common problem areas will facilitate a more thorough and complete physical examination that may help detect subtle abnormalities (Table 1.1).

Table 1.1 Occupation-related lameness problems.

TB, Thoroughbred; QH, Quarter horse; STD, Standardbred.

P1, first phalanx; P2, second phalanx; P3, third phalanx; MC/MT, metacarpus/metatarsus; C3, third carpal bone; PSD, proximal suspensory desmitis; SDFT, superficial digital flexor tendon; OA, osteoarthritis; PSSM, polysaccharide storage myopathy; DSLD, degenerative suspensory ligament desmitis.

Signalment and...