Induced and Baseline Lameness

It is important to understand the concept of induced lameness and the possible difference between lameness seen at this stage and baseline lameness. During lameness examination, baseline lameness is established before any form of manipulation is performed. This may be difficult if more than one limb is involved, if lameness is subtle or sub-clinical, or if lameness is bilaterally symmetrical, which causes a gait abnormality without overt lameness. Lameness provocation is performed to exacerbate the baseline lameness or to provoke a hidden gait abnormality and attempt to localize the source of pain within a limb or limbs.

Provocative tests create induced lameness that may not have any clinical relevance to the baseline lameness observed during initial movement. These tests are not sensitive or specific and often result in false-positive and false-negative findings. Many horses with palmar foot pain respond positively to the “fetlock flexion test” (or lower limb flexion test, see following text) and could erroneously be thought to have lameness of the metacarpophalangeal joint. Similarly, a horse with proximal suspensory desmitis (PSD) may show exacerbation of lameness after distal limb flexion. A racehorse with baseline lameness as the result of a carpal chip fracture may have preexisting low-grade osteoarthritis of the metacarpophalangeal joint and respond positively to lower limb flexion but response to carpal flexion may be equivocal. Diagnostic analgesia is essential to localize the source or sources of pain. In the hindlimb, hock flexion, the so-called “spavin test,” causes many false-positive reactions.

Flexion Tests

Flexion tests were first described early in the twentieth century, but information regarding the degree of flexion, force, or duration of the tests was lacking.¹ Variations in technique persist and produce variable responses that can be misleading. There appear to be more false-positive reactions to flexion than there are false negatives, but the latter do occur. Flexion tests are useful during prepurchase examinations because the horses being examined usually are relatively sound, and the tests are useful at uncovering hidden sources of pain. Flexion tests may be useful in exacerbating lameness, particularly when the primary or baseline lameness is in the region being flexed, but sensitivity is doubtful. Horses judged to be clinically sound underwent a “normal” and then a “firm” lower limb flexion test (fetlock flexion).² Of the 50 horses tested, 20 had a positive response to normal flexion, and 10 of these horses were judged to be lame while trotting for about 15 m (50 feet) or more. Forty-nine of 50 horses had a positive response to firm flexion, and 35 of these remained lame for a minimum of 15 m. In this study the force applied was not calibrated, 7 of the 50 horses developed lameness within 60 days of completion of the study, and 24 horses had radiological abnormalities that could have contributed to a positive response to flexion.² Although there may be an explanation for a positive response in some horses, the high percentage of positive results in the study is in agreement with my clinical impression. In a study using the Flextest (Krypton Electronic Engineering NV, Leuven, Belgium), an apparatus designed to control traction force and time during a lower limb flexion test, the optimal force and time for flexion were 100 N and 1 minute, respectively. There was a positive response to flexion in many horses that were considered sound, and a positive response in sound horses was more likely in those in active work than in horses that had been rested or turned out on pasture. Horses were more likely to manifest a positive response to flexion as the force used in the test was increased.³ A false-positive response to flexion can be observed in clinically normal horses and in those with unimportant low-grade problems. Lameness induced by flexion in these horses may have little clinical relevance.

However, other evidence suggests that a positive response to lower limb flexion in sound horses may be useful to predict future lameness. In a retrospective study, 151 initially sound horses were followed for 6 months. Twenty-one percent of horses with a positive forelimb flexion test result developed lameness in the area being flexed, whereas only 5% of horses with a negative flexion test result subsequently developed lameness. In young Swedish Warmbloods there was a positive correlation between a positive response to flexion and a subsequent insurance claim related to lameness.⁴

In a more recent study the predictive value of the lower limb flexion test was debated.⁵ Sixty percent of sound horses responded positively to the flexion test. There was no influence of body weight, height, or range of motion, but outcome of the flexion test increased significantly with age and in mares. Over a 6-month period the number of horses responding positively decreased significantly, a finding that casts doubt on the possible predictive value of the test.⁵

Flexion tests lack specificity because it is nearly impossible to flex a single joint without flexing other joints or nearby tissues, particularly in any hindlimb or distal forelimb flexion tests. Elevation of a limb without flexion in severely lame horses may exacerbate the baseline lameness, because horses guard the limb or need to warm out of the lameness for a number of steps while trotting, thus complicating interpretation. Hindlimb flexion tests are less specific than forelimb tests because the reciprocal apparatus prevents flexion of any joint without concomitant flexion of other joints. Hindlimb flexion tests are useful in exacerbating baseline lameness, but positive responses to individual lower limb and upper limb tests, in my opinion, only localize pain causing lameness to the entire hindlimb. I believe that flexion tests are useful in exacerbating lameness, and in some horses it is the baseline or relevant lameness that is being worsened. In general, unless the horse’s response is clearly pronounced and different from that of other manipulation, lameness cannot be localized based on response to flexion alone. Diagnostic analgesia should always be used, when possible, to localize pain.

Order, Duration, Force, and Venue during Flexion Tests

Consistency in technique is essential. Although force exerted by individuals varies, the flexion technique of experienced practitioners is sufficient to objectively assess response to flexion.⁶ Response to flexion can and should be compared with response in the contralateral limb. Ideally the flexion test should be performed in the contralateral sound limb first before being performed in the suspect limb, to determine the horse’s response. Accurate assessment of response to flexion in the contralateral nonlame limb may not be possible if the horse is severely lame after flexion of the lame limb and lameness persists. In some instances, baseline lameness is actually increased by forcing the horse to stand for the contralateral flexion test, a useful observation seen most commonly in horses with forelimb lameness (see following text). Horses with nearly bilaterally symmetrical lameness may have a similar positive test result in the less-lame contralateral limb.

Duration of flexion is somewhat controversial and may be an individual choice. In a study evaluating lower limb flexion, duration of 1 minute was considered ideal, because normal horses that underwent flexion at 100 N for 1 minute had few false-positive responses.³ Maintaining firm flexion for 1 minute while performing all flexion tests and repeating the tests in the contralateral limbs can make this portion of the lameness examination time-consuming. On the other hand, if the clinician takes the time to perform these tests, optimum chances of success are improved. A false-positive result is more useful than a false-negative test result. Some clinicians prefer to perform flexion tests with more force but for a shorter duration. This technique works well for lower limb flexion tests. Seldom is it possible to maintain some upper limb manipulative procedures for 1 minute. Thus some latitude is necessary. I believe that duration of flexion of 45 seconds to 1 minute is enough to elicit an accurate response in most horses.

Force used during flexion varies considerably, but excessive force induces lameness in most normal horses. Forces in the range of 100 to 150 N represent a moderate degree of force for lower limb flexion tests. In studies using a dynamometer, the maximum amount of force that could be used without a consistent withdrawal response in normal horses was 150 N.⁷ The amount of force also depends on the size of the horse or the joint being flexed. The amount of force used in adult horses cannot be used in foals. Horses with obvious osteoarthritis or articular fracture, or those with substantial soft tissue injury likely to be affected by the flexion test, do not tolerate the same force as horses with more mild conditions. In a study of healthy and injured Thoroughbred (TB) racehorses, a positive correlation existed between decreased range of motion and joint injury.⁸ Loss of joint motion was most likely caused by joint capsule fibrosis, but pain associated with increased intraarticular pressure from effusion or flexion also may have limited joint motion.⁸ I recommend flexing a joint as much as possible with an amount of force just slightly less than the force that consistently causes a withdrawal response. Consistency should be applied between horses, paying attention to how the horse reacts; obvious resistance to sustained passive flexion may be clinically significant.

Proper evaluation of the results of flexion tests requires that the horse be observed while trotting in a straight line on a firm, nonslip surface. Evaluation during trotting is important to differentiate those horses resistant to static flexion from those with an authentic positive flexion test result. Horses usually are trotted in hand, although occasionally a horse’s response to flexion is evaluated while it is being ridden. The horse should be trotted immediately after the limb is placed to the ground, with care taken to avoid scaring the horse or providing any excessive encouragement to trot, because many horses will slip initially, gallop off, or balk, all of which necessitate test repetition. If possible, the horse should be trotted away from the examiner for a minimum of 12 to 15 m.

Causes of Pain during Flexion and Positive Flexion Test Results

Forced flexion of a joint can induce pain in many potential sites. Force is being applied to both articular structures and surrounding soft tissues. The tissues on the flexion side of the joint are being compressed, whereas tissues on the extension side are under tension. During flexion, intraarticular pressure and intraosseous pressure in subchondral bone are increased.^3,8 Stretching or compression of the joint capsule, vascular constriction, and activation of pain receptors in the joints and surrounding soft tissues also can occur during flexion.³ It is rarely possible to attribute pain on static flexion or during movement after flexion to an individual articular surface. The “fetlock flexion test” is a misnomer because as it is commonly performed it includes the interphalangeal joints and stresses surrounding soft tissue. Thus the names lower limb flexion test or fetlock region flexion test are more appropriate.

Positive Responses to Flexion

Positive responses to flexion can be seen with static flexion (see Chapter 6) and when movement follows flexion. A positive flexion test result is defined as obvious lameness or an increase over baseline lameness that is observed for more than three to five strides while the horse trots in a straight line after flexion. A mild response, even in sound horses, often is seen in the first few strides, a finding that should be compared with the contralateral limb. Sound horses warm out of this mild response quickly. Of 100 sound horses 50% had a slight response, 35% had mild lameness and 15% had distinct lameness after a lower limb flexion test.⁵ A persistent, one- to two-grade increase over baseline lameness for several steps is a positive response. In horses with hindlimb lameness a marked positive response often is accompanied by reluctance to place the heel on the ground, and the horse may land only on the toe for several strides.