Behind the Flex

Recent research has found that relying on the flex test may not be as accurate as once thought.

Use of the flexion test in pre-purchase and insurance examinations may incorrectly indicate a problem exists in a sound horse, according to a team of Dutch researchers.

“A flexion test might indicate a subclinical problem in a clinically sound horse, but the outcome of the flexion test can also be influenced by many other factors,” said Evita Busschers, MVMr, of Utrecht University, the Netherlands. Busschers said horse-related and examiner-related factors have to be taken into consideration when interpreting the outcome of a flexion test. In fact, in a study of the horse-related factors that can influence the results, Busschers found that apparently sound horses may not fare well in the test.

“In a normal population of non-lame horses, many will have a positive flexion test, which raises questions regarding the value of the test during these examinations,” she said.

For the flexion test, the examining veterinarian stands in front of the horse and holds the foreleg by the hoof, so that the horse’s knee pushes against the examiner’s chest and the horse’s cannon bone is vertical. The fetlock is flexed and the lower limb is held with a certain amount of pressure, so pressure is exerted on the lower limb. This pressure affects all the joints and structures of the lower limb.

According to Busschers, this test has been used since at least 1850 to localize lameness. In the past twenty years, however, veterinarians have begun using the test on clinically sound horses as part of a pre-purchase or insurance examination.

“In these cases, the concept of the use of the flexion test is that an apparently sound horse with a positive flexion test may have a subclinical problem,” Busschers said. “But when used in this way, the test is given a predictive value,” she said. In other words, the horse that is sound during the exam but that has a positive response to the flexion test (shows lameness following the test), is thought to have a “hidden” or developing problem that may show up later.

But when Busschers looked for guidelines on how to perform a flexion test, she found either vague information or conflicting advice, all of it backed up with limited or no scientific data.

“Many different values have been advised,” she said, with the duration of the test ranging from 30 seconds to three minutes, with 60 seconds most commonly advised. The force to be exerted during the flexion test also ranges broadly, from no amount of force mentioned up to 300 Newtons. The reasons for the duration and force advised are rarely noted, Busschers said, and even when they are, they are based on opinion.

“The predictive value of the flexion test is rather unreliable…”

Busschers set out to determine the horse-related factors (age, gender, weight, height and fetlock joint range of motion) and their relationship to the outcome of the test by strictly controlling for examiner-related factors (duration of the test, amount of force applied and how the test is performed). She performed a standardized flexion test on 100 clinically sound horses. Over 60 percent of the 100 horses had a positive flexion test. About 50 percent of the horses with a positive test showed slight lameness as a result of the test, while 35 percent had mild lameness and 15 percent a distinct lameness. Busschers noted, however, that the lameness resolved within minutes after the test.

Weight, height and range of motion of the fetlock joint had no impact on the outcome. Not surprisingly, older horses scored higher (had more lameness in response to the test). So did mares, though it was not apparent why mares scored higher than geldings. Six months later, Busschers repeated the test in 23 of the horses at intervals of ten minutes, 30 minutes, 48 hours and six months to assess repeatability of the test. When Busschers repeated the test at ten-minute and 30- minute intervals, the lameness score increased significantly after the second test. A test repeated 48 hours later, however, had no significant effect on the score, and a test repeated six months later showed a significant decrease in the score, while range of motion of the fetlock joint increased significantly.

“The predictive value of the flexion test is rather unreliable, since the outcome of the flexion test may vary significantly over a long period of time,” said Busschers.

To standardize the test, Busschers used a force meter adapted by another researcher to fit around the hoof. She took care to keep the horse’s cannon bone vertical, and applied a force of 150 N ± 13 N for 60 seconds. Busschers performed the test first on the left front leg, then on the right. A horse that was obviously lame after the left leg test was walked for three to four minutes and checked for lameness again before she performed the test on the right leg. Immediately following completion of the test on each leg, the horse was trotted on a hard, even surface, and Busschers scored the degree of lameness using a modified nine-point scale. Zero indicates no visible lameness and four a non-weight-bearing lameness.

Busschers tested the repeatability of the test since the test is commonly repeated during a lameness examination or for repeat purchase and insurance examinations. According to Busschers, the increase in positive test results following three administrations, over all but the longer (48 hours and six month) intervals may be due to intermittent ischemia, or lack of blood to the region that causes pain in the tissues.

“It is known that forced flexion of the fetlock joint for more than 45 degrees, which normally happens during a flexion test, is associated with a marked increase in intra-articular pressure,” or pressure in the joint, Busschers said. She noted that previous researchers have suggested that the increase in intra-articular pressure may result in an obstruction of the vessels in the joint capsule, leading to an oxygen debt in the joint.

“It may be assumed that, when the oxygen debt is not eliminated before the next flexion test, a cumulative effect may occur,” she said. This may cause the horse pain in that joint and therefore lead to an increased score (greater lameness) following repeated flexion tests over short intervals.

“The results of this study suggest that such a situation may exist after two or more flexion tests,” she said.

Busschers said it is not clear why the test outcome decreased when repeated six months later but noted that long-term consistency of the test is limited.

“In fact, it seems that a (slightly) positive flexion test, if performed with a force of 150N and for 60 seconds, is the rule rather than the exception,” she said.

Busschers advises that the outcome of the test be interpreted in light of clinical and radiological findings if used as a part of a pre-purchase or insurance exam.






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