Limitations and Advances in Lameness Assessment

When it comes to diagnosing a horse's lameness, the veterinarian's eyes are no longer the ultimate diagnostic machine. In fact, in one study (Keegan 2010) only 52% of participating veterinarians agreed on which of a horse's limbs was lame while assessing him visually.

During the American Association of Equine Practitioners' Focus on the Sport Horse program, held July 20-22, in Louisville, Kentucky, Hilary Clayton, BVMS, PhD, MRCVS, Dipl. ACVSMR, McPhail Dressage Chair Emerita at Michigan State University and president of Sport Horse Science, in Mason, Michigan, discussed the human eye's limitations and described up-and-coming inertial sensor systems' usefulness for detecting lameness.

When veterinarians observe horses' movement at the trot, speed affects what they see. "Lameness appears to decrease at faster trotting speeds," Clayton said. "But, in fact, the lameness doesn’t change—it’s just that our eyes can't see it as well when the horse trots faster."

Thus, she recommended that veterinarians evaluate horses—particularly those with subtle lameness—at a slow and consistent trot.

When evaluating horses on a longe line, circular locomotion also affects what the clinician sees. As the horse turns, he leans inward to develop centripetal force. This inward lean naturally causes the left and right sides to become asymmetrical. This is why if a sound horse trots in a small enough circle, he will appear lame on the inside front and hind limbs.

"It's important to standardize circle size and speed between directions when evaluating these horses because these are the factors that influence the natural asymmetry seen when the horse trots on a small circle," Clayton said.

Flexion tests, another common method veterinarians use to detect lameness, also have their drawbacks. One flaw, said Clayton, is that each veterinarian's technique varies, and the greater the force or duration of the flexion, the more likely you are to see a positive response (lameness). Veterinarians must also take into consideration that even sound horses typically show a mild positive to lower limb flexion, which decreases as they trot away.

Clayton listed further confounding factors of flexion tests, including:

The likelihood of a positive test increases with the horse's age;
Mares are more likely to test positive than geldings or stallions; and
Contrary to some clinicians' beliefs, a horse's weight, height, and fetlock range of motion do not impact flexion results.

Finally, when evaluating a horse under saddle, Clayton warned practitioners of rein lameness: "When a horse appears lame only when ridden, it is usually due to the rider pulling rhythmically on the reins."

For the above reasons, inertial sensor systems (ISS) might be a useful addition to veterinarians' lameness diagnostic arsenal.

An inertial sensor system is an electronic device that combines information from a variety of sources, such as an accelerometer to measure acceleration, a gyroscope to measure rotation, a magnetometer to measure orientation, and GPS to measure speed and direction.

"In a research environment, motion analysis systems and force plates have been used to describe and analyze equine gaits and the effects of lameness," Clayton said. "In the field, inertial sensor systems … are more practical for detecting and monitoring lameness. They are able to 'see' a more subtle asymmetry than our eyes can recognize, and they allow the veterinarian to monitor changes in lameness over time"

Examples of existing ISS include:

Equimetrix, a 2-dimensional accelerometer that attaches to the girth;
Lameness Locator, which features sensors on the body and one limb that indicate the lame limb, degree of lameness, and affected stage of the stride.
Equigait, a new and not-yet-commercially-available wireless system that provides common symmetry measurements and also shows back motion.

In conclusion, Clayton said, "Inertial sensor systems are proving useful in quantifying lameness and monitoring changes and also in understanding the capabilities and limitations of the clinician's eye."