High-tech equipment using inertial measurement unit (IMU) sensors can detect lameness in horses by measuring uneven movement in the forelimbs or the hind-limbs. But that doesn’t mean all asymmetrical horses are lame.

According to French biomechanics experts, some asymmetry—the kind that might get picked up by highly sensitive IMUs—can be perfectly normal. Whether it’s from mild individual asymmetries in the muscular development, working on slightly uneven terrain, laterality, or even minor genetic differences between the left and right sides, horses can have slight asymmetry without negative health consequences.

“It’s not just because these systems are capable of measuring something extremely precise that these extremely precise measurements are clinically significant,” said Henry Chateau, DVM, PhD, of the National Veterinary School of Maisons Alfort Equine Biomechanics and Musculoskeletal Pathology department and the INRA, the French national agricultural research institution.

“Overdiagnosing” horses as lame when IMU sensors detect mild, non-pathological asymmetry could have significant financial consequences for the owner, Chateau said. The use of X-rays, magnetic resonance imaging (MRI), ultrasound, and other imaging techniques could be costly yet unnecessary, and the horse might be needlessly taken out of work. Interpreting all asymmetry as lameness could also cause issues with pre-purchase exams and erroneously block the sale of a healthy horse.

However, true lameness does cause asymmetry, he said. Thus, it’s critical to find thresholds of asymmetry that draw the line between soundness and lameness.

To that end, Chateau and his fellow researchers evaluated 224 sport horses, aged 2 to 20 years, for lameness and asymmetry at the Centre d’Imagerie et de Recherche sur les Affections Locomotrices Equines (CIRALE) clinic in Normandy. They placed IMU sensors (EQUISYM system) specifically designed to automatically measure asymmetries in horses on the head, withers, pelvis, and all four cannon bones. Meanwhile, the team’s trained locomotor experts evaluated each horse during a visual lameness exam as the animals were trotted in hand by their regular handlers in a straight line on asphalt.

The researchers determined that 62 horses had left forelimb lameness, 67 had right forelimb lameness, 23 had left hind-limb lameness, and 23 had right hind-limb lameness, Chateau said. The researchers excluded horses with multilimb lameness from the study.

The remaining 49 horses were sound, he said. To confirm their soundness, the experts examined the horses during a full locomotor exam, at a walk and a trot on a hard circle and a soft circle in both directions, on a hard straight line, and through a flexion test for each limb.

“Based on significant veterinary expertise, we were able to consider that these horses were sound and healthy if they presented no visible sign of locomotor anomalies throughout all these test conditions,” Chateau said.

The expert team concluded that all 49 horses were perfectly sound but not perfectly symmetrical, he said. None of the 49 sound horses in the study had 100% perfect symmetry between the right and left vertical movements of the head, withers, or croup for all recorded strides, said Chateau.

Specifically, the threshold with the best sensitivity and specificity (the least amount of false positives and false negatives) was 7% asymmetry on the right and 10% asymmetry on the left in the forelimbs and 18% asymmetry on the right and 7% asymmetry on the left in the hind-limbs, he said.

“Many horses, if not all, present at least some percentage of asymmetry, almost invisible to the naked eye … but it’s not systematically a sign of pain,” Chateau told The Horse. “Simply put, it’s like when humans are more right-sided or left-sided. We can all have a little bit of asymmetry without these being necessarily pathologies.”

Based on their findings, the team has proposed these asymmetries- namely -7% to +10% in front and -7% to +18% in back- as threshold values to distinguish soundness from lameness when using IMU sensors on a straight line, Chateau said. “Of course, natural asymmetries based on the physics of movement on a curve, for example, must also be taken into account,” he added. “The thresholds between physiological and pathological asymmetry in these circumstances will be even higher. This is the subject of ongoing work.”

Those thresholds are subject to evolve as more horses undergo IMU evaluations, said Chateau.

“As the use of IMUs increases, so will our precision in defining the limits of soundness versus lameness,” he said, adding that those limits are unlikely to ever be black and white.

“There will never be a straightforward dichotomy between normality and abnormality on a single criterion, each lameness being different and requiring the accumulation of a large amount of information and criteria to arbitrate,” Chateau said. “In this respect, it’s worth remembering that measuring devices are an accurate aid to decision-making but that they do not replace the expertise of the vet.”

References:

Macaire, C.; Hanne-Poujade, S.; De Azevedo, E.; Denoix, J.-M.; Coudry, V.; Jacquet, S.; Bertoni, L.; Tallaj, A.; Audigié, F.; Hatrisse, C.; et al. Investigation of Thresholds for Asymmetry Indices to Represent the Visual Assessment of Single Limb Lameness by Expert Veterinarians on Horses Trotting in a Straight Line. Animals 2022, 12, 3498. https://doi.org/10.3390/ani12243498