Using horse-mounted sensors to detect uneven movement suggestive of lameness may be subject to error if the horse is asymmetric to begin with, researchers say.
A study team in the United States used body-mounted inertial sensors in a study in which they introduced a hindlimb length difference by attaching one 12.5mm shoe on a hind hoof.
Jael Pitts and her colleagues, writing in the open-access journal PLOS ONE, say the rise and fall of the equine pelvis is generally the same between the right and left side when no lameness is present.
In horses with pain during hindlimb weight-bearing, the pelvic fall, the pelvic rise, or both, decrease for the lame compared to the non or less-lame hindlimb.
However, this assumes equivalent overall hindlimb length between right and left sides during normal weight bearing.
“But, is it reasonable to assume that not all horses will have equal hindlimb length, even if the horse is healthy and functioning normally?” they asked.
“Hindlimb length difference can result from growth discrepancies of the hooves or long bones between the right and left limbs.
“These discrepancies may have been caused by injuries that are healed and no longer painful, or differences in functional demand from specialized training or repetitive movement, leading to the development of a type of ‘leggedness’ similar to ‘handedness’ in humans.”
Such differences in hindlimb length would not be expected to cause pain, they said, but the resulting differences in pelvic height may be measured as pelvic height asymmetry and assessed as hindlimb lameness.
“Is lameness measured in horses with different hindlimb length due to pain or simply to the difference in length?” they asked. “Can lameness measurement be affected by inducing differences in hindlimb length?”
The researchers described a study using 16 sound horses, used in a range of disciplines, that were being ridden daily. They were assessed as showing no asymmetry when standing squarely.
They were fitted with sensors and then trotted in a straight line and on a lunge, on both hard and soft surfaces.
A hind-limb length difference was then introduced by attaching a single, rear 12.5mm glue-on shoe and the horses were reassessed.
The measurements of interest to the researchers were the changes in the minimum and maximum pelvic heights during strides.
Significant changes were seen. The pelvic minimum — the measure of impact-type hindlimb lameness — was consistently affected, increasing on the side of elevation in a straight line and at the lunge.
The pelvic maximum — the measure of pushoff-type hindlimb lameness — was also affected, but to a lesser degree than the minimum, during straight-line evaluation, but not during the lunge.
Results of the study suggest that, if the nature of hindlimb lameness is primarily lack of pushoff, the cause is less likely due to a pre-existing limb length asymmetry, but to actual differences in push-off force between the hindlimbs.
However, if the nature of the hindlimb lameness is primarily lack of impact, at least part of the cause may be due to a pre-existing hindlimb length difference.
“When using inertial sensors placed on the midline to measure hindlimb lameness, if the measured hindlimb lameness is of a purely impact nature, one should check for pre-existing hindlimb length differences and pelvic asymmetry.
“This may be done by carefully standing the horse squarely on both hindlimbs and observing from the rear.”
They concluded that objective measurement of vertical pelvic movement asymmetry to assess hindlimb lameness may be subject to error if there is a pre-existing reason for asymmetric pelvic height, such as differences in hindlimb length.
The long-term effect of compensation for limb length differences and pelvic asymmetry may be detrimental to the horse, they said.
“Standardbred trotters with hindquarter asymmetry had poorer racing performance than those without hindquarter asymmetry.
“In human studies, leg length differences have been shown to increase the incidence of a variety of abnormalities and pathologies, including low back pain, hip osteoarthritis, sacroiliac disease, stress fractures, loosening of hip prostheses, and running injuries.
“Whether these detrimental long-term effects are directly due to the asymmetry or to the compensatory efforts to adjust for this asymmetry are unknown.”
The study team comprised Pitts, Joanne Kramer, Shannon Reed, Paul Schiltz, Lori Thombs and Kevin Keegan.
Pitts is with Waller Equine Hospital in Texas, Kramer and Keegan are with the College of Veterinary Medicine at the University of Missouri, Reed is with the College of Veterinary Medicine at Ohio State University, Schiltz is with the Equestrian Studies Department at William Woods University in Missouri, and Thombs is with the Department of Statistics at the University of Missouri.
Pitts JB, Kramer J, Reed SK, Schiltz P, Thombs L, Keegan KG (2020) Effect of induced hindlimb length difference on body-mounted inertial sensor measures used to evaluate hindlimb lameness in horses. PLoS ONE 15(2): e0228872. https://doi.org/10.1371/journal.pone.0228872