After a multi-year, multi-organizational research effort, the Thoroughbred industry finally has data on the long-term impacts of two types of common radiographic findings.
With the advent of digital radiography and the radiograph repository at public auction, a horse's commercial value has become closely linked to the cleanliness of those radiographs. For years, consignors and buyers alike have wondered if certain common abnormalities on radiographs actually have any significant impact on a horse's long-term soundness or racing ability.
Thanks to a large-scale research project launched in 2016 by Colorado State University's Dr. Frances Peat, we have a few answers.
At the recent annual convention of the American Association of Equine Practitioners, researchers presented the results of two studies from the research – one looking at stifle lesions, the other looking at sesamoid bone findings.
Subchondral lucencies in stifles
Respected Colorado State University equine orthopedic researcher Dr. Wayne McIlwraith presented the results of a study of subchondral lucencies in the medial femoral condyle – a bone in the stifle. This finding has been a concern for people purchasing yearlings to race or to pinhook, as no one really knew whether a low-grade finding in a yearling would turn into something more problematic down the road.
McIlwraith and the research team looked at radiographs in the repository for 2,508 yearlings at the 2016 Keeneland September auction and 436 2-year-olds across all major North American 2-year-old sales the following year, matching together radiographs for horses who appeared at both yearling and 2-year-old sales. A group of four veterinarians who were not working to evaluate radiographs for clients at those auctions were asked to review the images and rate them Grade 0 through 3 for the presence of subchondral lucencies. Multiple vets were asked to analyze each image, and researchers ran statistical analyses to determine how frequently they agreed with each other. The analyzing vets agreed with each other nearly all the time when rating an image Grade 0 or Grade 3; Grades 1 and 2 produced a little more conflict, but the statistical rating for observer agreement was still considered moderate.
The vast majority of horses in both age categories were Grade 0 for lucencies in the medial femoral condyle. Of those who had some lucency (9.6 percent of yearlings and 11.2 percent of 2-year-olds), Grade 1 was the most common, followed by Grades 2 and 3 respectively.
Interestingly, McIlwraith pointed out that Grades 2 and 3 lesions were much more common in the right stifle than the left for horses who had them, though the reason for that remains unclear.
Most horses who had Grade 1 lucencies as yearlings saw them either stay the same (45 percent) as 2-year-olds or improve to become rated as Grade 0s by their next auction appearance (36 percent). There were smaller numbers of yearlings with Grade 2 and 3 lesions who also went to sale at two, but they saw a similar pattern – of 10 Grade 2 yearlings, six remained at a Grade 2, two improved to Grade 1 and two worsened to Grade 3. Of the yearlings with Grade 3 lucencies, only three went to sale the next year and two of those improved to Grade 2s.
Researchers found there was no significant difference in racing performance for horses with any subchondral lucencies in the medial femoral condyle; 85 percent of study yearlings started at least once by the end of their 4-year-old year. The probability of a horse with a Grade 3 lesion starting in a race was lower (77.6 percent) than the probability for a horse with any other grade (84.3 to 91.3 percent) but the difference was not statistically significant in mathematic analysis. Racing performance was assessed based on number of starts, age at first start, total earnings, earnings per start, listed or stakes appearances, and Class Performance I ndex.
The numbers came from yearlings who represented 10.9 percent of the 2015 foal crop and 36 percent of the yearlings sold in North America in 2016. The study of 2-year-old radiographs covered 19.7 percent of 2-year-olds sold at auction that year.
McIlwraith said that if anything, the results are probably an underrepresentation of reality for this type of radiograph issue.
“It's likely we underestimated the prevalence of severe lesions in the entire Thoroughbred crop born in 2015,” said McIlwraith. “This is because of two scenarios that I think all of you are probably familiar [with] — the onset of clinical lameness either between birth and yearling age or during sales preparation at yearling or 2-year-old age is going to be a reason to take the yearling out of the sale or take the 2-year-old out of the sale.
“Also, findings on screening radiographs that could potentially inhibit sale value often cause the consignor to withdraw their yearling or 2-year-old from the sale. Hopefully our results that I'm providing here will provide a reduction in such decision.”
Sesamoids
Dr. Chris Kawcak, also of Colorado State University, presented the results of a look at sesamoid bone findings.
There are a few findings in the sesamoid bones that have given buyers and sellers pause through the years. Unlike the lucencies in the stifle, which had been studied very sparingly prior to this research, there were a few studies out there on differed sesamoid bone findings. The trouble was they all had slightly conflicting results about what had a real impact on racing performance. They were also using slightly different grading scales for those issues.
There are a few commonly-identified issues with sesamoids on pre-sale radiographs – abnormalities of vascular channels, lucencies, abaxial bone formation, or abaxial margin concavity.
The research team used the same sets of radiographs from the same horses at the same sales, and applied the same statistical measures of inter-observer agreement and same parameters of racing performance.
To start with, this research group consulted with scientists who had previously studied sesamoid lucencies and vascular channels to create a standard grading system, with Grade 0 being normal and Grade 3 representing three or more vascular channels that were greater than two millimeters wide.
Using those standards, researchers found that most horses had Grade 0 sesamoids, while Grade 1 was the next most common finding; 14.4 percent of yearlings studied had Grade 1 vascular channels in the sesamoids, while 9.7 percent of sales 2-year-olds had Grade 1 vascular channels in their sesamoids. Few (less than 3 percent) of either group had any abaxial bone findings.
In yearlings who subsequently went on to 2-year-old sales, 72 percent of Grade 1 vascular channels had disappeared by the time they became 2-year-olds. The few horses who demonstrated abaxial concavity also saw it disappear in the vast majority of cases.
Only horses with Grade 3 lesions saw any significant impact on performance, with Grade 3 yearlings showing a significant reduction in the mean number of starts they made on the track, and a higher age at first start for yearlings who had Grade 3 vascular channels in hindlimbs. Signs of abaxial new bone in forelimbs also had a significant reduction in a yearling's likelihood of making it to the races later in life.
Other parameters – like comparing horses with less severe vascular channel gradings with horses with Grade 0 radiographs – saw no significant differences in likelihood of starting, age at first start
For Kawcak, the only puzzle is why the impact of Grade 3 vascular channels seems to linger so long.
“The question in my mind is why do the Grade 3s still have reduced race starts even though the lesions have improved?” he said. “I think that's where our ultrasound data will be of great importance.”
Kawcak said he is hopeful the rating system created for the purposes of the study could be used in the field to help vets and clients communicate about sesamoid findings in a standardized way that will better express a horse's actual performance limitations or lack thereof.
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