Orthopedics

How Useful Are Sesamoid Radiographs During Prepurchase Exams?

By Marie Rosenthal MS

Radiographs of the proximal sesamoid bones could be a useful tool in assessing a 1- or 2-year-old’s future racing career during the prepurchase exam—depending on the criteria used to assess bone changes, according to Christopher E. Kawcak, DVM, PhD, DACVS, DACVSMR, a professor of equine surgery at Colorado State University.

Dr. Kawcak and his colleagues fine-tuned several grading systems to see if they could develop a method that veterinarians can use when evaluating radiographs during the prepurchase examination.

“We do feel this is an easy-to-use grading scheme,” he said.

Consigners and commercial breeders worry that veterinarians often overinterpret radiographic changes without evidence that the changes will harm the horse’s career, which makes sesamoid bone grading contentious because it depends on expert opinion, Dr. Kawcak explained. “It’s hard to discredit opinion because most of these people have such vast experience,” he said.

They wanted to know if there was a way to make the interpretation of the radiographs more objective to better support a veterinarian’s recommendations, according to Dr. Kawcak.

“There is a lot of opinions about what vascular channels mean in the sesamoids, but we identified that we need more evidence to support our [a veterinarian’s] recommendations,” he said.

In 2003, Kane, et al., found no significant effects of sesamoid, lucencies, or vascular channels on the ability of a horse to race. But Drs. McClellan and Plevin later found that some changes that were considered normal might be pathogenic, and more than 2 abnormal linear lucencies in the sesamoid could decrease mean starts in earnings.

“Consequently, radiologic findings are contentious because of inconsistent classification,” Dr. Kawcak explained. “We saw an evolution of classification system since Kane and conflicting assignment of potential clinical significance, which hopefully we can address today. So, we identified this gap in our understanding about sesamoids and identified that we needed to characterize further the significance of sesamoid changes in yearlings and better understand the progression of those changes.

“The study aims wanted to evaluate radiographic appearance to sesamoids in a large number of sales yearlings, define the range of prevalence of sesamoid findings, analyze the association to racing performance, and evaluate changes in sesamoid between the yearling and 2-year-old year and determine which findings are associated with reduced performance,” he said.

They looked at the radiographs of 2,508 horses at the 2016 Keeneland September Yearling Sale and 436 horses presented at five 2017 Fasig-Tipton and Ocala Breeders’ 2-year-old sales.

First, they met with several experts to develop a grading system that would become the standard for the study and 1 that every observer would use.

The consensus was:
• Grade 0 had any number of vascular channels, but they were all <2 mm in width.
• Grade 1 is 1 vascular channel that’s >2 mm for least one-third of the length of the vascular channel.
• Grade 2 was 2 vascular channels >2 mm for one-third of the length.
• Grade 3 was 3 vascular channels >2 mm for one-third of the length.

They also looked at other bone changes:
• abaxial new bone,
• abaxial margin concavity,
• abaxial fragment and
• apical fragment.

“In the absence of vascular lesions, we wanted to evaluate those as a separate entity on their own,” he said.

He admitted that at first it sounded confusing, but once they were trained about what to look for, the landmarks were easy to find and measure.

“These became very easy to grade consistently,” he assured.

They used several statistical analyses to assess their scores and found their observations were consistent.

They viewed 20,000 sesamoids of yearlings and nearly 3,500 for 2-year-olds, and most were grade 0; 14.5% were grade 1, 3.4% were grade 2 and 0.8% for grade 3. There was a significant reduction in the number of grade 3 horses that raced as yearlings compared with grade 2 or lower lesions.

“We basically went from 80% plus chance of racing down to 67%. The grade 3s did appear to have an association with reduced racing,” he said.

They saw a similar reduction in horses with abaxial new bone formation of the forelimbs, which went from a nearly 85% chance of starting down to 77.7%.

“We also had a significant reduction in racing with apical or abaxial fragmentation combined. Most of those were due to the abaxial. We couldn’t pull out significant differences between the 2, but you can clearly see a reduced performance, a reduced chance of racing with those horses that had abaxial,” Dr. Kawcak said.

“And then for those horses that did race, we found some other significant findings in the hindlimb sesamoids. Those horses with grade 3 vascular channels, their mean age to first start was significantly higher than the other 3 categories. For forelimb sesamoid vascular channels, we found that those horses with grade 3 lesions had a significant reduction in their mean grade of number of starts,” he said.

For some horses, they had yearling and 2-year-old radiographs and that produced a surprising finding—many of the lesions improved in the second year, according to Dr. Kawcak.

“What we found is—and this was statistically significant—the lesion grade improved in general between yearling and 2-year-old years [P=0.008]. So as an example, in the grade 1 lesions as the yearlings, basically 72% of them became grade 0. Remember, all of these were graded blinded without knowing who exactly they were as yearlings. Of the grade 2 as yearlings, 48% became grade 0; 36% grade 2. And of the grade 3s, 40% became grade 0; 6.7% grade 1; and 40% grade 2. Also, the abaxial concavity went away—only 23% of those remained as 2-year-olds.

“In general, these lesions do improve from yearling to 2-year-old year,” he said.

“For those that did race, 3 vascular channels in the forelimb sesamoids were associated with fewer starts and a delayed start to racing careers. And abaxial new bone in the forelimb sesamoids was associated with lower total earnings and earnings per start,” Dr. Kawcak said. MeV

For more information:
Peat FJ, Kawcak CE, McIlwraith CW, et al. Radiological findings in the proximal sesamoid bones of yearling and 2-year-old Thoroughbred sales horses: Prevalence, progression and associations with racing performance. Equine Vet J. 2024 Jan 18. doi: 10.1111/evj.14051.
https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.14051