Ultrasound of equine deep digital flexor tendon

Horse hooves might be hard on the outside, but they contain many soft-tissue structures within. When horses sustain injuries to these structures, veterinarians rely on imaging to diagnose. Unfortunately, said Myra Barrett, DVM, MS, Dipl. ACVR, “the foot is particularly challenging to image—there are only so many ways we can see it.”

The gold standard is MRI. However, it’s cost-prohibitive for some owners and not all veterinarians have access to a unit. Radiography can reveal problems in the hoof, but it’s more useful for bony structures than soft tissue. And ultrasound is useful for imaging tendons and ligaments in other areas of the body, but it hasn’t been clear how it stacks up to MRI for imaging the hoof.

So Barrett, an assistant professor of veterinary diagnostic imaging at Colorado State University’s (CSU) College of Veterinary Medicine and Biomedical Sciences, in Fort Collins, and colleagues set out to compare the two. She presented the results of her prospective study at the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas.

The researchers performed ultrasound on horses that underwent MRI at CSU to compare visible injuries to the deep digital flexor tendon (DDFT), collateral sesamoidean ligament (CSL), and the navicular bursa (NB) within the hoof. They placed the ultrasound transducer between the horses’ heel bulbs to view the internal structures.

Barrett said they hypothesized that ultrasound would detect fewer lesions than MRI, which could cause veterinarians to underestimate the severity of the injuries.

MRI of equine deep digital flexor tendon

The team evaluated 70 ultrasound and MRI exams of 45 horses. Compared to MRI, they found that:

  • Ultrasound allowed veterinarians to diagnose DDFT tears with an 85% sensitivity (the ability of a test to correctly identify those with the ailment), 60% specificity (the ability of the test to correctly identify those without the injury), and 70% accuracy. With it they accurately diagnosed 36% of mild lesions, 74% of moderate lesions, and 100% of severe lesions;
  • For diagnosing NB effusion (swelling), ultrasound’s sensitivity was 65%, specificity was 69%, and accuracy was 67%;
  • For evaluating NB proliferation (excess tissue growth) its sensitivity was 47%, its specificity was 78%, and its accuracy was 61%;
  • Ultrasound allowed practitioners to diagnose CLS enlargement with had a 42% sensitivity, 78% specificity, and 61% accuracy; and
  • DDFT tearing below the navicular bone was evident on MRI in 27 limbs, 20 of which also had DDFT damage distal to (above) navicular bone that was apparent on both MRI and ultrasound.

Barrett noted that they did find what’s called a learning effect in one area—veterinarians’ accuracy in evaluating NB proliferation increased from 49% to 74% during the second half of the study, as they became familiar with the technique.

She also cautioned that it’s common for horses to sustain both soft-tissue and bony injuries within the hoof simultaneously, and it can be difficult to differentiate them on ultrasound.

Ultimately, Barrett said, the team determined that ultrasound is a useful screening tool for assessing DDFT lesions, especially moderate to severe ones and those that occur above the navicular bone, but it could cause veterinarians to underestimate NB and CSL lesions.

“There are just going to be some lesions we aren’t going to detect using ultrasound,” she said. “Those are the inherent limitations of ultrasound and limitations of imaging within the hoof capsule.”