In recent years more equine practitioners have gained access to cross-sectional imaging, including MRI and computed tomography (CT). These modalities create “slice” images of structures, providing a three-dimensional view and giving veterinarians the ability to diagnose issues that aren’t always visible on standard radiographs. Practitioners often assess show jumping horses’ fetlocks using MRI and CT because this high-motion joint is a common source of pain in these athletes. Cross-sectional images can reveal various fetlock joint, but determining whether these lesions hold clinical significance or simply reflect normal adaptive changes can be challenging.

Annamaria Nagy, DVM, PhD, of the Equine Department and Clinic at the University of Veterinary Medicine, in Budapest, Hungary, and Sue Dyson, MA, VetMB, PhD, conducted a study to document findings on low-field MRI, fan-beam CT, and radiographs (X rays) in nonlame show jumpers in full work. None of the horses they included had a history of fetlock joint disease.

The researchers most frequently noted CT and MRI changes consistent with the densification of the highly porous bone trabecular bone located in the sagittal ridge and/or condyles of the third metacarpal (cannon) bone. Both the sagittal ridge and condyles are located at the bottom (distal) aspect of the cannon bone where it articulates with the long pastern.

They noted this particular change in 53 (85.5%) of the 62 limbs they studied.

“Densification of the trabecular bone of the medial condyles in the dorsal and palmar aspects was very common, and the densification of the lateral condyle was more pronounced in the palmar aspect,” said Nagy. “Usually, the densification was bilaterally symmetrical, affecting both forelimbs.” In other words, Nagy and Dyson commonly saw clear patterns where this bony densification occurred within the fetlock.

On MRI and CT they also found:

• A focal hypoattenuating lesion (dark abnormality on CT) in the dorsoproximal aspect of the medial condyle in three horses that they were not able to detect on radiographs;  
• A focal hypoattenuating lesion in the dorsodistal aspect of the condyle in one limb that they also could not detect on radiographs;
• A focal hypoattenuating lesion in the palmar aspect of the lateral condyle of one horse that they did not detect on either MRI or radiographs;
• Subchondral (found beneath the cartilage) bone plate thickening in the proximal phalanx—the long pastern bone—in 61 limbs, typically in just one limb per horse;
• Sagittal groove indentation in both limbs of one horse; and
• A focal hypointense signal (low-signal intensity on MRI) in the medial aspect of the sagittal groove in one horse.

“Periarticular (around the joint) remodeling was more evident on CT than MRI and radiographs, and no significant soft-tissue lesions were identified in this study,” said Nagy.

“Trabecular bone densification of the third metacarpal bone was common in nonlame show jumpers,” she added. “While the trabecular bone densification likely reflects adaptive changes to exercise, horses could experience pain in those areas due to increased intraosseous (in the bone) pressure.”

Take-Home Message

Although all horses included in this study were sound, some of the lesions in the cannon bone condyles or long pasterns could cause lameness and become clinically significant over time.

“Future studies collecting longitudinal data (from the same subjects repeatedly over a period of time) on these horses and objective comparison of imaging findings for specific abnormalities are needed,” said Nagy. Other research could include performing objective assessments of bone thickening in these horses, she added.