Diagnosing Equine Knee and Upper Cannon Bone Injuries

Jumpers’ forelimbs withstand tremendous impact, so the bones of their knees and neighboring ligaments at the upper part of the cannon bone are prone to injury.

Photo: Erica Larson, News Editor

Show hunters’ and jumpers’ forelimbs withstand tremendous impact, so the bones of their knees and neighboring ligaments at the upper part of the cannon bone are prone to injury. It’s important that veterinarians be able to detect and diagnose such insults early, which requires skill and practice, noted one sport horse practitioner at the recent American Association of Equine Practitioners Convention.

At the 2013 meeting, held Dec. 7-11 in Nashville, Tenn., Kit Miller, DVM, Fédération Equestre Internationale-accredited veterinarian and founder of Miller and Associates, in Brewster, N.Y., described how ambulatory veterinarians can identify these lesions using routine imaging modalities.

Miller urged practitioners to use diagnostic anesthesia in order to rule out problems in the distal (lower) limb. Following sequential anesthesia, working from the bottom of the limb upward, if the veterinarian does not localize the horse’s lameness to the lower limb Miller suggested using a lateral palmar nerve block to isolate the source of lameness to the carpal (knee) area.

He listed some characteristics of injuries within the carpal sheath (which surrounds the superficial and deep digital flexor tendons in the back of the knee, extending several inches above and below the knee joint) and the proximal (upper) cannon bone area:

  • Bony lesions might impinge upon the carpal sheath and irritate the deep digital flexor tendon;
  • Not all lesions are centrally located, so imaging should also include medial and lateral (both sides) views, not just the midline, which focuses primarily on the center of the limb as the X ray beam shoots from front to back; and
  • Carpal sheath effusion (swelling) might not always be visible or be discernible with palpation, but it might be apparent on diagnostic ultrasound.

Miller said other imaging modalities besides radiography and ultrasound, including computed tomography (CT) scan, MRI, nuclear scintigraphy, or tenoscopy (which requires a surgical approach with a fiberoptic camera), can help veterinarians identify persistent or recurrent problems that are difficult to diagnose. He recommended taking serial images to compare changes over time.

Speaking to specific carpal and proximal cannon lamenesses, Miller noted that a horse with proximal suspensory desmitis (inflammation of the upper suspensory ligament) often places the affected limb a bit more to the outside of its body. Commonly, the lameness worsens in the days following the initial suspensory injury. Miller urged veterinarians to rapidly identify these injuries, ideally within 10 days, in order to improve horses’ chances of recovering successfully.

“Injury within the proximal superficial digital flexor tendon is especially common in older campaigners,” Miller said. Initially, an affected horse might show an intermittent lameness that worsens under tack until he warms up during rides. Miller said most horses are not sensitive on palpation, and the horse may or may not become more lame with carpal flexion. "This kind of injury tends to be progressive and degenerative,” he said.

Effusion in the carpal sheath is rarely a primary problem, said Miller, and it’s usually related to an underlying issue, either bony or soft tissue. Because of the anatomical proximity, veterinarians often assume this could be linked to a superficial digital flexor injury, but that might not be the case. These horses are markedly lame. He also described instances of soft tissue/tendon injury within the carpal canal (the depression running down the back of the knee) that were not accompanied by effusion.

In conclusion, Miller stressed that a solid working knowledge of carpal and proximal cannon bone region anatomy, coupled with thorough and methodical nerve blocks and imaging techniques, can help a practitioner pinpoint an accurate diagnosis.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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