With six bones articulating in close range and multiple tendons and ligaments controlling extension and flexion, the hock, or the horse equivalent of the human ankle, has many moving and shock-absorbing parts. Add to those the animal’s weight and the fact the joint is almost always in flexion, and you’ve got a recipe for a perplexing number of injuries. And, because each hock lameness case is as unique as the horse it beleaguers, it isn’t always easy to define the cause of the lameness, one veterinarian says.

Jerry Black, DVM, of Colorado State University, shared what he's learned about diagnosing unusual hock lameness cases at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn., by describing two scenarios he considers noteworthy.

Case 1: First, Black described a case involving a 2-year old Quarter Horse filly that suffered a kick-related injury to her right hock. The filly's attending veterinarians discovered mild soft tissue swelling and increased fluid within the hock joints, but they couldn't identify any specific injury on radiographs or ultrasound.

The filly's lameness worsened and her hock became more swollen by one month post-injury, so veterinarians opted to perform additional radiographs, on which they identified a chip fracture off the point of the hock. However, Black explained, such a chip fracture did not correlate well with the degree of lameness and swelling the horse exhibited. While chip fractures can cause lameness and some swelling, the filly's clinical signs were far more severe and progressive.

To further their diagnostic quest veterinarians sent the filly for an MRI, which provided a clearer picture of what was behind the lameness and swelling: multiple small fractures on the talus (the largest bone in the hock joint) with increased fluid in the bone from trauma, along with an area of bone resorption (breakdown) on a weight-bearing surface of the tarsocrural joint. They gave the filly a moderately poor prognosis for athletic function and she was sold as a breeding prospect.

Black asked the relevant rhetorical question in this case: “If there is no MRI is available, how does one arrive at a definitive diagnosis?”

Case 2: Next he described a case involving an 11-year old Warmblood dressage mare with a prior history of hock and stifle lameness. Veterinarians determined the mare had Grade 2 lameness (on a 5-point scale) in addition to moderate swelling in the stifle joint and moderate positive responses to full limb and stifle flexion tests.

Veterinarians had attempted treating with an anti-inflammatory stifle injection, however the mare didn’t improve. She didn’t tolerate regional limb anesthetic nerve blocks without endangering the practitioner's safety, so veterinarians opted to pursue nuclear scintigraphy.

At that point they identified a region of increased radiopharmaceutical uptake in the mare's upper cannon bone, a pattern that is suggestive of proximal suspensory disease. Diagnostic ultrasound confirmed a moderately enlarged suspensory ligament with possible avulsion (pulling away) of cannon bone material where the ligament attaches to the bone. The veterinarians identified moderate osteoarthritis of the tarsometarsal joint (a lower hock joint) on the radiographs, as well.

“Pain with proximal suspensory ligament disease is often bone-related pain,” Black relayed.

Veterinarians treated the mare with three weekly regional limb perfusion treatments using tiludronate (a drug designed to inhibit bone resorption) and injected stem cells into her proximal suspensory ligament using ultrasound guidance. After six months of rest following treatment, the mare regained soundness.

Black said this case is an example of the difficulty veterinarians sometimes face when diagnosing hind limb suspensory problems. Even in cases where a horse is accepting of diagnostic nerve blocks, “it’s still difficult to make this diagnosis because many nerve blocks are not as specific as hoped for," he said.

"Nuclear bone scan is a useful diagnostic tool for these cases.” Take-Home Message

Both these cases demonstrate that being open minded and persistent in tracking down a source of lameness can yield results. Black emphasized that practitioners might need to employ various imaging tools in order to reach accurate diagnoses. Then they are best able to discuss possible treatment plans and realistic prognoses with the owner.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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