Fri & Sat Feb 4,5. More on the hock

Vet tip of Two Days!
Key words: osteoarthritis, hock,chondoprotective, joint therapy, glucosamine, chondroitin

My apologies for this late vet tip. It's actually Sunday morning, but it is snowing again and I've get an hour before I go out to do a couple of calls this morning. One of them is to inject the carpus of an older rope horse with chronic osteoarthritis. Do you remember how many joints there are in the carpus and which joint on your body is similar anatomically to this equine joint? (for review see last Wednesday's blog).

Osteoarthritis of the lower hock joints (bone spavin) is the most common cause of chronic lameness that I see in my practice, with forelimb heel pain a close second. Bone spavin occurs in every type of athletic horse, and unfortunately it is not uncommon to see horses develop sore hocks at a relatively early age. The 3 or 4 year old cutting horse is the extreme example, but jumpers, dressage horses, foxhunters, endurance horses, and other western sport horses may have degenerative changes in their hocks before 10 years of age.

Why are these joints predisposed to arthritis more than other joints of the body? Probably because of their unique position of high stress and low motion. Watch your horse's hocks closely as he trots on the longe line. Most of the flexion that occurs is due to motion of the large tibiotarsal joint, which has a large joint space and round, ball bearing like sliding surface where the tibia sits on the knuckle shaped talus. This joint is an uncommon site for osteoarthritis secondary to athletic activity.

Now look at the radiograph from Thursday's blog (the normal hock), then look at this radiograph which demonstrates osteoarthritis in the distal intertarsal and tarsometarsal joints. These are the joints at the bottom of the hock that are complex and relatively rigid in construction. The joint space is the thin black line between the rows of tarsal bones. Imagine the force that flows through these joints when your jumper throws his thousand plus pound body into the air to clear an obstacle or your barrel horse digs in to accelerate out of the turn around the last barrel or your dressage horse sucks it up to sit back on his hocks and put that tremendous effort into a canter pirouette. There isn't much room for error in these joints, because there is very little room for movement, so over time, the stress of high force through tightly packed bone results in chronic inflammation.

Joint fluid lubricates and cushions the cartilage that lines bone surfaces. The inflammatory process causes this fluid to becomes thin and less able to absorb repeated impact. The cartilage, in turn, looses its flexibility and ultimately the bone is left to take up too much of the load of activity. It's response is to say "Hey, I'm working too hard, maybe if there is more of me this job will be easier." Unfortunately, the production of extra bone simply destroys the smooth architecture of the carefully constructed joint anatomy with the end result: pain.

That's how the process starts, now how do we treat it?

Many of the key words for this vet tip are related to the huge topic of joint therapies. Tomorrow's vet tip will be devoted to the subject of oral and intra-articular therapies, and trying to separate fact from fiction. To close today's entry, here is the bottom line: there is sound, repeatable scientific evidence that certain oral and injectable chondroprotective agents can help to alleviate the inflammation, and therefore the pain, associated with osteoarthritis of the hock. The efficacy of these products is variable, and the products themselves are variable. We will look at them in more detail tomorrow.

I hope you are enjoying these vet tips - we are in the process of updating the practice website - the blog name is probably going to change (high brow internet search engine stuff that is GREEK TO ME). I promise to keep everyone up to date as the High Desert Veterinarian continues to explore cyberspace in order to share the knowledge and wonderful life of a practicing veterinarian with you all.