Ah, hock injections. We all hear about them, know about them, possibly have even had them done on our show horses. Here’s the nuts and bolts of the situation:
The “hock joint” in the horse is actually comprised of several separate joint spaces – while they have fancy medical names, basically most vets refer to them for injection purposes as the lower, middle and upper hock joints.
Just briefly mentioning the upper hock joint, it is the one responsible for most of the motion of the hock joint – when the hock bends, this is the joint that makes that happen. It is pretty uncommon for show horses to have primary upper hock issues, but it can happen. OCD of the upper hock joint can also be a signifciant issue in young horses just coming into training – usually presenting as a very swollen hock joint once the training process has started. OCD’s should be surgically removed, and upper hock joint injections can be done to keep the horse comfortable.
In stock breed horses, the most common place that is injected when people say they are “having their horse’s hocks done” are the lower and middle hock joints. The lower and middle hock joints are two very low motion joint spaces. Basically, they are comprised of several tarsal (hock) bones that are stacked one on top of the other like Jenga blocks, and these bones happen to have some cartilage between them and synovial joint capsule surrounding them. While they are technically joint spaces, they only comprise between 2 and 4% of the motion of the entire hock joint. Basically, they have just enough cartilage and joint fluid to slide around on top of each other a little bit as the horse puts its foot on the ground.
If you can imagine, these lower and middle hock joints undergo a huge amount of stress and strain everytime the horse puts its hind leg into the ground. Genetics, the repeated forces, and the things that we as pleasure horse enthusiasts ask of our horses, make those tarsal bones twist, slide and grind over each other.
Cartilage is kinda like a Teflon coating. When a horse starts to have thinning and degeneration of the cartilage in the lower and middle joint spaces, pain is often the result because it is no longer the Teflon coating sliding over each other, but actual bone grinding on bone. Horses will also start to form bone spurs along the edges of the bones. This is the beginning of the fusion process in the lower and middle hock joints.
To help relieve pain during the degeneration process, injection of the lower and middle joint spaces is often done. Like I mentioned in the article, trying to encourage the fusion process in the lower and middle hock joints is a good thing – if we can get them to fuse, they don’t slide, and therefore they don’t hurt anymore.
All veterinarians will have different combinations of medicines to use in joint injections. In a young horse in training – say a 2 or 3 year old – if the xrays are showing no damage to the cartilage or spur formation, but the horse is sore in the hock joints, I will reach for an HA (hyaluronic acid) product possibly combined with triamcinalone (TA) which is more chondroprotective than the other steroid choices.
If a horse is older and already showing signs of the fusion process beginning, I may reach for Depo Medrol only. Depo is a great drug for these types of joints because it has longer acting pain killing properties and it is thought to speed up the cartilage degradation process and may help speed up the fusion process.
Horses that are not helped by the steroid injections may be helped with ethyl alcohol injections in fusing lower and middle hock joints. This is a relatively new procedure that is being done, and must be done after plain xrays and appropriate dye studies of the lower and/or middle hock joints are done, proving that the joint spaces are not connected to the upper (high motion) joint. In a small percentage of horses, they may have communicating joint spaces – so while getting a little steroid in the upper joint during a routine steroid or HA injection is not a problem, getting alcohol in that joint space would be devastating and possibly life ending for the horse.
The ethyl alcohol works in two ways – almost immediately it has an amazing pain-killing property and the horses are much more comfortable. The alcohol also causes a chemical reaction in the joint space and eats the cartilage, therefore speeding up the fusion process. More and more veterinarians are becoming familiar with and are performing or referring out for this procedure.
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