We as horse people usually call the Carpus of a horse the knee. The stifle of a horse would be better described as a knee, however the carpus of a horse is a big problem area for horses that re performing galloping work.
Horses performing fast work can often present with carpus pain, or more commonly known as knee pain. Not quite like our knee but more like our wrist the carpus is a very important structure to understand.
Young racehorses can present with knee pain often resulting in time out of training. Management is a combination of rest or medication depending on the time of the soreness develops.
Each of these xrays are of different horses and all have the common problem of pain originating from the distal radial carpal bone. The distal radial carpal bone is the bone that is subject to the most stress in the galloping horse. Remodelling at this site is something to take serious. In young horses with loss of bone density at this site it is a serious concern. Working horses at this stage can lead to spur formation and in the worst case a chip fracture.
I often find that the effusion (swelling) of a knee joint can be more subtle and often missed. Effusion of fetlocks is very easy to see. So the take home is knee pain is real and is a common problem seen in a lot of horses performing gallop work: Racehorses, Eventers, Polo Ponies and Barrel Racers.
So how do we assess these horses and prevent problems developing? Patience is the first step when you introduce horses to fast work, secondly regular assessment of these horses. Swelling of the knee joint is the first sign of problem but can be subtle. Examination by a vet will often pick this up. Thirdly is regular x-rays of the knees to assess any changes. I recommend all young racehorses have surveys performed of their knees before commencing fast work. A little investment now can help in the long-term soundness of your horse.
Another option to consider is to use a piece of equipment such as a Lameness Locator to regularly monitor your horse: http://www.tevs.com.au/diagnostics-treatments/lameness-locator/