If you have ever seen a horse with a large, movable swelling at the point of the elbow, it is likely a capped elbow. It's also called a shoe boil, although it does occur in barefoot horses. These swellings are filled with fluid and occur due to trauma from the heel of the foot hitting the back of the elbow when the horse lies down. The trauma of the heel causes a bursa or fluid-filled sac to develop at the back of the elbow, and a similar condition can occur at the point of the hock called a capped hock or the front of the knee. In fact, the same condition can occur in dogs at the elbow but is called a hygroma and is due to lying on hard ground.
Treatment of this condition is very difficult as draining the swelling with a needle will remove the fluid, but because a membrane is producing the fluid, it will generally recur. Also, sticking a needle in the swelling must be done with a sterile technique or the bursa will get infected and then the problem will be worse. Some veterinarians have opened the bursas and used drains to allow drainage while others have opened them and cauterized the tissue with strong iodine to prevent recurrence. The bursas can be removed surgically but it is difficult to keep the sutures in because if the horse lies down, the pressure on the suture line causes the sutures to come out. So after surgery, horses have to be tied up for at least three weeks to prevent the sutures from pulling out. One method to prevent the bursa from enlarging is to place a doughnut-shaped object called a shoe boil roll on the horse's pastern to prevent the heel from touching the elbow as the horse lies down. If your horse develops a small swelling at the point of the elbow, call your vet so treatment can be considered while the swelling is still small.