Why is my dog's ankle swollen?

Answer 1

There are many things that can cause a swollen joint in a dog. The most common, particularly for younger and active dogs, is mild trauma - spraining or straining the tendons or ligaments of the joint. This tends to resolve itself with rest and time, although if your dog is limping badly you should probably take him/her to your veterinarian.

Other causes include infection in the joint, degenerative disease such as Arthritis, auto-immune disease in some cases, and cancer. If you have a large breed older dog, I would be worried about osteosarcoma, a rather unpleasant and mostly untreatable cancer of the bone.