Well, in my case, there was obviously a problem other than just plain arthritis in the stifle,(or hock) he came up lame all of the sudden. Some people inject hocks just because they think they need to. (which I consider maintance) I don't know how they they specifically consider which is maintance or an injury, I guess that would be the vets call. My vet is the person who referred me to Wilkins because they do pay for injections.