My friend just bought a new horse so she and I could go trail riding together, and we have noticed that there is something strange about her legs. When she stands, her front right leg quivers. This only worsens when someone is on her. I have narrowed it down to something being wrong in her cannon bone or knee, but I'm not quite sure because whatever it is, it's a very old injury. What makes it worse is that we really don't know what this mare did in her previous home. My friend picked her up dirt cheap from a place that just takes horses and turns around and sells them... so there could really be any number of things. The leg does not produce heat and is not swollen. She is over at the knee and I know that this can cause a horse to trip and stumble... but she is only slightly over and has very upright pasterns and otherwise good conformation. Today she was trotting in the round pen with no one on her and her leg just gave out, causing her to fall down and hit her head pretty bad. I am not very familiar with common lower limb disorders that can be caused by sprung knees, so does anyone have any ideas what this could be? We were kinda thinking it could have resulted from a bowed tendon.... but there is no lameness or swelling. She has full rotation and no pain, however her knee does occasionally grind when manipulated and she has an injury on the cannon bone that resembles a splint or a callous on the bone. Any ideas would be great!