Join Date: Aug 2013
Location: New Brunswick Canada
How old is your horse? My old mare eventually had a knee twice, three times that size when she was in the late stages of arthritis. It started when she was about 15 years old with mild swelling in the knee, eventually looking like yours, all the time. The mild swelling came out of nowhere- no heat, no lameness. Vet said it's just one of those things, between genetics, her extremely hard, early started using life before us and her general hot, active lifestyle she had some arthritis coming somewhere, eventually. By the time she was 28 it had nearly completely calcified and she walked with a stiff-kneed hitch in her step but was generally still active and with meds, other vet interventions and supplements she was still pretty pain free. We did injections a few times and the vet opted out of it due to what he was assuming was some moderate allergic reactions and inflammation, followed by a weird lameness.
But from all I learned with her, and with a few other joint injuries- joint stuff definitely warrants a pretty quick vet visit. I would send the photo to your vet and see what they say.