Yes, I think the first person confused cannon bone with coffin bone. Definitely not cannon bone haha
Her cannon bones are NOT "dropping down through her hooves"!! Foundering is a form of laminitis which is when the hoof wall moves away from the pedal bone (little bone in the centre of the hoof) the pedal bone normally sits on the same angle as the hoof wall but most often with lamanitis the horses toe will be too long and pull the hoof wall away from the pedal bone, the pedal bone can often drop and put pressure on the sole of the hoof, this is what causes the severe lameness. Diet control is definitely hugely important. I would take your horse off all or most grass to start with but try to keep them in a paddock they can move around in. Also don't forget to provide lots of hay for them, only meadow hay as lucerne hay is very high sugar. Get a really good farrier, your horse won't be in work while he's foundered so I would take his shoes off, this will help him build up thicker souls and better hoof quality. It is difficult to look after a lamanitic horse, it is sort of like equine diabetes. I recommended low sugar diet and once he's sound lots of exercise 😊😊 good luck and I hope this helps 🙂
Her cannon bones are dropping down through her hooves. Most likely needs a special diet and shoeing.