This was a mare at our rescue, she had a canker that swelled up and broke off the heel of her hoof. Rood+Riddle came and did an experimental surgery to remove the remainder of the canker. It worked. We kept it in a boot and soaked it regularly in hydrogen peroxide. It grew back completely, the frog is a bit mishappen, but other than that it's a solid, working hoof. She's completely sound on it!