Your dog's overprotective ulna bone tries to protect the bony part of the elbow that hits the ground first. With every landing on a hard surface, the ulna tells the skin to get thicker and thicker, and pretty soon there is a callus. Enough hard landings can cause the callus to become cracked, bloody, infected, or ulcerated.To top it off, every time your dog slams down on a hard surface, the callus is made thicker. or recreated.