Ring bone is a type of osteoarthritis, an athritis of the
pastern or the coffin joints (which are in all 4 legs), or both.
This is also known as degenerative joint disease. If often occurs
in horses with poor conformation, or use on bad surfaces (such as
galloping on pavement), horses with high stress careers
(racers/jumpers) and can even occur from bad shoing. Its not a
death sentence but there is no real cure. The horse must be
"managed" for the rest of its life and best for it to stick to
light work on soft surfaces.