Not to pick brains - just to show progression: Left hind ringbone Xrays August 2009 and November 2011 August 2009 November 2011:
The Vet's best guess is at sometime in the past, he took a mis-step landing with the foot folded back and tore loose some ligament attachments at the point of ringbone. There is also some medial ringbone - but we did not Xray this medially in Nov for some reason.