Prognosis is guarded.
I've had two horses with collateral ligament damage. One had to be put down, the other is back on light work.
The first was my 8yr old TB eventer. He ruptured his medial collateral ligament. It took a long time to diagnose and this resulted in complications and the collapse of the coffin joint. After nine months box rest and much consultation with the vet we made the decision to call it a day. He wasn't even sound in hand in walk.
Fast forward five years and my new TB has been diagnosed with chronic wear of the collateral ligament. Prognosis is guarded but more positive than last time. He had six months off and is now back in light work. He can't jump and is unlikely to compete again but he is sound.
Prognosis depends on how quickly the prognosis was made; the level of injury and the individual horse. There can be problems of reoccurence although my vet is hopeful that now we've got mine sound as long as I don't push my luck he will stay that way for awhile.
My horse had this with a guarded pognosis. He did come sound again but I was advised only to ride him on good ground so he just does dressage now (used to event). There have been loads of different posts on this so may be worth doing a search.
I have a horse who had damage to the collateral ligament of the navicular bone, and fluid distension of the DIP joint. Not exactly the same as your case, but the horse did become sound again.
What I gleaned from the professionals involved was that key to the recovery of any collateral ligament damage in the foot, is completely perfect foot balance. If you have a farrier you trust, then fair enough, but in our case we put the horse in the hands of an Equine Podiatrist (DAEP) and went down the barefoot route.
I am assuming you have had an MRI scan, to have this diagnosis. Did the report make any recommendations? I have to admit ours recommended 3 more months box rest, followed by reassessment and possibly further box rest. As this was on top of the 6 months she had already been confined, I would rather have had her PTS than suffer any more, so once she was more comfortable I turned her out to the care of "Dr Green", and she made steady progress until she was back to moderate competition level.
My biggest problem with this is that trying to get a good farrier is proving difficult as we are not on a yard and therefore the attitude tends to be its not worth the bother to them as its only a max of 4 trims which is a real shame as that is exactly what my mare needs. Does anyone know of a good farrier in Edinburgh that can help? She was first diagnosed with this in 2005 before we bought her and she came sound enough to hack gently. However I am unsure what has happened to her to make this recur