just joint growth. Some vets will say box rest but it's better to keep foalie to about 3 hrs turnout every other day to allow joint to work but not be over extended (otherwise when you let foalie out it goes manic and re-does the damage)
you can get goldshield gel - it's a mix of arnica and witch hazel - about 5quid per tub - put some of that gel on each stifle twice per day.
it IS better if foalie rests the joints whilst swollen - so 24 hr turnout is NOT good idea.
also watch out for the protein in the diet - if you give too much protein you end up with joint problems in some breeds and stifle is a joint. I feed pasture mix and add a vit./mineral supplement and have no joint problems. Much of the 'Stud Mix' stuff is aimed at TBs and seriously large warmbloods, - i.e. not for 'normal' pony or riding horses.