Depends on whether it's an 'active' splint, i.e. has come up as a result of work or something and is changing size/shape, or if it's conformational.
My horse has 2 big splints under his knees (HUGE conformational no-no, I know, but I didn't buy him to show so makes no odds to me, they don't affect him any other way other than in appearance!) and he's had them since birth - I know the breeder and have now had him over a year and they've never changed so I've just left them well alone - they don't affect his performance or soundness.
If you're having soundness issues or they're changing or have suddenly appeared I'd get the vet to take a look. It also depends on how old the horse is, amount of work etc. Lots of variables.
I had a junmping mare last year, she had one splint - that was caused by to much work on hard ground before i got her. though it was alot smaller then. Whilest our menarge was being built i had to school + jump in the feild on her.
It did get a tiney bit bigger, then it just stopped dead. I didnt get the vet f=out for it, as i new it was just pure bone no swelling. Sometimes if she'd been doing alot of work i'd ice pack it for a few hours. But theres not much you can do with them.
I wouldnt worrie, there nothing much to worrie about. Though do be carefull.. I would never get a sj'er with one again,
Rest until sound and bring slowly back into work on soft surfaces. They cause no problem (usually) once they have formed. They are no issue at all except in a show horse. You can try a mild blister to remove it if so, like B-R linament.
Workalin was brilliant for splints (mid blister) but I dont think you can get hold of it now. I still have half a bottle at home which I use if needed.
Global Herbs have a cream and spray in a splint kit. Can't reccomend it enough my horse became sound overnight on it and it helped to reduce it in size as well. It is more effective the earlier it is used though.
Calc Fluor @ 30%, get it from your local health shop & treat splint from inside. Add a few drops to every feed. I was told about this by a lady who used to show. At the time my horse, age 5, had developed 2 huge splints on both front legs while he was turned away. Whether it was the calc fluor or not I don't know but after about 10 months the splints had disappeared & the horse had stayed sound & in work.