Duck With Leg Issues

We acquired two ducks from a friend that didn't know what they were doing.  They bought ducks from our local feed store and expected to just put them in their pond and be done with care.  The ducks are four weeks old and have not been given niacin at all.  We picked them up yesterday and after we saw the larger ones legs knew that it had not been given the proper nutrients.  We went straight to the store and got niacin.  My question is, at 4 weeks and already showing signs of leg issues are we too late to help?!  Is there something else I can do to help it out?  Thanks for any and all help that you guys can provide!

We had a Pekin years ago that had a leg broken, probably by a snapping turtle. We called him "Hoppy." He did fine hopping everywhere he went until something finally killed him.

We added niacin to duckling rations after seeing some leg issues, mostly quivering shakes. The ducklings came around nicely and recovered. We did give them higher doseage of the niacin for a few days initially. Found that the best way to give it to them was to mix it in their wet mash as they splashed way too much water around so never sure if they were getting enough by drinking.