Everything speckledhen says is true, but being the patron saint of lost causes myself, I'd suggest you try something a little crazy. Take a tiny plastic cup and an egg carton, and cut out a "seat" in eg carton where the legs can fit through in the right position. Put the chick down inside it and make a tiny waterer from a medicine cup, and a tiny feeder from another.
The goal here is to get the leg straight underneath her and let her put weight on it gently.
Two things should be obvious here:
#1. You will need to pay a great deal of care on positioning for heat.
#2. You will need to constantly attend to the water/heat needs. Also if the vent isn't clear pasty butt will be a constant problem.
You are probably better off following speckledhen's advice but I have been there, picking up runts and dragging half starved chickens home in the dead of winter because, well, because.
Remember that it is highly likely that your attempts will end in failure, regardless of your effort, and please, if the chick shows signs of pain (pinch the wing of one gently to hear the pain chirp) don't extend its pain to ease yours.
Originally Posted by speckledhen
I've been here a few times because I have one hen who will throw a percentage of her chicks as dwarfs and/or with slipped tendons and other serious hock issues. Let me try to be tactful here--after many times trying many things with several chicks with serious hock issues, we no longer try to fix them, if you get my drift. It is my considered and experienced opinion that 99% of these defects cannot be fixed. You are welcome to try, of course, but if nothing works, I don't want you to beat yourself up about it. The chick may also be in a fair amount of pain, which is sometimes hard to assess.
I have one D'Anver chick who hatched with what I believe is a slipped tendon. I'm giving it 24 hours to resolve on its own, in case it's just due to him being crammed in a tiny egg, however, if it does not, because of all our past experience with this, we will euthanize him tomorrow if he isn't up on his feet by then.
I have raised a bird who was injured as a young male and ended up with permanent tendon damage at the age of about 20 weeks old (Zane, who died suddenly about a week ago at going on 5 years old). I will not do it again. There is more to it than you realize. I do, however, wish you luck with the chick, sunny. Seems a shame for them to make it all the way to hatch, then to come into the world with a defect such as that, so I understand your desire to help.