What is the best thing to do when the leg joints got sprains? And how do the horses acquire these sprains? Is it true that they could get joint sprains from kicking walls or doors of the barn?

I think for an animal or a human if you misuse the joint even on accident it can cause a sprain, then again I am not a vet just a person that has been accident prone before. :D

Many things can cause a sprain, from overwork to a fall. I would imagine that kicking a wall would also be a risk as it would exert too much force on the joint.

Treatment would be rest, and cool packs if the joint it obviously hot and swollen. It can cause a fair sized problem if not treated.