Ferula is a charm used to bind and splint broken limbs. It apparently eases pain as well. It was used by Remus Lupin in 1993, who cast this spell on Ron Weasley. Lupin used the spell to conjure a wooden rod and bandages to help support Ron's broken leg. The spell is described as "bandages spinning ... snapping tightly to a splint".
Etymology
Latin ferula meaning "a stick".