This condition is observed only in young growing chickens, usually those under 6 weeks of age. The twisted leg condition is referred to as "perosis" and is caused by a nutritional deficiency of a vitamin named "choline." This vitamin is important in the formation of cartilage that maintains the stability of the hock or ankle joint.
Perosis appears in only one leg of each affected chick. It is a completely different problem than when both legs are damaged, as in "spraddle legs." Spraddle leg is commonly caused by injuries that a chick receives when other chicks pile upon it or when the chick is placed on slick flooring soon after hatching.
Older chickens do not suffer from perosis because the bodies of mature birds develop the ability to manufacture or synthesize choline, but young chicks cannot. It is important to provide starter and grower diets that contain adequate amounts of all vitamins, especially choline. Water containing vitamin supplements will ensure that perosis does not develop in young chicks."