There are different kinds of spica casts, but all of them are used to prevent the hip or leg from moving after an injury or surgery. Most start from just below the armpits and go down to the ankles on one or both legs. In a typical spica cast, your child will not be able to walk. However, the short-term challenges of this type of cast are worth the benefits. By preventing movement at the hip joint and thigh bone, spica casts allow the bones and/or hip to heal properly.