A crutch is a mobility aid, made out of wood or metal, that transfers load from the legs to the upper body, generally through the armpit or the forearm depending on the design and needs of the patient. They can be used to compensate for either a temporary disability (such as having a broken leg in a cast) or a permanent disability (such as a lack of muscle strength in the legs due to muscular dystrophy).
Chase has to use crutches after losing the use of his legs in Nobody's Fault.