The Medical billing code L3999 is a very loosely defined billing code. A doctor will most likely prescribe you an orthosis with this billing code if you have a condition with your fingers, wrists, or hands. The L3999 code is used to identify an upper limb orthosis. This type of brace is often used in cases where motion needs to be restricted. Common conditions include, arthritis, sprained thumb/finger, or setting a finger.