Sometimes, there are add-ones needed for prosthetics and orthotics that do not come with the initial prosthetic. The medical billing code L9900 is used for no specific condition. When a doctor prescribes something with an L9900 medical billing code it is likely an add-on to an orthotic to make the fit better. The medical billing code L9900 is used by insurance companies to identify specific aspects of each piece. To see more regulations for this L-code continue below.