Some students in programs led by teachers with special education degrees have physical disabilities that limit mobility. Also known as orthopedic impairments, these physical disabilities of the muscles or bones are severe enough that they can negatively affect a student's educational performance. There are many types of orthopedic impairments, including cerebral palsy, muscular dystrophy, and juvenile rheumatoid arthritis. Other physical impairments are caused by congenital anomalies or diseases like bone tuberculosis. One of the most common orthopedic impairments is cerebral palsy. This condition is caused by an injury to the part of the brain that controls the ability to use some muscles, if not the whole body. The injury is related to birth, and can happen before, during, or after the birthing process. There are degrees of cerebral palsy. A mild case means that someone has some balance or coordination problems, while a moderate case means that someone walks with a limp, has a special leg brace, or perhaps uses a cane or wheelchair. A severe case of cerebral palsy means that nearly all physical abilities are affected, and those afflicted often use a wheelchair. Sometimes, students with cerebral palsy suffer from a visual or hearing impairment, have intellectual disabilities, or have other learning problems. Lastly, muscular dystrophy is a hereditary condition that results in the degeneration of muscle fibers, which often affects the ability to walk and usually gets worse over time.