In 80 to 85 percent of people, the cause of scoliosis is unknown; this is called idiopathic scoliosis. Before concluding that a person has idiopathic scoliosis, the doctor looks for other possible causes, such as injury or infection. Causes of curves are classified as either nonstructural or structural.
Nonstructural (functional) scoliosis–A structurally normal spine that appears curved. This is a temporary, changing curve. It is caused by an underlying condition such as a difference in leg length, muscle spasms, or inflammatory conditions such as appendicitis. Doctors treat this type of scoliosis by correcting the underlying problem.
Structural scoliosis–A fixed curve that doctors treat case by case. Sometimes structural scoliosis is one part of a syndrome or disease, such as Marfan’s syndrome, an inherited connective tissue disorder. In other cases, it occurs by itself. Structural scoliosis can be caused by neuromuscular diseases (such as cerebral palsy, poliomyelitis, or muscular dystrophy), birth defects (such as hemivertebra, in which one side of a vertebra fails to form normally before birth), injury, certain infections, tumors (such as those caused by neurofibromatosis, a birth defect sometimes associated with benign tumors on the spinal column), metabolic diseases, connective tissue disorders, rheumatic diseases, or unknown factors (idiopathic scoliosis).
Source: National Institute of Arthritis and Musculoskeletal and Skin Diseases.