In as many as 80% of cases, doctors don't find the exact reason for a curved spine. Scoliosis without a known cause is what doctors call 'idiopathic.'
Some kinds of scoliosis do have clear causes. Doctors divide those curves into two types structural and nonstructural.
In nonstructural scoliosis, the spine works normally, but looks curved. Why does this happen? There are a number of reasons, such as one leg's being longer than the other, muscle spasms, and inflammations like appendicitis. When these problems are treated, this type of scoliosis often goes away.
In structural scoliosis, the curve of the spine is rigid and can't be reversed.