When does a curved spine need surgery?

45-50 degrees. In general, surgery for scoliosis becomes a consideration when the curve reaches 45-50 degrees.
Depends. This depends on multiple factors including: the kind of curve ie the cause, the location, the size or magnitude, neurologic status----your age, your underlying health including your bone density and nutritional status as well as underlying medical problems and the risks of no treatment v surgery.
Surgery. Depends on extent of curve, any complications, general health, etc. Consult your orthopedic surgeon for more info.