Scoliosis uses the 45 to 50° threshold as a result of the well-documented, plus or minus 5° measurement error that can occur while measuring Cobb angles. Surgeons who are specialized in spine surgery perform surgery for scoliosis. To completely straighten a scoliotic spine is usually impossible, but for the most part, significant corrections are achieved. The two main types of surgery are: One or both of these surgical procedures may be needed. The surgery may be done in one or two stages and, on average, takes four to eight hours. A 50-year follow-up study published in the "Journal of the American
Supernumerary body part considered to have lost much or all of their original function through evolution. These body parts can be classed as additional to the required functioning of the body. In human anatomy the vermiform appendix is sometimes classed as a vestigial remnant. Prosthesis is an artificial extension that replaces a body part, and cybernetics is the study of computer technology in relation to organisms which can include replacement or additional body parts. Body integrity identity disorder (BIID) is a psychiatric disorder in which a person thinks that they have one or more additional limbs than they should, despite having two arms