Most curves can be treated with a posterior procedure in which the surgeon uses segmental instrumentation and fusion. This type of surgery generally works best for thoracic curves and for double curves. A posterior approach is easier for the surgeon and offers the advantage of allowing two rods to be placed which obviates the need for external immobilization with a brace or other device. Traditionally, some special curves – such as thoracolumbar and lumbar deformities – have been treated with anterior approach because it offers the advantage of fusing the fewest levels possible, and the ability to derotate the spine and provide three-dimensional correction of spinal deformity. Spine surgeons have applied anterior instrumentation to thoracic curves, but this not a popular practice these days.