Causes of sagittal spinal imbalance and assessment of the extent of needed correction.

Most patients with spinal sagittal imbalance have a fusion mass that is either kyphotic or hypolordotic, with segments above or below the fusion that have subsequently degenerated. The four most common presentations include a patient who had a long fusion for adolescent idiopathic scoliosis with subsequent degeneration distally; a patient with degenerative sagittal imbalance in whom fusions have initially been performed in the distal lumbar spine in a somewhat hypolordotic or kyphotic position with subsequent degeneration of segments above the fusion; a patient with posttraumatic kyphosis; and a patient with ankylosing spondylitis. The surgical solutions usually involve a combination of osteotomies through the fusion mass and extension of the fusion to include degenerated segments. Most of the correction is accomplished by the osteotomies with additional correction achieved by adding the degenerated segments to the fusion in patients with idiopathic scoliosis, degenerative sagittal imbalance, or posttraumatic kyphosis. For patients with ankylosing spondylitis, the correction is achieved entirely with osteotomies. The usual goal is to normalize the regional segmental spinal alignment as much as possible and to achieve global balance. Global balance is confirmed when the C7 plumb falls over the lumbosacral disk on a standing long cassette lateral radiograph taken with the patient standing with knees fully extended in a natural, comfortable position. Most patients should have at least 10 degrees to 20 degrees more lumbar lordosis than thoracic kyphosis. Usually a Smith-Petersen osteotomy will achieve 10 degrees of correction and a pedicle subtraction osteotomy will produce 30 degrees to 35 degrees of lordization of the spine.