The Selection of L5 Versus S1 in Long Fusions for Adult Idiopathic Scoliosis

The treatment of adult spinal deformities often involves long thoracolumbar fusions into the lower lumbar spine, raising the debate of selecting L5 or S1 as the caudal extent of the fusion. The presence of significant deformity or degenerative pathologic findings at L5-S1 mandates fusion to the sacrum. Fusion to the sacrum is of larger magnitude than fusion to L5 and introduces a higher surgical complication rate. Advantages of ending the fusion at L5 include preservation of motion, avoiding the high complication rate associated with fusion to the sacrum, and possibly avoiding a second operation. Complications with fusion to L5 include possible loss of fixation and subsequent disc degeneration at L5-S1, however, leading to possible pain and loss of sagittal balance and the need for revision surgery. To date, the functional consequences of an open disc space beneath long constructs remain poorly defined, and there is no firm evidence in the literature guiding the surgeon's choice. The issues and evidence guiding the decision to fuse to L5 or S1 are examined in this article.