Lumbar fusion is commonly performed to relieve pain from degenerative conditions, including spinal stenosis and spondylolisthesis. While clinical studies have reported favorable fusion rates with limited complications, few have investigated the effect of fusion on the adjacent motion segment. A solid fusion alters the biomechanics at the adjacent level, resulting in increased mechanical demands. There have been reports of increased rates of adjacent-level pathologic lesions after fusion, but these have not accounted for the natural history of degenerative changes. Biomechanical and radiographic studies have shown increased forces, mobility, and intradiscal pressure in adjacent segments after fusion. It has been hypothesized that these changes lead to an acceleration in pathologic changes.