Adjacent-segment degeneration after lumbar fusion is one of the more serious long-term results being reported. Usually the segment above or below is affected. Studies report anywhere from four to 17 percent of the patients experience this problem.

Some doctors argue that this new degeneration is part of the ongoing aging process and would have happened anyway. Others claim that aging can only be blamed when a segment goes bad that isn't right next to the fusion. Still others say that aging may be part of the formula but that a change in the mechanical stresses on the low back after fusion is a major factor.

Other risk factors include fusion at more than one level, uneven spinal alignment after fusion, and injury to the nearby facet joint. The facet joint gives the spine its motion. Any deformity or changes in the joint can lead to degeneration and spinal instability.