How can a spinal fusion slip forward?

Pseudoarthrosis: If a bony fusion doesn't heal properly (pseudoarthrosis or failed fusion) then it may slip forward over time. It may also cause pain and result in broken hardware. This is more common in tobacco users, diabetics, obesity, and other conditions. A ct scan can be used to assess the fusion. Revision spinal fusion may be necessary.

Failure to fuse: The goal of fusion surgery is to cause bones of the spine to grow together. If a fusion is successful, then two or possibly more bones joined as one. Usually this is accomplished by removing the disc between each two bones being fused and replacing it with something to stimulate fusion of the bones. If the fusion does not occur, then the superior vertebrae may slip forward (called listhesis).

Varies: If a fusion is solid and the alignment of the spine is well maintained; it would be highly unusual for a fusion to "shift forward." The most common reasons are either if the fusion did not ever fully heal, which is known as a nonunion; or if the alignment was not restored, then the malalignment that can "bend" an otherwise solid fusion.