Pseudarthrosis is a non-union of two bones. In essence, it is a fracture that does not unite with nearby bone. During spinal surgery using fusion, the purpose of the tools and instruments used (such as screws, rods, etc.) is to help fuse and unite the vertebrae so that it forms into one solid column. Patients with pseudarthrosis of the spine often exhibit abnormal movement at the site resulting in the union to fail. Often there are other factors that lead to the non-union such as metabolic factors, choice of instrumentation, surgical technique, etc.