A fusion is designed to join two or more segments together permanently. It is indicated when the intervening disc is the source of back pain or if there is obvious instability or movement across the level. This procedure is commonly performed with titanium screws connected to rods. At the time of surgery, bone graft will be placed to enable a bony fusion. In time, this will provide the strength of the fusion. Alignment and curvatures can also be corrected with instrumented fusions.