The most common reason for performing a spinal fusion is low back pain caused by painful motion of the vertebrae. The goal of a spine fusion is to eliminate the motion at a painful motion segment, thus reducing the pain caused by the motion.
This abnormal and painful motion can be caused by painful discs (discogenic pain or degenerative disc disease), abnormal slippage and motion of the vertebra (spondylolisthesis or spondylolysis), or other degenerative spinal conditions, including but not limited to facet joint degeneration. In addition, a spine fusion may be indicated for any condition that causes excessive instability of the spine, such as certain fractures, infections, tumors, and spinal deformity (such as scoliosis).
Ingredients of a Spine Fusion
In order to obtain a fusion certain basic criteria must be present.
- A suitable graft must be available to serve as the bridge to connect the vertebra.
- An appropriate location must be present to lay the bone graft and allow it to heal to each vertebral segment on either end.
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- The bed which this bone graft is being laid in must be prepared correctly and the patient must have the appropriate biology for the graft to fuse. This bed can be prepared by removing the outside covering of the bone (the cortex) and exposing a bed with better blood supply (decortication). It is important that there is contact between the bone and the bone graft and no soft tissue (e.g. muscles, ligaments) in the way.
- There must be adequate fixation to immobilize this area while the bone graft heals to the vertebral segments. This immobilization is usually provided by internal fixation with metallic screws and rods and/or interbody devices such as cages.
Internal fixation of the spine (usually with metallic screws and rods/plates, or interbody cages) serves to immobilize the spine, while the bony bridge heals across the two vertebrae. The degree of immobilization afforded to the spine by internal fixation will not change when the bone graft matures and heals across the two vertebrae. However, if the fusion (the healing of the bone) does not occur, over time the implants will loosen, break or pull out of the bone. This occurs despite the strength of the metallic constructs which are being used today. The term used to describe the lack of fusion after a spinal fusion surgery is pseudoarthrosis.