Spine Anatomy
The spine consists of the spinal cord (which carries nerves from the brain throughout the body) and bones called vertebrae which surround the spinal cord and protect it. In between the vertebrae are structures called discs, which support and cushion the vertebrae. Neighboring vertebrae are connected via small bony outgrowths called facet joints. Thus, a vertebral joint consists of two vertebrae connected by facet joints and separated by the disc. Sometimes these discs, which normally have a lot of water in them, get dried out, which causes the joint to become stiff and painful. Other times one vertebrae will slip out of the facet joint, which can lead to pain, inflammation, and pinching of the spinal cord. One way of fixing this problem is with spinal fusion surgery.
Lumbar Fusion
Lumbar fusion is the process of fusing together two vertebrae at the bottom of the spine (the lumbar region). Essentially, lumbar fusion is a surgery in which two lumbar vertebrae are fused together using bone to eliminate the damaged joint. The bone may come from a cadaver (which is called an allograft) or from the patient's hip bone (known as an autograft). Sometimes a synthetic bone is used instead. Regardless of the graft material, it is put in between the two vertebrae and held in place using rods and screws. This eliminates the vertebral joint entirely and gives the surrounding structures more support.
Effects
Although spine fusion can help relieve pain and instability from a damaged vertebral joint, it also eliminates the flexibility at that joint. This means that the back can no longer bend at that point. When only two vertebrae are fused together it does not cause significant loss of motion in the back (particularly in the lumbar portion of the spine, which is not particularly flexible). However, fusion of more than two vertebrae can severely impair the spine's flexibility; for this reason, fusion of three or more vertebrae are rarely done except in cases of scoliosis or if the vertebrae themselves are deformed.
